Review of current progress in inorganic hole-transport materials for perovskite solar cells

Applied Materials Today 14 (2019) 175–200

Contents lists available at ScienceDirect

Applied Materials Today journal homepage: www.elsevier.com/locate/apmt

Review

Review of current progress in inorganic hole-transport materials for perovskite solar cells Rahul Singh a,∗ , Pramod K. Singh b , B. Bhattacharya c , Hee-Woo Rhee a,∗ a b c

Polymer Materials Lab, Department of Chemical and Biomolecular Engineering, Sogang University, 35 Baekbeom-Ro, Mapo-Gu, Seoul 121-742, Republic of Korea Material Research Laboratory, School of Basic Sciences and Research, Sharda University, G. Noida 201310, India Department of Physics (MMV), Banaras Hindu University [BHU], Varanasi 221005, India

a r t i c l e

i n f o

Article history: Received 20 August 2018 Received in revised form 3 December 2018 Accepted 10 December 2018 Keywords: Organic/inorganic perovskite Copper Nickel Molybdenum Graphene Inorganic materials

a b s t r a c t Plenty of options for inorganic electron transport materials (ETMs) for perovskite solar cells (PSCs) are available. However, most hole transport materials (HTMs) is of organic nature. Organic materials are less stable as they are easily degraded by water and oxygen. Developing more variants of inorganic HTM is a major challenge. Till date, many materials have been reported, but their performance has not superseded that of their organic counterparts. In this review article, we look into the various inorganic HTMs that are available and analyze their performance. Apart from stability, their performance is also a concern for reproducible parameters of device performance. CuSCN, NiOx and MoS2 based PSCs are highly stable devices, maintaining power conversion efficiency (PCEs) over 20% whereas, number of devices made from CuI, CuOx , CuS, CuGaO2 and MoOx but shows low PCEs below 20%. Recently, HTM-free carbon/CNTs/rGO based PSCs shows promises for commercialization. Inorganic HTMs is overcoming the stability and cost issue over organic HTMs, various techniques, their novelty is shown in this work which will contribute in paving a path for synthesizing the ideal inorganic HTM for PSCs. © 2018 Elsevier Ltd. All rights reserved.

Abbreviations: PSCs, perovskite solar cells; HTMs, hole transport materials; HTLs, hole transport layer; ETMs, electron transport materials; PCEs, power conversion efficiency; DCB, 1,2-dichlorobenzen; Spiro-MeOTAD, 2,2 ,7,7 -tetrakis-(N,N-di-pmethoxyphenylamine)-9,90-spiro-biuorene; t-BP, 4-tert-butylpyridine; DEH, 4-(diethylamino)-benzaldehydediphenylhydrazone; Al2 O3 , aluminium oxide; Al, aluminum; NH3 , ammonia; (NH4 )6 Mo7 O24 ·4H2 O, ammonium paramolybdate; Ar, argon; Eg, band gap/energy gap; Br, bromine anion; C, carbon; CNTs, carbon nanotube; CVD, chemical vapor deposition; Cl− , chlorine anion; H2 PtCl6−x H2 O, chloroplatinic acid hydrate; Co, cobalt; CB, conduction band; CuI, copper iodide; Cu2 , copper(I) oxide; CuSCN, copper(I) thiocyanate; Cu(NO3 )2 ·3H2 O, copper(II) nitrate trihydrate; CE, counter electrode; CuO, cupric oxide; DMSO, dimethyl sulfoxide; DMF, dimethylformamide; DSSCs, dye sensitized solar cells; QDSCs, quantum dot solar cell; FA=CH3 (NH2 )2 + , formamidinium cation; EDTA, ethylene diamine tetra acetic acid; FTO, fluorine doped tin oxide; g-GBL, gamma-butyrolactone; Au, gold; PF6 − , hexafluorophosphate anion; HiPco, high-pressure carbon monoxide method; HCl, hydrochloride; H2 , hydrogen; H2 O2 , hydrogen peroxide; ITO, indium doped tin oxide; I− , iodine anion; FeI2 , iron(II) iodide; IPA, isopropyl alcohol; KrF, Krypton difluoride; L–B, Langmuir–Blodgett; PbI2 , lead iodide; Li, lithium; LiTFS, lithium bis (trifluoromethanesulfonyl) imide; LiI, lithium iodide; MA=CH3 NH3 + , methyl ammonium cation; MAPbI3 , methylammonium lead triiodide; MoO3 , molybdenum trioxide; MWCNTs/MWNTs, multi-layer carbon nanotube; PDI, N,N -dialkylperylenediimide; NiO, nickel oxide; Ni(NO3 )2 ·6H2 O, nickel(II) nitrate hexahydrate; HNO3 , nitric acid; N2 , nitrogen; ODA, octadecylamine; C18 H37 N, oleylamine; PVK/PVSK, perovskite; PCBM, phenyl-C61/71-butyric acid methyl ester; Pt, platinum; PEDOT, poly(3,4-ethylenedioxythiophene); PEDOT:PSS, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate); PEO, poly(ethylene oxide); PMMA, poly(methyl methacrylate); PCDTBT, poly-[[9-(1-octylnonyl)-9H-carbazole2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]; P3HT, poly-[3-hexylthiophene-2,5-diyl]; PANI, polyaniline; PDMS, polydimethylsiloxane; PEG, polyethylene glycol; PPy, polypyrrole; PTFE, polytetrafluoroethylene; PTAA, poly-triarylamine; PVAc, polyvinyl acetate; KI, potassium iodide; KMnO4 , potassium permanganate; KSCN, potassium thiocyanate; PLD, pulsed laser deposition; RGO/r-GO, reduce graphene oxide; SiO2 , silicon dioxide; Ag, silver; Ag/AgCl, silver/silver chloride; SWCNT/SWNT, single-walled carbon nanotube; NaOH, sodioum hydroxide; NaI, sodium iodide; H2 SO4 , sulfuric acid; C14 H30 , tetradecane; BF4 − , tetrafluoroborate anion; THF, tetrahydrofuran; SCN− , thiocyanate anion; TiO2 , titanium dioxide; VB, valence band; H2 O, water; ZnO, zinc oxide; ZrO2 , zirconium dioxide. ∗ Corresponding authors. E-mail addresses: [email protected] (R. Singh), [email protected] (H.-W. Rhee). https://doi.org/10.1016/j.apmt.2018.12.011 2352-9407/© 2018 Elsevier Ltd. All rights reserved.

176

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

Contents 1. 2. 3.

4. 5. 6. 7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 Perovskite materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 Inorganic HTM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 3.1. Synthesis of copper based materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 3.1.1. Copper (I) thiocyanate (CuSCN) hole transport layer deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 3.1.2. Copper Iodide (CuI) hole transport layer deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 3.1.3. Copper oxides (CuOx ) hole transport layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 3.2. Nickel oxide (NiOx ) hole transport layer deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 3.2.1. Combustion method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 3.2.2. Pulsed Laser Deposition Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 3.2.3. Spin Coating Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 3.2.4. Spray Coating method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 3.2.5. Spray pyrolysis method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 3.2.6. Screen printing method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 3.2.7. RF Sputtering method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 3.2.8. e-beam evaporator method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 3.3. Molybdenum trioxide (MoO3 ) Hole Transport Layer Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 3.3.1. Thermal evaporation method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 3.3.2. Spin Coating method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 3.4. Carbon layer Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 3.4.1. Doctor-blade method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 3.4.2. Printing method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 3.4.3. Inkjet printing method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 3.4.4. Screen-printing method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 3.4.5. Drop casting method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 3.4.6. Spin coating method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 3.5. SWCNTs, MWCNTs and reduced graphene oxide layer deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 3.5.1. Dynamic spin-coating method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 3.5.2. Spin-coating method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 3.5.3. PDMS stamp method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 3.5.4. Printing Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 3.5.5. Chemical vapor deposition method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .190 3.6. Synthesis of Redox electrolyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 Future scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

1. Introduction Extensive research has been performed and still going on toward the development of highly efficient and stable perovskite solar cell, with a high efficiency of 15–21% achieved [1–3]. Traditional organic dye molecule-based sensitizers have been first replaced by perovskite as a sensitizer using TiO2 nanocrystalline for working electrode with iodide-based electrolytes. The prime advantage of the organotrihalide hybrid semiconducting perovskite (MAPbX3 ; MA = CH3 NH3 + and FA = CH3 (NH2 )2 + , X = Cl− , Br− , I− , BF4 − , PF6 − , SCN−) [4–10] absorbers is their direct band gap with large absorption coefficients over a wide range [11], that enables efficient light absorption in ultra-thin films, high efficiency and low-cost alternative to conventional silicon based solar cell with long diffusion length [12–14], high carrier mobility [15,16], low exciton binding energy [17,18], simple and easy preparation techniques [19]. At present stability of these devices are the main concerns for the researchers. Whereas, controlling the ionic radii of cations ‘A’ and ‘B’ in the perovskite structure ABX3 can make it stabilized by calculating corresponding tolerance factor. However, perovskite ABX3 structure is composed of ‘A’ site as organic components in cuboctahedral and ‘B’ site as an inorganic component in octahedral therefore, the molecular chemistry of the organic and inorganic components can be tailored to tune the optical, magnetic, electronic, and mechanical properties of hybrid materials [20–24]. Organometallic semiconducting perovskite materials can be

implemented either as n-i-p or p-i-n architecture types in solar cells. Two different cell structures can be constructed one is planar, which lacks the mesoporous layer and another is mesoscopic structure, in which the mesoporous layer is present. The working principle of perovskite solar cell is similar to dye-sensitized solid state solar cells but the working mechanism is totally different in both types of perovskite solar cell (planner and mesoscopic structure) [25–28]. Many modifications were made to enhance the charge transport of working electrode by using various materials such as ZnO/TiO2 nanorods [29–31], TiO2 nanoparticles, and TiO2 nanoparticle/graphene composites [32–34]. In this review article, we have focused on the inorganic hole-transporting materials (HTMs) and their synthesis. Previously efforts have been put in making the device stable, most of these materials shows good electrical properties but unfortunately fails when comes to stability of these devices. The most commonly used organic HTMs are 2,2 ,7,7 tetrakis-(N,N-di-pmethoxyphenylamine)-9,90-spiro-biuorene (spiro-MeOTAD) [35,36], poly-[3-hexylthiophene-2,5-diyl] (P3HT) [37,38], poly-[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) [39,40], poly-triarylamine (PTAA) [29,40], 4(diethylamino)-benzaldehyde diphenylhydrazone (DEH) [41] and N,N -dialkyl perylene diimide (PDI) [42], (PPy), polyaniline (PANI) [43], poly(3,4polypyrrole ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), poly(3,4-ethylenedioxythiophene) (PEDOT) [44] etc. As for the

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

177

Fig. 1. The crystal structures of the (a) orthorhombic, (b) tetragonal, and (c) cubic phases of CH3 NH3 PbI3 at two different orientations (001) and (100) respectively.

inorganic HTMs, some options are copper based HTMs (CuSCN [45], CuI [46], Cu2 O, CuO [47], CuS [48], CuGaO3 [49] and CuAlO2 [50]), NiOx [51], MoOx [52], MoS2 [53] and polymer electrolyte [43,54]. Carbon based materials (including carbon black, activated carbon, graphite powder, graphene, carbon nanotube (CNT) etc.) are also employed in HTM-free structures [55–58]. Many approaches have been made to make highly efficient and stable devices with their low cost, ease of synthesis, high mobility, high transparency in the visible region and good chemical stability; here we are presenting various types of solid and liquid types of HTMs which were employed in fabrication of PSCs. Rau et al. shows a comparative study between liquid and solid electrolytes and summarized that recombination is a critical issue in the solid-state devices as compare with liquid ones [59]. From commercialization point of view, the fabrication of solar cells using organic hole transport layer has faced a lot of obstacles, the most important obstacles are expensive material and its stability issue. Metals such as gold and platinum-based counter electrode are much cheaper than high purity spiro-OMeTAD. The cost of commercially available spiro-OMeTAD is almost ten times higher than that of gold and platinum. However, there are a lot of low-cost inorganic HTMs available like copper-based p-type semiconductors, such as CuSCN, Cu2 O, CuO and CuI that act as potential HTMs. Also, the use of copper, nickel, carbon, graphene, etc. based hole conductors have shown promise in conventional third generation solar cells (DSSCs and QDSCs) because they offer interesting properties such as suitable band alignment with perovskite active layer, high conductivity and solution-processability [54,60]. In the case of inorganic HTM, if demand is increased then it would strikingly lower the cost of any large-scale production, but in the case of spiroOMeTAD, it is likely to remain expensive due to the preparation methods and high purity that is required for solar cells applications. This is the main reason, why researchers have focused their attention toward the synthesis of a substitute to spiro-OMeTAD. There are lots of literatures available on varieties of different HTMs but few of them shows promises to improve the overall performance and stability of the perovskite solar cells. Therefore, search for ideal hole transport material is topic of great interest. While p-type dopant such as Co and Li based salts are commonly used

as additive in organic HTMs to enhance the conductivity but addition of these additives is hygroscopic in nature therefore affect the overall stability of the PSCs. Many approaches have been made for developing non-hygroscopic HTMs with high conductivity is to utilize inorganic p-type semiconductors material, example NiOx , Cu, etc. [61]. 2. Perovskite materials ABX3 perovskite structure is a basic building block of the organic-inorganic perovskite family (Fig. 1). It is the simplest structure made up of 3D network of corner-sharing BX6 octahedra, where metal cation is a represented by B atom and an anion is X with the appropriate charge to balance the A and B cations (where A > B) [62]. The A cations fill the large 12-fold coordinated holes between the octahedra. Generally, BX6 octahedra corner sharing with the A ion sit in the cuboctahedral interstices inside a perovskite structure and forming a cubic Pm3m crystal structure [63]. As discussed in previous section organometal halide perovskites, ‘A’ can be made up of an organic or inorganic cation (i.e., MA+ , FA+ , Cs+ , Rb+ and K+ ), some of the popular ‘B’ metal cation in perovskite (i.e., Sn2 + or Pb2 + ), where, ‘X’ is made up of a halide anion (i.e., Cl− , Br− , I− etc.) [63,64]. In general, ABX3−x Yx type hybrid organometal with mixed halides perovskites, for example, MAPbI3−x Clx and MAPbI3−x Brx are also a topic of interest and attracts the attention of researchers around the globe due to their high tunable optical properties that makes it easy to play with device performance and enhances the overall performance of PSCs [21,64]. Stability of perovskite device is a major challenge and in layered type of perovskite one can overcome this issue by producing thin perovskite films free from grain boundaries and high-quality single crystalline material, in which the crystallographic planes of the inorganic perovskite component have a strongly preferential outof-plane alignment with respect to the contacts in planar solar cells to facilitate efficient charge transport. The condition for achieving a perfectly packed cubic perovskite structure, √ that A, B, and X ions have to satisfy this formula, t = (RA + RX )/ 2(RB + RX ) where RA , RB and RX are the corresponding ionic radii and the tolerance factor, t = 1. Quantitatively it is found that 0.8 ≤ t ≤ 0.9 for most cubic

178

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

sector [67–69]. Inorganic HTMs have shown promises in replacing organic HTMs [62,70]. The following sub-section shows the different types of synthesis procedure used by researchers for preparing inorganic HTMs. In this review article, we have tried to focus our attention on synthesis of low-cost inorganic HTMs and its electrical parameter with its device configuration in single platform. 3.1. Synthesis of copper based materials

Fig. 2. The energy level diagram for Inorganic HTMs and other materials basically used for perovskite solar cell [71,72].

perovskite structures. If tolerance factor below ∼0.80, it forms different structures such as the ilmenite-type (FeTiO3 ) that is more stable due to the similar sizes RA and RB . If the values of tolerance factor greater than 1 therefore it forms hexagonal structures and layers of face-sharing octahedra are introduced into the structure [63,64]. If the value of t is smaller due to a lower symmetry it forms tetragonal or orthorhombic structure, however if the value of t is larger than it can destabilize the 3D network, creating a 2D structure [63]. The methylammonium cation shows an appropriate choice for producing highly efficient PSCs. But the methylammonium cation undergoes quick isotropic reorientation. Fig. 1 shows that varying the temperature can generate phase transition in MAPbI3 such as orthorhombic phase can be formed below temperature 162 K, tetragonal phase can be formed at temperature 327 K and above that cubic phase [63]. MAPbI3 is having tetragonal phase at ambient room temperature. Therefore, 2D perovskite material with methylammonium cation can be the material of interest for good device performance. 3. Inorganic HTM Inorganic hole transport material has been widely studied from past few decades. These materials have been shown potential application in the solar cell, LED and FET, etc. Still there is a lot of room available to focus on the materials properties for PSCs such as: band matching of HTMs and perovskite material should be compatible with each other. Fig. 2 shows energy level diagram for inorganic hole transport materials widely used in PSCs. It should have high mobility, high conductivity, excellent transmittance in IR and visible region. For long term stability, it is desirable to have HTMs that are stable thermally and in presence of small amount of moisture. Although, many inorganic hole transport materials have been investigated but the performance have not been optimized as the organic HTMs. Generally, the organic HTMs are doped with lithium or cobalt based salts with 4-tert-butylpyridine (t-BP) etc. for enhancing the device performance. Whereas, organic HTMs shows high efficiency in perovskite solar cells, but for commercialization point of view, these materials have several drawbacks such as high cost and instability in performance of devices. The degradation of perovskite material by oxygen and moisture causes the instability in the device; also due to the presence of hygroscopic materials such as lithium salt in many organic HTMs is hostile. Additionally, the other common additive, t-BP, has been reported to interact with the perovskite material [65] and organic polymers with ␲conjugated systems can be degraded by the presence of oxygen and light [62,66]. Therefore, the current challenge is to synthesize a low cost and highly stable HTM. Another option is to design an HTM free architecture. Many groups are focusing their attention toward this

Inorganic semiconducting copper-based HTMs, such as copper thiocyanate (CuSCN), copper iodide (CuI), copper oxide (CuO) and copper dioxide (Cu2 O), have been mostly used in dye-sensitized and quantum dot-sensitized solar cells. Solution process technique is widely used for depositing these materials uniformly and with good pore-filling. P-type inorganic Cu based HTMs, are economical and chemically stable. They exhibit suitable energy level, high hole mobility, high conductivity, and excellent transmittance. The challenge for solution processing of an inorganic material on top of perovskite is to find a suitable solvent that does not dissolve the perovskite material. The detailed descriptions of various copper-based materials are as follows. 3.1.1. Copper (I) thiocyanate (CuSCN) hole transport layer deposition It has good transparency in ultra violet, visible and infrared spectrum range with wide band-gap (Eg = 3.6 eV), high hole mobility (0.01–0.1 cm2 V−1 s−1 ), relatively good chemical stability, and can be synthesized by simple preparation process. Some methods to prepare CuSCN thin films are spray coating, doctor blading and electro-deposition. It can be deposited through a solutionprocess technique at low temperature, which makes it compatible with flexible substrates. CuSCN possesses adequate energy levels (EVB = −5.4 eV and ECB = −1.8 eV) which allow it to function as an excellent HTL for solar cell. Recently, in the PSCs field, CuSCN have shown promises for their use as an efficient material due to its proper band matching with PVK materials such as CH3 NH3 PbI3 . Device performance and its configuration is listed in Table 1. 3.1.1.1. Spin-coating. Organic–inorganic hybrid spiro-OMeTAD: CuSCN (or CuI) composite films were deposited by spin-coating by mixing various ratio (0–42 mol%) of CuSCN (or CuI) solutions (in propyl sulfide, 40 mg mL−1 ) with spiro-OMeTAD solution (in chlorobenzene, 80 mg mL−1 ). HTLs (spiro-OMeTAD, CuSCN-doped spiro-OMeTAD, and CuI-doped spiro-OMeTAD) in chlorobenzene solution were coated on the perovskite layer at 3000 rpm for 40 s. The device was transferred to vacuum chamber at 2 × 10−6 Torr for Ag electrode evaporation. The active area of cells was 7.25 mm2 [73]. The p-type doping is observed when CuSCN or CuI is incorporated in spiro-OMeTAD, resulting in enhanced charge transfer complex with improved film conductivity and hole mobility. Adding CuSCN or CuI into spiro-OMeTAD can reduce the film aggregation and crystallization of spiro-OMeTAD layer with reduced pinholes and voids. This slows down the perovskite decomposition due to the prevention of moisture infiltration to some extent [73]. The CuSCN film was spin coated on the substrate maintaining the thickness of ∼13 nm in a nitrogen filled glovebox and dried at 60 ◦ C for 2 min. The CuSCN solution was prepared by dissolving CuSCN in dipropyl sulfide at a concentration of 15 mg mL−1 . The solution was kept on stirring for 5 h at room temperature and then filtered with 0.2 mm PVDF filters before spin-coating [74]. 3.1.1.2. Electro-deposition. An electro-deposited CuSCN film as hole-transport layer on PVK results in 16.6% efficiency by using a single-step fast deposition crystallization method. Thin film of CuSCN was deposited by potentiostatic electro-deposition from

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

179

Table 1 The methods adopted for achieving highly efficient cell parameter using copper (I) thiocyanate (CuSCN) hole transport layer for perovskite solar cell. S.N

Architecture

Method

Cell structure for CuSCN

Voc (V)

Jsc (mA cm−2 )

FF

Eff. (%)

Ref.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Mesoscopic n-i-p Mesoscopic n-i-p Planar p-i-n Planar p-i-n Mesoscopic n-i-p Planar p-i-n Mesoscopic n-i-p Mesoscopic n-i-p Mesoscopic n-i-p Planar p-i-n Planar p-i-n Planar p-i-n Mesoscopic n-i-p Mesoscopic n-i-p Mesoscopic n-i-p Mesoscopic n-i-p Mesoscopic n-i-p Mesoscopic n-i-p

Spin coating Spin coating Spin coating Electro deposition Doctor blading Spin coating Spin coated Spray-coated Doctor blading Spin coating Spin coating Spin coating Spin coating Doctor blading Drop casting Spin coating Doctor blading Electro deposition

FTO/TiO2 /PVK CuSCN/rGO/Au FTO/TiO2 /PVK/spiro-OMeTAD:CuSCN/Ag ITO/CuSCN/PVK/C60 /Ag ITO/CuSCN/PVK/C60 /BCP/Ag FTO/bl-TiO2 /mp-TiO2 /PVK/CuSCN/Au ITO/CuSCN/PVK/PCBM/PDINO/Al FTO/bl-TiO2 /mp-TiO2 /PVK/CuSCN/Au FTO/Al2 O3 /CuSCN/Au/PMMA FTO/bl-TiO2 /mp-TiO2 /PVK/CuSCN/Au ITO/CuSCN/PVK/PCBM/Al ITO/CuSCN/PVK/PCBM/LiF-Ag ITO/CuSCN/PVK/PCBM/C60 /Ag FTO/SnO2 /PVK/CuSCN/Au FTO/bl-TiO2 /PVK/CuSCN/Au FTO/bl-TiO2 /PVK/CuSCN/Au FTO/cp-TiO2 /mp-TiO2 /Sb2 S3 /PVK/CuSCN/Au FTO/bl-TiO2 /mp-TiO2 /PVK/CuSCN/Au FTO/CuSCN/PVK/PCBM/Ag

1.13 1.06 1.09 1.00 1.1 1.05 1.06 0.98 1.016 1.0 1.06 1.07 0.96 0.97 0.727 0.57 0.63 0.67

23.35 22.01 22.3 21.9 21.8 21.4 20.8 21.07 19.7 19.1 15.76 12.2 18.99 18.42 18.53 17.2 14.5 8.8

0.77 0.77 0.71 0.758 0.692 0.731 0.701 0.64 0.62 0.647 0.632 0.76 0.45 0.40 0.617 – 0.53 –

20.8 18.02 17.2 16.6 16.6 16.4 15.43 13.3 12.4 12.4 10.5 10.22 8.38 7.19 6.4 5.12 4.85 3.8

[45] [73] [85] [75] [78] [76] [78] [83] [81] [76] [74] [86] [79] [82] [84] [87] [80] [77]

12 mM aqueous solution of copper sulfate (CuSO4 ), 12 mM ethylene diamine tetra acetic acid and 12 mM potassium thiocyanate (KSCN) at a fixed potential of −0.3 V versus Ag/AgCl (3 M KCl) electrode [52,54,75,76]. In another report, −0.4 V was applied with respect to the Ag/AgCl reference electrode, and a platinum rod was used as counter electrode with FTO coated glass as working electrode, resulting in formation of an orthorhombic CuSCN layer. In this case solution contained 0.04 M CuSO4 , 0.04 M titriplex (C10 H14 N2 Na2 O8 ·2H2 O), and 0.02 M KSCN, and pH 1.8 was maintained, no post heating is required in this method [77]. 3.1.1.3. Doctor-blading. This is one of the simplest and low-cost methods used for producing uniform layer. A solution was prepared containing a fixed amount of CuSCN in di-propyl sulphide solvent was stirred overnight and deposited by doctor-blade after which the substrate was annealed at 80 ◦ C. Sequential deposition is performed to produce required thickness of the film. An efficiency of 16.6% was reported for deposition via this method [78]. In other reports, though the technique of deposition and annealing temperature is same, a different solvent propyl sulfide was used [79,80]. In another reported article, the CuSCN HTM was deposited by doctor-blade and annealed at 65 ◦ C inside a glove box [81]. Similar experiment was performed in ambient atmosphere, though the result inside glove box was better [82]. 3.1.1.4. Spray coating. The spray coating method is widely known for large area production. In this report the coating speed was controlled at roughly 2 mL min−1 . Highly smooth and uniform layer of CuSCN was prepared by spray coating a solution in di-propylsulfide solvent stirred for 48 h with a clear solution concentration of 8 mg mL−1 on hot substrates set at 85 ◦ C [83].

