Recent progress in lead-free perovskite (-like) solar cells

Materials Today Energy 8 (2018) 157e165

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Recent progress in lead-free perovskite (-like) solar cells Qiaohui Zhang a, Hungkit Ting a, b, Shiyuan Wei a, Daiqiang Huang a, Cuncun Wu a, Weihai Sun a, Bo Qu a, Shufeng Wang a, Zhijian Chen a, *, Lixin Xiao a, c, * a

State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, PR China Technology Innovation Center, Dongguan Institute of Opto-Electronics, Peking University, Dongguan, Guangdong 523808, PR China c College of Physics and Electronics Engineering, Hengyang Normal University, Hengyang, Hunan 421008, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 January 2018 Received in revised form 24 February 2018 Accepted 4 March 2018

Organic-inorganic hybrid perovskite solar cells (SCs) have emerged as one of the most promising contenders to traditional silicon solar cells, due to their active layers outstanding photoelectric properties, such as appropriate direct bandgap, balanced high carrier mobility and long carrier diffusion length, the identified power conversion efficiency (PCE) has reached to 22.7%. But the toxic lead, a key component in the archetypical light harvesting material, is a large obstacle to commercialization. Herein, we reviewed the recent progress in lead-free perovskite (-like) SCs according to the valent difference of metal ions in absorber material, e.g., bivalent (Sn2þ, Ge2þ, Cu2þ, Sr2þ), trivalent (Bi3þ, Sb3þ), tetravalent (Sn4þ) and hybrid valent (e.g. Agþ and Bi3þ). Finally, we gave an outlook on the tactic to achieve high performance lead-free perovskite (-like) SCs. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Lead-free Perovskite Perovskite-like Solar cell Progress

1. Introduction Organic-inorganic lead halide perovskite (e.g., CH3NH3PbI3) was first used as visible-light sensitizer in photovoltaic cells with a power conversion efficiency (PCE) of 3.8% in 2009 [1], which is the prototype of archetypical perovskite SCs. In the past several years, the device structure and processing technique have been largely developed [2e12]. Current champion cell could achieve a PCE of 22.7% [13], which is comparable to polycrystalline SCs. While it is not qualified enough for commercial applications until overcoming the challenges of instability and toxicity. The stability of perovskite SCs in ambient air has been continuously promoted by composition engineering [14e16], interface passivating [17e19] or some other method [20]. However, excellent devices are all based on lead halide perovskite APbX3, where A is a cation, e.g., methylþ þ þ þ ammonium (CH3NHþ 3 , MA ), formamidinium (HC(NH2)2 , FA ), Cs ,    etc., X is a halogen anion, e.g., I , Br , Cl , etc., which is watersoluble and the lead is easily leaked into the soil. Lead is incapable of degrading, which could damage the nervous system and cause brain disorder even death. Therefore, it is essential to explore

* Corresponding authors. State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, PR China E-mail addresses: [email protected] (Z. Chen), [email protected] (L. Xiao). https://doi.org/10.1016/j.mtener.2018.03.001 2468-6069/© 2018 Elsevier Ltd. All rights reserved.

other nontoxic metal to replace lead, and the overall cell performance will not be seriously degraded. For lead halide perovskites, the stable corner-sharing structure of [PbX6] octahedron with mixed ionic-covalent bond enables excellent features, such as solution-processable technique, suitable direct bandgap close to ideal Shockley-Queisser gap for singlejunction solar cells (1e1.5 eV) [21], high optical absorption (a z 1.5  104 cm1 at 550 nm for CH3NH3PbI3) [22] and long carrier diffusion length (~100 nm for CH3NH3PbI3 and ~1 mm for CH3NH3PbI3-xClx) [23,24]. What's more, the features of dominant shallow point defects and intrinsically benign grain boundaries are prominent among other photovoltaic absorbers. Theoretical studies base on density functional theory (DFT) have demonstrated their unique photovoltaic properties are mainly attributed to the combination of structure symmetry and the existence of lone-pair 6 s2 orbitals in lead, which could establish direct-gap p-p transitions and appropriate bandgaps [25,26]. That is to say, making clear the role of lead plays in perovskite semiconductor would pave an avenue to lead-free research. Lead in perovskite type semiconductors is vital for photovoltaic properties: Structurally, the size of Pb2þ is capable of sustaining special symmetry structure according to the geometric tolerance factor (t) [27], t ¼ (rA þ rX)/[√2(rM þ rX)], where rA, rM, rX are the effective ionic radii of A, M, and X ions in the typical formula AMX3. While t is equal to 1, the structure belongs to Pm3m and has a

