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ScienceDirect Materials Today: Proceedings 19 (2019) 73–78
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NANOTEXNOLOGY2018
Mixing cations and halide anions in perovskite solar cells Konstantina E. Gkinia,b, Maria Antoniadoua, Nikolaos Balisa, Andreas Kaltzogloua, Athanassios G. Kontosa, Polycarpos Falarasa,* a
Institute of Nanoscience and Nanotechnology, NCSR Demokritos, 15341, Agia Paraskevi Attikis, Athens, Greece b Physics Department, School of Natural Sciences, University of Patras, 26504, Greece
Abstract In this work we synthesized triple cation (A = Cs, FA, MA), mixed halide (X = I, Br, Cl) APbX3 compounds to develop perovskite solar cells (PSCs) with improved thermal stability as well as high power conversion efficiency. The effects of the cation combination and the mixing of halides in mesostructured PSCs have been thoroughly investigated. The presence of cesium results in greater stability, less impurities and higher reproducibility. Power conversion efficiencies of ~17% and ~13.5% were recorded for PSCs with I-Br and I-Cl compounds, respectively. Despite their lower efficiency, the I-Cl containing PSCs exhibit greater stability in ambient conditions. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of International Conferences & Exhibition on Nanosciences & Nanotechnologies and Flexible Organic Electronics 2018, June 30th - July 6th, 2018 Keywords: perovskite solar cells; mixed cations; mixed halides; mesostructured solar cells; stability
1. Introduction The application of organic-inorganic metal halide perovskite materials on solar cells has attracted great attention worldwide due to their extraordinary optoelectronic properties, such as large absorption coefficient, high electronhole diffusion length, and high charge-carrier mobility [1–6]. Emerging from the solid state dye-sensitized solar cells, the PCS’s power conversion efficiency has increased from 3.8% to over 22% in only a few years [7-9]. However, stability issues in these devices seem to discourage further advancements towards their commercialization.
* Corresponding author. Tel.: +30 210 6503644; fax: +30 210 6511766. E-mail address:
[email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of International Conferences & Exhibition on Nanosciences & Nanotechnologies and Flexible Organic Electronics 2018, June 30th - July 6th, 2018
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The organic-inorganic lead halides adopt the ABX3 perovskite structure. “A” is typically an organic cation, methylammonium (MA) CH3NH3+ or formamidinium (FA) CH3(NH2)2+ [11,12], “B” is a divalent metal (Pb2+, Sn2+, Ge2+) [12,13], and “X” is an anion halogen or pseudohalogen (Cl-, Br-, I-, SCN-) [14,15]. Recently, perovskites with mixed cations and halogen anions have attracted the attention as their pure analogues, namely MAPbX3, FAPbX3 and CsPbX3 present several disadvantages [16]. A fundamental issue with the MAPbI3 based PSCs is their rapid degradation when exposed to light in an oxygen and humid atmosphere. This was attributed to a chemical reaction between oxygen and the MA cation in the presence of electronic charge in the perovskite [16-18]. Formamidinium (FA)-based perovskites are much more thermally stable than those based on methylammonium (MA). Although the use of FAPbI3 is advantageous due to the reduced band gap [17,18], one major problem is that this material is not thermodynamically stable as black perovskite phase and it undergoes a structural transition to yellow at room temperature [17-19]. It was found that replacing a fraction of MA+ or FA+ by Cs+ in perovskite materials could result in films with reduced trap state density, thus improving the performance of PSCs [19,20]. An attempt to study the triple cation mixed halide perovskite Csx(MA0.17FA0.83)1-xPb(I0.83Br0.17)3 has already been made, confirming