The selection of CuI as a HTM is on the basis of its suitable valence band position as shown in Fig. 2 and its high solubility in organic solvents as compared to CuSCN. It has excellent hole mobility 0.5–2 cm2 V−1 s−1 . High reduction in the J–V hysteresis was observed in the CuI-based solar cells as compared to the conventional spiro-OMeTAD-based solar cells; therefore, all these properties make this material of great interest. Solar cells parameters were listed in Table 2 with its structure and techniques. Several methods used by researches till now are described below. 3.1.2.1. Solution-process method. In this method solution of CuI is prepared in 1:39 ratio of di-n-propyl sulfide to chlorobenzene, respectively, resulting 0.1 M CuI solution after stirring overnight [88]. Several holes were drilled into the side of a syringe needle. The prepared solution was pumped at a constant rate using a syringe pump on the FTO substrate, the substrate was placed on a hot plate at 80 ◦ C with the deposition needle aligned parallel. At fixed rate, the syringe needle moved on the surface of the entire electrode and the solution penetrated through the TiO2 pores. Using this method CuI layer was spread all over the active area of device and was dried before the next film was deposited, producing 1.5–2 ␮m over layer. Excess CuI was removed before Au deposition, schematic diagram of experimental setup is shown in Fig. 3(i) [89]. 3.1.2.2. Spin coating. In this technique, the CuI films were prepared by simple spin coating of copper iodide solution made in acetonitrile solvent (10 mg mL−1 ) at 4000 rpm for 30 s inside glove box, followed by heating at 100 ◦ C for 10 min [90].

3.1.1.5. Drop-casting method. The drop-casting method was used for deposition of CuSCN film from a saturated solution of CuSCN in propylsulfide. Syringe pump was used for this method, in which the substrate was kept at 85 ◦ C and the prepared solution was pumped at a constant rate of 27 mL min−1 onto the substrate. 18 mL of CuSCN solution were drop casted per cm2 of the sample to obtain required film thickness [84].

3.1.2.3. Doctor blading. Thin film of CuI was deposited on the PVK layer by doctor blade method. 0.1 M CuI solution was prepared in 1:39 ratio of di-n-propylsulfide and chlorobenzene. Few drops of CuI solution were first placed on the one side of the substrate surface, at 70 ◦ C, for 30 s. The glass tube as shown in Fig. 3(ii) spread very quickly over the perovskite layer at fixed rate until a thin layer of solution was formed on the substrate. Once the solvent had evaporated, the above process was repeated several times until desired thickness was achieved, followed by drying on hot plate at 70 ◦ C for 60 min [91].

3.1.2. Copper Iodide (CuI) hole transport layer deposition CuI is a p-type semiconductor used as the hole conductor, which is an inexpensive and stable material. It offers large bandgap (3.1 eV), high p-type conductivity, solution-processible inorganic hole conductor. CuI based HTL shows good air stability than that of the conventional perovskite solar cells using PEDOT:PSS.

3.1.2.4. Spray casting method. Thin film of CuI was deposited by spray casting method using an airbrush through the nitrogen as shown in Fig. 3(iii). The solution was prepared by dissolving 1.25 g CuI in 1 mL propyl sulfide followed by dilution in 6 mL of chlorobenzene. The prepared solution was sprayed onto the surface of PVK coated substrate by spray casting method. The thickness of the CuI

180

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

Table 2 The methods adopted for achieving highly efficient cell parameter using copper iodide (CuI) HTMs deposition for perovskite solar cell. S.N

Type

Method

Cell structure of CuI

Voc (V)

Jsc (mA cm−2 )

FF

Eff. (%)

Ref.

1 2 3 4 5 6 7 8 9

Planar p-i-n Mesoscopic n-i-p Mesoscopic n-i-p Planar p-i-n Mesoscopic n-i-p Mesoscopic n-i-p Mesoscopic n-i-p Mesoscopic n-i-p Mesoscopic n-i-p

Spin coating Spin coating Thermally evaporated Spin coating Thermal evaporation Doctor blading Doctor blading Spray Coating Thermally evaporated

ITO/CuI/PVK/PCBM/BCP/Ag FTO/TiO2 /PVK/spiro-OMeTAD:CuI/Ag FTO/CuI/PVK/PCBM/PEI/Ag FTO/CuI/PVK/PCBM/Al FTO/TiO2 /PVK/CuI/Cu FTO/bl-TiO2 /PVK/CuI/graphite/Cu FTO/bl-TiO2 /mp-TiO2 /PVK/CuI/Au FTO/bl-TiO2 /mp-TiO2 /PVK/CuI/Au FTO/TiO2 /PVK/CuI/Au

0.99 1.06 1.04 1.04 0.85 0.78 0.55 0.64 0.66

22.6 21.52 20.3 21.06 22.99 16.7 17.8 22.6 22.8

0.71 0.73 0.68 0.62 0.47 0.57 0.62 0.39 0.29

16.8 16.67 14.3 13.58 9.24 7.5 6.0 5.6 4.38

[88] [73] [93] [90] [94] [91] [89] [46] [92]

Fig. 3. The methods adopted for preparing CuI layer. (i) Setup of automated drop-casting apparatus used for deposition of CuI solution onto mesoporous TiO2 /CH3 NH3 PbI3 films, reprinted from Ref. [89]. (ii) Schematic drawings of (a) setup for doctor blading method of CuI deposition and cross-section of the devices architectures with: (b) gold; (c) graphite counter electrodes, reprinted from Ref. [91]. (iii) Spray casting method of the CuI as a hole transporting layer, reprinted from Ref. [46]. (iv) The formation of CuI using the gas–solid transformation method with (a) architecture of perovskite device after deposition of a Cu thin film, (b) the gas–solid treatment using iodide gas, and (c) architecture of perovskite device after the iodization. (d) The Cu and CuI coated thin film, reprinted from Ref. [92].

layer was controlled by the amount of solution used for spraying onto the substrate [46].

film showed good air stability. The progress of CuOx based perovskite solar cell till now is tabulated in Table 3.

3.1.2.5. Thermal evaporation technique. CuI thin film was deposited by a thermal evaporation technique on the surface of perovskite layer. To achieve desired thickness of the layer, an oscillating quartz sensor was used for monitoring the deposition rate. The evaporation rate was 1 A˚ s−1 at 40 ◦ C. Transformation of thermally deposited Cu thin film to CuI was done by transferring the sample inside the closed petridish filled with I2 vapor. The change in the color was observed from reddish/orange color to transparent color, resulting in CuI formation, where no heating was required. The change in color was due to the iodization of the samples. The difference in substrate post iodization can be seen in Fig. 3(iv) [92].

3.1.3.1. Spin-coating method. The CuOx film was deposited by spincoating 1.5 mg mL−1 Cu(acac)2 in 1,2-dichlorobenzene solution at 4000 rpm for 60 s on the surface of ITO substrate followed by heating at 80 ◦ C for 20 min and then 15 min treatment under ultra-violet ozone [47]. Fabricated solar cells active area was 0.1 cm2 . For Cu2 O films, the solution of CuI in acetonitrile (1–40 mg mL−1 ) was spin coated onto the surface of ITO substrate at 2000 rpm for 30 s. The substrate was immersed in 10 mg mL−1 aqueous NaOH solution for 5 s, rinsed with distilled water and dried by N2 . Then the substrate was transferred into glovebox and heated at 100 ◦ C for 10 min. CuO film was prepared by heating Cu2 O film at 250 ◦ C for 30 min in air as shown in Fig. 4(i) [96].

3.1.3. Copper oxides (CuOx ) hole transport layer Stable copper oxides (CuOx ), namely, cupric oxide (CuO) and cuprous oxide (Cu2 O), are well-know p-type semiconductors. Due to the natural abundance of copper, CuOx has become a promising alternative for photovoltaic system. The first report of Cu2 O as HTM in perovskite solar cell showed 8.93% of PCE by using magnetron sputtering technique [95]. The solution-processed CuOx thin film showed a smooth and uniform surface with excellent transparency in the visible light. The solar devices based on CuOx thin

3.1.3.2. RF sputtering method. RF sputtering method provides a uniform thin film as compare to conventional methods. Direct path deposition of the Cu2 O thin film by rotating sputtering onto the surface of the PVK substrate showed pinholes on substrate surface therefore, possibility of short circuit in solar cells as shown in Fig. 4(ii). Pinhole free perovskite solar cell structure was obtained by tilting the substrate at 45◦ tilting angle against the sputtering target. It has been observed from this technique that a uniform

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

181

Table 3 The methods adopted for achieving highly efficient cell parameter using copper oxides (CuOx ) and various other Cu based HTMs for perovskite solar cell. S.N

HTM

Type

Method

Cell structure of other copper oxides based HTM

Voc (V)

Jsc (mA cm−2 )

FF

Eff. (%)

Ref.

1 2 3 4 5 6 7

CuOx Cu2 O CuO Cu:CrOx Cu2 O Cu2O CuxO

Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Mesoscopic n-i-p Planar p-i-n Planar p-i-n

Spin coating Spin coating Spin coating Spin coating Spin coating Spin coating Electro spray

ITO/CuOx /PVK/PCBM/BCP-Ag ITO/Cu2 O/PVK/PCBM/Ca-Al ITO/CuO/PVK/PCBM/Ca-Al FTO/Cu-CrOx /PVK/PCBM/Ag FTO/TiO2 /PVK/Cu2 O/Au ITO/Cu2 O/PVK/PCBM/Al ITO/CuxO/PVK/C60/BCP/Al

1.0 1.07 1.06 0.98 0.96 0.92 0.78

22.2 16.52 15.82 16.02 15.8 15.60 17.22

0.757 0.755 0.725 0.70 0.59 0.58 0.48

16.8 13.35 12.61 10.99 8.93 8.30 5.83

[47] [96] [96] [97] [95] [98] [99]

S.N

HTM

Type

Method

Cell structure Cu based other HTMs

Voc (V)

Jsc (mA cm−2 )

FF

Eff. (%)

Ref.

1 2 3 4 5 6

CuS CuGaO2 CuCrO2 CuAlO2 Cu3 SbS4 Cu2 ZnSnS4 (CZTS)

Planar p-i-n Mesoscopic n-i-p Planar p-i-n Planar p-i-n Mesoscopic n-i-p Mesoscopic n-i-p

Spin coating Spin coating Spin coating Vacuum sputter Spray deposited Spin coating

ITO/CuS/PVK/C60/BCP/Ag FTO/TiO2 /PVK/CuGaO2 /Au ITO/CuCrO2 /PVK/C60/BCP/Ag ITO/a:CuAlO2 /PEDOT:PSS/PVK/PCBM/Ag FTO/SnO2 /PVK/Cu3 SbS4 /Au FTO/TiO2 /PVK/Cu2 ZnSnS4 /Au

1.02 1.11 0.92 0.88 – 1.06

22.3 21.66 20.0 21.98 – 20.54

0.71 0.77 0.69 0.75 – 58.7

16.2 18.51 12.8 14.52 8.7 12.75

[48] [49] [100] [50] [101] [102]

Fig. 4. (i) Preparation process for Cu2 O and CuO films, reprinted from Ref. [96]. (ii) FTO/TiO2 /CH3 NH3 PbI3−x Clx /Cu2 O/Au cells containing pinhole and pinhole free perovskite layers, reprinted from Ref. [95].

and pinhole free Cu2 O thin film was achieved on the surface of Perovskite layer. DC reactive magnetron sputtering of metallic Cu target was used for deposition of Cu2 O thin film on the perovskite layer. Several other parameters have to be taken into consideration for making proper sputtering condition. Gas mixture of O2 and Ar was used. Change in the ratio O2 gas resulted in a phase mixture of CuO and Cu2 O. Hence, O2 gas ratio of 20% to reach a total Cu2 O phase was used for achieving pinhole free Cu2 O layer as shown in Fig. 4(ii) [95].

3.2. Nickel oxide (NiOx ) hole transport layer deposition NiO thin film have been widely studied and used as a cubic p-type semiconducting material-based substrate for p-i-n based PVK planar device, but overall performance remains behind that of n-i-p using thin mesoporous metal oxide layers. NiOx based HTL have shown great promise to PSCs due to its intrinsic p-type semiconductor properties, excellent transparency, wide bandgap and suitable work function with valence band (5.4 eV) that well matches with the VB of PVK. Inorganic NiO thin film based HTL is preferred for its superior stability in presence of ambient atmosphere with superior thermal and chemical stability. The importance of NiO is a low-cost, earth abundant material and can be easily synthesized by cost effective techniques. Several approaches have been reported for synthesizing NiO (NiOx ) layers, such as sol–gel method, electro-deposition, RF sputtering, e-beam evaporator, pulsed laser deposition (PLD), screen-printing technology etc. NiOx is an excellent material as compare to the conventional ones: PEDOT: PSS and

spiro-OMeTAD because NiOx does not contain acidic nature and hygroscopic characteristics, as these drawbacks were present in the conventional materials. The NiO-based p-i-n type perovskite solar cells often exhibit smaller fill factor (FF) than the PEDOT and spiro-OMeTAD based counterparts, whereas they generally show higher open-circuit voltage (Voc ). NiO can efficiently enhance charge extraction and prolong the charge lifetime. Various literatures have reported 14–15% PCE for PEDOT:PSS based PSC whereas, 18–19% PCE for spiro-OMeTAD based PSC, NiO based PSC shows PCE (highest 20.65%). with lower fill factor (FF, 0.81) and offers limited photocurrent density (Jsc ) with highest reported Voc is 1.12 V. A list of NiO based PSC is tabulated in Table 4. The inconsistent performance of NiO in photovoltaic’s strongly suggests that there is still room for improvement for this stable, low cost and effective hole selective interfacial material in inverted PSCs [103]. Detailed description of the methods adopted for the preparation of NiO thin film is mention below:

3.2.1. Combustion method In this method, combustion-derived Cu:NiOx , 276.3 mg of Ni(NO3 )2 ·6H2 O (0.95 mmol) and 12.1 mg of Cu(NO3 )2 ·3H2 O (0.05 mmol) were dissolved in 2-methoxyethanol (10 mL). The prepared solution was kept for stirring at 50 ◦ C for 1 h, followed by addition of 10 ␮L of acetyl acetone in the solution and again was kept for stirring for additional 1 hr. Spin coating method was used for coating Cu:NiOx thin film onto the surface of ITO-coated substrate, and then was kept in ambient atmosphere for 1 h at 150 ◦ C and 500 ◦ C for combustion and conventional process, respectively

182

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

Table 4 The methods adopted for achieving highly efficient cell parameter using nickel oxide (NiOx ) HTMs for perovskite solar cell. S.N Type

Method

Cell structure for NiOx

Voc (V) Jsc (mA cm−2 ) FF

Eff. (%) Ref.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

Spraying Spin coating Spray pyrolysis Spin coating Spin coating Spin coating Spin coating Spin coating Spin coating Combustion Pulsed laser deposition Spin coating Electrodeposition Spin coating Spin coating Spin coating Spin coating Vacuum deposition Drop casting Spin coating Screen printing Spray pyrolysis deposition Screen printing Spin coating Dip coating Spray pyrolysis deposition Spin coating Spin coating Dip coating Sputtering Screen printing Spin coating Vacuum thermal evaporation Sputtering Spin coating Spin coating Spin coating Spin coating e-beam evaporator Spin coating Electro deposition Screen printing Screen printing

FTO/NiOx /FAPbI3 /PCBM/TiOx /Ag ITO/NiOx /PVK/PCBM/ZrAcac/Al FTO/NiOx /PVK/PCBM/Ag FTO/Cs:NiOx /PVK/PCBM/ZrAcac/Ag ITO/NiOx /PVK/PCBM/ZrAcac/Al ITO/Cu:NiOx /PVK/PCBM/BCP/Ag FTO/NiOx /PVK/PCBM/Ag ITO/NiOx /PVK/PCBM/c-HATNA/Bis-C60 /Ag ITO/NiOx /PVK/PCBM/Al ITO/Cu:NiO/PVK/Bis-C60 /C60 /Ag ITO/PLD-NiO/PVK/PCBM/LiF-Al ITO/NiOx /PVK/PCBM/Ag ITO/NiOx /PVK/PCBM/Ag ITO/NiOx /PVK/PCBM/Ag ITO/NiOx /PVK/PCBM/Ag ITO/NiOx /PVK/ZnO/Al FTO/Cu:NiO/PVK/PCBM/Ag ITO/NiOx /PVK/PCBM/BCP/Ag FTO/c-TiO2 /m-TiO2 /PVK:NiO-MWCNTs ITO/NiO-PEDOT/PVK/PCBM/Ag FTO/bl-TiO2 /mp-TiO2 + PVK/mp-Al2 O3 + PVK/mp-NIO + PVK/C FTO/NiMgLiO/PVK/PCBM/Ti(Nb)Ox /Ag FTO/bl-TiO2 /mp-TiO2 + PVK/mp-ZrO2 + PVK/mp-NiO + PVK/C FTO/NiOx /PVK/PCBM/Ag FTO/c-TiO2 /m-TiO2 /m-ZrO2 + PVK/NIO -NS FTO/NiO/meso-AL2 O3 + PVK/PCBM/BCP/Ag ITO-PEN/NiOx -based flexible device/PVK/PCBM/Ag ITO/NiOx /PVK/PCBM/LiF/Al FTO/c-TiO2 /m-TiO2 /m-ZrO2 + PVK/NIO-NP ITO/bl-NiOx /nc-NiO/PVK/PCBM/BCP/Al FTO/bl-TiO2 /mp-TiO2 + PVK//mp-NIO+ PVK/C FTO/NiO NCs/PVK/PCBM (1.5 wt% PS)/Al ITO/NiOx /PVK/PCBM/C60 /BCP/Al FTO/NiO/PVK/PCBM/BCP/Au ITO/NiOx /nc-NiO/PVK/PCBM/BCP/Al FTO/NiO/PVK/PCBM/Au ITO/NiO/PVK/PCBM/Ag ITO/NiO/PVK/PCBM/BCP/Al ITO/NiOx /PVK/PCBM/BCP/Al ITO/NiO/PVK/PCBM/Al FTO/NiO/PVK/PCBM/Ag FTO/bl-NiO/nc-NiO/PVK/PCBM/Al FTO/NiOx /PVK/Pt/FTO

1.10 1.12 1.120 1.079 1.11 1.09 1.09 1.12 1.05 1.06 1.03 1.05 1.04 1.07 1.01 1.11 1.06 0.903 1.04 0.915 1.090 0.917 1.09 0.965 1.04 1.04 1.03 0.930 0.96 0.890 1.07 1.06 1.10 1.040 0.882 1.08 0.92 0.901 1.05 0.786 0.830 0.205

20.65 20.5 19.58 19.35 18.69 18.66 18.6 18.21 18.0 17.46 17.3 17.2 17.1 16.55 16.47 16.1 15.40 15.4 15.38 15.1 15.03 15.00 14.9 14.42 14.2 13.5 13.43 13.4 12.4 11.6 11.4 10.68 10.6 9.83 9.51 9.11 8.94 7.8 7.75 7.6 7.26 1.50 0.71

Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Mesoscopic p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Mesoscopic n-i-p Planar p-i-n Multi-layered n-i-p Planar p-i-n Multi-layered n-i-p Planar p-i-n Mesoscopic n-i-p Mesoscopic p-i-n Planar p-i-n Planar p-i-n Mesoscopic n-i-p Mesoscopic p-i-n Multi-layered n-i-p Planar p-i-n Planar p-i-n Planar p-i-n Mesoscopic p Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Mesoscopic n-i-p Mesoscopic n-i-p

[51]. After combustion process transparent Cu:NiOx coated thin film was be achieved as shown in Fig. 5 (i). 3.2.2. Pulsed Laser Deposition Method The Pulsed Laser Deposition (PLD) technique is used for depositing the NiO films on patterned ITO glass substrates. This method was performed in the presence of oxygen pressures (10 m Torr to 900 m Torr) at room temperature, with a KrF excimer laser (␭ = 248 nm) with a pulse energy density of 3.0 J·cm−2 and a repetition rate of 5 Hz. The NiO ceramic target, which ablates the NiO material in the form of a plasma plume by a focused pulsed laser, was prepared using high-purity NiO powders. The film thickness was controlled by the deposition time. The prepared films were heated at 200 ◦ C for 1 h, in air as shown in Fig. 5 (ii) [104]. 3.2.3. Spin Coating Method Chemical precipitation process is generally used for preparation of NiOx nanoparticles thin film. 12.885 g of NiCl2 ·6H2 O was dissolved in 100 mL of deionized water under magnetic stirring. 10 M NaOH was added into the solution drop by drop until the pH value reached 10. The obtained turbid green solution was centrifuged, and the precipitation was washed with deionized water two times, the powder was dried at 80 ◦ C overnight and annealed at different temperatures for 2 h. NiOx nanoparticle solution was prepared by dispersing the NiOx nanoparticles in deionized water with

