The Effect of Stoichiometry on the Stability of Inorganic Cesium Lead

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The Effect of Stoichiometry on the Stability of Inorganic Cesium Lead Mixed-Halide Perovskites Solar Cells Qingshan Ma, Shujuan Huang, Sheng Chen, Meng Zhang, Cho-Fai Jonathan Lau, Mark N. Lockrey, Hemant Kumar Mulmudi, Yuchao Shan, Jizhong Yao, Jianghui Zheng, Xiaofan Deng, Kylie R. Catchpole, Martin A. Green, and Anita W. Y. Ho-Baillie J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06268 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 24, 2017

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The Journal of Physical Chemistry

The Effect of Stoichiometry on the Stability of Inorganic Cesium Lead Mixed-Halide Perovskites Solar Cells Qingshan Ma†, Shujuan Huang*,†, Sheng Chen†, Meng Zhang†, Cho Fai Jonathan Lau†, Mark N. Lockrey‡, Hemant K. Mulmudi§, Yuchao Shan⊥, Jizhong Yao⊥, Jianghui Zheng†, Xiaofan Deng†, Kylie Catchpole§, Martin A. Green†, Anita W. Y. Ho-Baillie† †

Australian Centre for Advanced Photovoltaics, University of New South Wales, Sydney 2052, Austral-

ia ‡

Australian National Fabrication Facility ACT Node, Research School of Physics and Engineering, The

Australian National University, Canberra, ACT 2601, Australia §

Centre for Sustainable Energy Systems, Research School of Engineering, Australian National Univer-

sity, Canberra, ACT 2601, Australia ⊥ Microquanta

Semiconductor Co., LTD, Hangzhou, 311121, China

1

ACS Paragon Plus Environment


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ABSTRACT. Metal halide perovskite solar cells that use the inorganic cation Cs have been shown to have better thermal stability than the organic cation containing counterparts and CsPbI2Br has a more suitable (lower) bandgap than CsPbIBr2 as a photovoltaic energy harvesting material. However, increase in iodine content reduces structural stability due to the preference towards the non-perovskite orthorhombic phase when the film is exposed to air. In this work, the effect of varying stoichiometry of CsPbI2Br perovskite on film quality such as the grain size, presence of impurities and nature of impurity grains, photoluminescence, morphology, and elemental distribution are studied. Details on how to vary the stoichiometry during the dual source thermal evaporation process are reported. It is found that the air stability of CsPbI2Br film correlates with the CsBr to PbI2 deposition rate ratio, in which the CsBr-rich CsPbI2Br is the most stable upon air exposure while the stoichiometrically balanced CsPbI2Br perovskite film gives the best photovoltaic performance. The encapsulated device remains 90% of the initial performance after 240 hours damp and heat test at 85 °C and 85% relative humidity.

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1. INTRODUCTION The past few years have seen the exceptional progress in metal halide perovskite solar cells with photovoltaic performance comparable to the silicon and CIGS solar cells.1-3 However, instability of perovskite solar cells remains an issue for commercialisation. In the state of the art perovskite cells, organic cations such as HC(NH2)2 (FA) and CH3NH3 (MA) are used. Durability is compromised when the perovskite film is exposed to air and moisture

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due to decomposition. It has been shown that stability

could be improved by partially replacing the organic FA or/and MA cations with inorganic Cs or/and Rb.11,12 Saliba et al. have reported that by incorporating Rb cation into the FAMACs based perovskite, the device maintains 95% of its initial performance after 500 hours at 85°C.13 Zhang et al. have also reported that the addition of Rb into FAPbI3 can significantly improve its moisture stability.14 Completely replacing the organic cation by inorganic Cs in the perovskite structure has also been explored. Sutton et al. and Beal et al. have demonstrated CsPbI2Br perovskite solar cell by solution deposition methods with stabilised power conversion efficiency (PCE) of 5.6% and 6.5%, respectively.15,16 These perovskites showed improved thermal stability compared to the MA-based equivalents. CsPbI2Br perovskite solar cell fabricated by vacuum deposition method was also demonstrated with better long-term stability than the organic lead halide counterpart when encapsulated.17 Lau et al. reported CsPbIBr2 perovskite solar cell by spray assisted deposition in air with a stabilised conversion efficiency of 6.3%.18 In addition, we have previously demonstrated planar hole-transport-layer-free CsPbIBr2 solar cells with an efficiency of 4.7%. The CsPbIBr2 perovskite film fabricated by dual source thermal evaporation had shown to be stable for 12 hours at 200 °C in a nitrogen glovebox and for 2 hours at 150 °C in atmosphere condition respectively.19 According to the efficiency limits calculated by detailed balance approach for multi-junction solar cells,20 the bandgap of CsPbIBr2 is higher than the optimum necessitating the development of lower bandgap Cs-based perovskite solar cells. This can be achieved by increasing the iodine content. Howev3