that this concept is beneficial for the device characteristics [7,19]. Motivated by the consideration that Cl ion in organometal halide perovskite can boost the mobility of excitons and the charge carrier transport [20-23], we mix for the first time iodide with chloride salts. The resulting Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Cl0.17)3-based mesostructured solar cells are characterized for their structural, optical and electrochemical properties. Thusa power conversion efficiency of 13.5% was reached and more importantly, great stability and sturdiness under humid, daylight, room temperature and oxygen conditions were demonstrated. 2. Materials and Methods Triple cation mixed halide perovskites were incorporated in mesostructured solar cells with the following architecture: FTO / TiO2(CL) / TiO2(MP) / Perovskite / Spiro-MeOTAD / Ag. The mesostructured PSCs were fabricated by depositing the successive layers upon FTO conductive substrates (Dyesol, 7 Ohm/sqt). The FTO electrodes were patterned with a 2 mol L-1 aqueous HCl (Fisher) solution in combination with zinc powder (97.5%, Alfa Aesar) and were carefully cleaned and sonicated into a bath with TritonX-100 (Sigma), acetone (Fisher), 2propanol (Merck) and UV ozone in 15 min cycle each. 2.1. Preparation of TiO2 photoanode The TiO2 compact layer was spin coated at 2000 rpm for 60 s from a mildly acidic solution of titanium(IV) isopropoxide (Aldrich, 97%) in ethanol. The obtained films were annealed at 500 °C for 45 min (ramp rate of 5 °C min-1). A mesoporous TiO2 layer was then deposited onto the compact layer substrate, by spin coating a diluted paste [weight ratio of TiO2 (Dyesol paste, 18-NRT) and ethanol is 1:8] at 4000 rpm for 20 s. This was followed by sintering the substrates at 450 °C for 30 min in dry air flow. For Li treatment of the mesoporous TiO2 scaffold, 100 μL of LiTFSI (bis(trifluoromethane) - sulfonimide lithium salt) solution in acetonitrile (10 mg mL-1, freshly prepared) was spin coated at 3000 rpm for 20 s, after a loading time of 10 s, in ambient conditions. Thereafter, Litreated substrates were subjected to a second sintering step at 450 °C for 30 min in dry air. The substrates were transferred into a Ar-filled glove box (humidity and oxygen levels < 1 ppm) for the perovskite deposition. 2.2. Synthesis and deposition of perovskite solutions and hole transporting layer (HTL) For the synthesis of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Cl0.17)3 we used FAI (1.0 mol L-1) (Dyesol), PbI2 (1.1 mol L-1) (Acros), MACl (0.2 mol L-1) (99%, Acros) and PbCl2 (0.2 mol L-1) (Alfa Aesar). For the synthesis of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3, MACl was replaced by MABr (0.2 mol L-1) (Dyesol) and PbCl2 by PbBr2 (0.2 mol L-1) (99.9%, Alfa Aesar). In both cases, the reagents were diluted in a mixture of anhydrous dimethylformamide (99.8%, Acros) / dimethylsulphoxide (99.7%, Acros) (4:1 (v:v)). The precursor solution was completed by adding CsI (99.999%, trace metal basis, Acros) solution (5% volume, 1.5 mol L-1 DMSO). The perovskites were formed in the glove box, using single-step deposition method by spin-coating the precursors onto the mesoporous TiO2 films at 1000 and 6000 rpm for 10 and 30s, respectively. During the spin coating, 100 μL of chlorobenzene (99.8%, Acros)