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perfect cubic symmetry. Huge deviations would cause structural distortion then weaken photovoltaic properties, such as worse optical absorption, wider bandgap, bigger electron/hole effective masses [25]. Electronically, Pb2þ has 6 s2 electron orbitals, the so called lone-pair s electrons, which won't lose in halides. The conduction band minimum of CH3NH3PbI3 is derived from the unoccupied 6p orbital, while the valence band maximum is composed mainly of p orbital of halogen mixed with small component of the occupied s orbital of Pb2þ, which makes p-p transitions possible and induces strongly absorption. Recently electronic and structural dimensionality of corner-sharing perovskite and its derivatives, perovskite-like structure, i.e., edge-sharing, face-sharing or even isolated connection, have been discussed via statistics and calculations [28], and it indicated that a promising photovoltaic absorber ought to have both high electronic and structural dimensionality values. Considering the importance of ionic radius, electronic properties and fabrication technique, various candidates for lead halide perovskite have sprung up. In this review, we have summarized the lead-free perovskite (-like) SCs published until now and classified the various lead-free perovskites or perovskite-like absorbers according to the valence state of metal ions (M), in detail, bivalent (Sn2þ, Ge2þ, Cu2þ, Sr2þ, Mn2þ), trivalent (Bi3þ, Sb3þ), tetravalent (Sn4þ) and hybrid valent (e.g. Agþ and Bi3þ) (Fig. 1). We believe this review will present a step towards the realization of environmentally benign lead-free perovskite SCs. 2. Lead-free perovskite (-like) SCs 2.1. Bivalent metal ion based perovskite (-like) SCs The most direct way is to substitute Pb2þ in APbX3 with other nontoxic bivalent metal ions (M2þ), researchers focused on the group IIA and IVA metals (Sr2þ, Sn2þ, Ge2þ), and some other transition metals (Cu2þ). 2.1.1. Sn2þ based absorbers Among all the elements in periodic table, tin was regarded as the most potential metal to replace lead since they locate in the same IVA group and their bivalent ions (Pb2þ, Sn2þ) have similar ionic radii (1.19 Å and 1.02 Å, respectively) [28,29]. Sn2þ based perovskite

Fig. 1. Different perovskite (-like) structures.

SCs have been reported tremendously and we have classified them as organic-inorganic hybrid or all-inorganic one. MASnI3 has a lower bandgap (1.2e1.4 eV) [30,31] which induces broader absorption spectrum compared with benchmark MAPbI3 counterpart (1.55 eV). The first entirely lead-free Sn2þ based perovskite SC was made by Snaith et al. [32] They fabricated MASnI3 devices on mesoporous TiO2 scaffold and achieved a PCE of 6.40% with high open circuit voltage (Voc) of 0.88 V. Meanwhile Kanatzidis et al. [31] reported a halide-mixed absorber, MASnI3xBrx, while the PCE of device was 5.73%. Besides, they investigated solvent effects on the crystallization of MASnI3 where highly uniform, pinhole-free MASnI3 films could be obtained via a transitional SnI2:3DMSO intermediate phase [33] and the device without hole-transporting material (HTM) could achieve an unprecedented photocurrent density up to 21 mA cm2. They also pointed out the bad performance is due to high “background” carrier density. During the development of MASnI3 based perovskite SCs, short circuit behavior caused by poor MASnI3 morphology and undesired electrical properties of film often occurred, to solve this, Kanatzidis et al. [34] employed a low temperature vapor-assisted solution process (LT-VASP) which could achieve high surface coverage and excellent uniformity films and the device showed high reproducibility. In addition, Ogale et al. [35] employed pulsed laser deposition (PLD) process in room temperature to achieve high quality MASnI3 film, and Qi et al. [36] introduced two vapor deposition-based methods, i.e., co-evaporation (CE) and sequential evaporation (SE), to fabricate MASnBr3 films. Kanatzidis et al. [37] indicated the p-type defect states, mainly Sn4þ and Sn vacancies, in Sn2þ based perovskite would also cause poor device reproducibility and performance, they presented a hydrazine reducing vapor atmosphere (RVA) process to suppress the oxidized Sn4þ impurities and lower levels of defect states which enables a high PCE of 3.89%. As HC(CH2)þ 2 was introduced into lead perovskite devices with excellent performance, Sn2þ based analogues were also investigated. Seok et al. [38] fabricated FASnI3 SCs through SnF2-pyrazine complex with high reproducibility and high PCE of 4.8%, where homogeneous dispersible SnF2 is as an inhibitor of Sn4þ [39]. Moreover, Yan et al. [40] employed a simple inverted device architecture to avoid using salt-doped HTMs and used diethyl ether dripping in solvent engineering process besides SnF2 additives, which could achieve a PCE of 6.22%. Wu et al. [41] also demonstrated a “multichannel interdiffusion”(MI) strategy to create uniform FASnI3 films and achieved a PCE of 3.98% based on the inverted structure. Seo et al. [42] introduced bromide into FASnI3 lattice to significantly lower the carrier density and achieved a PCE of 5% with high stability over 1000 h when encapsulated. Bian et al. [43] have reported (FA)0.75(MA)0.25SnI3 based SCs with a PCE of 8.12% which originates from the improved photoactive film morphology and reduced recombination process. In order to solve the issue of instability in ambient air, Sn2þ based perovskite-like absorbers were also investigated, Ning et al. [44] presented highly oriented low-dimensional (PEA)2(FA)8Sn9I28 perovskitelike SCs, and Kanatzidis et al. [45] also reported “hollow” {en} MASnI3 (en is ethylenediammonium) based SCs with much enhanced stability, they showed PCE of 5.94% and 6.63%, respectively. Recently, Loi et al. achieved the highest PCE of 9% until now by mixing very small amount of layered perovskite(Sn) with FASnI3 [46] (Fig. 2). As for all-inorganic perovskite SCs, Csþ is the most promising cation according to t [47,48], and lead-free CsSnI3 was also intensively investigated as a potential photovoltaic semiconductor [49]. Among three phases, B-a, B-b and B-g, Kenney et al. [50] first experimentally demonstrated the black CsSnI3 (B-g CsSnI3) was a