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er, increase in iodine content in Cs based perovskite affects its structural stability as the films convert to non-perovskites phase rapidly when exposed to air. Previous work has shown that as x increases from 1 to 2 in CsPbIxBr(3-x) film, air-stability deteriorates requiring the films and devices to be encapsulated for characterizations.15,16,19 When x=3, CsPbI3 has even poorer stability17,21 requiring quantum dot–induced phase stabilization.22 This shows that more work needs to be done towards stabilization of inorganic Cs based perovskite films. In this work, we have investigated the effect of the stoichiometry on the stability of CsPbI2Br thin films and devices fabricated by dual source thermal evaporation. The films were deposited by coevaporation of CsBr and PbI2 followed by a post annealing at 300°C on a hot plate for 10 min in a N2 glovebox. The film stoichiometry and morphology were analyzed by the energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM). Interestingly, we found that slight changes in CsPbI2Br stoichiometry can affect air stability significantly. The CsBr-rich film shows the best stability while the PbI2-rich sample is the worst. We compared the electrical performance of planar Glass/FTO/c-TiO2/perovskite/P3HT/Au perovskites solar cells using CsPbI2Br with different stoichiometry. The stoichiometrically balanced CsPbI2Br cell achieved a PCE of 7.7% for an active area of 0.159cm2 and 6.8% for a 1.2cm2 cell under reverse scan condition. The 1.2cm2 active area is the largest in inorganic caesium lead halide perovskites solar cells reported to date.

2. METHODS 2.1. Device substrate preparation. FTO-coated glass (TEC15, 15Ω/☐sheet resistivity) was patterned by laser scribing, followed by cleaning by sonication in solutions of 2% Hellmanex in deionized water, acetone and isopropanol for 15 minutes respectively. After drying, the substrate was treated by UV ozone cleaner for 10min. To form the compact TiO2 layer, a solution of titanium diisopropoxide

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bis(acetylacetonate) in ethanol was deposited on the cleaned substrate by spray pyrolysis at 450°C and the substrate was subsequently annealed on a hot plate at 450°C for 10 min. 2.2. Dual source thermal evaporation of the perovskite absorber. The dual source thermal evaporation of the two precursors caesium bromide (CsBr) and lead iodide (PbI2), were carried out in a thermal evaporation system (Kurt J. Lesker Mini Spectros) integrated in a glove box. CsBr and PbI2 (Alfa Aesar) were loaded in separate crucible heaters and the sample substrates were fixed on a rotatable substrate holder with the compact TiO2 side facing towards the precursor sources. After the pressure of the evaporator chamber was pumped down to 10-6 mbar, CsBr and PbI2 were then heated to the set temperature of 300°C and 160°C, respectively. Once the temperatures were reached, the shutter for each source was opened to commence deposition. To produce a stoichiometrically balanced CsPbI2Br thin film, the deposition rates of CsBr and PbI2 are at 0.14 Å s−1 and 0.22 Å s−1 respectively. To achieve different stoichiometry, we fix the CsBr deposition rate at 0.14 Å s−1 and vary the PbI2 deposition rate from 0.22 Å s−1 to 0.17 Å s−1 or 0.27 Å s−1 to produce CsBr-rich and PbI2-rich CsPbI2Br film, respectively. After the evaporation, the samples were annealed on a hot plate at 300°C for 10min in the glove box. The hole transporting layer was deposited on CsPbI2Br by spin coating a 15mg/ml P3HT (Sigma Aldrich) solution in chlorobenzene (Sigma Aldrich) at 2000rpm for 60s. To complete device fabrication, 100nm gold was thermally evaporated on the samples to form rear contacts. Some of the samples were encapsulated to minimise degradation for some of the characterisations, e.g. photo-luminescence (PL) imaging, time-resolved PL spectroscopy (TRPL) and current-voltage (I-V) measurement of completed photovoltaic devices. 2.3. Characterization. Energy dispersive X-ray spectroscopy (EDS) measurements were carried out using a FEI Verios equipped with an Oxford EDX detector and the data were collected with an accelerating voltage of 10 kV.