23.09 22.68 21.77 22.17 20.76 22.8 21.25 21.79 21.60 20.2 21.4 22.6 21.22 20.58 21.0 1 18.75 18.6 22.38 20.1 21.62 20.96 21.36 17.93 20.4 18.0 18.74 19 19.5 19.8 18.2 15.62 18 15.17 13.24 16.27 0.58 12.43 13.16 15.4 14.2 4.94 9.47

0.81 0.77 0.79 0.78 0.81 0.75 0.79 0.74 0.77 0.81 0.78 0.72 0.75 0.748 0.760 0.72 0.78 0.76 0.72 0.76 0.668 0.76 0.738 0.72 0.72 0.689 0.69 0.68 0.61 0.71 0.64 0.56 0.59 0.69 0.635 14.13 0.68 0.65 0.48 0.65 0. 35 0.36

[121] [122] [123] [124] [125] [126] [127] [128] [129] [51] [104] [130] [131] [132] [105] [112] [108] [133] [134] [110] [106] [116] [135] [107] [118] [103] [105] [136] [118] [120] [137] [138] [114] [117] [113] [115] [108] [109] [119] [111] [77] [139] [140]

sonication. 150 mg of NiOx nanoparticles was added into 5 mL of deionized water, and then this mixture was ultrasonicated in an ultrasonic cleaner at a power of 100 W. The total time for the ultrasonication was about 8 h. The resulting solution was filtered through a polytetrafluoroethylene filter (0.45 ␮m). NiOx films were deposited on ITO-coated glass substrates and PEN substrates by using a spin coating method, at 2000 rpm for 30 s. The NiOx coated substrates were then baked at 130 ◦ C for 20 min in air [105]. In another report, the NiO nanoparticle powder was used as received with particle size of 20 nm (73.22 wt% Ni). Screen-printing pastes were prepared by mixing ethyl cellulose solution in 44 wt% of ethanol, 46 wt% of terpinol and 10 wt% of NiO powder [106]. In another literature, pre-crystallized NiO nanoparticles as a HTL was prepared by spin coating method for p-i-n-type planarstructured PSC. The thickness of the NiO thin film was adjusted by changing the concentration of the NP solution and rpm of the spincoater. In a simple procedure, Ni(II) acetylacetonate (C10 H14 NiO4 ) (1 mmol) was dissolved in 15 mL of oleylamine (C18 H37 N) with and without 1 mmol of oleic acid (C18 H34 O2 ) and was kept for heating at 110 ◦ C for 30 min followed by cooling at 90 ◦ C. 2.4 mmol of borane–triethylamine complex [(C2 H5 )3 N·BH3 ] mixed with 2 mL of oleylamine was added into the solution followed by cooling after 1 h to room temperature. Adding 30 mL ethanol to the prepared solution and then centrifuged at 6000–7000 rpm for 15 min to collect the NiO NPs followed by washing with

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

183

Fig. 5. (i) Photographs of (a) PEDOT:PSS anode on glass substrate and (b) comparison of PEDOT:PSS degradation after formation of Cu:NiOx from different methods, reprinted from Ref. [51]. (ii) Shows the schematic diagram of the effect on the morphology and growth of the PLD-NiO films as a hole-transporting layer on the solar cell performance. Schematic images of thin, nanostructured, and disordered NiO films fabricated using PLD as an electron blocking layer/hole extraction layer in the MAPbI3 /PCBM heterojunction solar cells, Reprinted from Ref. [104]. (iii) Schematic diagram of device. (a) Schematic representation of the TiO2 /Al2 O3 /NiO/carbon (MAPbI3 ) based device. (b) PSCs under 100 mW cm−2 illuminations and a device photograph, reprinted from Ref. [106]. (iv) Schematic illustration of the sequential deposition of a CH3 NH3 PbI3 layer onto a planar NiO film, reprinted from Ref. [111]. (v) Architecture of PSC:FTO/NiO/meso-Al2 O3 /CH3 NH3 PbI3 /PCBM/BCP/Ag, reprinted from Ref. [103]. (vi) Carbon based counter electrode for PSC device using NiO nanosheets as a hole collector, reprinted from Ref. [118]. (vii) FE-SEM images shows the surface morphology of NiO compact layers by (a) RMS and (b) spray pyrolysis methods, reprinted from Ref. [117]. (viii) Device structures of NiO/perovskite hetero-junction solar cells, reprinted from Ref. [120].

ethanol and were dispersed in tetradecane (C14 H30 ) by ultrasonication. Schematic architecture of the p-i-n structure is shown in Fig. 5 (iii) [106]. Autoclave process was used for synthesizing the NiOx nanoparticle thin film. In this process, 1.262 g diethanolamine and 3.084 g nickel (II) acetylacetonate were added to 30 mL ethanol and kept for stirring at 60 ◦ C for 12 h. The solution was kept inside an autoclaving at 150 ◦ C for 10 h and then cooled down to room temperature. This resulted in dark green color solution which was filtered and centrifuged. The prepared green solution was mixed with 400 mL of cyclohexane and stirred for 10 min. The prepared solution was again centrifuged and the precipitate was collected. The precipitate was then dispersed in ethanol to make NiOx nanoparticles solutions. Spin coating method was used for depositing NiOx thin film at 3000 rpm for 30 s by using the prepared NiOx nanoparticle solution, followed by annealing at 150 ◦ C for 10 min and then again further annealing was done at 500 ◦ C in air for 1 h. The thickness of the NiOx films was controlled by changing the concentration of the NiOx nanoparticle [107]. Solution process is attracting immense interest due to their simple synthesis procedure for materials. Copper (Cu)-doped NiOx (Cu:NiOx ) based thin film have been synthesized by solution process in this article [108]. The dense NiOx thin film consists of 50 to 100 nm sized nanoparticles. This NiOx film was prepared by using nickel nitride salt as a precursor such as nickel formate dihydrate [109] or nickel acetate tetrahydrate [110,111]. It has been found that nitride salt precursors can produce high-quality metal oxide films. In this method, 0.5 M nickel formate dihydrate in ethylene glycol with 1 molar equivalents of ethylenediamine was filtered with 0.45 ␮m nylon filters. The prepared solution was spin coated on surface of the ITO substrate at 4000 rpm for 90 sec followed by post annealing at 300 ◦ C for 1 h. Further, it was kept under UV-Ozone treatment before using for device fabrication [109,112]. In another report,

slurry of NiO was prepared by mixing 3 g of NiO nanopowder in 80 mL ethanol. 15 g of 10 wt% ethyl cellulose (in ethanol) in 10 g of terpineol was subsequently added to the previous solution followed by stirring. The final solution was sonicated and evaporated in a rotatory evaporator for ethanol removal. The resultant slurry of mesoporous NiO was dispersed in ethanol in ratio 1:7 was used for spin coating. NiOx electrode-interlayer, was spin-casted onto the substrates at 4000 rpm for 90 s and was then heated at 300 ◦ C for 1 h. Afterwards, the prepared substrates were spin-coated with NiO (nanocrystalline) at 4000 rpm for 30 s and heated at 400 ◦ C for 30 min, the resulting mesoporous NiO film formation [113]. In another method, the NiOx thin film was prepared by dissolving 0.1 M nickel (II) acetate tetra hydrate in ethyl alcohol by sonification. The prepared nickel oxide solution was coated on the ITO glass substrates by spin-coating method at 4000 rpm for 45 s and annealed at 300 ◦ C for 1 h [110]. NiOx thin film was prepared by spin-coating a 100 mL of methoxyethanol solution containing 1 wt% of nickel (II) acetate onto the surface of the ITO coated substrate at 3000 rpm for 30 sec followed by heating at 300 ◦ C in air for 10 min [114]. In another report, the NiO precursor solution was prepared by adding nickel acetate tetrahydrate and monoethanolamine in methoxyethanol and was stirred overnight. Precursor solutions with nickel acetate weight percentages of 1%, 2%, 5%, 10%, and 15% were prepared to fabricate NiO film of different thickness. Nickel acetate solution was spin-coated at 4000 rpm for 30 s on the ITO substrates, then was annealed on a hot plate at 150 ◦ C for 5 min, 250 ◦ C for 10 min, and 350 ◦ C for 60 min, all in air as shown in Fig. 5 (iv) [111]. Sol–gel process is used for synthesizing NiO nanocrystalline (NC) thin film. In a typical synthesis process of the NiO NC thin film, the sol solution was prepared by dissolving nickel (II) acetylacetonate in diethanolamine. The precursor was heated at 150 ◦ C to form the sol suspension, NiO nanoparticles were uniformly spin coated on the surface of substrate followed by annealing at 500 ◦ C [115].

184

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

3.2.4. Spray Coating method This is a coating processes in which solution of materials is sprayed onto a surface. 30 mL of an acetonitrile: ethanol (95:5 volume ratio) solution of nickel acetylacetonate (or with 15 mol% magnesium acetate tetrahydrate and 5 mol% lithium acetate) was sprayed for 10 min by an air nozzle (with 0.2 mm caliber) onto the hot FTO glasses. A distance of 20 cm was maintained between the substrate and the nozzle. The total metal ion concentration was set at 0.02 molL−1 . After spraying, the film was further treated at 500 ◦ C for another 20 min to promote NiO crystallization. NiO film thickness could be tuned from 10 to 40 nm by varying the solution volume from 15 to 60 mL [116]. 3.2.5. Spray pyrolysis method It is a process in which NiO layer was deposited by spraying a solution on a hot substrate. In this process, the FTO substrate was kept on a hotplate at 500 ◦ C. 10 mL of acetonitrile solution of nickel acetylacetonate was sprayed within 10 min by an air nozzle onto the FTO substrate. Afterwards, the coated FTO substrate was kept at 500 ◦ C for 30 min to promote crystallization followed by preparation of transparent meso-Al2 O3 film, as shown in Fig. 5 (v) [103]. In another reported method, the solution prepared by adding 0.2 mol L−1 nickel acetylacetonate in acetonitrile. The substrate was kept at 450 ◦ C for 30 min. Oxygen gas was used for purging out the solution through a spraying head. Spray was controlled by a ball valve for optimizing the flow rate and followed by heating at 450 ◦ C for 15 min and then cooled down to room temperature [117]. 3.2.6. Screen printing method It is a printing technique in which a mesh is used to transfer solution onto a substrate. In NiO nanosheet synthesis process, 0.582 g nickel (II) nitrate hexahydrate was added in 100 mL DI water, 50 mL of solution made from 4 mM oxalic acid dihydrate and 20 mM hexamethylenetetramine was added drop wise in a three-neck flask kept under stirring for 30 min followed by heating at 100 ◦ C with refluxing for 6 hr. The green precipitate was collected by filtration followed by washing with water and ethanol three times. It was then dried at 60 ◦ C in a vacuum oven for 12 h, followed by heating at 400 ◦ C for 2 h resulting in NiO nanosheets gray powder. For screen printing process, paste of NiO nanosheets was prepared by mixing ethyl cellulose solution in 44 wt% of ethanol, 46 wt% of terpineol and 10 wt% of NiO nanosheets powder, followed by ultrasonication and vigorous stirring. Excess ethanol was evaporated by rotary evaporation. 1 ␮m thick NiO thin film was screen printed on the FTO/compact-TiO2 /mesoporous-TiO2 /ZrO2 substrate and was sintered at 500 ◦ C. Cross-section SEM image of NiO nanosheet layer is shown in Fig. 5 (vi) [118]. 3.2.7. RF Sputtering method High vacuum of <5 × 10−4 Pa was maintained for sputtering process to achieve uniform layer of metal oxide on the substrate. Ni metal was used as target and oxygen was used as a carrier gas at fixed pressure. A deposition rate of 2 nm/min was monitored, variation in the thickness of the NiO films were produced by controlling the growth time [117]. A comparison of the surface morphology of NiO using RF sputtering and spray pyrolysis method for NiO deposited film is shown in Fig. 5 (vii). 3.2.8. e-beam evaporator method In this method, wide range of deposition rate can be controlled from 1 nm per min to few micrometers per min. In this literature, ebeam evaporator was used for depositing 10-nm thick Ni metal on the surface of ITO substrate. The substrate was annealed in presence of O2 with varying annealing temperatures for 20 min. This step

is essential for oxidation of Ni to form NiOx thin film. Schematic diagram NiOx layer is shown in Fig. 5(viii) [119]. 3.3. Molybdenum trioxide (MoO3 ) Hole Transport Layer Deposition In recent years, transition metal oxides (MoOx ) have been used as either an interlayer or buffer layer for a variety of optoelectronic devices to improve either hole injection or hole extraction (photovoltaic’s). The use of MoOx /Al layer in perovskite solar cell provides good device performance as compared to Ag layer. Varying the thickness of MoOx can directly affect the cell performance. MoO3 is one of the most promising HTL materials because of its non-toxicity and stability in ambient conditions. MoOx thin films showed high density and good coverage. Thin molybdenum oxide (MoOx ) layer enables the use of Al as a cost-effective electrode alternative. This type of contact uses an oxygen-deficient molybdenum trioxide layer, which has a high work function that enables efficient hole extraction. Table 5 shows the PSC device performance which was fabricated using MoO3 based HTL. 3.3.1. Thermal evaporation method Vacuum thermal evaporator was used to fabricate the ultrathin MoO3 layers on the graphene- or ITO-coated glass substrates with deposition rate of 0.1 Ås−1 . The thickness of MoO3 layer ranges from 1 to 4 nm, substrate was further kept for annealing at 150 ◦ C for 10 min. Quartz crystal sensor is used for monitoring the deposition rate and the thickness of the material [52]. The MoOx /Au/Ag/MoOx multilayer top electrode as shown in Fig. 6 (i) was deposited subsequently with thermal evaporation method, with the thickness of 10, 1, 10, and 40 nm and deposition rate of 0.1, 0.02, 0.1, and 0.1 nm/s respectively [100,141]. In another literature, the depo˚ sition rate was calibrated using a profilometer. A rate of 0.5 A/s was optimized for deposition of MoO3 material [142]. Similarly, 10 nm of MoOx and WOx were deposited using thermal evaporator [143] whereas, 0–20 nm of MoOx were deposited [144]. In another system, the MoOx interlayer was deposited on spiro-OMeTAD at a ˚ at a base pressure lower than 4 × 10−6 torr. The rate of 0.2–1.0 A/s thickness of the MoOx was varied from 8 to 200 nm [145]. 3.3.2. Spin Coating method Thermal decomposition solution process: MoO3 solutions were prepared by a milder thermal decomposition solution method. Under this method ammonium heptamolybdate (NH4 )6Mo7 O24 .4H2 O was dissolved in deionized water in open round-bottom flask so that NH3 can volatilized in air and was kept for stirring at 80 ◦ C for 60 min. The prepared solution was diluted in deionized water (0.2, 0.5, 1.0, and 2.0 wt%) and were further used for making MoO3 layer. Hot ITO substrate was used for spin coating process; the MoO3 solutions with varying concentrations were spun on substrate at 4000 rpm for 40 s and then annealed at 100 ◦ C for 10 min [146]. Nano particle synthesis process: In this method, the solution of 2.0 mg mL−1 ammonium paramolybdate (NH4 )6Mo7 O24 .4H2 O was synthesized first. A fixed amount of TiO2 (P25) was kept under ultra-sonication with 8:1 ratio of MoO3 to TiO2 followed by stirring in an open round-bottom flask at 80 ◦ C for 60 min, removing excess water and resulting TiO2 /MoO3 core/shell nanoparticles solution with a concentration of 5 mg mL−1 . The prepared mixture was mixed with PEDOT: PSS solution to form the precursor with varying TiO2 :MoO3 NPs concentrations and was subsequently spin coated on the ITO coated substrates at 4000 rpm for 40 s, and heated at 120 ◦ C for 10 min. This method obtained thickness of 40 nm as shown in Fig. 6 (ii) [147].

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

185

Table 5 The methods adopted for achieving highly efficient cell parameter using molybdenum trioxide (MoO3 ) and other oxides based HTMs for perovskite solar cell. S.N

HTM

Type

Method

Cell structure for MoOx

Voc (V)

Jsc (mA cm−2 )

FF

Eff. (%)

Ref.

1 2 3 4 5 6 7 8 9 10 11 12 13

MoOx MoOx MoOx MoO3 MoO3 MoO3 MoOx MoO3 MoOx MoOx MoOx MoOx MoOx

Planar p-i-n Planar p-i-n Tandem Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Planar p-i-n Mesoporous n-i-p Planar p-i-n Mesoporous n-i-p Mesoporous n-i-p Planar p-i-n

Vacuum thermal evaporation Vacuum thermal evaporation Vacuum thermal evaporation Spin coating Vacuum thermal evaporation Vacuum thermal evaporation Vacuum thermal evaporation Spin Coating – Vacuum thermal evaporation Vacuum thermal evaporation Vacuum thermal evaporation

ITO/MoO3 /PEDOT:PSK/PVK/C60 /BCP/LiF Graphene/MoO3 /PEDOT:PSK/PVK/C60 /BCP/LiF CIGS/ITO/TiOx /PVK/spiro-OMeTAD/MoOx ITO/MoO3 /PEDOT:PSS/PVK/C60 /Bphen/Ag ITO/MoO3 /NPB/PVK/C60 /BCP/Al ITO/ZnO/PVK/spiro-OMeTAD/MoO3 /Ag ITO/MoO3 /PVK/PCBM/Ag ITO/PEDOT:PSS:TiO2 /MoO3/ PVK/C60 /Bphen/Ag FTO/TiO2 /PVK/spiro-MeTAD/MoOx /Ag FTO/TiO2 /PVK/spiro-OMeTAD/MoOx /Al FTO/TiO2 /PVK/Spiro-OMeTAD/MoO3 FTO/TiO2 /PVK/spiro-MeTAD/MoOx /ITO (transparent) ITO/MoOx /PVK/PCBM/C60 /BCP/Al

0.97 1.03 1.00 1.12 1.04 0.99 0.96 0.938 0.990 0.93 0.821 0.94

22.6 21.9 21.49 18.1 22.4 18.8 18.39 18.5 19.55 17 14.5 16.1

0.83 0.72 0.69 0.68 0.574 0.71 0.67 0.67 0.59 0.6 0.519 0.39

18.2 16.1 15.5 14.87 13.7 13.4 13.1 11.75 11.6 11.42 9.5 6.2 5.9

[52] [52] [141] [146] [142] [148] [143] [147] [149] [144] [145] [149] [114]

S.N

HTM

Type

Method

Cell structure of other oxides

Voc (V)

Jsc (mA cm−2 )

FF

Eff. (%)

Ref.

1 2 3 4 5 6 7 8 9 10

MoS2 MoS2 MoS2 MoS2 WO3 WOx WO3 WO3 WO3 VOx

Mesoporous n-i-p Planar p-i-n Mesoporous n-i-p Planar p-i-n Planar p-i-n Mesoscopic n-i-p Mesoscopic n-i-p Mesoscopic n-i-p Mesoscopic n-i-p Planar p-i-n

Spray coating Theoretically Spin coating Centrifugal casting Spin coating Spin coating Spin coating Electro spraying Spin coating Vacuum thermal evaporation

FTO/TiO2 /PVK/MoS2 -QD/Au ITO/MoS2 /PVK/TiO2 /Ag FTO/TiO2 /PVK/MoS2 /spiro-OMeTAD/Au ITO/MoS2 /PVK/PCBM ITO/WO3 /PVK/PCBM/Al FTO/WO3 /TiO2 /PVK/spiro-OMeTAD/Ag FTO/WO3 /PVK/spiro-OMeTAD/Au FTO/WO3 /TiO2 /PVK/spiro-OMeTAD/Ag FTO/WO3 /Cs2 CO3 /PCBM/PVK/P3HT/Au ITO/VOx /PVK/PCBM/C60 /BCP/Al

1.11 0.93 0.93 0.84 0.92 0.71 0.66 0.87 0.84 1.02

22.81 26.25 21.09 12.60 18.10 21.77 18.00 17.00 20.4 20.8

0.79 0.84 66.2 0.57 0.64 9.58 0.61 0.76 0.61 0.56

20.12 20.53 13.09 6.01 7.68 8.99 7.04 11.24 10.49 11.7

[150] [151] [152] [153] [154] [155] [156] [157] [158] [114]

Fig. 6. (i) The schematic diagram and TEM cross-sectional image of the photovoltaic device, reprinted from Ref. [141]. (ii) (a) Architecture of PSC and (b) cross-sectional SEM image of PSC, reprinted from Ref. [147].