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was dropped on the spinning substrate 10 s prior to the end of the program. This was followed by annealing of the films at 100 °C for 45 min. A 7 wt% Spiro-MeOTAD (Solaronix) solution in chlorobenzene was deposited as the hole transport layer, containing additives of lithium bis(trifluoromethanesulfonyl)imide lithium salt ≥99%, Aldrich) in acetonitrile, and 4-tertbutylpyridine (96%, Aldrich). The deposition was realized at 3000 rpm for 30 s. Finally, the films were transferred outside the glove-box and the devices were prepared by thermal evaporation of 100 nm silver electrodes under vacuum of 10-6 Torr, at a rate of ~1 Å s-1. 2.3. Characterization methods and instrumentation The crystallinity of the perovskite film was examined by X-ray powder diffraction (XRPD), using a Siemens D-500 diffractometer, that operates with Cu Kα1 (λ = 1.5406 Å) and Cu Kα2 (λ = 1.5444 Å) radiation. Raman analysis was performed using a Renishaw in Via Reflex microscope with solid state laser (λ = 514.4 nm) excitation source. The laser light was focused on the samples using a ×100 objective lens of a Leica DMLM microscope at power density equal to 0.035 mW μm-2. Prior to the Raman mapping, steady state micro-photoluminescence (PL) maps were carried out on a larger area, using the Raman microscope. Excitation was carried at 785 nm adjusting the power density to 5 10-5 mW μm-2 and the PL signal was recorded through integration of the PL tail, above 788 nm. The absorbance measurements of the perovskite film in the ultraviolet-visible spectral region were carried on a UV-Vis Hitachi 3010. In order to observe the morphology and the roughness of the perovskite surface, Scanning Electron Microscopy analysis was conducted with a field emission scanning electron microscope (JSM 7401F, JEOL Ltd., Tokyo, Japan), equipped with Gentle Beam mode, and AFM images were obtained with an AFM: Digital Instruments Nanoscope III, operating in tapping mode. J-V curves were obtained by illuminating the solar cells under a Solar Light Co. 300W Air Mass Solar Simulator Model 16S-300 (1 sun, 1000 W m-2) calibrated using an Optopolymer Si reference cell. The curves were recorded with an Autolab PG-STAT-30 potentiostat under a 200 mV s-1 reverse scan rate. The measurements were carried out using Ossila’s Push-Fit Test Board for Photovoltaic Substrates with a 0.12 cm2 aperture mask which defines the active area of the PSCs. 3. Results and Discussion 3.1. X-Ray Diffraction
Fig. 1. XRD diagram for Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Cl0.17)3 perovskite film on FTO/TiO2 substrate.
Fig. 1 shows the XRPD pattern of the I-Cl based perovskite films, grown on mesoporous TiO2 substrate. For comparison of the reflection intensities, the theoretical histograms of the MAPbI3 and FAPbCl3 phases are shown. The sample contains mainly MAPbI3 and to smaller extent FAPbCl3, instead of a solid solution of the two phases. Apart from the perovskite reflections, weak peaks of unreacted PbI2 impurities phase and the SnO2 substrate are observed, too. No cesium-containing salt was detected.
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3.2. Raman Spectroscopy The structural stability for the I-Cl based films was examined on films exposed to ambient conditions, including humidity (60%), oxygen and daylight for several days. Firstly, micro-PL images were recorded on large 60 × 50 μm2 areas of the films and show non uniform lateral distribution of the PL intensity (Fig. 2A). A subarea of the film (marked in blue outline) was further examined by Raman spectroscopic mapping (Fig. 2B and 2C). PbI2 signal at 205 cm-1 was detected in certain domains of the film area (a), while other domains (b) seem to be free of PbI2. By comparison of the PL and Raman map images from the same area, domains which present strong PL signal are rather correlated with parts of the sample where no PbI2 was formed [24].
Fig. 2. (A) Photoluminescence map (0-1200 a.u. in chromatic scale), (B) Raman signal map at 205 cm-1 (0-5000 counts) and (C) Raman spectra from different areas of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Cl0.17)3/FTO after several days exposed to ambient conditions.
3.3. UV-vis spectroscopy Fig. 3 presents the UV-vis absorbance spectra of I-Cl based perovskite films grown on mesoporous TiO2 substrate. Band gap extends to 800 nm (1.55 eV) due to the high content of FA in the composition.