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X-ray diffraction (XRD) patterns were measured using a PANalytical Xpert Materials Research diffractometer system with a Cu Kα radiation source (λ=0.1541 nm) at 45 kV and 40mA. Top view scanning electron microscopic (SEM) images were obtained at 5KV using a field emission SEM (NanoSEM 230). The SEM images in Figure 6 were obtained by the EDS settings as described above. The optical absorption spectra were measured using Perkin Elmer Lambda1050 UV/Vis/NIR spectrophotometer. Photoluminescence (PL) spectra were measured using a back scattering geometry with 405 nm laser excitation and a thermo-electrically cooled Si-CCD detector. The PL images were obtained by Leica TCS SP5 microscopy. The time-resolve PL (TRPL) decay traces were measured by the Microtime-200 (PicoQuant). Both PL imaging and TRPL were measured with 470 nm excitation and detection through a 620/40 nm band pass filter. The current density–voltage (J–V) measurements were performed using a solar cell I–V testing system from Abet Technologies, Inc. (using class AAA solar simulator) under an illumination power of 100 mW cm-2 and a scan rate of 40mV s-1. All measurements were undertaken at room temperature in ambient condition.

3.

RESULTS AND DISCUSSION

Details of material preparation, test structure and cell fabrication are listed in the Experimental Section. The stoichiometry of the CsPbI2Br perovskite film is adjusted by varying the evaporation rate of PbI2 with the evaporation of CsBr fixed at 0.14 Å/s. The elemental composition of the evaporated films was analysed by EDS. The EDS results of three films deposited with different PbI2 evaporation rates are listed in Table 1. When PbI2 is evaporated at a rate of 0.17 Å/s, a CsBr-rich CsPbI2Br film is produced. At a rate of 0.22 Å/s for PbI2, the CsPbI2Br film is stoichiometrically balanced. The slightly higher than expected iodine and bromine values is partly attributed to error in the EDS measurement. When PbI2 is 6


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evaporated at a rate of 0.27 Å/s, a PbI2-rich CsPbI2Br film is produced. Figure 1 shows the absorption and photoluminescence (PL) spectra of these three films. All of the films have the same absorption onset at around 645 nm (1.9 eV) which is consistent with the reported bandgap of CsPbI2Br.15,23 The three films also have similar PL emissions at around 643nm. These results confirm that the change of PbI2 evaporation rate in the range of 0.17 Å/s to 0.27 Å/s does not change the optical absorption and PL emission position of the CsPbI2Br perovskite. However the absorption intensities of the three films are slightly different possibly due to the different surface roughness. Table 1. Average Atomic Ratio of The Samples Deposited at Different Rates of PbI2 with The Evaporation Rate of CsBr Fixed at 0.14 Å/s Sample CsBr-rich Stoichiometrically balanced PbI2-rich

PbI2 rate ( Å/s) 0.17

Cs

Pb

I

Br

1.3

1.0

2.0

1.6

0.22

1.0

1.0

2.6

1.2

0.27

0.7

1.0

2.4

1.3

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Figure 1. (a) and (b) The PL and absorption spectra of CsBr-rich, stoichiometrically balanced and PbI2-rich CsPbI2Br perovskite films with a thickness around 230nm. However, we have observed that these three films manifest very different air stability. Photos of the CsBr-rich, stoichiometrically balanced and PbI2-rich CsPbI2Br films showing different rates of film degradation in air are shown in Figure 2a. PbI2-rich sample degrades completely in 5 min in ambient. The stoichiometrically balanced film starts to discolour after 10 min when left in air and becomes colourless after 40 min. The CsBr-rich film on the other hand does not change colour after 40 min in air, indicating its better air stability. The X-ray diffraction (XRD) patterns of un-encapsulated fresh films and air-exposed-films are shown in Figure 2b. The main diffraction peaks at 14.70°，20.95° and 29.65° correspond to the (100), (110) and (200) planes for the CsPbI2Br perovskite cubic phase.16,23,24 The perovskite crystallinity of the fresh 8


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PbI2-rich CsPbI2Br sample is not as good as the fresh CsBr-rich and stoichiometrically balanced samples. Impurity peaks at 11.18° and 28.18° are present in the PbI2-rich CsPbI2Br film, and impurity peaks at 12.11° and 27.63° are present in the CsBr-rich CsPbI2Br film. These impurity peaks don’t correspond to any single compound of CsBr, PbI2 or CsPbI2Br and therefore they could be from the mixtures of these compounds. Any grains containing these impurities may not be radiative and not contribute to the photovoltaic performance, as will be discussed later. It can be seen that the CsBr-rich and stoichiometrically balanced samples both have better perovskite phase initially. After 40 min of air-exposure, the stoichiometrically balanced and PbI2-rich samples experienced transition to non-perovskite orthorhombic phase which is found to be reversible up on heating at 300°C, while the CsBr-rich sample remains essentially the same, indicating better air stability.

