Relationship between Lattice Strain and Efficiency for Sn-Perovskite

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Organic Electronic Devices

Relationship between Lattice Strain and Efficiency for Sn-perovskite Solar Cells Kohei Nishimura, Daisuke Hirotani, Muhammad Akmal Kamarudin, Qing Shen, Taro Toyoda, Satoshi Iikubo, Takashi Minemoto, Kenji Yoshino, and Shuzi Hayase ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09564 • Publication Date (Web): 06 Aug 2019 Downloaded from pubs.acs.org on August 6, 2019

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ACS Applied Materials & Interfaces

Relationship between Lattice Strain and Efficiency for Sn-perovskite Solar Cells Kohei Nishimura†,#, Daisuke Hirotani#, Muhammad Akmal Kamarudin†,#, Qing Shen†, Taro Toyoda†, Satoshi Iikubo#, Takashi Minemoto¶, Kenji Yoshino§ and Shuzi Hayase†,# †Faculty

of Informatics and Engineering, The University of Electro-Communications, 1-5-1

Chofugaoka, Chofu, Tokyo 182-8585, Japan #Graduate

School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4

Hibikino, Wakamatsu-ku, Kitakyushu-shi, Fukuoka-ken 808-0196, Japan ¶Department

of Electrical and Electronic Engineering, Faculty of Science and Engineering,

Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu-shi, Shiga 525-8577, Japan §Department

of Electrical and Electronic Engineering, Miyazaki University, 1-1

Gakuenkibanadainishi, Miyazaki-shi, Miyazaki 889-2192, Japan

KEYWORDS. Perovskite solar cells, Pb-free, Lattice strain, Sn-perovskite, Multi-cation

ABSTRACT. In the composition of Q0.1(FA0.75MA0.25)0.9SnI3, Q is replaced with Na+, K+, Cs+, Ethylammonium+ (EA+) and Butylammonium+ (BA+) respectively, and the relationship between

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actually measured lattice strain and photovoltaic performances are discussed. The lattice strain evaluated by Williamson-hall plot of XRD data decreased as the tolerance factor was close to one. The efficiency of the Sn-perovskite solar cell was enhanced as the lattice strain decreased. Among them, EA0.1(FA0.75MA0.25)0.9SnI3 having lowest lattice strain gave the best result of 5.41%. Since the carrier mobility increased with a decrease in the lattice strain, these lattice strains would disturb carrier mobility and decrease the solar cell efficiency. Finally, the results that the efficiency of the SnGe-perovskite solar cells was gradually enhanced from 6.42% to 7.60% during storage, was explained by the lattice strain relaxation during the storage.

INTRODUCTION The certified efficiency of Pb-based perovskite solar cells (PSCs) with area more than 1 cm2 has been reported to be 20.9%1 which is close to those of polycrystalline inorganic solar cells such as CIGS, poly-Si, and CdTe solar cells.2,3 However, the use of Pb is limited by EU's Restriction of Hazardous Substances (RoHS) Directive (RoHS directive) due to the toxic nature of the material. This prompted researcher to find alternative candidates with lower toxicity. Bismuth halide compounds such as Cs3Bi2I9,4 MA3Bi2I9,5 Ag3BiI6,6 AgBi2I7,7 Cs2AgBiBr6,8 antimony halide compounds such as Rb3Sb2I9,9 titanium halide compounds such as Cs2TiBr6,10 and copper halide compounds such as MA2CuI411 have been reported to replace lead. However, the solar cell efficiency based on these materials were less than 5% and was not satisfactory. Among all leadfree perovskite materials, Sn-based perovskite is one of the most promising candidates as the light harvesting layer for Pb-free PSCs, because they have perovskite structure similar to Pb perovskite, have a band gap close to the Shockley-Queisser limit and show the promise to harness hot electron.12-16 So far, the most efficient Sn-based PSCs has been reported to be around 9.6% which consists of 2D and 3D structures.17 This lo12w efficiency compared to Pb-based perovskite is due


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to the tendency of Sn2+ to oxidize into Sn4+ upon exposure to air.18-20 We have previously reported SnGe PSCs with 7.9% through addition of Ge ion into Sn perovskite in order to enhance the efficiency and stabilize the solar cell.21 The Ge ion retards the oxidation of Sn2+ to Sn4+, which decreases intrinsic carrier concentration. In addition, Ge2+ is partially oxidized to form GeOx, which passivates the grain boundary and decreases the carrier trap density. Another approach is to make the tolerance factor closer to one by optimizing the cation size. The efficiency of Pb-PVK solar cells has been enhanced by the introduction optimized sized of the A site cation of ABX3 perovskite structure.22-25 Similar to Pb-based perovskite solar cells, the introduction of Cs cation and Guanidinium cation into Sn perovskite material leading to higher device performance has been reported.17,