3.4. Carbon layer Deposition The uniqueness of this material is its outstanding physical and chemical properties such as its attractive nanostructure, high electrical conductivity, thermal stability, high optical transparency and excellent electro-catalytic activity [159]. As compared with Au or Pt based CEs, Carbon CEs are very cheap, earth abundant and easy to synthesize, especially for commercial purpose. Hence, using carbon CEs in perovskite solar cells will reduce material and production costs. A simple manufacturing process is required for large area production; the vacuum thermal evaporation processes of Au, Ag and Pt as noble metal electrodes are consuming very high energy and are raising the cost of the large-scale production of the solar cell. Mesoscopic perovskite solar cells structure has proved to be better in both performance and stability as compared to planar solar cell [160–165]. Replacing the expensive noble metal-based CEs with cheap and easily available earth abundant materials is the importance of carbon-based CE. Although several literatures have shown that carbon act as a promising candidate for PSCs [166–174], relevant reports on employing carbon CEs in PSCs remain scarce. List of carbon-based PSCs with its electrical parameters are provided in Table 6. The work function, homogeneity and average pore size of Carbon CEs have to be investigated in great detail for understanding of the physics behind devices. Generally, the Carbon CEs employed

carbon black and graphite as the important components, and also act as excellent candidate for the PSCs [175,176]. 3.4.1. Doctor-blade method Firstly, carbon black powder and carbon pastes were prepared from different weight ratios of graphite flake. The weight content ratio of PVAc and carbon materials was 20:80, respectively. Initially, the blended carbon powder was mixed into the ethyl acetate solution of PVAc followed with 2 h ball-milling to obtain carbon pastes. Teflon film was used for coating of the prepared carbon paste by doctor-blade method as shown in Fig. 7 (i). The prepared carbon film was directly hot-pressed with Al foil at varying pressure (0.15, 0.25 and 0.40 MPa) onto the surface of perovskite at 85 ◦ C for 15 s [177]. Graphite is also used for CE for PSC; graphite with CuI-based devices is already discussed in Section 2.1.2 [91]. Doctor blade method was used for coating conductive carbon ink on a PVK layer [178–182]. The substrates were then annealed at 70 ◦ C for 40 min [183]. Similarly, commercial carbon paste heat treatment was done at 100 ◦ C for 30 min. Preparation of carbon CEusing doctor-blade method shown in Fig. 7 (ii) [184]. The thickness of the film is controlled by the adhesive tape. Low-cost commercial conductive-carbon paste was dried at 120 ◦ C for 1 h to remove the solvent, which can destroy the structure of perovskite. Subsequently, 5 g of dried conductive-carbon sample and 4 g of

186

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

Table 6 The methods adopted for achieving highly efficient cell parameter using carbonas an HTM-free perovskite solar cell. S.N HTM

Type

Method

Cell structure

Voc (V) Jsc (mA cm−2 ) FF

Eff. (%) Ref.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59

Mesoporous p-i-n Mesoporous n-i-p Multi-layered n-i-p Mesoporous n-i-p Mesoporous n-i-p Multi-layered n-i-p Mesoporous n-i-p Multi-layered n-i-p Mesoporous n-i-p Mesoporous n-i-p Multi-layered n-i-p Mesoporous n-i-p Multi-layered n-i-p Mesoporous p-i-n Multi-layered n-i-p Mesoporous n-i-p Multi-layered n-i-p Mesoporous n-i-p Multi-layered n-i-p Multi-layered n-i-p Mesoporous n-i-p Multi-layered n-i-p Mesoporous n-i-p Multi-layered n-i-p Multi-layered n-i-p Planar p-i-n Mesoporous n-i-p Multi-layered n-i-p Mesoporous n-i-p Multi-layered n-i-p Mesoporous n-i-p Mesoporous n-i-p Multi-layered n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Multi-layered n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Multi-layered n-i-p Mesoporous n-i-p Multi-layered n-i-p Planar p-i-n Mesoporous n-i-p Multi-layered n-i-p Mesoporous n-i-p Mesoporous n-i-p Multi-layered n-i-p Multi-layered n-i-p Mesoporous n-i-p Mesoporous n-i-p Planar p-i-n Mesoporous n-i-p

e-beam Doctor blading Screen printing Doctor blading Doctor blading Screen printing Screen printing Blade coating Paint Printing Screen printing Screen printing Screen printing e-beam Screen printing Hot press Screen printing Screen printing Screen printing Screen printing Screen printing Printed Doctor blading Printing Screen printing Inkjet printing Spin coating Screen printing – Screen printing Rolling transfer Doctor blading Screen printing Blade coating Doctor blading Printing Hot press Screen printing Spin coating Screen printing Doctor blading Doctor blading Doctor blading Spin coating Doctor blading Screen printing Printing Doctor blading Doctor blading Doctor blading Screen printing Doctor blading Doctor blading Screen printing Screen printing Screen printing printing Doctor blading Soot of burning candle

FTO/PEDOT:PSS/MAPbI3−x Clx /PCBM/C FTO/TiO2 /mp-TiO2 /PVK/CuPc nanorods/C FTO/TiO2 /mp-TiO2 /ZrO2 /perovskite/C FTO/TiO2 /mp-TiO2 /PVK/doped-TPDI/C FTO/TiO2 /mp-TiO2 /PVK/Spiro-OMeTAD/C FTO/c-TiO2 /TiO2 /Al2 O3 /NiO/C/PVK FTO/TiO2 /mp-TiO2 /PVK/Al2 O3 /C FTO/TiO2 /mp-TiO2 /PVK/ZrO2 /NiO/C/graphite FTO/TiO2 /mp-TiO2 /PVK/C FTO/TiO2 /mp-TiO2 /PVK/C FTO/TiO2 /mp-TiO2 /ZrO2 /PVK/C FTO/TiO2 /mp-TiO2 /PVK/NiO nanosheets/C FTO/m-TiO2 /ZrO2 /C (perovskite infiltration) FTO/PEDOT:PSS/PVK/PCBM/C60 FTO/TiO2 /mp-TiO2 /ZrO2 /PVK/C FTO/TiO2 /PVK/Carbon/Al FTO/TiO2 /mp-TiO2 /ZrO2 /PVK/C FTO/TiO2 /mp-TiO2 /PVK/ZrO2 /C FTO/bl-TiO2 /mp-TiO2 + PVK/mp-ZrO2 + PVK/C FTO/bl-TiO2 /mp-TiO2 + PVK/mp-ZrO2 + PVK/C FTO/TiO2 /mp-TiO2 /PVK/Al2 O3 /C FTO/m-TiO2 /Al2 O3 /NiOx /C (dip-coating PVK) FTO/TiO2 /mp-TiO2 /PVK/C FTO/m-TiO2/2D/3D-PVK/ZrO2 /C FTO/bl-TiO2 /mp-TiO2 + PVK/mp-ZrO2 + PVK/C FTO/TiO2 /PVK/Carbon FTO/TiO2 /PVK/Carbon FTO/TiO2 /NiO/PVK/C FTO/TiO2 /mp-TiO2 /PVK/mp-ZrO2 /C FTO/TiO2 /mp-TiO2 /Al2 O3 /PVK/C FTO/TiO2 /PVK/carbon (cathode soot) FTO/TiO2 /mp-TiO2 /PVK/C/PDMS FTO/d-TiO2 /mp-TiO2 /PVK/ZrO2 /C/graphite FTO/d-TiO2 /W-doped mp-TiO2 /PVK/C FTO/TiO2 /mp-TiO2 /PVK/C FTO/TiO2 /mp-TiO2 /PVK/C FTO/TiO2 /PVK/carbon/graphite sheet FTO/c-TiO2 /silver contact/mp-TiO2 /PVK/ZrO2 /C FTO/TiO2 /PVK/carbon FTO/TiO2 /mp-TiO2 /ZrO2 /PVK/C FTO/TiO2 /PVK/carbon FTO/C-ZnO/PVK/C FTO/TiO2 /mp-TiO2 /PVK/Spiro-OMeTAD/C FTO/TiO2 /PVK/carbon FTO/TiO2 /PVK/carbon FTO/d-ZnO/PVK/ZnO NR layer/ZrO2 /C FTO/TiO2 /mp-TiO2 /PVK/C FTO/TiO2 /mp-TiO2 /porous Al2 O3 /PVK/C FTO/TiO2 /PVK/Carbon FTO/TiO2 /mp-TiO2 /PVK/C FTO/bl-TiO2 /mp-TiO2 + PVK/mp-ZrO2 + PVK/C FTO/TiO2 /mp-TiO2 /PVK/C FTO/TiO2 /mp-TiO2 /PVK/boron and phosphorus co doped C/Al FTO/bl-TiO2 /mp-TiO2 + PVK/mp-ZrO2 + PVK/C FTO/d-ZnO/PVK/(ZnO/TiO2 NR layer)/ZrO2 /C FTO/TiO2 /mp-TiO2 /PVK/Al2 O3 /C FTO/TiO2 /mp-TiO2 /PVK/C ITO/PEN/ZnO/PVK/C FTO/TiO2 /mp-TiO2 /PVK/spiro-OMeTAD/C/FTO

0.96 1.05 0.94 1.03 1.12 0.915 0.893 0.917 1.0 1.04 1.00 0.965 0.863 0.97 0.92 1.002 0.867 0.957 0.900 0.858 0.846 0.945 1.00 0.857 0.894 0.95 0.89 0.89 – 1.04 0.90 0.97 0.868 0.857 0.803 0.97 0.952 0.861 0.90 0.88 0.80 0.810 1.08 1.01 0.90 0.960 1.35 0.78 0.77 0.81 0.867 0.85 0.90 0.878 0.843 0.457 1.29 0.76 0.82

16.2 16.1 15.60 15.5 15.29 15.03 15.0 14.9 14.58 14.38 14.35 14.2 14.3 14.0 13.89 13.53 13.41 13.24 13.14 12.84 12.3 12.12 12.02 11.9 11.63 11.60 11.44 11.4 11.07 11.03 11.02 10.8 10.64 10.53 10.4 10.19 10.20 9.53 9.35 9.10 9.08 8.73 8.70 8.61 8.31 8.23 8.09 8.0 8.07 7.29 7.08 6.88 6.78 6.64 5.56 5.13 5.0 4.29 4.24

Carbon Carbon Carbon Carbon Carbon Carbon/graphite Carbon Carbon Carbon Carbon Carbon Carbon Carbon C60 Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon/graphite Carbon Carbon Carbon/graphite Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon Carbon/graphite Carbon Carbon

zirconium dioxide pearl were dispersed in 15 mL of chlorobenzene and milled for 2 hours in electro-mill. The carbon electrodes were prepared as discussed in [183] and followed by drying at room temperature for 1 hour [179]. 3.4.2. Printing method The screen-printing method was used for printing carbon film of varying thickness ranging from 500 nm to 10 ␮m, followed with heating at 400 ◦ C for 30 min and then cooled down to room temperature. Carbon film with 1 ␮m shows 10.3% power conversion efficiency [185]. In another system, printing of 10 ␮m thick carbon black/graphite composite slurry on ZrO2 layer and was annealed at 400 ◦ C for 30 min as shown in Fig. 7 (iii) [186]. Similarly, carbon

23.69 20.8 21.45 20.1 20.42 21.62 22.43 21.36 21.83 21.27 19.31 20.4 21.5 22.47 19.21 21.30 22.93 18.15 20.45 22.8 20.04 17.22 22.67 23.60 18.06 17.20 21.43 18.2 – 15.37 17.00 23.5 20.1 20.79 20.1 19.1 18.73 16.57 15.98 15.10 21.02 19.98 18.42 14.20 16.78 14.82 8.35 15.1 18.56 18.40 15.24 18.3 12.35 12.4 14.19 19.63 5.7 13.38 12.30

0.71 0.74 0.77 0.75 0.67 0.76 0.75 0.76 0.67 0.65 0.74 0.72 0.77 0.64 0.78 0.634 0.67 0.76 0.72 0.66 0.723 0.69 0.53 0.587 0.72 0.71 0.60 0.71 – 0.69 0.72 0.474 0.61 0.59 0.644 0.55 0.572 0.667 0.65 0.68 0.54 0.54 0.429 0.60 0.55 0.58 0.72 0.68 0.56 0.50 0.54 0.44 0.61 0.61 0.46 0.572 0.68 0.42 0.42

[55] [72] [193] [194] [195] [106] [196] [106] [197] [198] [199] [137] [200] [55] [159] [177] [160] [161] [185] [186] [162] [163] [164] [165] [187] [188] [189] [137] [166] [167] [91] [168] [169] [170] [171] [172] [173] [174] [191] [199] [183] [179] [195] [192] [184] [175] [176] [178] [179] [180] [190] [181] [182] [201] [175] [67] [68] [179] [69]

paste was synthesized as discussed earlier. 2 g of 30 nm size carbon black powders mixed with 6 g of graphite powders in a 30 mL of terpineol solvent, followed by adding 1 g of 20 nm sizes ZrO2 nano-powder and 1 g of hydroxypropyl cellulose into solution. The prepared solution was kept for stirring using ball milling for 2 h. Subsequently, printed a 9 ␮m mesoscopic carbon layer on the substrate, and annealed at 400 ◦ C for 30 min, as shown in Fig. 7 (iv) [187]. 3.4.3. Inkjet printing method In this technique two different processes were used. In first process, carbon black ink was prepared by dispersing carbon black into isopropanol solution and was then used for printing

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

187

Fig. 7. (i) (a) Images of conductive carbon ink (b) conductive carbon film prepared on a plastic substrate at 70 ◦ C, reprinted from Ref. [183]. (ii) Carbon electrodes are prepared by doctor-blade method, reprinted from Ref. [184]. (iii) Cross-section schematic drawing shows the three-different layer perovskite based fully printable mesoscopic PSC with mesoporous of TiO2 , ZrO2 and carbon layers, reprinted from Ref. [186]. (iv) The schematic architecture of a CCE based PSCs with SEM image of the devices, reprinted from Ref. [187]. (v) Schematic representation shows inkjet printing of the C/CH3 NH3 PbI3 PSCs and different method also shows for comparison in step 3 and step 4, reprinted from Ref. [188]. (vi) HTM-free PSCs on flexible electrode, reprinted from Ref. [173].

on the PbI2 layer, followed by immersion into a CH3 NH3 I bath to obtain the final TiO2 /CH3 NH3 PbI3 /C planar PSC. In second process, Carbon black and CH3 NH3 I were mixed in isopropanol for synthesizing a reactive ink. The PbI2 layer converted into CH3 NH3 PbI3 instantly, resulting in improved interfaces of TiO2 /CH3 NH3 PbI3 /C. The end of both processes was followed by 1 h annealing. All steps are shown in a schematic diagram of Fig. 7 (v) [188]. Similarly, carbon CE were prepared using carbon paste processed at 70–100 ◦ C and coated on the surface of perovskite/TiO2 substrate resulting HTM free PSCs. Preparation of carbon layer by doctor blade method using low-temperature conductive carbon ink onto the surface of perovskite layer, followed by annealing at 100 ◦ C for 30 min [189].

3.4.4. Screen-printing method Initially, carbon powder was prepared by mixing solution of butadiene-styrene rubber and ethyl cellulose in ethyl acetate followed by ball milling at 350 rpm for 4 hr. Subsequently carbon paste was screen printed onto the perovskite layer by two-step process, as shown in Fig. 7 (vi). A desired shape of graphite paper and carbon paste were pressed together to obtain electrode followed by drying at the room temperature to completely evaporate the solvent [173]. In a different system for preparation of carbon paste, desired amount of graphite powder was mixed with ethyl cellulose in terpineol. Similarly, Graphite/carbon black paste (20% content of carbon black) was prepared. All the mixtures were ball milled for 2 h. followed by printing of the prepared carbon material on the surface

188

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

of the nanoporous TiO2 /ZrO2 layer and was annealed at 400 ◦ C for 30 min [190]. 3.4.5. Drop casting method Three carbon materials (carbon black, multi-layer carbon nanotube (MWCNTs), and graphite) were well dispersed in chlorobenzene at a concentration of 10 mg mL−1 with a probe ultrasonic processor. The carbon material solution was then drop casted on the top of the TiO2 /PbI2 substrates. After coating, the substrates were heated at 50 ◦ C for another 5 min and then cooled down to room temperature [191]. 3.4.6. Spin coating method Nanocarbon coated substrate was synthesized by spin coating method. 15 mg mL−1 of nanocarbon particles dispersed in chlorobenzene was spin coated onto the surface of TiO2 /PbI2 substrate followed by drying at room temperature and annealing at 100 ◦ C for 5 min [192]. 3.5. SWCNTs, MWCNTs and reduced graphene oxide layer deposition Carbon based nanomaterials, like carbon nanoparticles, carbon nanotubes (CNT) and graphene flakes, are one of the most attractive alternative HTM and CE materials. They have been used in different generations of solar cells such as dye sensitized solar cells [202], quantum dot solar cell [202], heterojunction solar cells [203], polymer solar cells, and perovskite solar cell. However, as scope of this review article is limited to the perovskite solar cells, we will focus on the use of allotropes of carbon with context to perovskite solar cells. The progress of PSCs in terms of electrical performances and fabrication process has been listed in Table 7. Carbon is an earth abundant raw material, low-cost and possesses excellent chemical and physical properties. Carbon based nanomaterial thin films, showed high electrical conductivity and are semi-transparent and flexible. Single-walled carbon nanotube (SWCNTs) based thin films are attractive due to the excellent optical properties with superior conductivity however, these properties are not available in MWCNTs. Also, the conductivity of SWCNTs thin films can be improved by improving the synthesis processes, as shown in Fig. 8 (i) [56]. 3.5.1. Dynamic spin-coating method High-pressure carbon monoxide method (HiPco) was used to synthesize powdered singled-walled carbon nanotubes (SWCNTs) with lengths 100–1000 nm. The samples used in this work were purchased as ‘purified’ tubes (<15 wt % iron catalyst impurities). 3.0 mg of r-P3HT were dissolved in 5 mL of chlorobenzene followed with sonication for 60 min. 2.5 mg purified HiPco SWCNTs was mixed with polymer solution and dispersed by using an ultrasonic probe for 10 min followed with centrifuge. From the surfactant excess polymer was removed by adding 15 mL of toluene. The prepared mixture was annealed at low temperature for 60 min. to produce aggregation of the functionalized SWCNTs. The resultant material was centrifuged for 4 min at 16,000 gto collect the precipitate. The precipitate was dispersed in toluene and followed with sonication for 15 min to fully re-dissolve and was annealed at low temperature for 15 min to produce aggregation and then centrifuged. The supernatant was then discarded. This procedure from bath sonication to centrifugation was repeated three times to remove all excess polymers resulting in the final supernatant being fully transparent. Finally, pellet consisting of 2.0–2.2 mg of functionalized nanotubes, were mixed in chloroform followed with sonication for homogenous mixing before spin-coating. 200 ␮l of the solution was spin coating onto the perovskite layer, at 3000 rpm for 90 s as shown in Fig. 8 (ii) [204]. In another system, Dynamic

drop-by-drop spin-coating from a chloroform solution was found to be the most effective and reproducible method for producing dense SWCNT films. Non-uniformity of the SWCNT film was observed by use of other solvents such as chlorobenzene and toluene. Films produced by non-dynamic spin-coating did not exhibit a comparable degree of uniformity as the dynamic drop-by-drop method as shown in Fig. 8 (iii) [205]. 3.5.2. Spin-coating method The GO (as purchased) dispersed solution was prepared by mixing 3 mg of GO powder with 1 mL chlorobenzene, via shaking for 2 min and filtering with a 220-nm filter, resulting in 340 mg L−1 concentration. As prepared solution was further diluted to 85 mg L−1 in chlorobenzene and was spin coated onto the surface perovskite substrate at 7000 rpm for 30 s. The substrates were treated by chlorobenzene via spin coating. After 30 min, spiroOMeTAD solution as a HTM was spin coating at 4000 rpm for 30 s [206]. Whereas in case of MWNTs, nano solution of ∼5 nm MWNT was additionally added and stirred for 4 h to prepare the spiro- OMeTAD/MWNT solution [207]. In another system Hummers method is used for synthesis of grapheme oxide. Graphene oxide films were transferred on the substrate using Langmuir-Blodgett (L-B) technique. Finally, two type of graphene paste were prepared for HTM-free perovskite solar cell, firstly, 15 mg thermally reduced graphene was dispersed in 2 mL chlorobenzene to make a protective paste, which was spin-coated on PbI2 film to retard the dissolving of PbI2 in the subsequent reactive paste. CH3 NH3 I was firstly prepared according to a reported procedure. The reactive paste composed of 15 mg of thermally reduced graphene and 15 mg CH3 NH3 I in 2 mL isopropanol with ultrasonic techniques, which was drop-cast on to the photoanode [208]. GO was synthesized by reported literature [209], in which, a mixture of 10 mg of GO and 100 mg of ODA were dissolved in 5 mL of DCB under ultra-sonication. 3 mL of prepared dispersion was mixed with 15 mL of THF, followed by sonication and then was centrifuged at 15,000 rpm for 1 h. The separated, functionalized GO was washed once with 10 mL ethanol and several times with 1 mL THF. It was then dried under vacuum chamber. The chemically modified GO with ODA were then dissolved in chlorobenzene. 40 mg mL−1 of PMMA solution in chlorobenzene was also prepared. The both solutions were sequentially spin-coated onto the surface of the substrate at 4000 rpm for 30 s, followed by heating at 70 ◦ C for 30 min [210]. In another approach, Hummer’s and Offeman’s method was used to synthesize GO from natural graphite. For dispersion of RGO/GO, 50 mg as prepared RGO or GO powder was mixed in 50 mL chlorobenzene and kept for sonication for 2 hr followed by centrifugation for 25 min at 10,000 rpm to remove large RGO or GO particles. The recovered supernatant from the solution was again sonicated. Aggregates of RGO were removed by centrifugation and the supernatant was recovered. The resultant solution with a concentration of about 0.05–0.08 mg mL−1 was used as stock solution for spin-coating. Finally, RGO was deposited dynamically onto the surface of perovskite substrate by spin-coating 600 ␮L RGO or GO dispersion at 2000 rpm for 120 s. Schematic diagram is shown in Fig. 8 (iv) [211]. 3.5.3. PDMS stamp method SWCNTs were transferred by a PDMS stamp from strips of millipore filter paper to the top of the device [212]. GO was obtained from a modified Hummer’s method using flake expandable graphite as reported elsewhere [213–215] 3.5.4. Printing Method For preparation of HTM 1 g of SWCNTs (as purchased) was mixed in 20 mL ethyl alcohol and was kept for sonication for 30 min, followed by addition 0.588 g ZrO2 (50 nm), 4 g graphite and 1 g

Table 7 The methods adopted for achieving highly efficient cell parameter using SWCNT/MWCNT/graphene as HTM or HTM-free for perovskite solar cell. HTM

Type

Method

Cell structure

Voc (V)

Jsc (mA cm−2 )

FF

Eff. (%)

Ref.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

SWCNTs:spiro-OMeTAD SWCNTs SWCNTs MWCNTs GO MWCNTs SWCNTs-carbon SWCNTs CNTs SWCNTs/GO/PMMA Thiolatednano graphene SWCNTs MWCNTs Graphene GO MWCNTs multi-layered graphene CNTs GO r-GO CNTs CNTs MWCNTs Graphene SWCNTs/GO CNTs film CNTs Iodide-r-GO SWCNTs CNTs CNTs CNTs Bamboo-structured CNTs CNTs film CNTs Graphene Single-walled CNTs Graphene Graphite SWCNTs CNTs sheet

Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Planar n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Planar p-i-n Planar n-i-p Multi-layered n-i-p Mesoporous n-i-p Planer p-i-n Planar p-i-n Planar p-i-n Mesoporous n-i-p p-i-n Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p Planar n-i-p Planar n-i-p Mesoporous n-i-p Mesoporous n-i-p Mesoporous n-i-p

CVD Spin coating Spin coating Drop casting Spin coating Spin coating Spin coating Press transfer Dripping/screen printing Spin coating Spin coating Screen printing Drop cast Chemical vapor deposition Spin coating Doctor blading Spin coating PDMS stamp Spin coating Spin coating Transfer

FTO/TiO2 /PVK/SWCNTs:Spiro-OMeTAD FTO/TiO2 /mp-Al2 O3 + PVK/SWCNTs+/spiro-OMeTAD/Ag FTO/TiO2 /mp-Al2 O3 + PVK/SWCNTs + P3HT-PMMA/Ag FTO/TiO2 /mp-TiO2 /PVK/Al2 O3 /B-doped MWNTs FTO/TiO2 /PVK/grapheme oxide/spiro-OMeTAD/Au FTO/TiO2 /PVK/MWCNT + spiro-OMeTAD/Au FTO/TiO2 /Al2 O3 //PVK/SWCNTs-carbon FTO/TiO2 /mp-TiO2 /PVK/spiro-OMeTAD/SWCNTs FTO/b-TiO2 /mp-TiO2 /PVK/CNT/C FTO/TiO2 /PVK/SWCNTs/GO/PMMA FTO/TiO2 /PVK/thiolatednanographene FTO/TiO2 /mp-TiO2 /Al2 O3 /NiO/SWNCTs FTO/TiO2 /PVK/MWCNTs FTO/TiO2 /PVK/spiro-OMeTAD/PEDOT:PSS/graphene ITO/(mixed with organic HTM) FTO/d-TiO2 /(TiO2 /SiO2 )/PVK/C FTO/TiO2 /PVK/multi-layered graphene ITO/PEDOT/PVK/PCBM/CNTs ITO/graphene oxide/PVK/PCBM/ZnO/Al ITO/reduced graphene oxide/PCBM/PCB/Ag FTO/TiO2 /mp-TiO2 /PVK/Al2 O3 /CSCNTs/PMMA ITO/PEDOT:PSS/PVK/PCBM/CNTs FTO/TiO2 /PVK/MWCNTs FTO/TiO2 /(PVK/3DHG)/3DHG FTO/TiO2 /PVK/SWCNTs/GO FTO/TiO2 /PVK/CNT film + spiro OMeTAD FTO/TiO2 /mp-TiO2 /PVK/Al2 O3 /CSCNTs/PMMA FTO/TiO2 /PVK/iodide-reduced graphene oxide + spiro-OMeTAD/Au FTO/TiO2 /PVK/SWCNTs TO/TiO2 /mp-TiO2 /PVK/Al2 O3 /CSCNTs/PMMA FTO/TiO2 /mp-TiO2 /PVK/Al2 O3 /CSCNTs/PMMA Ti foil/TiO2 nanotube/PVK/spiroOMeTAD + CNTs FTO/TiO2 /PVK/bamboo-structured CNTs+P3HT/Au FTO/TiO2 /PVK/CNTs film FTO/TiO2 /mp-TiO2 /PVK/Al2 O3 /CSCNTs/PMMA FTO/TiO2 /PVK bilayer/SG Single-walled-CNTs/PEDOT:PSS/PVK/PCBM/Al FTO/TiO2 /PVK/spiro-OMeTAD/graphene FTO/TiO2 /PVK/Graphite FTO/TiO2 /PVK/SWCNTs Stainless Steel/TiO2 /PVK/spiro-OMeTAD/CNTs sheet