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Fig. 2. Schematic crystal structure of (a) 3D FASnI3, (b) 2D/3D mixture, and (c) 2D PEA2SnI4; (d) JeV curves for the champion devices containing pure 3D and 2D/3D perovskite (the inset shows the device structure); (e) JeV curves of the champion 2D/ 3D perovskite cell at different directions and rates. Reprinted with permission from Ref. [46].

direct bandgap semiconductor with a bandgap of ~1.3 eV. It is a ptype orthorhombic perovskite with intrinsic Sn vacancy, and has a carrier concentration of ~1017 cm3 with a considerable high hole mobility of ~585 cm2V1 s1 [51] and the exciton binding energy is ~18 meV [52]. Mathews et al. [39] firstly utilized CsSnI3 as an absorber in perovskite SCs and got a PCE of 2.02%. In order to avoid metallic conductivity caused by intrinsic defects of Sn-cation vacancies [51], they added 20 mol% SnF2 into the stoichiometric precursor (CsI, SnI2), and the film preparation only need a low annealing temperature (70 C). They also further investigated the impact of anionic Br substitution on Voc in CsSnI3-xBrx with and without SnF2 [53], and the Voc could increase even without SnF2. Besides, Wang et al. [54] investigated CsSnIBr2 thin film with suppressed bulk recombination by the addition of hypophosphorous acid (HPA) to speed up nucleation process and inhibit the Sn2þ oxidation. The device showed superior thermal stability up to 473 K and could achieve a PCE of 3.0%. Sun et al. [55] demonstrated planar heterojunction-depleted perovskite SC showed PCE of 3.31% with device structure of ITO/ NiOx/CsSnI3/PCBM/Al. Hatton et al. [56] further investigated the difference among the addition of SnX2(X ¼ F, Cl, Br, I). Device without HTM based on co-depositing the perovskite precursors with SnCl2 achieved a PCE of 3.56%, which showed higher stability than devices with the same architecture using MAPbI3. Recently Kanatzidis et al. [57] used excess SnI2 in CsSnI3 (CsI:SnI2 ¼ 0.4:1) combined with a reducing atmosphere to stabilize the Sn2þ state, and achieved a PCE of 4.81%. The device performances of Sn2þ based perovskite(-like) SCs are summarized in Table 1. Although Sn2þ based perovskites showed great electrical and optical properties for photovoltaic applications. They often exhibit metallic behavior caused by a strong “selfdoping” effect, that is, the easy oxidation of Sn2þ to Sn4þ and the low formation energy of Sn vacancies. Moreover, how to fabricate pinhole-free film with high air stability and reproducibility is still a significant challenge. Although many researchers have prompted various methods to suppress and the intrinsic defects, the efficiency was still much lower than lead based analogue, and it is a long way to achieve high performance Sn2þ based SCs.