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We were interested in the lattice strain and disordering, because the strain and

disordering may scatter carrier diffusion and become the center for charge recombination. During the research, we accidentally found that the efficiency of Sn-perovskite solar cell increased with the decrease in the lattice strain during the storage of the same solar cell in the glovebox, which is discussed in this text. This prompted us to examine the relationship between the lattice strain and the photovoltaic performances in detail. Zhu Kai and his coworkers have already reported that decreasing strains in Pb-perovskite layer improved the carrier extractions and enhanced the efficiency.27 However, in the case of Sn-based perovskites, the relation between photovoltaic performances and actually measured crystal lattice distortion relating to tolerance factor has not discussed yet. Our purpose is to clarify the relationship between the lattice strain from XRD data and the solar cell efficiency. In addition, the result that the efficiency of the solar cell was enhanced during storage, is discussed from the view point of the lattice strain relaxation.

MATERIALS AND METHODS


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Materials. Tin (II) iodide (SnI2, 99.99%, Sigma Aldrich), tin (II) fluoride (SnF2, 99%, Sigma Aldrich), germanium (II) iodide (GeI2, >99.8%, Sigma Aldrich), formamidinium iodide (FAI, >98.0%, TCI), methylammonium iodide (MAI, >98.0%, TCI), ethylammonium iodide (EAI, >98%, Sigma Aldrich), potassium iodide (KI, >99.99%, Sigma Aldrich), sodium iodide (NaI, >99.99%, Sigma Aldrich), cesium iodide (CsI, >99.99%, Sigma Aldrich), butylammonium iodide (BAI, >97.0%, TCI), poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS, Clevious PVP AI 4083), N,N-Dimethylformamide (DMF, 99.8%, Sigma Aldrich), dimethyl sulfoxide (DMSO, ≥99.9%, Sigma Aldrich), chlorobenzene (CB, 99.8%, Sigma Aldrich) were used without further purification. Preparation of Perovskite films. FA0.75MA0.25SnI3 solution was prepared by dissolving SnI2 (745.0 mg), SnF2 (31.3 mg), FAI (258.0 mg), and MAI (79.0 mg) in mixed solvent of DMF (2000 µl) and DMSO (500 µl). The QSnI3 (Q = Na+, K+, Cs+, EA+, BA+) solution was prepared by dissolving SnI2 (298.0 mg), SnF2 (12.5 mg), A site materials (NaI 119.9 mg, KI 132.8 mg, CsI 207.8 mg, EAI 138.4 mg, BAI 160.8 mg) in mixed solvent of DMF (800 µl) and DMSO (200 µl). The Q0.1(FA0.75MA0.25)0.9SnI3 perovskite solution was prepared by mixing the FA0.75MA0.25SnI3 and QSnI3 (Q = Na+, K+, Cs+, EA+, BA+) solution at a volume ratio of 9:1. The BA0.05(FA0.75MA0.25)0.95SnI3 perovskite solution was prepared by mixing the FA0.75MA0.25SnI3 and BASnI3 solution at a volume ratio of 9.5:0.5. SnGe-perovskite solution was prepared by mixing the FA0.75MA0.25SnI3 solution (1600 µl) and GeI2 solution (400 µl). FA0.75MA0.25SnI3 solution was prepared by dissolving SnI2 (670.5 mg), SnF2 (28.2 mg), FAI (232.2 mg), and MAI (71.1 mg) in mixed solvent of DMF (1344 µl) and DMSO (256 µl). GeI2 solution was prepared by dissolving GeI2 (146.9 mg) in solvent of DMF (2000 µl). Before mixing, GeI2 solution was filtered using a 0.45µm PTFE filter.15 These solutions were prepared in a nitrogen-purged glovebox.