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Figure 2. (a) Photos showing different rates of degradation for un-encapsulated CsBr-rich, stoichiometrically balanced and PbI2-rich CsPbI2Br films when exposed to air. (b) XRD patterns of the fresh and air-exposed CsBr-rich, stoichiometrically balanced and PbI2-rich CsPbI2Br films. The films are deposited on FTO glass substrates. “*”corresponds to peaks from CsPbI2Br perovskite cubic phase, “#”corresponds to peaks from CsPbI2Br non-perovskite orthorhombic phase, and “ ”corresponds to peaks from the FTO substrates. The morphology of the CsPbI2Br films are analysed by top view SEM, as shown in Figure 3. The grain size of the CsBr-rich is notably smaller. The CsBr-rich films also have more brighter impurity grains.25 The impurity grains in the PbI2-rich films have a different appearance. They are darker in the centre surrounded by brighter perimeter. The size of impurity grains is also different in CsBr-rich and PbI2-rich samples. Those in the CsBr-rich sample have a size of few hundred nanometres while those in the PbI2-rich sample are around 1µm in size. Upon air-exposure, both the stoichiometrically balanced and the PbI2-rich film experiences morphology change while the morphology of the CsBr-rich remains the same. The impurity grains in PbI2-rich sample become darker and bright perimeters disappear upon air-exposure. Upon closer examination of the PbI2-rich film, see high resolution SEM image in Figure S1, pin holes can be seen to emerge upon air-exposure. These results show that the CsPbI2Br films with different stoichiometry have different morphology and the CsBr-rich film have better air stability than the stoichiometrically balanced and PbI2-rich films. Figure S2 is a cross sectional SEM of a CsBr rich sample showing that the impurity grains are not limited to the surface only, but are in the bulk of the film.

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Figure 3. Top view SEM images of un-encapsualted fresh (top row) and air-exposed (bottom row) CsBr-rich, stoichiometrically balanced and PbI2-rich CsPbI2Br films. The films are deposited on FTO glass substrates. To further understand the relationship between the impurity grains and the normal grains in the CsPbI2Br films, PL imaging was performed on the samples by using confocal PL microscopy (Leica TCS SP5). Figure 4 shows the PL images detected through a 620/40 nm band pass filter, corresponding to the emission peak of the CsPbI2Br perovskite films. As shown in the figure, the stoichiometrically balanced film exhibits uniform PL emission across the film and the size of the grains observed under PL imaging is very similar to that observed under SEM. On the other hand, both CsBr-rich and PbI2-rich films exhibit non-uniform PL emission across the film. The brightest regions correspond to highest PL efficiency and they are typically grains with the smallest size. The less bright regions correspond to lower PL efficiency which are typically larger grains. There are regions that appear dark that are PL inactive and they correspond to impurity grains as observed under the SEM. The dark regions in the PbI211



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rich films may correspond to the impurity peaks at 11.18° and 28.18° in the XRD pattern and those in the CsBr-rich films may correspond to the impurity peaks at 12.11° and 27.63°. As these impurity peaks cannot be assigned to the typical cubic phase of CsPbI2Br perovskite and are non-radiative at 620/40 nm, they do not contribute to the photovoltaic performance.

Figure 4. PL images the CsBr-rich, stoichiometrically balanced and PbI2-rich CsPbI2Br film detected at 640 nm. The intensity bar on the left is in the range of 0 to 4095 a.u. and the indicator in the bar is at 350 a.u. We also performed the time-resolved PL (TRPL) measurement on the samples. Results are shown in Figure 5. For the PL decay traces, a triple exponential function (Equation 1) allows the best fit. The effective lifetime (Equation 2) is then extracted for the following quantitative analysis.26,27

y A exp A exp A exp

τ

(1) (2)