1.1 1.02 1.02 0.92 1.04 – 1.01 1.1 1.00 0.95 0.95 0.945 0.88 0.960 0.88 0.926 0.943 0.97 0.99 0.98 0.853 0.79 0.88 0.63 0.89 1.00 0.840 0.910 0.97 0.849 0.823 0.99 0.86 0.88 0.878 0.54 0.79 0.90 0.93 0.73 0.66

20.3 21.4 22.71 20.2 – 21.26 20.3 18.97 19.4 20.56 20.7 18.00 19.17 19.18 21.3 16.7 19.5 15.59 15.4 17.22 18.1 15.60 0.89 19.1 18.1 16.21 16.73 20.3 14.91 15.81 14.36 18.75 15.46 14.43 0.878 14.9 12.56 10.30 10.47 10.2

0.61 0.71 0.66 0.77 0.73 – 0.69 0.61 0.71 0.72 0.657 0.64 0.80 19.17 0.705 0.59 0.73 0.59 0.72 0.716 0.71 0.73 0.75 18.11 0.57 0.55 0.69 0.61 0.46 0.68 0.64 0.68 0.52 0.51 0.53 14.2 0.54 0.55 0.64 0.64 0.48

15.5 15.4 15.3 15.23 15.1 15.1 14.7 13.6 13.57 13.3 12.81 12.7 12.67 12.37 11.90 11.6 11.5 11.16 11.11 10.8 10.54 10.5 10.30 10.06 9.8 9.90 9.37 9.31 9.1 8.60 8.35 8.31 8.3 6.87 6.81 6.7 6.32 6.2 6.13 4.9 3.3

[56] [204] [205] [224] [206] [207] [216] [56] [225] [210] [57] [226] [191] [227] [217] [228] [208] [212] [213] [229] [230] [231] [191] [232] [210] [219] [230] [211] [56] [230] [230] [233] [234] [219] [230] [230] [235] [223] [191] [210] [236]

Spin coating Doctor blading Spin coating Attaching Transfer Spin coating CVD Transfer Transfer Chemical vapor deposition and transferred Spin coating Attaching Transfer Spin coating Chemical vapor deposition and transferred Chemical vapor deposition Spin coating Spin coating dip-coated

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

S.N

189

190

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

Fig. 8. (i) A schematic representation of the SWCNT:Spiro-OMeTAD PSCs fabrication. Steps (1) and (2) are common method for all PSCs; only Steps (3) and (4) show HTM and CE deposition for PSCs, reprinted from Ref. [56]. (ii) (a) Device structure shows undoped spiro-OMeTAD and P3HT/SWNTs and (b) device structure shows with a P3HT/SWNT layer underneath the hole transport material (spiro-OMeTAD) matrix, reprinted from Ref. [204]. (iii) Schematic diagram of PSCs with a carbon nanotube/polymer composite as hole-transporting structure. Device architecture shows the sequential layers of FTO as transparent electrode, a TiO2 compact layer, a mesostructured layer of Al2 O3 coated with CH3 NH3 PbI3−x Clx and the hole transporting structure composed of a P3HT/SWNT layer in-filled with a PMMA matrix, reprinted from Ref. [205]. (iv) (a) Device architecture and (b) energy levels of hole transport in PSC. Blue sheet aggregates in (a) represent RGO, arrows indicate the hole transport pathways in RGO/dopant-free spiro-OMeTAD HTM, reprinted from Ref. [211]. (v) (a) Device architecture consisting of FTO as a transparent substrate, a compact TiO2 layer, a mesoporous TiO2 layer, a Al2 O3 layer coated with SWCNT added graphite/carbon black CE, (b) cross-section SEM image of a mesoscopic PSC, reprinted from Ref. [216]. (vi) (a) Shows freestanding CNT film. (b) Architecture of PSCs with CNT film electrode, reprinted from Ref. [219]. (vii) Schematic architecture of four-terminal tandem solar cell made up of a graphene-based PSC as top cell and an amorphous/crystalline silicon bottom cell, reprinted from Ref. [223].

carbon black. Ultrasonic probe treatment was carried out on the obtained solution, followed by stirring for 10 min. This process was repeated several times followed with centrifugation just to remove non-functionalized SWCNTs and other carbonaceous particles. The recovered precipitate was mixed in ethyl alcohol, and 17.6 mL terpineol. Further treatment by ultrasonic probe was done for 5 min and kept for stirring for 10 min. Ball-milling was carried out for 16 h followed by addition of 2.94 g ethyl cellulose to the remove the extra ethyl alcohol resulting in 9∼10 ␮m carbon paste. Printing mixture of graphite and carbon black were prepared in 4:1 weight ratio and

were subsequently printed onto the surface of Al2 O3 substrate as shown in Fig. 8 (v) [216]. 3.5.5. Chemical vapor deposition method In this technique, a single-layered GO sheet was deposited onto the surface of Cu foils by CVD method. The GO/Cu sheets were then transplanted onto the surface of SiO2 /Si and quartz substrates. On the surface of prepared graphene oxide/Cu thin film, PMMA layer was spin coated. Aqueous 0.1 M ammonium persulfate solution (NH4 )2 S2 O8 was used to remove the Cu foil. The PMMA/graphene

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

oxide layers were placed on the substrate and annealed at 180 ◦ C for 2 h. In the end, PMMA layer was removed using acetone [217]. In another system, carbon nanotubes (CNT) films were prepared using a previously reported procedure inside a tube furnace [218]. The perovskite film was deposited on the CNT with the help of a tape. The CNT film and perovskite surface were wetted with drops of toluene. On vaporization of toluene, excess CNT on unwanted region were wiped off with a cotton bud. Metal contact was soldered onto the CNT and FTO. Schematic diagram is shown in Fig. 8 (vi) [219]. In another report, Large-area graphene sheets were prepared via catalytic decomposition of methane on a hot 25 ␮m thick copper foil [220–222]. Previously, the copper substrate was cleaned by ultrasonication in acetone and isopropyl alcohol for 10 minutes each. Residual oxides on the surface were removed using acetic acid. The deposition of graphene by CVD was performed by the following method. First, re-crystallization of copper substrates was performed in a hydrogen flow of 2 sccm at a temperature of 1000 ◦ C. Then, the hydrogen flow was reduced to 0.7 sccm. Finally, 11.4 sccm of the carbon precursor- methane was introduced in the chamber. The deposition was carried out at a pressure of 0.5 mbar for 1 h. The deposited graphene sheets had dimension of several square centimeters. Smaller pieces of about 1 cm2 were cut from the larger sheet, and then used as electrodes. The transparent graphene electrode kept in contact with Au grid finger, deposited via thermal evaporation onto spiro-OMeTAD. Fabricated device has been shown in Fig. 8 (vii) [223].

3.6. Synthesis of Redox electrolyte Electrolytic materials are in demand since they possess high ionic conductivity which is essential for electrochemical devices (like batteries, supercapacitors, solar cell, fuel cells etc.). Inorganic HTM, organic HTM and organic conducting polymers have been studied widely for device application. Many efforts have been made to make less volatile electrolyte with high conduction. Redox couple is a main component to make an electrolyte. There are several redox couple reported for electrochemical devices such as Iodide/triiodide (I– /I3– ), copper (I/II) and series of cobalt (II/III) and nickel (III/IV) complexes, Br– /Br3– , SCN– /(SCN)3– , ferrocene/ferricenium (Fc/Fc+ ), SeCN– /(SeCN)3– etc. As this review article is basically on the inorganic hole transport layer for perovskite solar cell, so we have restricted our discussion to the topic of our interest. The redox electrolyte was synthesized by dissolving 0.9 M LiI, 0.45 M I2 , 0.5 M tert-butylpyridine (t-BP) 0.05 M urea in ethyl acetate, which was introduced into the space of the sealed electrodes [60,237]. In another system, 0.4 M LiBr and 0.04 M Br2 dissolved in acetonitrile, however, 0.15 M LiI and 0.075 M I2 dissolved in methoxyacetonitrile [54]. A Li-salt solution prepared with different solvent, 0.032 M Li-TFSI, 0.032 M LiI, and 0.2 M t-BP in the mixed solvent of chlorobenzene and acetonitrile (chlorobenzene: acetonitrile 1/4 1:0.1 v/v) was prepared [43]. In a common preparation method of polymer electrolyte, a fixed amount of Poly (ethylene oxide) (Mw ∼5,000,000, sigma Aldrich) and Poly (ethylene glycol) (Mw∼ 200, sigma Aldrich) were dissolved in acetonitrile (A2) (20 mL) in a beaker with continuous stirring at room temperature. This ratio was used as a stock solution for the overall experiment (written here as B1). Stoichiometry ratio of potassium iodide (KI) and Iodine (I2 ) were then dissolved in A2 acetonitrile (5 mL) in another beaker (written here as B2). Adding B2 (drop by drop) in B1 formed the solid polymer electrolyte solution. During this process, the polymer salt solution was continuously stirred. Finally, solution is ready to be used in PSC [238].

191

4. Discussion Significant progress has been shown in the device architecture, synthesis of material, thin film formation, etc. of hybrid perovskite materials. We have collected the low cost, simple design and easy synthesis of inorganic-HTMs for PSCs. It has been found that the most efficient perovskite solar cells follow a p-i-n type of architecture with lot of possible p and n-type materials for efficient carrier extraction. In this review article, different types of inorganic hole transport materials have been discussed. Also describe the unique synthesis techniques with compatibility on other materials. The main purpose of HTM in PSCs is to extract holes from perovskite sensitizer layer. Due to the natural abundance of copper, Cu based HTMs has good transparency in ultra violet, visible and infrared spectrum range with wide band-gap, high hole mobility (CuSCN 0.01–0.1, CuI 0.5–2, Cu2 O 100, CuO 0.129, CuGaO2 0.01–10, CuAlO2 3.6 and CuCrO2 7.7 cm2 V−1 s−1 ) [71], relatively good chemical stability, and can be synthesized by simple preparation process and remain as an inexpensive material for PSCs. Cu based inorganic HTMs have become a promising alternative for photovoltaic system. Cu based HTMs like CuI, CuSCN, copper oxides, CuS, CuGaO2 , CuCrO2 , CuAlO2 , Cu3 SbS4 , and CZTS have been extensively used owing to their band matching and good efficiency. Inorganic HTM such as CuI, CuSCN and Copper oxides are the popular for both n-i-p and p-i-n type of devices Whereas, handful of literatures available on CuS, CuGaO2 , CuCrO2 , CuAlO2 , Cu3 SbS4 , and CZTS inorganic HTMs type of PSCs as shown in Fig. 9a. Number of PSCs made up of CuSCN, CuI and Copper oxides based inorganic HTM has been reported with excellent device performance as compare to the other Cu based inorganic HTMs. Spin coating, doctor blading and thermal evaporation are the popular methods used for deposition of inorganic HTMs. The device performance in terms of reproducibility crossing 15% PCEs from different laboratory around the globe can be easily estimated as shown in Fig. 9b. The highest PCE reported for CuSCN based PSC (Cs0.05 (MA0.17 FA0.83 )0.95 Pb(I0.83 Br0.17 )3 ) is greater than 20% with stability of 1000 h at 60 ◦ C at nitrogen atmosphere [45]. Whereas, poor performance of the device depends on various parameters such as type of perovskite sensitizer used, Interface layer and synthesis condition. Spin coating, doctor blading, Spray coating, thermal evaporation and vacuum sputtering method forms a smooth and uniform thin film with high electrical performance (Jsc ∼ 22 mA cm−1 and Voc ∼ 1.1 V). However, stability of the device is the major concern of these PSC’s, other Cu oxides such as CuGaO2 shows high electrical performance but device stability is not as good as in case of CuSCN. The highest PCE of CuGaO2 based PSC (MAPbI3−x Clx ) shows above 18% PCE with slight drop stability at 25 ◦ C [49]. Low J–V hysteresis was observed in the Cu based PSCs as compared to the conventional spiro-OMeTAD-based PSCs therefore all these properties make these materials of great interest. Bar graph and point graphs shows the electrical parameter reported in various literature (Fig. 9a–c). Oxides such as NiOx , MoOx , WO3 and VOx also show good device performance. One can easily achieve ∼15% PCE with NiOx based any device structure, whereas; the highest PCE is reported above 20% for FAPbI3 /MACl based perovskite with good thermal and light soaking stability of 500 h [121]. The best cell made up of MoS2 had also attained 20% PCE with loss of 8.8% out of 100% till 1000 h [150]. On other hand, MoOx type of oxides shows good device performance with planar p-i-n structure (∼18% PCE), whereas; other device structure such as mesoscopic n-i-p and tandem shows above 10 and 15% PCE respectively. However, only few literatures are available on WO3 and VOx based PSCs, the popular devices structure for WO3 are planar p-i-n and mesoscopic n-i-p with highest PSC crossing 10% as shown in Fig. 10a. Here spin coating and vacuum thermal evaporation are widely used methods for preparation of oxides films. Out of all oxides, NiOx is widely studied material

192

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

Fig. 9. Cu thin film-based PSCs (a) mesoscopic n-i-p and planar p-i-n based devices, (b) different method used for synthesizing thin film and it PCEs, (c) thin film synthesis method and Voc and (d) thin film synthesis method and Jsc .

for PSCs and shows excellent electrical results crossing 20% PCE, Voc ∼ 1.1 V and Jsc > 22 mA cm−1 by using spraying and spin coating techniques. NiOx based PSCs shows better results in terms of stability as compare to MoOx , WO3 and VOx based devices, Other methods such as spray pyrolysis, combustion, PLD, electro deposition, vacuum deposition, screen printing and dip coating method hold a lot of promise as their usage in the device show good and reproducible results with >15% PCE, Voc ∼ 1.1 V and Jsc > 20 mA cm−1 as shown in Fig. 10b–d. However, the methods like RF sputtering and e-beam evaporation shows poor device performance as compare with other techniques and also raising the cost of device fabrication. Highly uniform films of MoO3 oxides were mainly synthesized by vacuum thermal evaporation technique and shows good device performance as compare with other methods. As compare to other oxides, Inorganic NiOx based PSCs is showing promising stability results in presence of ambient atmosphere with superior thermal and chemical stability. As discussed in the carbon, SWCNTs, MWCNTs and reduced graphene oxide sections that these materials have remarkable electrical, physical and chemical properties. Using simple manufacturing process one can easily fabricate low-cost large area PSCs. Lot of literatures available on the material and its device fabrication. Several type of device structures is compatible with these materials such as multi-layer n-i-p, planar (p-i-n and n-i-p) and mesoscopic (n-i-p and p-i-n). However, n-i-p type of structure is well studied structure and shows reliable and excellent cell performance.

Carbon based multi-layer n-i-p and mesoscopic (n-i-p and p-i-n) types of structures are crossing 15% PCE whereas, SWCNTs, MWCNTs and reduced graphene oxide based mesoscopic (n-i-p) and planar (n-i-p) types of structures are crossing 15% PCE as shown in Fig. 11a and b. These materials are widely used in the PSCs by many groups using various methods but doctor blading, screen printing CVD and spin coating are showing excellent results, crossing 15% PCE. Carbon based HTM free PSC shows highest PCE above 16% with stability of 600 h using CH3 NH3 PbI3 perovskite via doctor blading method [72]. However, composite of spiro-OMeTAD with CNT based PSC shows highest PCE above 15% with stability of 500 h using Spin coating method [204]. Among so many methods dip-coating and soot transfer method shows poor device performance, Whereas, PDMS stamp, inkjet printing, hot press, paint, press and rolling transfer shows promising results and need more work to carried out as compare to other techniques as shown in Fig. 11b. Electrical properties of the carbon-based PSCs are good as compare to the CNT/r-GO based devices. One can easily obtain Jsc = 22 mA cm−1 and Voc = 1 V from these materials, Fig. 11c and d shows Jsc and Voc values obtained using various method reported by several research groups. 5. Challenges Long-term device stability during operation under stressed conditions such as humidity, air, elevated temperature and under ultra

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

193

Fig. 10. Oxides NiOx , MoO3 , WOx , VOx and MoS2 based PSCs (a) tandem, planar p-i-n, mesoscopic n-i-p and p-i-n based devices, (b) different method used for synthesizing thin film and it PCEs, (c) thin film synthesis method and Voc and (d) thin film synthesis method and Jsc .

violet light has yet to be well studied. Perovskite solar cells have proven for producing high efficiency but are not commercialized till now due to the poor stability. Real understanding of physics behind these devices is have to examined in depth for understanding of how structural and electronic properties determine, stable short-circuit current (Jsc ) and open-circuit voltage (Voc ) are still lacking. However, the important issues and challenges that limited the commercialization of perovskite solar cells still exist. Most of the inorganic HTMs already studied in QDSCs and DSSCs

application, these materials have remarkable electrical properties like wide bandgap, high conductivity, and easy processability. Whereas, role of charge transfer states in the perovskite solar cell is still topic of debate. Still J–V hysteresis is observed in the J–V characteristic of perovskite solar cell which limits the standardized characterization of device performance. There are lot of concern about the environment impact of these devices; the toxicity of leadbased perovskite is main concern for commercialization. There are only few reports on lead-free perovskite solar cell but this class of

194

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

Fig. 11. Carbon, SWCNTs, MWCNTs and reduced graphene oxide layer based PSCs (a) multi-layer n-i-p, planar p-i-n and n-i-p, mesoscopic n-i-p and p-i-n based devices, (b) different method used for synthesizing thin film and it PCEs, (c) thin film synthesis method and Voc and (d) thin film synthesis method and Jsc .

solar cell doesn’t show efficient device performance. Still the manufacturing, operational, and disposal phases of perovskite solar cells are needed to be taken into consideration. Contamination is also related with the perovskite which leads to the poor performance of the materials in terms of producing reproducible solar devices. Although the complexity of the diverse material preparation methods and device architectures make it more difficult to address these issues, recent progress has provided insights into these issues and the corresponding material properties. 6. Future scope Much progress has been made in Perovskite solar cell area over the past few years. Researchers are looking for cost-effective, clean and reliable energy devices. From a commercialization point

of view, the large-scale implementation of perovskite solar cells requires toxicity and stability issues to be resolved. Recently, work on lead-free perovskite solar cell has been started but still results obtained are expected to improve in terms of stability and toxicity [239,240]. Instability under air and moisture conditions are the challenges for researchers to work on. Recently, few studies have shown good moisture and air stable solar devices based on 2D/3D layered perovskite solar cell. These materials are truly remarkable absorbers and should surely be considered for commercialization. However, the perovskite material has inherent stability and toxicity problem. Material design and interface engineering is required for achieving more stable devices. Stability is a major challenge for these solar cells, improving stability of perovskite solar cell with sealant such as glass and photocurable fluoropolymer on the top of perovskite devices also needed to focus. Due to tunable band

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

195

Fig. 12. PSCs with high PCE as a function of time (a) stability of CuSCN based PSCs and CuSCN with rGO based PSCs at 60 ◦ C under N2 atmosphere, reprinted from Ref. [45]; (b) stability of CuGaO2 -based PSCs, reprinted from Ref. [49]; (c) stability of MoS2 -QD based PSCs and spiro-OMeTAD based PSCs, reprinted from Ref. [150]; (d) light soaking and thermal stability of NiOx -FAPbI3 -based PSC and NiOx -FAPbI3 /MACl based PSC at 85 ◦ C (triangle dot lines, at dark), reprinted from Ref. [121]; (e) stability of carbon-based PSCs and spiro-OMeTAD based PSCs, reprinted from Ref. [72]; (f) stability of SWNTs-spiro-OMeTAD based PSCs encapsulated with epoxy resin with glass slide, reprinted from Ref. [204].

gap from 1.5 to 2.3 eV and shows high Voc , which is great interest in implementing them in perovskite tandem devices with silicon or CIGS or perovskite cell [241–243]. The Shockley–Queisser limit in terms of efficiency of the monolithic perovskite tandem cells can exceed 35% in coming future. There are few articles which talk about the sealant materials for perovskite layer to enhance the stability of the device. Fig. 12 shows some of the best inorganic HTM or HTM free PSCs performance as a function of time. A novel approach and methods are needed to the design the device architecture for perovskite solar cells which should be motivated from low-cost, light weight and flexible substrates. These devices are of commercial interest for cheap, large-scale roll-to-roll processing and for technological advancement applications. Lot of effort has been made on perovskite based solar devices on flexible, conductive substrates, such as poly(ethylene terephthalate), polyethylene naphthalate, and Ti-foils. Additionally, high-transparency and colorful perovskite PV for building integration have also been studied with great interest and have lot of scope in future. The main advantage for fabrication of perovskite solar cell is its simple and easy processing such as solution cast technique, vapor deposition, thermal evaporation, hot casting technique and large area processing. In a very shot spam of time it has reaches efficiencies that other technologies took decades to achieve. Now researchers are proving that perovskite solar cell is an emerging type of technology soon will be the face of the future.

Cells employing CuSCN HTM especially have given high efficiency among the family of Cu based inorganic HTMs. The various methods used for deposition of inorganic HTMs are spin coating; spray coating, drop coating, doctor blading and electro-deposition. These methods are preferred because of their easy and economic processability. Oxides such as NiOx , MoOx and MoO3 also hold a lot of promise as their usage in the device show good and reproducible results. MoO3 in inverted structure has delivered an efficiency of 18.2%, which is a remarkable achievement. Though the methods like thermal evaporation, RF sputtering, and e-beam evaporation are employed for deposition such oxides, these methods require high vacuum level, which increase the cost of preparation of the solar cells. The improvement in the stability of the cells employing inorganic materials surpass those using organic HTMs. Carbon based hole transport materials using SWCNTs, MWCNTs and rGOs, are also a good alternative, owing to their stability against moisture and heat. These devices are also giving good efficiency of above 15%. Comparison tables of the materials, Challenges, Improvement and Future Trends have been discussed in this review article so that a clear picture can be created which will definitely help to open up the new way in the photovoltaic field. Though much work has been done in the field of inorganic HTM, we still have a long way to go in order to match the efficiency delivered by the conventional devices employing organic HTMs. If we are able to do that, inorganic HTMs will definitely be the answer for the addressing the stability issue of PSCs.

7. Conclusion Acknowledgments In this review paper, we have explored the various inorganic HTMs available as an alternative to the conventional organic HTMs being used in PSC. Careful selections of the most promising materials, their processing procedure and advantages have also been looked into. Cu based HTMs like CuI, CuSCN and CuOx have been extensively used owing to their band matching and good efficiency.