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2.1.2. Other M2þ based absorbers In addition to Sn2þ, some other M2þ were also investigated to substitute lead while pursuing friendly light harvester in perovskite SCs. There are a few reports of working SCs based on Ge2þ, Cu2þ based lead-free perovskite (-like) with a poor PCE lower than 1%, and several researches focused on the Sr2þ based perovskites. The device performances of Ge2þor Cu2þ based perovskite (-like) SCs are summarized in Table 2. Germanium was reasonably considered because it belongs to the same group as lead. However, AGeI3 (A ¼ MA, FA, etc.) have larger bandgap than Pb2þ or Sn2þ analogues, ranging from 1.63 to 2.8 eV [59], which is due to the structural distortion. The much smaller ionic radius (0.73 Å) of Ge2þ [29] couldn't sustain the regular [GeI6] octahedra. Experimentally, CsGeI3 and MAGeI3-based perovskite SCs were reported with extremely low PCE of 0.11% and 0.20%, respectively. 2D layered Cu-based perovskites have been previously studied mainly for their fascinating magnetic performance [60]. Despite that, the synthesis and optoelectronic properties of the 2D perovskite series MA2CuClxBr4-x have been discussed [61]. It indicates that improving Br/Cl ratio could tune bandgap from 2.48 eV (x ¼ 4) to 1.80 eV (x ¼ 0.5), and the device using MA2CuCl2Br2 as absorber could achieve only a PCE of 0.017% limited by the low absorption and heavy mass for holes. In addition, other organic layer spacing cations have been studied [62], and a PCE of 0.63% was achieved by using (CH3(CH2)3NH3)2CuBr4 as absorber. It is considered that the poor efficiency is due to the intrinsic low conductivity in the 2D single layered perovskites. Recently, Han et al. [63] reported a highly stable C6H4NH2CuBr2I compound, which exhibited extraordinary hydrophobic behavior and the printable mesoscopic SCs employed it as absorber achieved PCE of 0.46% (Fig. 3). Strontium was also regarded as one of the most promising alternatives according to the Goldschmidt's rules together with quantum mechanical analysis [64]. The ionic radius of Sr2þ (1.18 Å) and Pb2þ are nearly identical [29], implying that the substituting won't greatly affect the crystal structure, but bandgap of MASrI3 is estimated to 3.6 eV, which is rather higher than lead-based analogue due to a higher degree of ionic interaction in metalhalogen connections deriving from the low electronegativity of Sr. It also indicates that Sr2þ based perovskite has the potential to be a top cell in tandem architectures after decreasing bandgap by changing the organic cation. 2.2. Trivalent metal ion based perovskite-like SCs The VA group metals (Bi3þ, Sb3þ) were also considered to form the perovskite-like derivatives, A3M2X9, which has two structures. One is two-dimensional (2-D) layered structure, termed as “vacancy-ordered perovskite”, with space group P3m1, and it could be identified as the removal of every third B3þ sites along the 〈111〉 direction of the perovskite structure AMX3 [65]. The other is zerodimensional (0-D) structure with space group P63/mmc, where Aþ cations and [M2X9] dimers formed by face-sharing [MX6] octahedra are isolated distributed. 2.2.1. Bi3þ based absorbers Bismuth ion (Bi3þ) has the same 6s26p0 electronic configuration as Pb2þ [66], and its low toxicity and stability in atmosphere make it a promising substitute for Pb in photovoltaic field. However, compared to Pb2þ based perovskites, Bi3þ based analogues have a higher bandgap. Johansson et al. [67] reported the bandgap of 0-D dimer form Cs3Bi2I9 and MA3Bi2I9 is 2.2 eV and 2.1 eV, respectively. Besides, introducing Cl into it would increase the bandgap to 2.4 eV (MA3Bi2I9Clx). They fabricated films by one step spin-coating method, and achieved PCE of 1.09% and 0.12% in Cs3Bi2I9 and

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Table 1 Summary of device performance of Sn2þ based perovskite(-like) SCs. Types

Device structure

Eg(eV)

PCE(%)

Jsc(mA/cm2)

Voc(V)

FF

Ref.

MASnI3 MASnIBr2 MASnI3 MASnBr3 (CE) MASnBr3 (SE) MASnI3 (RVA) FASnI3 FASnI3 FASnI3 (MI) FASnI3 (FA)0.75(MA)0.25SnI3 FASnI3 (2D/3D) (PEA)2(FA)8Sn9I28 {en}MASnI3 CsSnI3 CsSnI2Br CsSnBr3 CsSnIBr2 (HPA) CsSnI3 CsSnI3 (SnCl2) CsSnI3

FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Au FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Au FTO/bl-TiO2/mp-TiO2/PV/Au FTO/bl-TiO2/PV/P3HT/Au FTO/bl-TiO2/PV/P3HT/Au FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Au FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Au ITO/PEDOT:PSS/PV/C60/BCP/Ag ITO/PEDOT:PSS/PV/C60/BCP/Ag FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Au ITO/PEDOT:PSS/PV/C60/BCP/Ag ITO/PEDOT:PSS/PV/C60/BCP/Al ITO/NiOx/PV/PCBM/Al FTO/bl-TiO2/mp-TiO2/PV/PTAA/Au FTO/bl-TiO2/mp-TiO2/PV/m-MTDATA/Au FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Au FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Au FTO/bl-TiO2/mp-TiO2/Al2O3/PV/C ITO/NiOx/PV/PCBM/Al ITO/PV/PCBM/BCP/Al FTO/bl-TiO2/mp-TiO2/PV/PTAA/Au

1.23 1.75 1.30 2.20 2.30 1.30 e 1.40 1.40 1.63 1.33 e e e 1.30 1.37 1.75 1.63 1.30 1.30 1.30

6.40 5.73 3.15 0.35 1.12 3.89 4.80 6.22 3.98 5.50 8.12 9.00 5.94 6.63 2.02 1.67 2.17 3.00 3.31 3.56 4.81