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Figure 5. PL decay traces of the CsBr-rich, stoichiometrically balanced and PbI2-rich CsPbI2Br film detected at 640 nm. It is calculated that PL effective lifetimes of the CsBr-rich, stoichiometrically balanced and PbI2-rich films are 1.15 ns, 7.15 ns and 4.05ns, respectively. The long lifetime observed in the stoichiometrically balanced film is likely due to the absence of the non-radiative impurity grains. It is later found out that stoichiometrically balanced film also produces higher device performance, as shall be discussed later. To investigate the non-perovskite phase/impurities in the CsPbI2Br films, EDS mapping was carried out to analyse the chemical composition of the different grains. As shown in Figure 6, the brighter impurity grains in the SEM image of CsBr-rich film contain more Cs element and less Pb element. EDS point analysis on the grains as shown in Figure S3a also confirms that the impurities grains (position 1 and 2) are Cs-rich. The stoichiometrically balanced film has uniform distribution of Cs and Pb. This is also confirmed by the point analysis as shown in Figure S3b. The impurity grains of PbI2-rich film are rich in Pb element and poor in Cs element (see Figure 6c and grains 3 and 4 in Figure S3c).

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Figure 6. SEM images and the corresponding EDS mappings of Pb and Cs elements in (a) CsBr-rich, (b) stoichiometrically balanced, and (c) PbI2-rich samples. Combining the SEM, PL imaging and the degradation images, it can be concluded that as the CsBr-toPbI2 evaporation rate ratio increases for the CsPbI2Br film deposition, the grains become smaller while the air stability improves. This is consistent with the findings in previous work, 21,24 that reported higher number of smaller grains in inorganic Cs based perovskite result in more stable film with higher instances of black perovskite phase. A closer look at the XRD pattern of CsBr-rich film (Figure 2b and Figure S4) shows that the (110) peak at 20.95° has a small split peak at 21.58° and the (200) peak at 29.65° has a shoulder peak at 29.22°. These split peak and shoulder peaks are not present in the XRD pattern of PbI2-rich and stoichiometrically balanced samples. This is consistent with the peak splitting for the (110) and (200) peaks observed by Eperon et al.21 This suggests that the smaller grain size observed in the CsBr-rich film is caused by the crystal lattice strain which contributes to better air stability in the CsBr-rich film. To compare the effect of varying the stoichiometry of CsPbI2Br absorber on cell performance, planar devices of Glass/FTO/c-TiO2/CsPbI2Br/P3HT/Au were fabricated and encapsulated. Device perfor14


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mance for the 0.159cm2 and 1.2cm2 cells are shown in Table 2 and Figure 7. Although the CsBr-rich cells have respectable performances, they have poorer stabilised efficiencies. Cells made of stoichiometrically balanced CsPbI2Br film give the best performance with highest currents. On the other hand, PbI2rich CsPbI2Br cells produce the lowest currents. The poorer performance in the CsBr-rich and PbI2-rich devices are due to the presence of non-perovskite grains that do not contribute to the photovoltaic performance as reflected in the lower carrier lifetime measured by TRPL previously. All of the 1.2cm2 devices have lower fill factor due to higher resistance as the cell area increases. It can be concluded that the instability of the stoichiometrically balanced CsPbI2Br cells can be overcome by encapsulations as shown in Figure S5 in the Supporting Information. Damp heat test has been performed on the stoichiometrically balanced encapsulated CsPbI2Br cell. Results are shown in Figure 8. It is found that CsPbI2Br cells can retain 90% of their initial performance after 240 hours damp heat testing at 85℃ and 85% relative humidity. The drop in fill factor is the main reason for performance drop, although the VOC has a slightly increasing trend over time. Further work will be carried out to understand the cause for VOC increase. Table 2. Electrical Performance of CsBr-rich, Stoichiometrically Balanced and PbI2-rich CsPbI2Br Devices.

Sample CsBr-rich Stoichiometrically balanced PbI2-rich

Active area [cm2] 0.159 1.2 0.159 1.2 0.159 1.2

Jsc [mA/cm2]

Voc [mV]

FF [%]

Eff [%]

11.2 10.2 11.5 11.5 9.9 9.4

970 1043 1005 1019 1002 1004

63 57 67 58 69 60

6.8 6.1 7.7 6.8 6.8 5.7

Stabilized Eff [%] 4.4 4.2 6.7 5.5 5.4 4.1

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Figure 7. (a) Light current density-voltage (JV )curves under reverse scan (VOC to JSC) at a scan speed of 40mV/s and (b) stabilized efficiencies of 0.159cm2 (solid lines) and 1.2cm2 (dash lines) CsBr-rich (red), stoichiometrically balanced (black) and PbI2-rich (blue) CsPbI2Br planar cells.