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Device fabrications. The ITO glass substrates were cleaned with detergent, deionized water, acetone, and isopropyl alcohol in an ultrasonic bath for 15 min., respectively. The cleaned ITO substrates were treated with oxygen plasma for 5 min. PEDOT:PSS filtered with a 0.45 µm PVDF filter was spin-coated on the ITO substrate at 500 r.p.m for 9 s followed by 5000 r.p.m for 50 s. After that, the substrate was annealed at 140 °C for 20 min. The perovskite precursor solution was spin-coated at 5000 r.p.m for 50 s with Chlorobenzene used as the anti-solvent. All perovskite films were annealed on a hot plate at 70 °C for 10 min. PCBM in 1,2-dichlorobenzene (5 mg/ml) solution was spin-coated at 2000 r.p.m for 60 s and the sample was kept in a nitrogen-purged glovebox for 12 hours. Subsequently, C60 (20 nm), BCP (6 nm), Ag (60 nm), Au (30 nm) were sequentially evaporated by thermal evaporation under vacuum, resulting in an active area of 0.405 cm2. Characterization. The XRD patterns were obtained by a Rigaku Smartlab X-ray diffractometer with monochromatic Cu-Kβ irradiation (45 kV/200 mA). The UV-vis spectrum measurement was performed using a JASCO V-670 Spectrophotometer. Photoelectron yield spectroscopy (PYS) was used to determine the valence band, using a Bunkoukeiki KV205-HK ionization energy measurement system with -5.0 V of applied voltage. The thickness of the film was measured using a 3D surface profile measurement system (Nikon BW-A501). The solar cell measurement was performed, using a Keithley 4200 source meter and a solar simulator under 100 mW/cm2 AM 1.5G in air (Bunkoukeiki CEP-2000SRR). The measured area was fixed to be 0.1 cm2 with nonreflective metal mask. Photoluminescence-spectroscopy and fluorescence lifetime were measured by time correlated single photon counting fluorescence lifetime measurement (Hamamatu photonics K.K. Co., Japan., Quantaurus-Tau). Carrier density, mobility and conductivity were


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measured by Hall-effect measurement (Ecopia Co.). Samples for Hall effect measurement were prepared on platinum (Pt) sputtered glass having a cross slit (width: 2 μm).

RESULT AND DISCUSSION In this paper, we prepared Sn based perovskites with the structure of Qx(FA0.75MA0.25)1-xSnI3, where, Q was replaced with Na+, K+, Cs+, Ethylammonium+ (EA+) and Butylammonium+ (BA+), respectively. For all cations, the substitution concentration was fixed at x=0.1, with the except for BA+, where x=0.05 and 0.1 were studied. FA0.75MA0.25SnI3 and Q- substituted Sn perovskites are denoted as FAMA, Na-0.1, K-0.1, Cs-0.1, EA-0.1, BA-0.05 and BA-0.1 for simplification. Figure 1 shows the energy diagram of perovskite materials employed in this work. The valence band was measured by Photoelectron Yield Spectroscopy method (PYS) (Figure S1). The conduction band was estimated by adding the value of the valence band energy and the band gap energy determined from the absorption spectrum (Figure S2) and the corresponding Tauc plots (Figure S3). Large band gap difference among them was not observed when the cation of the Q site was varied. Cs0.1 has a little narrow bandgap, compared with others as shown in Figure S1 and S2. The conduction band and the valence band shifted toward deeper energies when the Q cation was changed from the small Na+ to the larger EA+. However, 5 mol% and 10 mol% addition of BA+ made the conduction band and the valence band shallower, which was results different from the other Q cations. The relationship between the energy level and the lattice strain will be discussed later.


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Figure 1. Energy level of conduction band and valence band for various Q-substituted Sn perovskite. Q is Na+, K+, Cs+, EA+ and BA+, respectively. For all cations, the substitution concentration was fixed at x = 0.1, with the exception of x = 0.05 and 0.1 for BA+.

Figure 2 shows the XRD spectra. The peak position of XRD for Na-0.1, K-0.1, FAMA, BA-0.05, and BA-0.1 was almost the same (2θ = 14.05°, Figure 2a, 2b, and 2c). The peak position for Cs0.1 shifted to 14.08°, suggesting that the lattice parameter became slightly shorter (Figure S4).28 In addition, the peak for EA-0.1 shifted to 14.03° suggesting that the lattice parameter for EA-0.1 became slightly longer. This will be also discussed from the view point of A+ site later. XRD spectra for Na-01, K-01, Cs-01, EA-01 and BA-0.05 shows the typical peaks corresponding to (100), (120), (200), (211), (222) and (300) facets, demonstrating that they have 3D perovskite structure. When BA+ with a larger ionic radius was introduced at a ratio of 10 mol%, a peak appeared at 2θ = 9° which is assigned to 2D structure (Figure 2c and 2d).13 This was not observed for BA-0.05. In the case of EA+ whose radius is between FA+ and BA+, the 2D structure was not observed. This will be discussed from the view point of the lattice strain later. It has been reported