16.80 12.30 21.40 2.05 4.27 19.92 23.70 22.07 17.78 19.80 21.20 24.10 14.44 24.28 22.70 15.06 9.10 16.70 10.21 10.44 25.71

0.88 0.82 0.32 0.41 0.49 0.37 0.32 0.46 0.33 0.41 0.61 0.52 0.59 0.42 0.24 0.28 0.42 0.33 0.52 0.51 0.38

0.42 0.57 0.46 0.41 0.49 0.51 0.63 0.60 0.67 0.66 0.62 0.71 0.69 0.63 0.37 0.38 0.57 0.53 0.62 0.69 0.49

[32] [31] [33] [36] [36] [37] [38] [40] [41] [42] [43] [46] [44] [45] [39] [53] [58] [54] [55] [56] [57]

PV means photovoltaic material; Eg means the value of bandgap.

Table 2 Summary of device performance of Ge2þor Cu2þ based perovskite (-like) SCs. Types

Device Structure

Eg(eV)

PCE(%)

Jsc(mA/cm2)

Voc(V)

FF

Ref.

CsGeI3 MAGeI3 MA2CuCl2Br2 (p-F-C6H5C2H4NH3)2CuBr4 (CH3(CH2)3NH3)2CuBr4 C6H4NH2CuBr2I

FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Au FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Au FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Au FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Ag FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Ag FTO/bl-TiO2/mp-TiO2/ZrO2/PV/C

1.63 2.00 2.12 1.74 1.76 1.64

0.11 0.20 0.01 0.51 0.63 0.46

5.70 4.00 0.21 1.46 1.78 6.20

0.07 0.15 0.25 0.87 0.88 0.20

0.27 0.30 0.32 0.40 0.40 0.46

[59] [59] [61] [62] [62] [63]

Fig. 3. (a) Digital photograph of the contact angle with water (inset shows a photograph of a water drop on the film surface after 5 min); (b) Tauc plot of C6H4NH2CuBr2I from UV/vis spectroscopy to determine Eg and photoluminescence spectra; (c) scheme of the device structure; (d) JeV curves of the best performing device. Reprinted with permission from Ref. [63].

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MA3Bi2I9 based devices. (NH4)3Bi2I9 was also investigated experimentally and theoretically by Cheetham et al. [68], the 2-D layered film was prepared by solution and showed a bandgap of 2.04 eV. Moreover, Miyasaka et al. [69] investigated the effect of electron transporting layer (ETL) on MA3Bi2I9 based devices, they obtained a PCE of 0.26% by employing anatase TiO2 as ETL and all the devices were stable in ambient conditions for more than 10 weeks. Recently, Gao et al. [70] have produced fully compact 0-D MA3Bi2I9 (MBI) films by employing a novel two-step, high-low vacuum deposition method, besides, the devices based on it could achieved a PCE of 1.64% (Fig. 4). 2.2.2. Sb3þ based absorbers Antimony (Sb) is a congener of bismuth with smaller atomic radius. Similar to Bi3þ, ternary halide semiconductor Cs3Sb2I9 also has 0-D dimer and 2-D layered form, while the general solution processing is favorable to 0-D dimer one with poor transporting properties. Mitzi et al. [65] achieve large grain (>1 mm) and continuous thin films of layered Cs3Sb2I9 by co-evaporated CsI and SbI3 with a lower bandgap of 2.05 eV. However, the PCE of corresponding devices was lower than 1%. They pointed out the Ii, ISb and VI defects could produce deep levels, which act as the centers of nonradiative recombination. Kirchartz et al. [71] reported 0-D dimer form MA3Sb2I9 based devices with a PCE of 0.49%, and the absorber film showed a peak absorption coefficient of az105 cm1, an optical bandgap of 2.14 eV and a much lower exciton binding energy than Bi3þ based analogues. Rbþ (1.72 Å) was also investigated by Mathews et al. [72] to form layered form Rb3Sb2I9, which has a direct and indirect bandgap of 2.24 and 2.10 eV, respectively. Comparing to Csþ (1.88 Å) based analogues, the desired layered Rb3Sb2I9 could be fabricated irrespective of processing methodologies, and the corresponding device achieved a PCE of 0.66%. A family of perovskite-like light absorbers, (NH4)3Sb2IxBr9-x, with good solubility in ethanol was investigated [73]. And the (NH4)3Sb2I9 based SCs achieved a PCE of 0.51% with a higher Voc of 1.03 V, while the poor PCE results from the bad crystallinity of absorber film. Recently, Chu et al. [74] employed HI as additive in precursor to prepare 0-D Cs3Sb2I9 films by one step solution

Fig. 5. (a) Schematic plot of the Cl addition induced transformation from the 0-D dimer phase of A3Sb2I9 to the 2-D layered phase of A3Sb2ClxI9-x; (b) scheme of the device structure; (c) JeV curves at different scan directions. Reprinted with permission from Ref. [76].