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Figure 8. Results of damp heat test on the encapsulated stoichiometrically balanced CsPbI2Br cell. Test condition: 85℃ and 85% relative humidity.

4.

CONCLUSIONS

We have investigated the effect of varying the stoichoimetry of CsPbI2Br on material property and photovoltaic performance. By varying the deposition rate ratio of CsBr to PbI2 during dual source thermal evaporation, CsPbI2Br films that are CsBr-rich, stoichiometrically balanced or PbI2-rich films can be produced. While the absorption onset remains the same for these films, the grain property, level of impurities, elemental distribution, morphology and stability of these films are very different as the stoichiometry is varied. The presence of impurities in the CsBr-rich and PbI2-rich films result in lower photoluminescence and PL lifetimes and consequently lower cell performance. In terms of air stability, 17



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CsBr-rich sample is the best likely due to smaller grain size that result in crystal lattice strain hampering degradation. However, the less stable stoichiometrically balanced CsPbI2Br devices once encapsulated produce the best cell performance compared to CsBr and PbI2 rich CsPbI2Br devices, with a PCE of 7.7% under reverse scan and a stabilized PCE of 6.7% for a 0.159cm2 device. A PCE at 6.8% under reverse scan and stabilized PCE at 5.5% are also obtained for the 1.2cm2 device. This is the largest inorganic caesium lead halide perovskite solar cell reported to date. This works enriches the understanding of air stability of CsPbI2Br perovskite film and the effectiveness of stoichiometry engineering on film property.

ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:.” Top view SEM images of fresh and air-exposed PbI2-rich CsPbI2Br films, cross section SEM of CsBr rich CsPbI2Br on compact TiO2 and FTO substrate, EDS point analysis on the CsBr-rich, stoichiometrically balanced and PbI2-rich CsPbI2Br films, XRD pattern of the fresh and air-exposed CsBr-rich CsPbI2Br films, photos showing the lack of degradation for encapsulated stoichiometrically balanced CsPbI2Br films upon exposure to air (PDF)

AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected]

ACKNOWLEDGMENT

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The Australian Centre for Advanced Photovoltaics (ACAP) encompasses the Australian-based activities of the Australia-US Institute for Advanced Photovoltaics (AUSIAPV) and is supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). We thank the Electron Microscopy Unit and the BioMedical Imaging Facility at UNSW for SEM and fluorescence imaging supports. We acknowledge the support of the ANFF ACT Node for EDS measurements.

REFERENCES (1) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat. Photon-

ics 2014, 8, 506–514. (2) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D.; Levi, D. H.; Ho-Baillie, A. W. Y. Solar Cell Efficiency Tables (version 49). Prog. Photovolt: Res. Appl. 2016, 25, 3–13. (3) Green, M. A.; Ho-Baillie, A. Perovskite Solar Cells: The Birth of a New Era in Photovoltaics. ACS Energy Lett. 2017, 2, 822–830. (4) Wang, Z.; Shi, Z.; Li, T.; Chen, Y.; Huang, W. Stability of Perovskite Solar Cells: A Prospective on the Substitution of the A Cation and X Anion. Angew. Chem. Int. Ed. 2017, 56, 1190–1212. (5) Rong, Y.; Liu, L.; Mei, A.; Li, X.; Han, H. Beyond Efﬁciency: the Challenge of Stability in Mesoscopic Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1501066. (6) Leijtens, T.; Eperon, G. E.; Noel, N. K.; Habisreutinger, S. N.; Petrozza, A.; Snaith, H. J. Stability of Metal Halide Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1500963. (7) Berhe, T. A.; Su, W.; Chen, C.; Pan, C.; Cheng, J.; Chen, H.; Tsai, M.; Chen, L.; Dubale, A. A.; Hwang, B. Organometal Halide Perovskite Solar Cells: Degradation and Stability. Energy Environ. Sci. 2016, 9, 323—356. 19