Fig. 3. Absorption spectra of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Cl0.17)3 perovskite film, grown on mesoporous TiO2 substrate
3.4. Scanning Electron Microscopy and Atomic Force Microscopy To investigate the surface morphology and the grains that are formed in the perovskite film, we fabricated films of each perovskite compound with the structure of FTO / TiO2 (CL) / TiO2 (MP) / perovskite and obtained the topview SEM images (Fig. 4A and Fig. 4B). For the I-Cl based films, we also got an AFM image (Fig. 4C). The I-Br based films show a 3-D morphology with evenly distributed grains and mean size of 500 nm. The I-Cl based films show greater dispersion in the size of the grains (200 – 800 nm). Similar grain size is extracted by the AFM image, which is analyzed with a mean square roughness of Rrms = 27.9 nm.
A
B
C
Fig. 4. SEM images of A) Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 and B) Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Cl0.17)3, and C) AFM image of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Cl0.17)3.
3.5. Current density-voltage characteristics A series of devices with the structure of FTO / TiO2(CL) / TiO2(MP) / Perovskite / Spiro-MeOTAD/ Ag, as shown in Fig. 5, was fabricated to obtain the photovoltaic properties of the Csx(MA0.17FA0.83)1-xPb(I0.83Cl0.17)3 based PSCs and to compare them to the Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 based ones. The characteristics of the champion devices in each case and the corresponding J-V curves are presented in Fig. 5 and the inset summarizes the corresponding cell parameters.
Fig. 5. Left: J-V curves of perovskite solar cells A) with iodine and chlorine, and B) with iodine and bromine. Right: A typical mesostructured architecture for a PSC.
The overall photovoltaic performance is higher for I-Br containing perovskites. The compounds are less sensitive to processing conditions leading to power output of ~17% and ~13.5%, for I-Br and I-Cl analogues, respectively. Despite their lower efficiency, the PSCs based on I-Cl exhibit greater stability in ambient conditions. In addition, the open-circuit voltage (Voc) is stable reaching almost 1.05 V for all the devices based on the different perovskite films. Works are underway to expand the novel approach to planar PSCs [25], where the use of cations and anions mixtures in hybrid perovskite absorbers is also expected to increase the performance (efficiency and stability) of the devices.
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4. Conclusions In this work, we mixed for the first time I and Cl atoms in triple cation perovskite solar cells. By adding a fraction of Cs to (FA/MA)PbI3 and using the mixed cation and mixed halide perovskite in solar cells with structure of FTO / TiO2(CL) / TiO2(MP) / Perovskite / Spiro-MeOTAD / Ag, we overcome the perovskite structural instability, while achieving a high power conversion efficiency. The Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3-based PSCs present better photovoltaic performance reaching a power conversion efficiency of ~17%, with short–circuit photocurrents of 23.15 mA cm-2. Open circuit voltages for both perovskite compounds were relatively high reaching ~1.05 V. The Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Cl0.17)3 films exhibited great stability for several days exposed in ambient conditions (oxygen, humidity and daylight). Analysis of the device stability vs. time is under way. Acknowledgements The authors acknowledge financial support by European Union’s Horizon 2020 Marie Curie Innovative Training Network 764787 “MAESTRO” project. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc., 131(2009), 6050-6051. [2] J. Burschka, N. Pellet, S.J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Grätzel, Nature, 499 (2013), 316-319. [3] M. Antoniadou, E. Siranidi, N. Vaenas, A.G. Kontos, E. Stathatos, P. Falaras, J. Surf. Interfac. Mater., 2 (2014) 323-327. [4] D. Fabini, G. Laurita, J. Bechtel, C. Stoumpos, H. Evans, A. Kontos, Y. Raptis, P. Falaras, A. Van der Ven, M. Kanatzidis, and R. Seshadri, J. Am. Chem. Soc., 138 (2016) 11820–11832. [5] D. H. Fabini, C. C. Stoumpos, G. Laurita, A. Kaltzoglou, A. G. Kontos, P. Falaras, M. G. Kanatzidis, and R. Seshadri, Angew. Chem. Int. Ed. 55 (2016) 15392–15396. [6] A. Kaltzoglou, M. M. Elsenety, I. Koutselas, A. G. Kontos, K. Papadopoulos, V. Psycharis, C. P. Raptopoulou, D. Perganti, T. Stergiopoulos, P. Falaras, Polyhedron, 140 (2018) 67-73. [7] N. Arora, M.I. Dar, A. Hinderhofer, N. Pellet, F. Schreiber, S.M. Zakeeruddin, M. Grätzel, Science 358 (2017) 768-771. [8] 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. Grätzel, Energy Environ. Sci., 9 (2016) 1989-1997. [9] W. S. Yang, B. W. Park, E. H. Jung, N. J. Jeon, Y. C. Kim, D. U. Lee, S. S. Shin, J. Seo, E. K. Kim, J. H. Noh, S. I. Seok, Science, 348(2015), 1234-1237. [10] M. Konstantakou, D. Perganti, P. Falaras and T. Stergiopoulos, Crystals, 7(2017), 294. [11] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J. Snaith, Science, 338(2012), 643-647. [12] H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, S. J. Moon, R. Humphry-Baker, J. H. Yum, J. E. Moser, M. Gratzel and N. G. Park, Sci Rep, 2(2012), 591. [13] F. Hao, C. C. Stoumpos, D. H. Cao, R. P. H. Chang and M. G. Kanatzidis, Nat Photonics, 8(2014), 489-494. [14] C. H. Hendon, R. X. Yang, L. A. Burton and A. Walsh, J Mater Chem A, 3(2015), 9067-9070. [15] J. H. Heo, D. H. Song and S. H. Im, Adv Mater, 26(2014), 8179-8183. [16] N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. Seo and S. I. Seok, Nature, 517(2015), 476-480. [17] C. C. Stoumpos, C. D. Malliakas and M. G. Kanatzidis, Inorg Chem, 52(2013), 9019-9038. [18] J. W. Lee, D. J. Seol, A. N. Cho and N. G. Park, Adv Mater, 26(2014), 4991-4998. [19] H. Choi, J. Jeong, H.-B. Kim, S. Kim, B. Walker, G.-H. Kim, J.Y. Kim, Nano Energy, 7(2014) 80-85. [20] C. Wang, D. Zhao, Y. Yu, N. Shrestha, C.R. Grice, W. Liao, A.J. Cimaroli, J. Chen, R.J. Ellingson, X. Zhao, Y. Yan, Nano Energy, 35(2017) 223-232. [21] M. Zhang, H. Yu, M.Q. Lyu, Q. Wang, J.H. Yun, L.Z. Wang, Chem. Commun., 50(2014), 11727-30. [22] S. Colella, E. Mosconi, P. Fedeli, A. Listorti, F. Gazza, F. Orlandi, P. Ferro, T. Besagni, A. Rizzo, G. Calestani, G. Gigli, D.F. Angelis, R. Mosca, Chem. Mater., 25(2013), 4613–4618. [23] R. G. Niemann, A.G. Kontos, D. Palles, E.I. Kamitsos, A. Kaltzoglou, F. Brivio, P. Falaras, P.J. Cameron, J. Phys. Chem. C., 120(2016), 2509–2519. [24] N. Balis, A. Verykios, A. Soultati, V. Constantoudis, M. Papadakis, F. Kournoutas, C. Drivas, M.-C. Skoulikidou, S. Gardelis, M. Fakis, S. Kennou, A. G. Kontos, A. G. Coutsolelos, P. Falaras and M. Vasilopoulou, ACS Appl. Energy Mater., 1 (2018), 3216–3229. [25] N. Balis, A.A. Zaky, D. Perganti, A. Kaltzoglou, L. Sygellou, F. Katsaros, T. Stergiopoulos, A.G. Kontos, and P. Falaras, ACS Appl. Energy Mater., DOI: 10.1021/acsaem.8b01221