method, and the device based it has a PCE of 2.04%, then, they reported a vapor-assisted solution-processed 2-D layered polymorph of Cs3Sb2I9 [75], which achieved a PCE as high as 1.49%. Chlorine was also incorporated into MA3Sb2I9 to induce the formation of desired 2-D layered phase. Moreover, Zhou et al. [76] have achieved a record-high PCE over 2% by employing the as-obtained MA3Sb2ClxI9-x as absorber (Fig. 5). The device performance of Bi3þ or Sb3þ based perovskite-like SCs are summarized in Table 3. A3M2X9 perovskite-like absorbers have a huge bandgap (~2 eV) which is unfit for photovoltaic applications. Moreover, the general solution process tends to form 0D phase with poor charge transporting properties. It is worth mentioning that a series of all-inorganic silver bismuth halides

Fig. 4. (a) The film fabrication process diagram; (b) SEM morphology of the low vacuum-deposited BiI3 films (with RMS ¼ 7.47 nm); (c) scheme of the device structure (D-TiO2: dense TiO2; M-TiO2: mesoporous TiO2); (d) energy alignment of the devices. Reprinted with permission from Ref. [70].

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Table 3 Summary of device performance of Bi3þ or Sb3þ based perovskite-like SCs. Types

Device structure

Eg(eV)

PCE(%)

Jsc(mA/cm2)

Voc(V)

FF

Ref.

Cs3Bi2I9 MA3Bi2I9 MA3Bi2I9Clx MA3Bi2I9 MA3Bi2I9 Cs3Sb2I9 MA3Sb2I9 Rb3Sb2I9 (NH4)3Sb2I9 MA3Sb2I9 Cs3Sb2I9 MA3Sb2ClxI9-x Ag3BiI6 AgBi2I7 Ag2BiI5

FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Ag FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Ag FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Ag FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Au FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Au FTO/bl-TiO2/PV/PTAA/Au ITO/PEDOT:PSS/PV/PCBM/ZnO-NP/Al FTO/bl-TiO2/mp-TiO2/PV/Poly-TPD/Au ITO/PEDOT:PSS/PV/PCBM/Al ITO/PEDOT:PSS/PV/PCBM/C60/BCP/Al ITO/PEDOT:PSS/PV/PCBM/BCP/Al FTO/bl-TiO2/mp-TiO2/PV/Spiro-OMeTAD/Au FTO/bl-TiO2/mp-TiO2/PV/PTAA/Au FTO/bl-TiO2/mp-TiO2/PV/P3HT/Au FTO/bl-TiO2/mp-TiO2/PV/P3HT/Au

2.20 2.10 2.40 e e 2.05 2.14 2.24 2.27 1.95 2.05 1.79 1.83 1.87 1.85

1.09 0.12 <0.01 0.25 1.64 <1.00 0.49 0.66 0.51 2.04 1.49 2.19 4.30 1.22 2.10

2.15 0.52 0.18 0.83 2.95 e 1.00 2.11 1.15 5.41 5.31 5.04 10.70 3.30 6.80

0.85 0.68 0.04 0.56 0.81 0.25e0.30 0.90 0.55 1.03 0.62 0.72 0.69 0.63 0.56 0.49

0.60 0.33 0.38 0.56 0.69 e 0.55 0.56 0.42 0.60 0.38 0.63 0.64 0.67 0.63

[67] [67] [67] [69] [70] [65] [71] [72] [73] [74] [75] [76] [77] [78] [79]

based on formula AaBbXx (x ¼ a þ 3b), named rudorffites, whose prototype is NaVO2 oxide, they have gained great attention recently, where AX6 and BX6 octahedra are edge shared. They showed lower direct bandgap (1.79e1.83 eV) [77] and better device performance, such as AgBi2I7 [78], Ag2BiI5 [79], Ag3BiI6 [77], what's more, the SCs employed Ag3BiI6 as absorber could achieve a highest PCE of 4.3% with great stability. 2.3. Tetravalent tin ion (Sn4þ) based perovskite-like SCs Considering the air sensitivity and rather inert atmosphere while fabricating Sn2þ based absorber in perovskite SCs, researchers turn attention to Cs2SnI6, which is more stable exposing to ambient environment [80,81] and owes similar photoelectric properties to MAPbI3. Moreover, researchers have gained a few computational and experimental achievements for its photovoltaic application. Tetravalent metal compound A2MX6 adopts the K2PtCl6 structure type [82] with a space group of Fm3m. In detail, the Aþ cation lies in the void net of the discrete [BX6]2þ octahedra, then Cs2SnI6 can be seen as a defect-variant of removing half of Sn in CsSnI3. In addition, the lattice constant (LC) of A2BX6 has been verified as a linear function of ionic radii and electronegativity of its constituting ions by Brik et al. [83], and the LC of Cs2SnI6 is 11.650 Å. The vacancy-ordered Cs2SnI6 owns excellent photoelectric properties, e.g., direct bandgap (1.26e1.62 eV) and high absorption coefficient of 105 cm1 in 1.7 eV which depends on deposition methods [80e82,84e87] as shown in Table 4; ambipolar transport capability [81], Hall effect demonstrated intrinsic bulk Cs2SnI6 behaves as a ntype semiconductor (electron mobility, me ¼ 310 cm2/V s) and the Seebeck coefficient of Sn2þ(as SnI2) doped one is positive sign (hole mobility, mh ¼ 42 cm2/V s). However, Cs2SnX6 (X ¼ Cl, Br, I) are regarded as molecular salts due to their rather ionic character, and “radius ratio” [82] of Aþ cation to 12-coordinate void in A2BX6 (Cs2SnI6 is 0.94) was adopted to predict structure stability of them which is different from tolerance factor used in ABX3. Nevertheless, the DFT calculations have revealed that the real oxidation state of