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that not only the size of A site cation but also facet orientation is related to the efficiencies.28 They reported that preferred facet orientation facilitates photocarrier transport. In our case, doping with Na, K, Cs and EA did not give changes in facet growth direction, which was observed by XRD patters. When BA was added, a peak assigned to 001 facet seems to be enhanced, which is consistent with the previous report.13 However, carrier mobility of BA-0.1 was lower than that of Cs-0.1 with the same strain as BA-0.1 as discussed later. Facet orientation should be considered as a future study.


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Figure 2. a) XRD spectra for Sn perovskite with Na+, K+, Cs+, No addition and EA+ by 10 mol%, respectively. b) XRD spectra for various Q-substituted Sn perovskite around 2θ = 14.0°. c) XRD spectra for Sn perovskite with BA+ at varied proportions – 0, 5 and 10 mol%. d) XRD spectra for Sn perovskite with BA+ around 9°.

The steady-state photoluminescence spectra are shown in Figure S5. The peak position for these perovskite layers was 916 nm (Na-0.1), 914 nm (K-0.1), 926 nm (Cs-0.1), 912 nm (FAMA), 904 nm (EA-0.1), 914 nm (BA-0.05), 918 nm (BA-0.1), respectively. Cs-0.1 had a little redshifted


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emission, and EA-0.1 had a little blue shifted emission which agreed to the red shifted absorption spectrum of the Cs-0.1 and EA-0.1. The lattice strain was estimated by Williamson-Hall method (Supporting Information, Figure S6, and S7)30-33. The tolerance factor (t) was calculated by the following equation.34

𝑡=

𝑟𝐴 + 𝑟𝑋 2 ( 𝑟𝐵 + 𝑟𝑋)

eq.(1)

where 𝑟𝐵 and 𝑟𝑋 are the effective ionic radii of B divalent metals and X halide ions, respectively.34 When the t value lies in the range of 0.8 - 1.06, cubic perovskite structures with high stability are formed.34 𝑟𝐴 for these mixed cation site was calculated by the following equation.34

𝑟𝐴 = 𝑥 ∗ 𝑟𝑄 + (1 ― 𝑥) ∗ (0.75 ∗ 𝑟𝐹𝐴 + 0.25 ∗ 𝑟𝑀𝐴)

eq.(2)

where 𝑟𝐹𝐴, 𝑟𝑀𝐴, and 𝑟𝑄 refer to the effective ionic radii of FA+, MA+ and Q cation, respectively, and x represents the ratio of Q cation substituted in A site for the Sn perovskite.34-36 Figure 3 shows the relationship between the lattice strain and the tolerance factor for Q-substituted Sn perovskites. An inverse relationship between the lattice strain and the tolerance factor was observed. Among them, Na-0.1 had the lowest tolerance factor of 0.949 and the highest lattice strain of 3.46%. When the radius of Q increased in the order from K+, Cs+, and to EA+, the lattice strain decreased as the tolerance factor becomes close to 1.0. The lowest lattice strain was observed for EA-0.1 of 0.85%. This demonstrates that the lattice strain obtained by Williamson-Hall plot method are reasonable. When 5 mol% of BA+ with larger ionic radius was introduced, the tolerance factor became larger than 1.0 with increased lattice strain of 1.17% which is higher than that of EA-0.1 (Figure 3). When


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the ratio of BA+ increased to 10 mol% (BA-0.1), the tolerance factor increased to values larger than 1.0 with the decrease in the lattice strain. As mentioned above, XRD in Figure 2 shows that BA-0.05 had 3D perovskite structure and did not have 2D structure. For BA-0.1, a peak assigned to 2D structure appeared at near 2θ= 8°.13 It has been reported that the tolerance factor of less than 1.0 is favorable for perovskite structure.34 The 3D structure of BA-0.1 was partially transferred to 2D structure in order to relax the large strain of 3D-perovskite with BA+.

Figure 3. Relationship between the lattice strain and the tolerance factor (t) for various Qsubstituted Sn perovskite. Abbreviation: see Figure 1.