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 60

Page 20 of 31

(8) Li, B.; Li, Y.; Zheng, C.; Gao, D.; Huang, W. Advancements in The Stability of Perovskite Solar Cells: Degradation Mechanisms and Improvement Approaches. RSC Adv. 2016, 6, 38079–38091. (9) Niu, G.; Li, W.; Meng, F.; Wang, L.; Dong, H.; Qiu, Y. Study on The Stability of CH3NH3PbI3 Films and The Effect of Post-Modification by Aluminum Oxide in All-Solid-State Hybrid Solar Cells. J. Mater. Chem. A 2014, 2. 705–710. (10) Clegg, C.; Hill, I. G. Systematic Study on The Impact of Water on The Performance and Stability of Perovskite Solar Cells. RSC Adv. 2016, 6, 52448–52458. (11) Lee, J.; Kim, D.; Kim, H.; Seo, S.; Cho, S. M.; Park, N. Formamidinium and Cesium Hybridization for Photo- and Moisture-Stable Perovskite Solar Cell. Adv. Energy Mater. 2015, 5, 1501310. (12) Saliba, M.; Matsui, T.; Seo, J.; Domanski, K.; Correa-Baena, J.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Tress, W.; Abate, A.; Hagfeldt, A. et al. Cesium-Containing Triple Cation Perovskite Solar Cells: Improved Stability, Reproducibility and High Efficiency. Energy Environ. Sci. 2016, 9, 1989— 1997. (13) Saliba, M.; Matsui, T.; Domanski, K.; Seo, J.; Ummadisingu, A.; Zakeeruddin, S. M.; CorreaBaena, J.; Tress, W. R.; Abate, A.; Hagfeldt, A. et al. Incorporation of Rubidium Cations into Perovskite Solar Cells Improves Photovoltaic Performance. Science 2016, 354, 206-209. (14) Zhang, M.; Yun, J. S.; Ma, Q.; Zheng, J.; Lau, C. F. J.; Deng, X.; Kim, J.; Kim, D.; Seidel, J.; Green, M. A. et al. High Efficiency Rubidium Incorporated Perovskite Solar Cells by Gas Quenching. ACS Energy Lett. 2017, 2, 438–444.

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Page 21 of 31

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 60


(15) Sutton, R. J.; Eperon, G. E.; Miranda, L.; Parrott, E. S.; Kamino, B. A.; Patel, J. B.; Hörantner, M. T.; Johnston, M. B.; Haghighirad, A. A.; Moore, D. T. et al. Bandgap-Tunable Cesium Lead Halide Perovskites with High Thermal Stability for Efficient Solar Cells. Adv. Energy Mater. 2016, 6, 1502458. (16) Beal, R. E.; Slotcavage, D. J.; Leijtens, T.; Bowring, A. R.; Belisle, R. A.; Nguyen, W. H.; Burkhard, G. F.; Hoke, E. T.; McGehee, M. D. Cesium Lead Halide Perovskites with Improved Stability for Tandem Solar Cells. J. Phys. Chem. Lett. 2016, 7, 746−751. (17) Chen, C.; Lin, H.; Chiang, K.; Tsai, W.; Huang, Y.; Tsao, C.; Lin, H. All-Vacuum-Deposited Stoichiometrically Balanced Inorganic Cesium Lead Halide Perovskite Solar Cells with Stabilized Efficiency Exceeding 11%. Adv. Mater. 2017, 29, 1605290. (18) Lau, C. F. J.; Deng, X.; Ma, Q.; Zheng, J.; Yun, J. S.; Green, M. A.; Huang, S.; Ho-Baillie, A. W. Y. CsPbIBr2 Perovskite Solar Cell by Spray-Assisted Deposition. ACS Energy Lett. 2016, 1, 573−577. (19) Ma, Q.; Huang, S.; Wen, X.; Green, M. A.; Ho-Baillie, A. W. Y. Hole Transport Layer Free Inorganic CsPbIBr2 Perovskite Solar Cell by Dual Source Thermal Evaporation. Adv. Energy Mater. 2016, 6, 1502202. (20) Bremner, S. P.; Yi, C.; Almansouri, I.; Ho-Baillie, A.; Green, M. A. Optimum Band Gap Combinations to Make Best Use of New Photovoltaic Materials. Solar Energy 2016, 135, 750–757. (21) Eperon, G. E.; Paterno, G. M.; Zampetti, R. J. S. A.; Haghighirad, A. A.; Caciallibc, F.; Snaith, H. J. Inorganic Caesium Lead Iodide Perovskite Solar Cells. J. Mater. Chem. A 2015, 3, 19688. (22) Swarnkar, A.; Marshall, A. R.; Sanehira, E. M.; Chernomordik, B. D.; Moore, D. T.; Christians, J. A.; Chakrabarti, T.; Luther, J. M. Quantum Dot–Induced Phase Stabilization of A-CsPbI3 Perovskite for High-Efficiency Photovoltaics. Science 2016, 354, 92-95. 21