Sn in Cs2SnI6 is þ2 similar to CsSnI3 caused by ligand holes in [SnI6] unit, actually [I66Lþ2]4, and Cs2SnI6 owes better stability than CsSnI3 to the shorter and stronger Sn-I bonds [88]. Morphology of Cs2SnI6 depends on deposition method extremely influences device performance [89]. Cs2SnI6 was originally employed as solid hole transport materials in dye-sensitized solar cells (DSSCs) [81] and the PCE is near 8% with mixture of porphyrin dyes. Qiu et al. [86] adopted Cs2SnI6 as a light absorber in nanostructured ZnO-based mesoscopic perovskite-like SCs firstly with PCE near 1%. Almost simultaneously they reported Cs2SnI6 film formed from unstable CsSnI3 and the device PCE was also about 1% [85]. Mitzi et al. [87] reported a vacuum-assisted deposition approach for preparing high quality films of Cs2SnI6, and they indicated the poor device performance was due to the dominant VI and Sni defects and bad electron transport property. In addition, electrospraying was also employed to fabricate Cs2SnI6 film by Hayase et al. [90] (Fig. 6), and they pointed out the unbalanced carrier mobility inside Cs2SnI6. Chang et al. studied Cs2SnI6-xBrx for photovoltaic application and the device based on compound of x ¼ 2 could achieve a PCE of 2.1% [89], which is the highest PCE of Sn4þ based perovskite-like SCs. The device performances of Sn4þ based perovskite-like SCs are summarized in Table 4. In conclusion, it is apparent that Cs2SnX6 owns feasible photoelectric properties and shows excellent performance as HTM in DSSCs, while the application in perovskite-like SCs seems inferior. Maybe the photogenerated carriers couldn't be extracted efficiently due to the huge internal resistance and scanty diffusion length of Cs2SnI6 caused by intrinsic defects. Moreover, the poor solubility in polar solvent and rapid crystallization of CsI is a major challenge for fabricating compact Cs2SnI6 films by conventional solution spinning process. 2.4. Hybrid valent metal ions based perovskite-like SCs Exploration hybrid valent substitution of two Pb2þ by Mþ I and M3þ II cations to form elpasolite structure type AMIMIIX6, which also named “double perovskite”, was also a promising research

Table 4 Summary of device performance of Sn4þ based perovskite-like SCs. Types

Device structure

Eg(eV)

PCE(%)

Jsc(mA/cm2)

Voc(V)

FF

Ref.

Cs2SnI6 (vacuum evaporation) Cs2SnI6 Cs2SnI6 Cs2SnI6 (from CsSnI3) Cs2SnI4Br2

e e FTO/bl-ZnO/nanorods/PV/P3HT/Ag FTO/bl-TiO2/PV/P3HT/Ag FTO/bl-TiO2/mp-2 wt% Sn-TiO2/PV/solid state Cs2SnI6 based HTM/LPAH/FTO

1.62 1.25a 1.48 1.48 1.40

e e 0.86 0.96 2.02

e e 3.20 5.41 6.22

e e 0.52 0.51 0.56

e e 0.51 0.35 0.57

[87] [82] [86] [85] [89]

a

Means powder state.

Q. Zhang et al. / Materials Today Energy 8 (2018) 157e165

Fig. 6. (a) The film fabrication process diagram; SEM images of (b) spin-coated film and (c) spray-coated film. Reprinted with permission from Ref. [90].