The authors are thankful to the Material Research Laboratory, Sharda University, India, School of Electrical and Computer Engineering, Purdue University, USA, Centre for Ionics University of Malaya (CIUM), Universiti Malaya, Malaysia and Polymer Material Laboratory, Sogang University, S. Korea. Also thank to Samsung

196

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

Display Research Center, Sogang University, Seoul, S. Korea. Gratitude to the Department of Science and Technology, Govt. of India and IUSSTF.

References [1] M. Saliba, T. Matsui, J.Y. Seo, K. Domanski, J.P. Correa-Baena, M.K. Nazeeruddin, S.M. Zakeeruddin, W. Tress, A. Abate, A. Hagfeldt, M. Gratzel, Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency, Energy Environ. Sci. 9 (2016) 1989–1997. [2] J. Burschka, N. Pellet, S.J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Gratzel, Sequential deposition as a route to high-performance perovskite-sensitized solar cells, Nature 499 (2013) 316–319. [3] NREL best research-cell efficiency chart, https://www.nrel.gov/pv/assets/ pdfs/pv-efficiencies-07-17-2018.pdf. [4] C.H. Hendon, R.X. Yang, L.A. Burton, A. Walsh, Assessment of polyanion (BF4 − and PF6 − ) substitutions in hybrid halide perovskites, J. Mater. Chem. A 3 (2015) 9067–9070. [5] S. Nagane, U. Bansode, O. Game, S. Chhatre, S. Ogale, CH3 NH3 PbI(3−x) (BF4 )x : molecular ion substituted hybrid perovskite, Chem. Commun. (Camb.) 50 (2014) 9741–9744. [6] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites, Science 338 (2012) 643–647. [7] T.M. Koh, K.W. Fu, Y.N. Fang, S. Chen, T.C. Sum, N. Mathews, S.G. Mhaisalkar, P.P. Boix, T. Baikie, Formamidinium-containing metal-halide: an alternative material for near-IR absorption perovskite solar cells, J. Phys. Chem. C 118 (2014) 16458–16462. [8] J.H. Heo, D.H. Song, S.H. Im, Planar CH3 NH3 PbBr3 hybrid solar cells with 10.4% power conversion efficiency, fabricated by controlled crystallization in the spin-coating process, Adv. Mater. 26 (2014) 8179–8183. [9] P. Prajongtat, T. Dittricht, Precipitation of CH3 NH3 PbCl3 in CH3 NH3 PbI3 and its impact on modulated charge separation, J. Phys. Chem. C 119 (2015) 9926–9933. [10] Q. Jiang, D. Rebollar, J. Gong, E.L. Piacentino, C. Zheng, T. Xu, Pseudohalide-induced moisture tolerance in perovskite CH3 NH3 Pb(SCN)2 I thin films, Angew. Chem. Int. Ed. Engl. 54 (2015) 7617–7620. [11] F. Hao, C.C. Stoumpos, D.H. Cao, R.P.H. Chang, M.G. Kanatzidis, Lead-free solid-state organic–inorganic halide perovskite solar cells, Nat. Photonics 8 (2014) 489–494. [12] Q. Dong, Y. Fang, Y. Shao, P. Mulligan, J. Qiu, L. Cao, J. Huang, Electron–hole diffusion lengths >175 ␮m in solution-grown CH3 NH3 PbI3 single crystals, Science 347 (2015) 967–970. [13] S.D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J. Alcocer, T. Leijtens, L.M. Herz, A. Petrozza, H.J. Snaith, Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber, Science 342 (2013) 341–344. [14] G. Xing, N. Mathews, S. Sun, S.S. Lim, Y.M. Lam, M. Gratzel, S. Mhaisalkar, T.C. Sum, Long-range balanced electron- and hole-transport lengths in organic–inorganic CH3 NH3 PbI3 , Science 342 (2013) 344–347. [15] T.M. Brenner, D.A. Egger, A.M. Rappe, L. Kronik, G. Hodes, D. Cahen, Are mobilities in hybrid organic–inorganic halide perovskites actually “high”? J. Phys. Chem. Lett. 6 (2015) 4754–4757. [16] C. Motta, F. El-Mellouhi, S. Sanvito, Charge carrier mobility in hybrid halide perovskites, Sci. Rep. 5 (2015) 12746. [17] A. Miyata, A. Mitioglu, P. Plochocka, O. Portugall, J.T.W. Wang, S.D. Stranks, H.J. Snaith, R.J. Nicholas, Direct measurement of the exciton binding energy and effective masses for charge carriers in organic–inorganic tri-halide perovskites, Nat. Phys. 11 (2015) 582–594. [18] Q.Q. Lin, A. Armin, R.C.R. Nagiri, P.L. Burn, P. Meredith, Electro-optics of perovskite solar cells, Nat. Photonics 9 (2015) 106–112. [19] N.G. Park, Methodologies for high efficiency perovskite solar cells, Nano Converg. 3 (2016) 15. [20] Y. Ogomi, A. Morita, S. Tsukamoto, T. Saitho, N. Fujikawa, Q. Shen, T. Toyoda, K. Yoshino, S.S. Pandey, T. Ma, S. Hayase, CH3 NH3 Snx Pb(1−x) I3 perovskite solar cells covering up to 1060 nm, J. Phys. Chem. Lett. 5 (2014) 1004–1011. [21] J.H. Noh, S.H. Im, J.H. Heo, T.N. Mandal, S.I. Seok, Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells, Nano Lett. 13 (2013) 1764–1769. [22] W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Solar cells. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science 348 (2015) 1234–1237. [23] N. Pellet, P. Gao, G. Gregori, T.Y. Yang, M.K. Nazeeruddin, J. Maier, M. Gratzel, Mixed-organic-cation perovskite photovoltaics for enhanced solar-light harvesting, Angew. Chem. Int. Ed. Engl. 53 (2014) 3151–3157. [24] C.C. Stoumpos, C.D. Malliakas, M.G. Kanatzidis, Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties, Inorg. Chem. 52 (2013) 9019–9038. [25] Z.N. Song, S.C. Watthage, A.B. Phillips, M.J. Heben, Pathways toward high-performance perovskite solar cells: review of recent advances in organo-metal halide perovskites for photovoltaic applications, J. Photon Energy 6 (2016) 022001–022023.

[26] N.-G. Park, High Efficiency Mesoscopic Organometal Halide Perovskite Solar Cells, The Royal Society of Chemistry, Cambridge, UK, 2016. [27] D.Y. Liu, T.L. Kelly, Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques, Nat. Photonics 8 (2014) 133–138. [28] L. Meng, J. You, T.F. Guo, Y. Yang, Recent advances in the inverted planar structure of perovskite solar cells, Acc. Chem. Res. 49 (2016) 155–165. [29] D.Y. Son, J.H. Im, H.S. Kim, N.G. Park, 11% efficient perovskite solar cell based on ZnO nanorods: an effective charge collection system, J. Phys. Chem. C 118 (2014) 16567–16573. [30] J.X. Duan, J.M. Wu, J. Zhang, Y. Xu, H. Wang, D. Gao, P.D. Lund, TiO2 /ZnO/TiO2 sandwich multi-layer films as a hole-blocking layer for efficient perovskite solar cells, Int. J. Energy Res. 40 (2016) 806–813. [31] X. Xu, H.Y. Zhang, J.J. Shi, J. Dong, Y.H. Luo, D.M. Li, Q.B. Meng, Highly efficient planar perovskite solar cells with a TiO2 /ZnO electron transport bilayer, J. Mater. Chem. A 3 (2015) 19288–19293. [32] G.M. Peng, J.M. Wu, S.Q. Wu, X.Q. Xu, J.E. Ellis, G. Xu, A. Star, D. Gao, Perovskite solar cells based on bottom-fused TiO2 nanocones, J. Mater. Chem. A 4 (2016) 1520–1530. [33] F. Giordano, A. Abate, J.P. Correa Baena, M. Saliba, T. Matsui, S.H. Im, S.M. Zakeeruddin, M.K. Nazeeruddin, A. Hagfeldt, M. Graetzel, Enhanced electronic properties in mesoporous TiO2 via lithium doping for high-efficiency perovskite solar cells, Nat. Commun. 7 (2016) 10379. [34] H. Yu, J. Roh, J. Yun, J. Jang, Synergistic effects of three-dimensional orchid-like TiO2 nanowire networks and plasmonic nanoparticles for highly efficient mesoscopic perovskite solar cells, J. Mater. Chem. A 4 (2016) 7322–7329. [35] W. Wang, Z. Zhang, Y. Cai, J. Chen, J. Wang, R. Huang, X. Lu, X. Gao, L. Shui, S. Wu, J.M. Liu, Enhanced performance of CH3 NH3 PbI3−x Clx perovskite solar cells by CH3 NH3 I modification of TiO2 -perovskite layer interface, Nanoscale Res. Lett. 11 (2016) 316. [36] S. Wang, W. Yuan, Y.S. Meng, Spectrum-dependent spiro-OMeTAD oxidization mechanism in perovskite solar cells, ACS Appl. Mater. Interfaces 7 (2015) 24791–24798. [37] H.W. Chen, T.Y. Huang, T.H. Chang, Y. Sanehira, C.W. Kung, C.W. Chu, M. Ikegami, T. Miyasaka, K.C. Ho, Efficiency enhancement of hybrid perovskite solar cells with MEH-PPV hole-transporting layers, Sci. Rep. 6 (2016) 34319. [38] J.Y. Xiao, J.J. Shi, H.B. Liu, Y.Z. Xu, S.T. Lv, Y.H. Luo, D.M. Li, Q.B. Meng, Y.L. Li, Efficient CH3 NH3 PbI3 perovskite solar cells based on graphdiyne (GD)-modified P3HT hole-transporting material, Adv. Energy Mater. 5 (2015). [39] J.H. Heo, S.H. Im, J.H. Noh, T.N. Mandal, C.S. Lim, J.A. Chang, Y.H. Lee, H.J. Kim, A. Sarkar, M.K. Nazeeruddin, M. Gratzel, S.I. Seok, Efficient inorganic–organic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors, Nat. Photonics 7 (2013) 487–492. [40] J. Seo, J.H. Noh, S.I. Seok, Rational strategies for efficient perovskite solar cells, Acc. Chem. Res. 49 (2016) 562–572. [41] D. Bi, L. Yang, G. Boschloo, A. Hagfeldt, E.M. Johansson, Effect of different hole transport materials on recombination in CH3 NH3 PbI3 perovskite-sensitized mesoscopic solar cells, J. Phys. Chem. Lett. 4 (2013) 1532–1536. [42] H. Zhang, L.W. Xue, J.B. Han, Y.Q. Fu, Y. Shen, Z.G. Zhang, Y.F. Li, M.K. Wang, New generation perovskite solar cells with solution-processed amino-substituted perylene diimide derivative as electron-transport Layer, J. Mater. Chem. A 4 (2016) 8724–8733. [43] Y.M. Xiao, G.Y. Han, Y.Z. Chang, H.H. Zhou, M.Y. Li, Y.P. Li, An all-solid-state perovskite-sensitized solar cell based on the dual function polyaniline as the sensitizer and p-type hole-transporting material, J. Power Sources 267 (2014) 1–8. [44] C.H. Chiang, C.G. Wu, Bulk heterojunction perovskite-PCBM solar cells with high fill factor, Nat. Photonics 10 (2016) 196–200. [45] N. Arora, M.I. Dar, A. Hinderhofer, N. Pellet, F. Schreiber, S.M. Zakeeruddin, M. Gratzel, Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies greater than 20, Science 358 (2017) 768–771. [46] M. Huangfu, Y. Shen, G. Zhu, K. Xu, M. Cao, F. Gu, L. Wang, Copper iodide as inorganic hole conductor for perovskite solar cells with different thickness of mesoporous layer and hole transport layer, Appl. Surf. Sci. 357 (2015) 2234–2240. [47] W. Sun, Y. Li, S. Ye, H. Rao, W. Yan, H. Peng, Y. Li, Z. Liu, S. Wang, Z. Chen, L. Xiao, Z. Bian, C. Huang, High-performance inverted planar heterojunction perovskite solar cells based on a solution-processed CuOx hole transport layer, Nanoscale 8 (2016) 10806–10813. [48] H. Rao, W. Sun, S. Ye, W. Yan, Y. Li, H. Peng, Z. Liu, Z. Bian, C. Huang, Solution-processed CuS NPs as an inorganic hole-selective contact material for inverted planar perovskite solar cells, ACS Appl. Mater. Interfaces 8 (2016) 7800–7805. [49] H. Zhang, H. Wang, W. Chen, A.K. Jen, CuGaO2 : a promising inorganic hole-transporting material for highly efficient and stable perovskite solar cells, Adv. Mater. 29 (2017). [50] F. Igbari, M. Li, Y. Hu, Z.K. Wang, L.S. Liao, A room-temperature CuAlO2 hole interfacial layer for efficient and stable planar perovskite solar cells, J. Mater. Chem. A 4 (2016) 1326–1335. [51] J.W. Jung, C.C. Chueh, A.K. Jen, A low-temperature, solution-processable, Cu-doped nickel oxide hole-transporting layer via the combustion method for high-performance thin-film perovskite solar cells, Adv. Mater. 27 (2015) 7874–7880.

R. Singh et al. / Applied Materials Today 14 (2019) 175–200 [52] H. Sung, N. Ahn, M.S. Jang, J.K. Lee, H. Yoon, N.G. Park, M. Choi, Transparent conductive oxide-free graphene-based perovskite solar cells with over 17% efficiency, Adv. Energy Mater. 6 (2016) 1501873. [53] E. Singh, K.S. Kim, G.Y. Yeom, H.S. Nalwa, Atomically thin-layered molybdenum disulfide (MoS2 ) for bulk-heterojunction solar cells, ACS Appl. Mater. Interfaces 9 (2017) 3223–3245. [54] Rahul, B. Bhattacharya, P.K. Singh, R. Singh, Z.H. Khan, Perovskite sensitized solar cell using solid polymer electrolyte, Int. J. Hydrogen Energy 41 (2016) 2847–2852. [55] A.A. Mamun, T.T. Ava, K. Zhang, H. Baumgart, G. Namkoong, New PCBM/carbon based electron transport layer for perovskite solar cells, Phys. Chem. Chem. Phys. 19 (2017) 17960–17966. [56] K. Aitola, K. Sveinbjornsson, J.P. Correa-Baena, A. Kaskela, A. Abate, Y. Tian, E.M.J. Johansson, M. Gratzel, E.I. Kauppinen, A. Hagfeldt, G. Boschloo, Carbon nanotube-based hybrid hole-transporting material and selective contact for high efficiency perovskite solar cells, Energy Environ. Sci. 9 (2016) 461–466. [57] J. Cao, Y.M. Liu, X. Jing, J. Yin, J. Li, B. Xu, Y.Z. Tan, N. Zheng, Well-defined thiolated nanographene as hole-transporting material for efficient and stable perovskite solar cells, J. Am. Chem. Soc. 137 (2015) 10914–10917. [58] R. Singh, H.K. Jun, A.K. Arof, Activated carbon as back contact for HTM-free mixed cation perovskite solar cell, Phase Transit. 91 (2018) 1268–1276. [59] G. Kron, T. Egerter, J.H. Werner, U. Rau, Electronic transport in dye-sensitized nanoporous TiO2 solar cells-comparison of electrolyte and solid-state devices, J. Phys. Chem. B 107 (2003) 3556–3564. [60] J.H. Im, C.R. Lee, J.W. Lee, S.W. Park, N.G. Park, 6.5% efficient perovskite quantum-dot-sensitized solar cell, Nanoscale 3 (2011) 4088–4093. [61] Z. Yu, L.C. Sun, Recent progress on hole-transporting materials for emerging organometal halide perovskite solar cells, Adv. Energy Mater. 5 (2015) 1500213. [62] B.V. Lotsch, New light on an old story: perovskites go solar, Angew. Chem. Int. Ed. Engl. 53 (2014) 635–637. [63] D.B. Mitzi, Synthesis, structure, and properties of organic–inorganic perovskites and related materials, in: K.D.K. (Ed.), Progress in Inorganic Chemistry, John Wiley & Sons Inc., New York, 2007, pp. 1–122. [64] S.F. Hoefler, G. Trimmel, T. Rath, Progress on lead-free metal halide perovskites for photovoltaic applications: a review, Monatsh. Chem. 148 (2017) 795–826. [65] H. Zhang, Y. Shi, F. Yan, L. Wang, K. Wang, Y. Xing, Q. Dong, T. Ma, A dual functional additive for the HTM layer in perovskite solar cells, Chem. Commun. (Camb.) 50 (2014) 5020–5022. [66] L. Calio, S. Kazim, M. Gratzel, S. Ahmad, Hole-transport materials for perovskite solar cells, Angew. Chem. Int. Ed. Engl. 55 (2016) 14522–14545. [67] C.M. Tsai, H.P. Wu, S.T. Chang, C.F. Huang, C.H. Wang, S. Narra, Y.W. Yang, C.L. Wang, C.H. Hung, E.W.G. Diau, Role of tin chloride in tin-rich mixed-halide perovskites applied as mesoscopic solar cells with a carbon counter electrode, ACS Energy Lett. 1 (2016) 1086–1093. [68] X. Chang, W. Li, L. Zhu, H. Liu, H. Geng, S. Xiang, J. Liu, H. Chen, Carbon-based CsPbBr3 perovskite solar cells: all-ambient processes and high thermal stability, ACS Appl. Mater. Interfaces 8 (2016) 33649–33655. [69] N.N. Zhang, Y.J. Guo, X. Yin, M. He, X.P. Zou, Spongy carbon film deposited on a separated substrate as counter electrode for perovskite-based solar cell, Mater. Lett. 182 (2016) 248–252. [70] S. Ameen, M.A. Rub, S.A. Kosa, K.A. Alamry, M.S. Akhtar, H.S. Shin, H.K. Seo, A.M. Asiri, M.K. Nazeeruddin, Perovskite solar cells: influence of hole transporting materials on power conversion efficiency, ChemSusChem 9 (2016) 10–27. [71] J.Z. Chen, N.G. Park, Inorganic hole transporting materials for stable and high efficiency perovskite solar cells, J. Phys. Chem. C 122 (2018) 14039–14063. [72] F.G. Zhang, X.C. Yang, M. Cheng, W.H. Wang, L.C. Sun, Boosting the efficiency and the stability of low cost perovskite solar cells by using CuPc nanorods as hole transport material and carbon as counter electrode, Nano Energy 20 (2016) 108–116. [73] M. Li, Z.K. Wang, Y.G. Yang, Y. Hu, S.L. Feng, J.M. Wang, X.Y. Gao, L.S. Liao, Copper salts doped spiro-OMeTAD for high-performance perovskite solar cells, Adv. Energy Mater. 6 (2016) 1601156. [74] K. Zhao, R. Munir, B. Yan, Y. Yang, T. Kim, A. Amassian, Solution-processed inorganic copper(i) thiocyanate (CuSCN) hole transporting layers for efficient p-i-n perovskite solar cells, J. Mater. Chem. A 3 (2015) 20554–20559. [75] S. Ye, W. Sun, Y. Li, W. Yan, H. Peng, Z. Bian, Z. Liu, C. Huang, CuSCN-based inverted planar perovskite solar cell with an average PCE of 15.6%, Nano Lett. 15 (2015) 3723–3728. [76] B.B. Wang, Z.G. Zhang, S.Y. Ye, L. Gao, T.H. Yan, Z.Q. Bian, C.H. Huang, Y.F. Li, Solution-processable cathode buffer layer for high-performance ITO/CuSCN-based planar heterojunction perovskite solar cell, Electrochim. Acta 218 (2016) 263–270. [77] A.S. Subbiah, A. Halder, S. Ghosh, N. Mahuli, G. Hodes, S.K. Sarkar, Inorganic hole conducting layers for perovskite-based solar cells, J. Phys. Chem. Lett. 5 (2014) 1748–1753. [78] V.E. Madhavan, I. Zimmermann, C. Roldan-Carmona, G. Grancini, M. Buffiere, A. Belaidi, M.K. Nazeeruddin, Copper thiocyanate inorganic hole-transporting material for high-efficiency perovskite solar cells, ACS Energy Lett. 1 (2016) 1112–1117. [79] G. Murugadoss, H. Kanda, S. Tanaka, H. Nishino, S. Ito, H. Imahoric, T. Umeyama, An efficient electron transport material of tin oxide for planar structure perovskite solar cells, J. Power Sources 307 (2016) 891–897.