Figure 4a shows the current-voltage curves for various Q-substituted Sn perovskite. The solar cell has an inverted structure consisting of glass/ ITO/ PEDOT:PSS (40 nm)/ Q-substituted Sn Perovskites (160-180 nm)/ PCBM (20 nm)/ C60 (20 nm)/ BCP (6 nm)/ Ag (60 nm)/ Au (30 nm) layer as shown in Figure S8. The efficiency had a good correlation with the lattice strain as shown in Figure 4b. EA-0.1 with the lowest lattice strain gave the higher efficiency than other Q cations. The lattice strain decreased in the following order; EA-0.1 >Cs-0.1 >BA-0.1 (with 2D structure)


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>BA-0.05 (without 2D structure), and the efficiency showed the similar trend. The efficiency for K-0.1 and Na-0.1 with higher lattice strain was lower than those of other perovskite solar cells. It has been reported that the introduction of 2D structure to Sn-perovskite solar cells enhanced the efficiency of the perovskite solar cells.13 In our case, EA-0.1 with 2D/3D structure gave a little better results than BA-0.05 with 3D structure. However, the efficiency was almost the same as that of FAMA and lower than EA-0.1. The Jsc vs. strain and Voc vs. strain curves are shown in S9 in Supporting Information. All of photovoltaic parameters such as Jsc, Voc and FF had tendency to decrease as the lattice strain increased. In addition, the carrier mobility increased with decrease in the lattice strain as it is discussed later. These results suggest that the lattice strain works as carrier trapping sites which becomes charge recombination centers and decreased Voc, Jsc and FF.

Figure 4. a) Current-voltage curves for various Q-substituted Sn perovskite and b) the relationship between the lattice strain and the efficiency. Abbreviation: see Figure 1.

Table 1 summarizes the lattice strain and electrical properties such as carrier concentration and carrier mobility. A clear relationship between the lattice strain and carrier concentration was not


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observed as shown in Figure S10. Carrier concentration was highest for FAMA (3.48 × 1017 /cm3) and decreased according to the following order of Cs-0.1 >BA-0.05 >Na-0.1 >K-0.1 >BA-0.1 >EA-0.1. The lowest was EA-0.1.

Table 1. Electronic performances for various Q-substituted Sn perovskite. Abbreviation: see Figure 1.

Q cations

Lattice strain [%]

Carrier concentration [/cm3]

Mobility [cm2/Vs]

Na-0.1

3.46

7.67×1016

4.62

K-0.1

3.11

4.55×1016

11.47

Cs-0.1

1.09

2.76×1017

43.96

FAMA

1.41

3.48×1017

27.07

EA-0.1

0.85

4.60×1015

43.03

BA-0.05

2.02

1.32×1017

18.25

BA-0.1

1.19

1.21×1016

33.77

The mobility has good correlation with the lattice strain, as shown in Figure 5. Cs-0.1 and EA0.1 have the highest mobility which decreased with an increase in the relative lattice strain in the following order of BA-0.1 >FAMA >BA-0.1 >K-0.1 >Na-0.1. From the above results, it seems that the lattice strain is associated with carrier mobility, implying that the lattice strain disturbs the carrier collection. Zhu Kai and his coworkers have already reported that decreasing strains in Pbperovskite layer improved the carrier extractions and enhanced the efficiency.27 Carrier mobility


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is one of the items which the lattice strain affects. The strain may make the carrier life time shorter, because the disordering site may become charge recombination center. The PL life time for some of the Sn-perovskite thin films was too short to measure correctly. The clarification of the relationship is our future work. Lattice parameter did not change among FAMA, K-0.1, Na-0.1, BA-0.1 and BA-0.05 although the ionic diameter changed. On the other hand, EA-0.1 having larger ionic diameter had longer lattice parameter than that of FAMA, and that for Cs-0.1 having smaller ionic diameter was shorter than that of FAMA. If other K, Na, and BA cations were introduced in the A site of the lattice, lattice size change should be observed, depending on the A site cation size. From the results, in the case of Sn-PVKs, Cs ion and EA ion were introduced in A site of the main lattice, but the K, Na, and BA cations may be in the A site of the main lattice, and most probably on grain boundary working as surface passivation. Even if they are not in the A site of the mail lattice, the lattice strain may be affected by the surface passivation, because surface defects also affect strain structure of the inside. Figure 6a and 6b shows the current-voltage curves, and corresponding IPCE curves for EA-0.1 with 5.41 % efficiency after storage for 4 days in glove box, which did not show hysteresis.