163

Karunadasa et al. [95] reconstructed the band-edge by doping highly toxic Tl3þ to Bi3þ based double perovskite and reduced the bandgap to 1.57 eV. Mitzi et al. [96] have gained 1.86 eV through trivalent metal alloy where 37.5% of Bi3þ was replaced by Sb3þ. Moreover, organic-inorganic hybrid double perovskite (MA)2AgBiBr6 was synthesized [97], which has a lower bandgap of 2.02 eV. Recently, Zhang et al. [98] have presented two direct-gap materials, Cs2InSbCl6 and Cs2InBiCl6 with an appropriate bandgap ground 1.0 eV, which are calling for experimentally synthesis. The double perovskite showed great photovoltaic properties, while how to fabricate high quality absorber films is still a huge challenge as other kinds of lead-free perovskite (-like) materials. Until now, only Cs2AgBiBr6 based SCs have been reported. Recently, we have fabricated Cs2AgBiBr6 film with the highest quality so far by a low-pressure assisted (LPA) solution processing, and the corresponding planar structure device presents a PCE of 1.44% [99] (Fig. 7). Meanwhile, Bein et al. [100] reported a mesoporous structure Cs2AgBiBr6 based device with a PCE of 2.43%. The devices above all showed great stability even without encapsulation. What's more, the device architecture and functional layers need optimizing later to enhance the device performance. 3. Conclusion and outlook

direction. Originally, Zhang et al. [91] have theoretically proposed a new group of CuIn-based halide perovskite, which showed direct tunable bandgap and offered a new routine for designing lead-free perovskite materials. Then, a series of double perovskite was researched theoretically and experimentally. However, the SCs based it still showed poor performance. Cs2AgBiBr6 was first reported by Karunadasa et al. [92], they confirmed that Cs2AgBiBr6 has characteristics of an indirect bandgap as 1.95 eV with an assisting phonon of 0.12 eV, and a long carrier recombination decay about 660 ns, which indicates that Cs2AgBiBr6 could be used for photovoltaic devices. Woodward et al. [93] demonstrated Cs2AgBiX6 (X ¼ Br, Cl) are crystallizing in the structure of elpasolite, which belongs to the Fm3m space group. Bandgap of 2.19 eV for Cs2AgBiBr6 and 2.77 eV for Cs2AgBiCl6 were measured by the UVeVis diffuse reflectance spectra, the result of which corresponds to another report for Cs2AgBiBr6(1.95 eV) [92], and Cs2AgBiCl6 (2.2 eV) [94]. Cs2AgBiX6 are more stable than Pb-based perovskite, but showed larger bandgap. In order to reduce it, several researchers had focused on the bandgap engineering, such as,

Fig. 7. (a) The film fabrication process diagram; (b) SEM images of film from 250  C; (c) absorption spectrum of Cs2AgBiBr6 film. Reprinted with permission from Ref. [99].

In conclusion, the exploration of lead-free perovskite (-like) SCs involved different valent metal ions, such as Agþ, Sn2þ, Ge2þ, Cu2þ, Sr2þ, Bi3þ, Sb3þ and Sn4þ, which formed various structures comþ þ    bined with CH3NHþ 3 , HC(CH2)2 or Cs cation and Cl , Br , I halogen anion. Among all the lead-free light absorber materials. (i) Sn2þ based one shows high density of intrinsic defects, which results in poor stability exposure to ambient condition, but the corresponding devices present the highest PCE of 9% among all the lead-free perovskite (like) SCs, in addition, the low-dimensional stable perovskite-like materials should not be ignored. Other bivalent ions based perovskite SCs showed much lower PCE (<1%) owing to the poor absorber film and photovoltaic properties, which could be enhanced by exploring novel film fabricating methods. (ii) Sn4þ containing material, e.g., Cs2SnI6, has good stability and an ideal bandgap, but the low electronic dimensionality (0D) and the dominant VI and Sni defects which act as centers of nonradiative recombination would induce huge internal resistance and scanty diffusion length. Moreover, CsI, one of the reactants, which has poor solubility in polar solvent and rapid crystallization caused the huge difficulty of fabricating high-quality film by general solution process. (iii) M3þ with (or without) Mþ ions formed various perovskitelike structures with different dimensionality, namely, double perovskite (3-D), vacancy-ordered perovskite (2-D) or 0D dimer form, they all show large bandgap (~2 eV) which need embedded composition optimization. Moreover, the film preparing process calls for in-depth study, for example, the general solution process is favorable for 0-D dimer Cs3Sb2I9 with poor charge transporting properties, rather than 2-D layered phase, which has 2-D electronic dimensionality. Beyond that, the so-called rudorffites (AaBbXx, x ¼ a þ 3b), also show great potential of photovoltaic application. Generally, lead-free perovskite-like research has showed much achievement, but it is still a long way to explore the novel film fabricating methods and develop new type materials with high stability and excellent photoelectric properties for photovoltaic applications.

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Q. Zhang et al. / Materials Today Energy 8 (2018) 157e165

Acknowledgements This work was supported by the National Key R&D Program of China (No. 2016YFB0401003), the National Natural Science Foundation of China (61575005, 61775004, 11574009, 11574013, U1605244) and the Science and Technology Planning Project of Guangdong Province, China (Grant Nos. 2014B010105002 and 2015A070713001).

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