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 60

Page 22 of 31

(23) Niezgoda, J. S.; Foley, B. J.; Chen, A. Z.; Choi, J. J. Improved Charge Collection in Highly Efficient CsPbBrI2 Solar Cells with Light-Induced Dealloying. ACS Energy Lett. 2017, 2, 1043−1049. (24) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I) Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692−3696. (25) Yonezawa, K.; Yamamoto, K.; Shahiduzzaman, Md.; Furumoto, Y.; Hamada, K.; Ripolles, T. S.; Karakawa, M.; Kuwabara, T.; Takahashi, K.; Hayase, S. et al. Annealing Effects on CsPbI3-Based Planar Heterojunction Perovskite Solar Cells Formed by Vacuum Deposition Method. Jpn. J. Appl. Phys. 2017, 56, 04CS11. (26) Wen, X.; Yu, P.; Toh, Y.; Lee, Y.; Huang, K.; Huang, S.; Shrestha, S.; Conibeer, G.; Tang, J. Ultrafast Electron Transfer in The Nanocomposite of The Graphene Oxide-Au Nanocluster with Graphene Oxide As A Donor. J. Mater. Chem. C 2014, 2, 3826-3834. (27) Chen, S.; Wen, X.; Yun, J. S.; Huang, S.; Green, M.; Jeon, N. J.; Yang, W. S.; Noh, J. H.; Seo, J.; Seok, S. Il. et al. Spatial Distribution of Lead Iodide and Local Passivation on Organo-Lead Halide Perovskite. ACS Appl. Mater. Interfaces 2017, 9, 6072-6078.

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TOC GRAPHICS

********************************* ACS Paragon Plus Environment


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Figure 1. (a) and (b) The PL and absorption spectra of CsBr-rich, stoichiometrically balanced and PbI2-rich CsPbI2Br perovskite films with a thickness around 230nm. 79x113mm (300 x 300 DPI)


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Figure 2. (a) Photos showing different rates of degradation for un-encapsulated CsBr-rich, stoichio-metrically balanced and PbI2-rich CsPbI2Br films when exposed to air. (b) XRD patterns of the fresh and air-exposed CsBr-rich, stoichiometrically balanced and PbI2-rich CsPbI2Br films. The films are de-posited on FTO glass substrates. “*”corresponds to peaks from CsPbI2Br perovskite cubic phase, “#”corresponds to peaks from CsPbI2Br non-perovskite orthorhombic phase, and “ ”corresponds to peaks from the FTO substrates. 159x151mm (300 x 300 DPI)



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Figure 3. Top view SEM images of un-encapsualted fresh (top row) and air-exposed (bottom row) CsBr-rich, stoichiometrically balanced and PbI2-rich CsPbI2Br films. The films are deposited on FTO glass substrates. 175x102mm (300 x 300 DPI)


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Figure 4. PL images the CsBr-rich, stoichiometrically balanced and PbI2-rich CsPbI2Br film detected at 640 nm. The intensity bar on the left is in the range of 0 to 4095 a.u. and the indicator in the bar is at 350 a.u. 170x63mm (300 x 300 DPI)



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Figure 5. PL decay traces of the CsBr-rich, stoichiometrically balanced and PbI2-rich CsPbI2Br film detected at 640 nm. 80x56mm (300 x 300 DPI)


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Figure 6. SEM images and the corresponding EDS mappings of Pb and Cs elements in (a) CsBr-rich, (b) stoichiometrically balanced, and (c) PbI2-rich samples. 80x75mm (300 x 300 DPI)



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Figure 7. (a) Light current density-voltage (JV )curves under reverse scan (VOC to JSC) at a scan speed of 40mV/s and (b) stabilized efficiencies of 0.159cm2 (solid lines) and 1.2cm2 (dash lines) CsBr-rich (red), stoichiometrically balanced (black) and PbI2-rich (blue) CsPbI2Br planar cells. 79x136mm (300 x 300 DPI)


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Figure 8. Results of damp heat test on the encapsulated stoichiometrically balanced CsPbI2Br cell. Test condition: 85℃ and 85% relative humidity. 170x126mm (300 x 300 DPI)


The Effect of Stoichiometry on the Stability of Inorganic Cesium Lead

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