197

[80] S. Ito, S. Tanaka, H. Vahlman, H. Nishino, K. Manabe, P. Lund, Carbon-double-bond-free printed solar cells from TiO2 /CH3 NH3 PbI3 /CuSCN/Au: structural control and photoaging effects, Chemphyschem 15 (2014) 1194–1200. [81] P. Qin, S. Tanaka, S. Ito, N. Tetreault, K. Manabe, H. Nishino, M.K. Nazeeruddin, M. Gratzel, Inorganic hole conductor-based lead halide perovskite solar cells with 12.4% conversion efficiency, Nat. Commun. 5 (2014) 3834. [82] S. Ito, S. Tanaka, H. Nishino, Lead-halide perovskite solar cells by CH3 NH3 I dripping on PbI2 -CH3 NH3 I-DMSO precursor layer for planar and porous structures using CuSCN hole-transporting material, J. Phys. Chem. Lett. 6 (2015) 881–886. [83] J.W. Liu, S.K. Pathak, N. Sakai, R. Sheng, S. Bai, Z.P. Wang, H.J. Snaith, Identification and mitigation of a critical interfacial instability in perovskite solar cells employing copper thiocyanate hole-transporter, Adv. Mater. Interfaces 3 (2016) 1600571. [84] S. Chavhan, O. Miguel, H.J. Grande, V. Gonzalez-Pedro, R.S. Sanchez, E.M. Barea, I. Mora-Sero, R. Tena-Zaera, Organo-metal halide perovskite-based solar cells with CuSCN as the inorganic hole selective contact, J. Mater. Chem. A 2 (2014) 12754–12760. [85] N. Wijeyasinghe, A. Regoutz, F. Eisner, T. Du, L. Tsetseris, Y.H. Lin, H. Faber, P. Pattanasattayavong, J.H. Li, F. Yan, M.A. McLachlan, D.J. Payne, M. Heeney, T.D. Anthopoulos, Copper(I) thiocyanate (CuSCN) hole-transport layers processed from aqueous precursor solutions and their application in thin-film transistors and highly efficient organic and organometal halide perovskite solar cells, Adv. Funct. Mater. 27 (2017) 1701818. [86] J.W. Jung, C.C. Chueh, A.K.Y. Jen, High-performance semitransparent perovskite solar cells with 10% power conversion efficiency and 25% average visible transmittance based on transparent CuSCN as the hole-transporting material, Adv. Energy Mater. 5 (2015) 1500486. [87] S. Ito, S. Tanaka, K. Manabe, H. Nishino, Effects of surface blocking layer of Sb2 S3 on nanocrystalline TiO2 for CH3 NH3 PbI3 perovskite solar cells, J. Phys. Chem. C 118 (2014) 16995–17000. [88] W. Sun, S. Ye, H. Rao, Y. Li, Z. Liu, L. Xiao, Z. Chen, Z. Bian, C. Huang, Room-temperature and solution-processed copper iodide as the hole transport layer for inverted planar perovskite solar cells, Nanoscale 8 (2016) 15954–15960. [89] J.A. Christians, R.C. Fung, P.V. Kamat, An inorganic hole conductor for organo-lead halide perovskite solar cells. Improved hole conductivity with copper iodide, J. Am. Chem. Soc. 136 (2014) 758–764. [90] W.Y. Chen, L.L. Deng, S.M. Dai, X. Wang, C.B. Tian, X.X. Zhan, S.Y. Xie, R.B. Huang, L.S. Zheng, Low-cost solution-processed copper iodide as an alternative to PEDOT:PSS hole transport layer for efficient and stable inverted planar heterojunction perovskite solar cells, J. Mater. Chem. A 3 (2015) 19353–19359. [91] G.A. Sepalage, S. Meyer, A. Pascoe, A.D. Scully, F. Huang, U. Bach, Y.-B. Cheng, L. Spiccia, Copper(I) iodide as hole-conductor in planar perovskite solar cells: probing the origin of J–V hysteresis, Adv. Funct. Mater. 25 (2015) 5650–5661. [92] S. Gharibzadeh, B.A. Nejand, A. Moshaii, N. Mohammadian, A.H. Alizadeh, R. Mohammadpour, V. Ahmadi, A. Alizadeh, Two-step physical deposition of a compact CuI hole-transport layer and the formation of an interfacial species in perovskite solar cells, ChemSusChem 9 (2016) 1929–1937. [93] H.X. Wang, Z. Yu, X. Jiang, J.J. Li, B. Cai, X.C. Yang, L.C. Sun, Efficient and stable inverted planar perovskite solar cells employing CuI as hole-transporting layer prepared by solid–gas transformation, Energy Technol. 5 (2017) 1836–1843. [94] F.A.P. Nazari, B. Abdollahi Nejand, V. Ahmadi, M. Payandeh, M. Salavati-Niasar, Physicochemical interface engineering of CuI/Cu as advanced potential hole-transporting materials/metal contact couples in hysteresis-free ultralow-cost and large-area perovskite solar cells, J. Phys. Chem. C 121 (2017) 21935–21944. [95] B.A. Nejand, V. Ahmadi, S. Gharibzadeh, H.R. Shahverdi, Cuprous oxide as a potential low-cost hole-transport material for stable perovskite solar cells, ChemSusChem 9 (2016) 302–313. [96] C. Zuo, L. Ding, Solution-processed Cu2 O and CuO as hole transport materials for efficient perovskite solar cells, Small 11 (2015) 5528–5532. [97] P.L. Qin, H.W. Lei, X.L. Zheng, Q. Liu, H. Tao, G. Yang, W.J. Ke, L.B. Xiong, M.C. Qin, X.Z. Zhao, G.J. Fang, Copper-doped chromium oxide hole-transporting layer for perovskite solar cells: interface engineering and performance improvement, Adv. Mater. Interfaces 3 (2016) 1500799. [98] S. Chatterjee, A.J. Pal, Introducing Cu2 O thin films as a hole-transport layer in efficient planar perovskite solar cell structures, J. Phys. Chem. C 120 (2016) 1428–1437. [99] I.Y.Y. Bu, Y.S. Fu, J.F. Li, T.F. Guo, Large-area electrospray-deposited nanocrystalline CuX O hole transport layer for perovskite solar cells, RSC Adv. 7 (2017) 46651–46656. [100] W.A. Dunlap-Shohl, T.B. Daunis, X.M. Wang, J. Wang, B.Y. Zhang, D. Barrera, Y.F. Yan, J.W.P. Hsu, D.B. Mitzi, Room-temperature fabrication of a delafossite CuCrO2 hole transport layer for perovskite solar cells, J. Mater. Chem. A 6 (2018) 469–477. [101] Q. Zeng, Y.X. Di, C. Huang, K.W. Sun, Y. Zhao, H.P. Xie, D.M. Niu, L.X. Jiang, X.J. Hao, Y.Q. Lai, F.Y. Liu, Famatinite Cu3 SbS4 nanocrystals as hole transporting material for efficient perovskite solar cells, J. Matr. Chem. C 6 (2018) 7989–7993.

198

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

[102] Q. Wu, C. Xue, Y. Li, P. Zhou, W. Liu, J. Zhu, S. Dai, C. Zhu, S. Yang, Kesterite Cu2 ZnSnS4 as a low-cost inorganic hole-transporting material for high-efficiency perovskite solar cells, ACS Appl. Mater. Interfaces 7 (2015) 28466–28473. [103] W. Chen, Y.Z. Wu, J. Liu, C.J. Qin, X.D. Yang, A. Islam, Y.B. Cheng, L.Y. Han, Hybrid interfacial layer leads to solid performance improvement of inverted perovskite solar cells, Energy Environ. Sci. 8 (2015) 629–640. [104] J.H. Park, J. Seo, S. Park, S.S. Shin, Y.C. Kim, N.J. Jeon, H.W. Shin, T.K. Ahn, J.H. Noh, S.C. Yoon, C.S. Hwang, S.I. Seok, Efficient CH3 NH3 PbI3 perovskite solar cells employing nanostructured p-type NiO electrode formed by a pulsed laser deposition, Adv. Mater. 27 (2015) 4013–4019. [105] X. Yin, P. Chen, M. Que, Y. Xing, W. Que, C. Niu, J. Shao, Highly efficient flexible perovskite solar cells using solution-derived NiOx hole contacts, ACS Nano 10 (2016) 3630–3636. [106] K. Cao, Z. Zuo, J. Cui, Y. Shen, T. Moehl, S.M. Zakeeruddin, M. Grätzel, M. Wang, Efficient screen printed perovskite solar cells based on mesoscopic TiO2 /Al2 O3 /NiO/carbon architecture, Nano Energy 17 (2015) 171–179. [107] X.T. Yin, M.D. Que, Y.L. Xing, W.X. Que, High efficiency hysteresis-less inverted planar heterojunction perovskite solar cells with a solution-derived NiOx hole contact layer, J. Mater. Chem. A 3 (2015) 24495–24503. [108] J.H. Kim, P.W. Liang, S.T. Williams, N. Cho, C.C. Chueh, M.S. Glaz, D.S. Ginger, A.K. Jen, High-performance and environmentally stable planar heterojunction perovskite solar cells based on a solution-processed copper-doped nickel oxide hole-transporting layer, Adv. Mater. 27 (2015) 695–701. [109] J.Y. Jeng, K.C. Chen, T.Y. Chiang, P.Y. Lin, T.D. Tsai, Y.C. Chang, T.F. Guo, P. Chen, T.C. Wen, Y.J. Hsu, Nickel oxide electrode interlayer in CH3 NH3 PbI3 perovskite/PCBM planar-heterojunction hybrid solar cells, Adv. Mater. 26 (2014) 4107–4113. [110] I.J. Park, M.A. Park, D.H. Kim, G.D. Park, B.J. Kim, H.J. Son, M.J. Ko, D.-K. Lee, T. Park, H. Shin, N.-G. Park, H.S. Jung, J.Y. Kim, New hybrid hole extraction layer of perovskite solar cells with a planar p-i-n geometry, J. Phys. Chem. C 119 (2015) 27285–27290. [111] L. Hu, J. Peng, W.W. Wang, Z. Xia, J.Y. Yuan, J.L. Lu, X.D. Huang, W.L. Ma, H.B. Song, W. Chen, Y.B. Cheng, J. Tang, Sequential deposition of CH3 NH3 PbI3 on planar NiO film for efficient planar perovskite solar cells, ACS Photonics 1 (2014) 547–553. [112] J. You, L. Meng, T.B. Song, T.F. Guo, Y.M. Yang, W.H. Chang, Z. Hong, H. Chen, H. Zhou, Q. Chen, Y. Liu, N. De Marco, Y. Yang, Improved air stability of perovskite solar cells via solution-processed metal oxide transport layers, Nat. Nanotechnol. 11 (2016) 75–81. [113] K.C. Wang, J.Y. Jeng, P.S. Shen, Y.C. Chang, E.W. Diau, C.H. Tsai, T.Y. Chao, H.C. Hsu, P.Y. Lin, P. Chen, T.F. Guo, T.C. Wen, p-type mesoscopic nickel oxide/organometallic perovskite heterojunction solar cells, Sci. Rep. 4 (2014) 4756. [114] M.D. Xiao, M. Gao, F.Z. Huang, A.R. Pascoe, T.S. Qin, Y.B. Cheng, U. Bach, L. Spiccia, Efficient perovskite solar cells employing inorganic interlayers, Chemnanomat 2 (2016) 182–188. [115] Z. Zhu, Y. Bai, T. Zhang, Z. Liu, X. Long, Z. Wei, Z. Wang, L. Zhang, J. Wang, F. Yan, S. Yang, High-performance hole-extraction layer of sol–gel-processed NiO nanocrystals for inverted planar perovskite solar cells, Angew. Chem. Int. Ed. Engl. 53 (2014) 12571–12575. [116] W. Chen, Y. Wu, Y. Yue, J. Liu, W. Zhang, X. Yang, H. Chen, E. Bi, I. Ashraful, M. Gratzel, L. Han, Efficient and stable large-area perovskite solar cells with inorganic charge extraction layers, Science 350 (2015) 944–948. [117] J. Cui, F. Meng, H. Zhang, K. Cao, H. Yuan, Y. Cheng, F. Huang, M. Wang, CH3 NH3 PbI3 -based planar solar cells with magnetron-sputtered nickel oxide, ACS Appl. Mater. Interfaces 6 (2014) 22862–22870. [118] Z.H. Liu, M. Zhang, X.B. Xu, F.S. Cai, H.L. Yuan, L.L. Bu, W.H. Li, A.L. Zhu, Z.X. Zhao, M.K. Wang, Y.B. Cheng, H.S. He, NiO nanosheets as efficient top hole transporters for carbon counter electrode based perovskite solar cells, J. Mater. Chem. A 3 (2015) 24121–24127. [119] L. Wei-Chih, L. Kun-Wei, G. Tzung-Fang, L. Jung, Perovskite-based solar cells with nickel-oxidized nickel oxide hole transfer layer, IEEE Trans. Electron. Devices 62 (2015) 1590–1595. [120] K.C. Wang, P.S. Shen, M.H. Li, S. Chen, M.W. Lin, P. Chen, T.F. Guo, Low-temperature sputtered nickel oxide compact thin film as effective electron blocking layer for mesoscopic NiO/CH3 NH3 PbI3 perovskite heterojunction solar cells, ACS Appl. Mater. Interfaces 6 (2014) 11851–11858. [121] F.X. Xie, C.C. Chen, Y.Z. Wu, X. Li, M.L. Cai, X. Liu, X.D. Yang, L.Y. Han, Vertical recrystallization for highly efficient and stable formamidinium-based inverted-structure perovskite solar cells, Energy Environ. Sci. 10 (2017) 1942–1949. [122] S.Z. Yue, K. Liu, R. Xu, M.C. Li, M. Azam, K. Ren, J. Liu, Y. Sun, Z.J. Wang, D.W. Cao, X.H. Yan, S.C. Qu, Y. Lei, Z.G. Wang, Efficacious engineering on charge extraction for realizing highly efficient perovskite solar cells, Energy Environ. Sci. 10 (2017) 2570–2578. [123] Y. Wu, F. Xie, H. Chen, X. Yang, H. Su, M. Cai, Z. Zhou, T. Noda, L. Han, Thermally stable MAPbI3 perovskite solar cells with efficiency of 19.19% and area over 1 cm2 achieved by additive engineering, Adv. Mater. 29 (2017). [124] W. Chen, F.Z. Liu, X.Y. Feng, A.B. Djurisic, W.K. Chan, Z.B. He, Cesium doped NiOx as an efficient hole extraction layer for inverted planar perovskite solar cells, Adv. Energy Mater. 7 (2017) 1700722. [125] W. Chen, L. Xu, X. Feng, J. Jie, Z. He, Metal acetylacetonate series in interface engineering for full low-temperature-processed, high-performance, and

[126]

[127]

[128]

[129]

[130]

[131]

[132]

[133]

[134]

[135]

[136]

[137]

[138]

[139]

[140]

[141]

[142]

[143]

[144]

[145]

[146]

[147]

[148]

stable planar perovskite solar cells with conversion efficiency over 16% on 1 cm2 scale, Adv. Mater. 29 (2017). Q. He, K. Yao, X. Wang, X. Xia, S. Leng, F. Li, Room-temperature and solution-processable Cu-doped nickel oxide nanoparticles for efficient hole-transport layers of flexible large-area perovskite solar cells, ACS Appl. Mater. Interfaces 9 (2017) 41887–41897. C. Hu, Y. Bai, S. Xiao, T. Zhang, X.Y. Meng, W.K. Ng, Y. Yang, K.S. Wong, H. Chen, S. Yang, Tuning the A-site cation composition of FA perovskites for efficient and stable NiO-based p-i-n perovskite solar cells, J. Mater. Chem. A 5 (2017) 21858–21865. Z. Zhu, D. Zhao, C.-C. Chueh, X. Shi, Z. Li, A.K.Y. Jen, Highly efficient and stable perovskite solar cells enabled by all-crosslinked charge-transporting layers, Joule 2 (2018) 168–183. W. Nie, H. Tsai, J.C. Blancon, F. Liu, C.C. Stoumpos, B. Traore, M. Kepenekian, O. Durand, C. Katan, S. Tretiak, J. Crochet, P.M. Ajayan, M. Kanatzidis, J. Even, A.D. Mohite, Critical role of interface and crystallinity on the performance and photostability of perovskite solar cell on nickel oxide, Adv. Mater. 30 (2018). H.-S. Kim, J.-Y. Seo, H. Xie, M. Lira-Cantu, S.M. Zakeeruddin, M. Grätzel, A. Hagfeldt, Effect of Cs-incorporated NiOx on the performance of perovskite solar cells, ACS Omega 2 (2017) 9074–9079. I.J. Park, G. Kang, M.A. Park, J.S. Kim, S.W. Seo, D.H. Kim, K. Zhu, T. Park, J.Y. Kim, Highly efficient and uniform 1 cm2 perovskite solar cells with an electrochemically deposited NiOx hole-extraction layer, ChemSusChem 10 (2017) 2660–2667. J. Ciro, D. Ramirez, M.A. Mejia Escobar, J.F. Montoya, S. Mesa, R. Betancur, F. Jaramillo, Self-functionalization behind a solution-processed NiOx film used as hole transporting layer for efficient perovskite solar cells, ACS Appl. Mater. Interfaces 9 (2017) 12348–12354. S.R. Pae, S. Byun, J. Kim, M. Kim, I. Gereige, B. Shin, Improving uniformity and reproducibility of hybrid perovskite solar cells via a low-temperature vacuum deposition process for NiOx hole transport layers, ACS Appl. Mater. Interfaces 10 (2018) 534–540. Y.L. Yang, H.N. Chen, X.L. Zheng, X.Y. Meng, T. Zhang, C. Hu, Y. Bai, S. Xiao, S.H. Yang, Ultrasound-spray deposition of multi-walled carbon nanotubes on NiO nanoparticles-embedded perovskite layers for high-performance carbon-based perovskite solar cells, Nano Energy 42 (2017) 322–333. X. Xu, Z. Liu, Z. Zuo, M. Zhang, Z. Zhao, Y. Shen, H. Zhou, Q. Chen, Y. Yang, M. Wang, Hole selective NiO contact for efficient perovskite solar cells with carbon electrode, Nano Lett. 15 (2015) 2402–2408. U. Kwon, B.G. Kim, D.C. Nguyen, J.H. Park, N.Y. Ha, S.J. Kim, S.H. Ko, S. Lee, D. Lee, H.J. Park, Solution-processible crystalline NiO nanoparticles for high-performance planar perovskite photovoltaic cells, Sci. Rep. 6 (2016) 30759. Z. Liu, M. Zhang, X. Xu, L. Bu, W. Zhang, W. Li, Z. Zhao, M. Wang, Y.B. Cheng, H. He, p-Type mesoscopic NiO as an active interfacial layer for carbon counter electrode based perovskite solar cells, Dalton Trans. 44 (2015) 3967–3973. Y. Bai, H. Yu, Z.L. Zhu, K. Jiang, T. Zhang, N. Zhao, S.H. Yang, H. Yan, High performance inverted structure perovskite solar cells based on a PCBM:polystyrene blend electron transport layer, J. Mater. Chem. A 3 (2015) 9098–9102. H. Tian, B. Xu, H. Chen, E.M. Johansson, G. Boschloo, Solid-state perovskite-sensitized p-type mesoporous nickel oxide solar cells, ChemSusChem 7 (2014) 2150–2153. H. Wang, X. Zeng, Z. Huang, W. Zhang, X. Qiao, B. Hu, X. Zou, M. Wang, Y.B. Cheng, W. Chen, Boosting the photocurrent density of p-type solar cells based on organometal halide perovskite-sensitized mesoporous NiO photocathodes, ACS Appl. Mater. Interfaces 6 (2014) 12609–12617. Y.M. Yang, Q. Chen, Y.T. Hsieh, T.B. Song, N.D. Marco, H. Zhou, Y. Yang, Multilayer transparent top electrode for solution processed perovskite/Cu(In,Ga)(Se,S)2 four terminal tandem solar cells, ACS Nano 9 (2015) 7714–7721. B.S. Kim, T.M. Kim, M.S. Choi, H.S. Shim, J.J. Kim, Fully vacuum-processed perovskite solar cells with high open circuit voltage using MoO3 /NPB as hole extraction layers, Org. Electron. 17 (2015) 102–106. Z.-L. Tseng, L.-C. Chen, C.-H. Chiang, S.-H. Chang, C.-C. Chen, C.-G. Wu, Efficient inverted-type perovskite solar cells using UV-ozone treated MoOx and WOx as hole transporting layers, Sol. Energy 139 (2016) 484–488. Y.X. Zhao, A.M. Nardes, K. Zhu, Effective hole extraction using MoOx –Al contact in perovskite CH3 NH3 PbI3 solar cells, Appl. Phys. Lett. 104 (2014) 213906. E.M. Sanehira, B.J.T. de Villers, P. Schulz, M.O. Reese, S. Ferrere, K. Zhu, L.Y. Lin, J.J. Berry, J.M. Luther, Influence of electrode interfaces on the stability of perovskite solar cells: reduced degradation using MoOx /Al for hole collection, ACS Energy Lett. 1 (2016) 38–45. F. Hou, Z. Su, F. Jin, X. Yan, L. Wang, H. Zhao, J. Zhu, B. Chu, W. Li, Efficient and stable planar heterojunction perovskite solar cells with an MoO3 /PEDOT:PSS hole transporting layer, Nanoscale 7 (2015) 9427–9432. C.Y. Liu, Z.S. Su, W.L. Li, F.M. Jin, B. Chu, J.B. Wang, H.F. Zhao, C.S. Lee, J.X. Tang, B.N. Kang, Improved performance of perovskite solar cells with a TiO2 /MoO3 core/shell nanoparticles doped PEDOT:PSS hole-transporter, Org. Electron. 33 (2016) 221–226. L. Liang, Z. Huang, L. Cai, W. Chen, B. Wang, K. Chen, H. Bai, Q. Tian, B. Fan, Magnetron sputtered zinc oxide nanorods as thickness-insensitive cathode interlayer for perovskite planar-heterojunction solar cells, ACS Appl. Mater. Interfaces 6 (2014) 20585–20589.

R. Singh et al. / Applied Materials Today 14 (2019) 175–200 [149] P. Loper, S.J. Moon, S.M. de Nicolas, B. Niesen, M. Ledinsky, S. Nicolay, J. Bailat, J.H. Yum, S. De Wolf, C. Ballif, Organic–inorganic halide perovskite/crystalline silicon four-terminal tandem solar cells, Phys. Chem. Chem. Phys. 17 (2015) 1619–1629. [150] L. Najafi, B. Taheri, B. Martin-Garcia, S. Bellani, D. Di Girolamo, A. Agresti, R. Oropesa-Nunez, S. Pescetelli, L. Vesce, E. Calabro, M. Prato, A.E. Del Rio Castillo, A. Di Carlo, F. Bonaccorso, MoS2 quantum dot/graphene hybrids for advanced interface engineering of a CH3 NH3 PbI3 perovskite solar cell with an efficiency of over 20, ACS Nano (2018), http://dx.doi.org/10.1021/ acsnano.8b05514. [151] S. Kohnehpoushi, P. Nazari, B.A. Nejand, M. Eskandari, MoS2 : a two-dimensional hole-transporting material for high-efficiency, low-cost perovskite solar cells, Nanotechnology 29 (2018) 205201. [152] A. Capasso, F. Matteocci, L. Najafi, M. Prato, J. Buha, L. Cinà, V. Pellegrini, A.D. Carlo, F. Bonaccorso, Few-layer MoS2 flakes as active buffer layer for stable perovskite solar cells, Adv. Energy Mater. 6 (2016) 1600920. [153] U. Dasgupta, S. Chatterjee, A.J. Pal, Thin-film formation of 2D MoS2 and its application as a hole-transport layer in planar perovskite solar cells, Sol. Energy Mater. Sol. Cells 172 (2017) 353–360. [154] Z.W. Li, Stable perovskite solar cells based on WO3 nanocrystals as hole transport layer, Chem. Lett. 44 (2015) 1140–1141. [155] K. Wang, Y. Shi, Q. Dong, Y. Li, S. Wang, X. Yu, M. Wu, T. Ma, Low-temperature and solution-processed amorphous WOx as electron-selective layer for perovskite solar cells, J. Phys. Chem. Lett. 6 (2015) 755–759. [156] J. Zhang, C. Shi, J. Chen, C. Ying, N. Wu, M. Wang, Pyrolysis preparation of WO3 thin films using ammonium metatungstate DMF/water solution for efficient compact layers in planar perovskite solar cells, J. Semicond. 37 (2016) 033002. [157] K. Mahmood, B.S. Swain, A.R. Kirmani, A. Amassian, Highly efficient perovskite solar cells based on a nanostructured WO3 –TiO2 core–shell electron transporting material, J. Mater. Chem. A 3 (2015) 9051–9057. [158] C.M. Chen, Z.K. Lin, W.J. Huang, S.H. Yang, WO3 nanoparticles or nanorods incorporating Cs2 CO3 /PCBM buffer bilayer as carriers transporting materials for perovskite solar cells, Nanoscale Res. Lett. 11 (2016) 464. [159] J.Z. Chen, Y.L. Xiong, Y.G. Rong, A.Y. Mei, Y.S. Sheng, P. Jiang, Y. Hu, X. Li, H.W. Han, Solvent effect on the hole-conductor-free fully printable perovskite solar cells, Nano Energy 27 (2016) 130–137. [160] Y. Yang, K. Ri, A.Y. Mei, L.F. Liu, M. Hu, T.F. Liu, X. Li, H.W. Han, The size effect of TiO2 nanoparticles on a printable mesoscopic perovskite solar cell, J. Mater. Chem. A 3 (2015) 9103–9107. [161] J. Chen, Y. Rong, A. Mei, Y. Xiong, T. Liu, Y. Sheng, P. Jiang, L. Hong, Y. Guan, X. Zhu, X. Hou, M. Duan, J. Zhao, X. Li, H. Han, Hole-conductor-free fully printable mesoscopic solar cell with mixed-anion perovskite CH3 NH3 PbI(3−x) (BF4 )x , Adv. Energy Mater. 6 (2016) 1502009. [162] C.Y. Chan, Y.Y. Wang, G.W. Wu, E.W.G. Diau, Solvent-extraction crystal growth for highly efficient carbon-based mesoscopic perovskite solar cells free of hole conductors, J. Mater. Chem. A 4 (2016) 3872–3878. [163] F. Behrouznejad, C.M. Tsai, S. Narra, E.W. Diau, N. Taghavinia, Interfacial investigation on printable carbon-based mesoscopic perovskite solar cells with NiOx /C back electrode, ACS Appl. Mater. Interfaces 9 (2017) 25204–25215. [164] E. Nouri, M.R. Mohammadi, P. Lianos, Inverted perovskite solar cells based on lithium-functionalized graphene oxide as an electron-transporting layer, Chem. Commun. (Camb.) 53 (2017) 1630–1633. [165] G. Grancini, C. Roldan-Carmona, I. Zimmermann, E. Mosconi, X. Lee, D. Martineau, S. Narbey, F. Oswald, F. De Angelis, M. Graetzel, M.K. Nazeeruddin, One-year stable perovskite solar cells by 2D/3D interface engineering, Nat. Commun. 8 (2017) 15684. [166] L. Xu, F. Wan, Y. Rong, H. Chen, S. He, X. Xu, G. Liu, H. Han, Y. Yuan, J. Yang, Y. Gao, B. Yang, C. Zhou, Stable monolithic hole-conductor-free perovskite solar cells using TiO2 nanoparticle binding carbon films, Org. Electron. 45 (2017) 131–138. [167] K. Cao, J. Cui, H. Zhang, H. Li, J.K. Song, Y. Shen, Y.B. Cheng, M.K. Wang, Efficient mesoscopic perovskite solar cells based on the CH3 NH3 PbI2 Br light absorber, J. Mater. Chem. A 3 (2015) 9116–9122. [168] Z.Y. Liu, B. Sun, T.L. Shi, Z.R. Tang, G.L. Liao, Enhanced photovoltaic performance and stability of carbon counter electrode based perovskite solar cells encapsulated by PDMS, J. Mater. Chem. A 4 (2016) 10700–10709. [169] Y. Rong, Z. Ku, A. Mei, T. Liu, M. Xu, S. Ko, X. Li, H. Han, Hole-conductor-free mesoscopic TiO2 /CH3 NH3 PbI3 heterojunction solar cells based on anatase nanosheets and carbon counter electrodes, J. Phys. Chem. Lett. 5 (2014) 2160–2164. [170] Y.Q. Xiao, N. Cheng, K.K. Kondamareddy, C.L. Wang, P. Liu, S.S. Guo, X.Z. Zhao, W-doped TiO2 mesoporous electron transport layer for efficient hole transport material free perovskite solar cells employing carbon counter electrodes, J. Power Sources 342 (2017) 489–494. [171] J. Li, J.X. Yao, X.Y. Liao, R.L. Yu, H.R. Xia, W.T. Sun, L.M. Peng, A contact study in hole conductor free perovskite solar cells with low temperature processed carbon electrodes, RSC Adv. 7 (2017) 20732–20737. [172] H.N. Chen, Z.H. Wei, X.L. Zheng, S.H. Yang, A scalable electrodeposition route to the low-cost, versatile and controllable fabrication of perovskite solar cells, Nano Energy 15 (2015) 216–226. [173] Y.Y. Yang, J.Y. Xiao, H.Y. Wei, L.F. Zhu, D.M. Li, Y.H. Luo, H.J. Wu, Q.B. Meng, An all-carbon counter electrode for highly efficient hole-conductor-free organo-metal perovskite solar cells, RSC Adv. 4 (2014) 52825–52830.