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Figure 5. Relationship between the lattice strain and carrier mobility for various Q-substituted Sn perovskite. Abbreviation: see Figure 1.

Figure 6. a) Current-voltage curves, and b) corresponding IPCE curves, for EA-substituted Sn perovskite by 10 mol% when stored under N2 atmosphere.

We have already reported that the efficiency of SnGe mixed metal perovskite solar cells increased gradually during storage in a glove box.15 Similar phenomena have been reported in other paper.13,17 Eric and coworkers also reported that efficiency increased from 8.5 % to 9.6 % during


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storage of 2000h in the glove box. Figure 7a and 7b shows the current-voltage curves, and the relationship between the lattice strain and the efficiency when the SnGe-perovskite solar cell was stored. The lattice strain decreased from 1st day (ɛ= 1.56%) to 5th day (ɛ= 0.26%). The efficiency increased from 6.42% to 7.60% as the lattice strain decreased during the storage. The photovoltaic properties are summarized in Table S2. When the perovskite layer was coated and crystallized by anti-solvent methods, the lattice has strain and disordering because of the quick crystallization process and the lattice is a kind of metastable state. During the storage at room temperature, the lattice strain is relaxed. Since the strain and disordering retard carrier collections and may be the center for charge recombination, the strain relaxation should enhance the solar cell efficiency. The frequently observed phenomena that the efficiency increase during the storage, is explained by the relaxation of the lattice strain which was created initially as a metastable structure.

Figure 7a) Current-voltage curves, and b) the relationship between the lattice strain and the efficiency for SnGe-perovskite when stored under N2 atmosphere.


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CONCLUTIONS It was proved that lower lattice strain gave higher photovoltaic performances. The lattice strain influenced the charge carrier mobility. The high mobility was obtained in the perovskite with low lattice strain, suggesting that the lattice strain disturbs carrier mobility. Efficiency enhancement during the storage was explained by the relaxation of the lattice strain during the storage. These results will provide one of design directions to Sn-based PSCs with high efficiency.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. PYS plot, absorption spectra and the expanded spectra in the area from 800-1000 nm, tauc plots, lattice strain and lattice spacing, photoluminescence spectra, Williamson-Hall plot, lattice strain and crystallite size, device structure, photovoltaic parameter, and lattice strain and carrier concentration, for various Q-substituted Sn-perovskite. Photovoltaic parameter for SnGeperovskite. AUTHOR INFORMATION Corresponding Author †E-mail:

[email protected]

†E-mail:

[email protected]

Author Contributions


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The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. / #, ¶, § These authors contributed equally. Notes This authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by JST Mirai Program. (JPMJMI17EA) REFERENCES (1) Green, M. A.; Hishikawa, Y.; Dunlop, E. D.; Levi, D. H.; Ebinger, J. H.; Baillie, A. W. Y. H. Solar Cell Efficiency Tables (Version 52). Prog Photovolt Res Appl. 2018, 26, 427-436. (2) Jackson, P.; Hariskos, D.; Lotter, E.; Paetel, S.; Wuerz, R.; Menner, R.; Wischmann, W.; Powalla, M. New World Record Efficiency for Cu (In, Ga) Se2 Thin-film Solar Cells beyond 20%. Prog Photovolt Res Appl. 2011, 19, 894-897. (3) Ferekides, C. S.; Marunskiy, D.; Viswanathan, V.; Tetali, B.; Palekis, V.; Selvaraj, P.; Morel, D. L. High Efficiency CSS Cd Te Solar Cells. Thin Solid Films 2000, 361-362, 520-526. (4) Bai, F.; Hu, Y.; Hu, Y.; Qiu, T.; Miao, X.; Zhang, S. Lead-free, Air-stable Ultrathin Cs3Bi2I9 Perovskite Nanosheets for Solar Cells. Solar Energy and Solar Cells 2018, 184, 15-21. (5) Zheng, Z.; Li, X.; Xia, X.; Wang, Z.; Huang, Z.; Lei, B.; Gao, Y. High-quality (CH3NH3)3Bi2I9 Film-based Solar Cells: Pushing Efficiency Up to 1.64%. J. Phys. Chem. Lett. 2017, 8, 4300-4307.


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Relationship between Lattice Strain and Efficiency for Sn-Perovskite

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