199

[174] S.G. Hashmi, D. Martineau, X. Li, M. Ozkan, A. Tiihonen, M.I. Dar, T. Sarikka, S.M. Zakeeruddin, J. Paltakari, P.D. Lund, M. Gratzel, Air processed inkjet infiltrated carbon based printed perovskite solar cells with high stability and reproducibility, Adv. Mater. Technol.-Us 2 (2017) 1600183. [175] B.X. Wang, T.F. Liu, Y.B. Zhou, X. Chen, X.B. Yuan, Y.Y. Yang, W.P. Liu, J.M. Wang, H.W. Han, Y.W. Tang, Hole-conductor-free perovskite solar cells with carbon counter electrodes based on ZnO nanorod arrays, Phys. Chem. Chem. Phys. 18 (2016) 27078–27082. [176] H.N. Chen, X.L. Zheng, Q. Li, Y.L. Yang, S. Xiao, C. Hu, Y. Bai, T. Zhang, K.S. Wong, S.H. Yang, An amorphous precursor route to the conformable oriented crystallization of CH3 NH3 PbBr3 in mesoporous scaffolds: toward efficient and thermally stable carbon-based perovskite solar cells, J. Mater. Chem. A 4 (2016) 12897–12912. [177] H.Y. Wei, J.Y. Xiao, Y.Y. Yang, S.T. Lv, J.J. Shi, X. Xu, J. Dong, Y.H. Luo, D.M. Li, Q.B. Meng, Free-standing flexible carbon electrode for highly efficient hole-conductor-free perovskite solar cells, Carbon 93 (2015) 861–868. [178] X. Xu, H. Zhang, K. Cao, J. Cui, J. Lu, X. Zeng, Y. Shen, M. Wang, Lead methylammonium triiodide perovskite-based solar cells: an interfacial charge-transfer investigation, ChemSusChem 7 (2014) 3088–3094. [179] H. Zhou, Y. Shi, K. Wang, Q. Dong, X. Bai, Y. Xing, Y. Du, T. Ma, Low-temperature processed and carbon-based ZnO/CH3 NH3 PbI3 /C planar heterojunction perovskite solar cells, J. Phys. Chem. C 119 (2015) 4600–4605. [180] G.Q. Yue, D. Chen, P. Wang, J. Zhang, Z.Y. Hu, Y.J. Zhu, Low-temperature prepared carbon electrodes for hole-conductor-free mesoscopic perovskite solar cells, Electrochim. Acta 218 (2016) 84–90. [181] Z. Liu, T. Shi, Z. Tang, B. Sun, G. Liao, Using a low-temperature carbon electrode for preparing hole-conductor-free perovskite heterojunction solar cells under high relative humidity, Nanoscale 8 (2016) 7017–7023. [182] M. Chen, R.H. Zha, Z.Y. Yuan, Q.S. Jing, Z.Y. Huang, X.K. Yang, S.M. Yang, X.H. Zhao, D.L. Xu, G.D. Zou, Boron and phosphorus co-doped carbon counter electrode for efficient hole-conductor-free perovskite solar cell, Chem. Eng. J. 313 (2017) 791–800. [183] H. Zhou, Y. Shi, Q. Dong, H. Zhang, Y. Xing, K. Wang, Y. Du, T. Ma, Hole-conductor-free, metal-electrode-free TiO2 /CH3 NH3 PbI3 heterojunction solar cells based on a low-temperature carbon electrode, J. Phys. Chem. Lett. 5 (2014) 3241–3246. [184] F. Zhang, X. Yang, H. Wang, M. Cheng, J. Zhao, L. Sun, Structure engineering of hole-conductor free perovskite-based solar cells with low-temperature-processed commercial carbon paste as cathode, ACS Appl. Mater. Interfaces 6 (2014) 16140–16146. [185] T. Liu, L. Liu, M. Hu, Y. Yang, L. Zhang, A. Mei, H. Han, Critical parameters in TiO2 /ZrO2 /Carbon-based mesoscopic perovskite solar cell, J. Power Sources 293 (2015) 533–538. [186] A. Mei, X. Li, L. Liu, Z. Ku, T. Liu, Y. Rong, M. Xu, M. Hu, J. Chen, Y. Yang, M. Gratzel, H. Han, A hole-conductor-free, fully printable mesoscopic perovskite solar cell with high stability, Science 345 (2014) 295–298. [187] L.J. Zhang, T.F. Liu, L.F. Liu, M. Hu, Y. Yang, A.Y. Mei, H.W. Han, The effect of carbon counter electrodes on fully printable mesoscopic perovskite solar cells, J. Mater. Chem. A 3 (2015) 9165–9170. [188] Z. Wei, H. Chen, K. Yan, S. Yang, Inkjet printing and instant chemical transformation of a CH3 NH3 PbI3 /nanocarbon electrode and interface for planar perovskite solar cells, Angew. Chem. Int. Ed. Engl. 53 (2014) 13239–13243. [189] C.X. Zhang, Y.D. Luo, X.H. Chen, Y.W. Chen, Z. Sun, S.M. Huang, Effective improvement of the photovoltaic performance of carbon-based perovskite solar cells by additional solvents, Nano-Micro Lett. 8 (2016) 347–357. [190] H. Wang, X.Y. Hu, H.X. Chen, The effect of carbon black in carbon counter electrode for CH3 NH3 PbI3 /TiO2 heterojunction solar cells, RSC Adv. 5 (2015) 30192–30196. [191] Z.H. Wei, H.N. Chen, K.Y. Yan, X.L. Zheng, S.H. Yang, Hysteresis-free multi-walled carbon nanotube-based perovskite solar cells with a high fill factor, J. Mater. Chem. A 3 (2015) 24226–24231. [192] H. Chen, Z. Wei, K. Yan, Y. Yi, J. Wang, S. Yang, Liquid phase deposition of TiO2 nanolayer affords CH3 NH3 PbI3 /nanocarbon solar cells with high open-circuit voltage, Faraday Discuss. 176 (2014) 271–286. [193] Y. Rong, X. Hou, Y. Hu, A. Mei, L. Liu, P. Wang, H. Han, Synergy of ammonium chloride and moisture on perovskite crystallization for efficient printable mesoscopic solar cells, Nat. Commun. 8 (2017) 14555. [194] F. Zhang, X. Yang, M. Cheng, J. Li, W. Wang, H. Wang, L. Sun, Engineering of hole-selective contact for low temperature-processed carbon counter electrode-based perovskite solar cells, J. Mater. Chem. A 3 (2015) 24272–24280. [195] S. Gholipour, J.P. Correa-Baena, K. Domanski, T. Matsui, L. Steier, F. Giordano, F. Tajabadi, W. Tress, M. Saliba, A. Abate, A.M. Ali, N. Taghavinia, M. Gratzel, A. Hagfeldt, Highly efficient and stable perovskite solar cells based on a low-cost carbon cloth, Adv. Energy Mater. 6 (2016). [196] C.M. Tsai, G.W. Wu, S. Narra, H.M. Chang, N. Mohanta, H.P. Wu, C.L. Wang, E.W.G. Diau, Control of preferred orientation with slow crystallization for carbon-based mesoscopic perovskite solar cells attaining efficiency 15%, J. Mater. Chem. A 5 (2017) 739–747. [197] X. Chang, W. Li, H. Chen, L. Zhu, H. Liu, H. Geng, S. Xiang, J. Liu, X. Zheng, Y. Yang, S. Yang, Colloidal precursor-induced growth of ultra-even CH3 NH3 PbI3 for high-performance paintable carbon-based perovskite solar cells, ACS Appl. Mater. Interfaces 8 (2016) 30184–30192.

200

R. Singh et al. / Applied Materials Today 14 (2019) 175–200

[198] H.N. Chen, Z.H. Wei, H.X. He, X.L. Zheng, K.S. Wong, S.H. Yang, Solvent engineering boosts the efficiency of paintable carbon-based perovskite solar cells to beyond 14%, Adv. Energy Mater. 6 (2016) 1502087. [199] X.M. Hou, Y. Hu, H.W. Liu, A.Y. Mei, X. Li, M. Duan, G.A. Zhang, Y.G. Rong, H.W. Han, Effect of guanidinium on mesoscopic perovskite solar cells, J. Mater. Chem. A 5 (2017) 73–78. [200] S.G. Hashmi, D. Martineau, M.I. Dar, T.T.T. Myllymaki, T. Sarikka, V. Ulla, S.M. Zakeeruddin, M. Gratzel, High performance carbon-based printed perovskite solar cells with humidity assisted thermal treatment, J. Mater. Chem. A 5 (2017) 12060–12067. [201] Z. Ku, Y. Rong, M. Xu, T. Liu, H. Han, Full printable processed mesoscopic CH3 NH3 PbI3 /TiO2 heterojunction solar cells with carbon counter electrode, Sci. Rep. 3 (2013) 3132. [202] E. Singh, H.S. Nalwa, Graphene-based bulk-heterojunction solar cells: a review, J. Nanosci. Nanotechnol. 15 (2015) 6237–6278. [203] E. Singh, H.S. Nalwa, Stability of graphene-based heterojunction solar cells, RSC Adv. 5 (2015) 73575–73600. [204] S.N. Habisreutinger, T. Leijtens, G.E. Eperon, S.D. Stranks, R.J. Nicholas, H.J. Snaith, Enhanced hole extraction in perovskite solar cells through carbon nanotubes, J. Phys. Chem. Lett. 5 (2014) 4207–4212. [205] S.N. Habisreutinger, T. Leijtens, G.E. Eperon, S.D. Stranks, R.J. Nicholas, H.J. Snaith, Carbon nanotube/polymer composites as a highly stable hole collection layer in perovskite solar cells, Nano Lett. 14 (2014) 5561–5568. [206] W.Z. Li, H.P. Dong, X.D. Guo, N. Li, J.W. Li, G.D. Niu, L.D. Wang, Graphene oxide as dual functional interface modifier for improving wettability and retarding recombination in hybrid perovskite solar cells, J. Mater. Chem. A 2 (2014) 20105–20111. [207] J. Lee, M.M. Menamparambath, J.Y. Hwang, S. Baik, Hierarchically structured hole transport layers of spiro-OMeTAD and multiwalled carbon nanotubes for perovskite solar cells, ChemSusChem 8 (2015) 2358–2362. [208] K. Yan, Z. Wei, J. Li, H. Chen, Y. Yi, X. Zheng, X. Long, Z. Wang, J. Wang, J. Xu, S. Yang, High-performance graphene-based hole conductor-free perovskite solar cells: Schottky junction enhanced hole extraction and electron blocking, Small 11 (2015) 2269–2274. [209] G.-K. Lim, Z.-L. Chen, J. Clark, R.G.S. Goh, W.-H. Ng, H.-W. Tan, R.H. Friend, P.K.H. Ho, L.-L. Chua, Giant broadband nonlinear optical absorption response in dispersed graphene single sheets, Nat. Photonics 5 (2011) 554. [210] F. Wang, M. Endo, S. Mouri, Y. Miyauchi, Y. Ohno, A. Wakamiya, Y. Murata, K. Matsuda, Highly stable perovskite solar cells with an all-carbon hole transport layer, Nanoscale 8 (2016) 11882–11888. [211] Q. Luo, Y. Zhang, C.Y. Liu, J.B. Li, N. Wang, H. Lin, Iodide-reduced graphene oxide with dopant-free spiro-OMeTAD for ambient stable and high-efficiency perovskite solar cells, J. Mater. Chem. A 3 (2015) 15996–16004. [212] K. Mielczarek, A.A. Zakhidov, Perovskite based hybrid solar cells with transparent carbon nanotube electrodes, MRS Proc. 1667 (2014). [213] Z. Wu, S. Bai, J. Xiang, Z. Yuan, Y. Yang, W. Cui, X. Gao, Z. Liu, Y. Jin, B. Sun, Efficient planar heterojunction perovskite solar cells employing graphene oxide as hole conductor, Nanoscale 6 (2014) 10505–10510. [214] Z. Liu, J.T. Robinson, X. Sun, H. Dai, PEGylated nanographene oxide for delivery of water-insoluble cancer drugs, J. Am. Chem. Soc. 130 (2008) 10876–10877. [215] X. Sun, Z. Liu, K. Welsher, J.T. Robinson, A. Goodwin, S. Zaric, H. Dai, Nano-graphene oxide for cellular imaging and drug delivery, Nano Res. 1 (2008) 203–212. [216] H. Li, K. Cao, J. Cui, S. Liu, X. Qiao, Y. Shen, M. Wang, 14.7% efficient mesoscopic perovskite solar cells using single walled carbon nanotubes/carbon composite counter electrodes, Nanoscale 8 (2016) 6379–6385. [217] T. Liu, D. Kim, H. Han, A.R. Yusoff, J. Jang, Fine-tuning optical and electronic properties of graphene oxide for highly efficient perovskite solar cells, Nanoscale 7 (2015) 10708–10718. [218] A. Kaskela, A.G. Nasibulin, M.Y. Timmermans, B. Aitchison, A. Papadimitratos, Y. Tian, Z. Zhu, H. Jiang, D.P. Brown, A. Zakhidov, E.I. Kauppinen, Aerosol-synthesized SWCNT networks with tunable conductivity and transparency by a dry transfer technique, Nano Lett. 10 (2010) 4349–4355. [219] Z. Li, S.A. Kulkarni, P.P. Boix, E. Shi, A. Cao, K. Fu, S.K. Batabyal, J. Zhang, Q. Xiong, L.H. Wong, N. Mathews, S.G. Mhaisalkar, Laminated carbon nanotube networks for metal electrode-free efficient perovskite solar cells, ACS Nano 8 (2014) 6797–6804. [220] Z. Li, Y. Jia, J. Wei, K. Wang, Q. Shu, X. Gui, H. Zhu, A. Cao, D. Wu, Large area, highly transparent carbon nanotube spiderwebs for energy harvesting, J. Mater. Chem. 20 (2010) 7236–7240. [221] M.A. Gluba, D. Amkreutz, G.V. Troppenz, J. Rappich, N.H. Nickel, Embedded graphene for large-area silicon-based devices, Appl. Phys. Lett. 103 (2013).

[222] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S.K. Banerjee, L. Colombo, R.S. Ruoff, Large-area synthesis of high-quality and uniform graphene films on copper foils, Science 324 (2009) 1312–1314. [223] F. Lang, M.A. Gluba, S. Albrecht, J. Rappich, L. Korte, B. Rech, N.H. Nickel, Perovskite solar cells with large-area CVD-graphene for tandem solar cells, J. Phys. Chem. Lett. 6 (2015) 2745–2750. [224] X. Zheng, H. Chen, Q. Li, Y. Yang, Z. Wei, Y. Bai, Y. Qiu, D. Zhou, K.S. Wong, S. Yang, Boron doping of multiwalled carbon nanotubes significantly enhances hole extraction in carbon-based perovskite solar cells, Nano Lett. 17 (2017) 2496–2505. [225] J. Ryu, K. Lee, J. Yun, H. Yu, J. Lee, J. Jang, Paintable carbon-based perovskite solar cells with engineered perovskite/carbon interface using carbon nanotubes dripping method, Small 13 (2017). [226] S. Liu, K. Cao, H. Li, J. Song, J. Han, Y. Shen, M. Wang, Full printable perovskite solar cells based on mesoscopic TiO2 /Al2 O3 /NiO (carbon nanotubes) architecture, Sol. Energy 144 (2017) 158–165. [227] P. You, Z. Liu, Q. Tai, S. Liu, F. Yan, Efficient semitransparent perovskite solar cells with graphene electrodes, Adv. Mater. 27 (2015) 3632–3638. [228] N. Cheng, P. Liu, F. Qi, Y.Q. Xiao, W.J. Yu, Z.H. Yu, W. Liu, S.S. Guo, X.Z. Zhao, Multi-walled carbon nanotubes act as charge transport channel to boost the efficiency of hole transport material free perovskite solar cells, J. Power Sources 332 (2016) 24–29. [229] J.S. Yeo, R. Kang, S. Lee, Y.J. Jeon, N. Myoung, C.L. Lee, D.Y. Kim, J.M. Yun, Y.H. Seo, S.S. Kim, S.I. Na, Highly efficient and stable planar perovskite solar cells with reduced graphene oxide nanosheets as electrode interlayer, Nano Energy 12 (2015) 96–104. [230] Q. Luo, H. Ma, Y. Zhang, X.W. Yin, Z.B. Yao, N. Wang, J.B. Li, S.S. Fan, K.L. Jiang, H. Lin, Cross-stacked superaligned carbon nanotube electrodes for efficient hole conductor-free perovskite solar cells, J. Mater. Chem. A 4 (2016) 5569–5577. [231] I. Jeon, S. Seo, Y. Sato, C. Delacou, A. Anisimov, K. Suenaga, E.I. Kauppinen, S. Maruyama, Y. Matsuo, Perovskite solar cells using carbon nanotubes both as cathode and as anode, J. Phys. Chem. C 121 (2017) 25743–25749. [232] W. Wei, B.Y. Hu, F.M. Jin, Z.Z. Jing, Y.X. Li, A.A.G. Blanco, D.J. Stacchiola, Y.H. Hu, Potassium-chemical synthesis of 3D graphene from CO2 and its excellent performance in HTM-free perovskite solar cells, J. Mater. Chem. A 5 (2017) 7749–7752. [233] X.Y. Wang, Z. Li, W.J. Xu, S.A. Kulkarni, S.K. Batabyal, S. Zhang, A.Y. Cao, L.H. Wong, TiO2 nanotube arrays based flexible perovskite solar cells with transparent carbon nanotube electrode, Nano Energy 11 (2015) 728–735. [234] M.L. Cai, V.T. Tiong, T. Hreid, J. Bell, H.X. Wang, An efficient hole transport material composite based on poly(3-hexylthiophene) and bamboo-structured carbon nanotubes for high performance perovskite solar cells, J. Mater. Chem. A 3 (2015) 2784–2793. [235] I. Jeon, T. Chiba, C. Delacou, Y. Guo, A. Kaskela, O. Reynaud, E.I. Kauppinen, S. Maruyama, Y. Matsuo, Single-walled carbon nanotube film as electrode in indium-free planar heterojunction perovskite solar cells: investigation of electron-blocking layers and dopants, Nano Lett. 15 (2015) 6665–6671. [236] L. Qiu, J. Deng, X. Lu, Z. Yang, H. Peng, Integrating perovskite solar cells into a flexible fiber, Angew. Chem. Int. Ed. Engl. 53 (2014) 10425–10428. [237] J.H. Im, J. Chung, S.J. Kim, N.G. Park, Synthesis, structure, and photovoltaic property of a nanocrystalline 2H perovskite-type novel sensitizer (CH3 CH2 NH3 )PbI3 , Nanoscale Res. Lett. 7 (2012). [238] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051. [239] F. Hao, C.C. Stoumpos, D.H. Cao, R.P.H. Chang, M.G. Kanatzidis, Lead-free solid-state organic–inorganic halide perovskite solar cells, Nat. Photonics 8 (2014) 489. [240] Rahul, P.K. Singh, R. Singh, V. Singh, B. Bhattacharya, Z.H. Khan, New class of lead free perovskite material for low-cost solar cell application, Mater. Res. Bull. 97 (2018) 572–577. [241] H. Chung, R. Singh, L. Kumar, M.A. Alam, P. Bermel, Characterization and redesign of perovskite/silicon tandem cells, in: 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC), 2016, pp. 3625–3628. [242] H.J. Chung, X.S. Sun, A.D. Mohite, R. Singh, L. Kumar, M.A. Alam, P. Bermel, Modeling and designing multilayer 2D perovskite/silicon bifacial tandem photovoltaics for high efficiencies and long-term stability, Opt. Express 25 (2017) A311–A322. [243] F. Sahli, J. Werner, B.A. Kamino, M. Bräuninger, R. Monnard, B. Paviet-Salomon, L. Barraud, L. Ding, J.J. Diaz Leon, D. Sacchetto, G. Cattaneo, M. Despeisse, M. Boccard, S. Nicolay, Q. Jeangros, B. Niesen, C. Ballif, Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency, Nat. Mater. 17 (2018) 820–826.