CsPbIBr2 Perovskite Solar Cell by SprayAssisted Deposition Cho Fai Jonathan Lau, Xiaofan Deng, Qingshan Ma, Jianghui Zheng, Jae S. Yun, Martin A. Green, Shujuan Huang, and Anita W. Y. Ho-Baillie* Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales (UNSW), Sydney 2052, Australia S Supporting Information *
ABSTRACT: In this work, an inorganic halide perovskite solar cell using a sprayassisted solution-processed CsPbIBr2 film is demonstrated. The process allows sequential solution processing of the CsPbIBr2 film, overcoming the solubility problem of the bromide ion in the precursor solution that would otherwise occur in a single-step solution process. The spraying of CsI in air is demonstrated to be successful, and the annealing of the CsPbIBr2 film in air is also successful in producing a CsPbIBr2 film with an optical band gap of 2.05 eV and is thermally stable at 300 °C. The effects of the substrate temperature during spraying and the annealing temperature on film quality and device performance are studied. The substrate temperature during spraying is found to be the most critical parameter. The best-performing device fabricated using these conditions achieves a stabilized conversion efficiency of 6.3% with negligible hysteresis. Cesium metal halide perovskites remain viable alternatives to organic metal halide perovskites as the cesium-containing perovskites can withstand higher temperature.
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large area cell or module manufacturing as well as its ability to coat conformally. The utilization rate of the material will also be higher for spraying compared to that for spin coating. Spraydeposited perovskite solar cells have been demonstrated for organic−inorganic perovskite solar cells such as FA0.9Cs0.1PbI3 solar cells with a PCE at 14.2%.17 Spray deposition is yet to be demonstrated for solar cells using inorganic halide perovskites. In this work, we report a two-step method of spray-assisted solution-processed CsPbIBr2 (with a band gap of 2.05 eV) solar cells. Apart from the advantages of spraying mentioned above, this two-step method overcomes the issue faced by one-step solution processing of the CsPbIBr2 perovskite film due to the poor solubility of the bromide ion in the precursor solution. The method can be scaled up when the PbBr2 is deposited by spraying instead of spin coating or a physical deposition process such as evaporation. In this work, Au/spiro-OMeTAD/CsPbIBr2/mp-TiO2/blTiO 2/FTO/glass solar devices are fabricated. For the deposition of the perovskite film, cesium iodide (CsI) is sprayed in air on a spin-coated lead bromide film. A series of experiments investigating the effects of substrate temperature during spraying and the post-annealing temperature on the perovskite quality are conducted in this work. The optimized device produces a stabilized PCE of 6.3%. When measured
rganic−inorganic halide perovskites have emerged as competitive light harvesters due to their excellent optical absorption, good carrier mobility, and lifetime.1 The power conversion efficiency (PCE) of organic− inorganic hybrid halide perovskite solar cells has improved rapidly from 9.7 to over 20% in a few years.2−6 These cells typically use organic cations such as methylammonium (MA) and formamidinium (FA). Although high PCE can be achieved, these cells are not thermally stable at high temperatures, limiting module processing temperatures. One possible way to improve the thermal stability is to replace the organic cation with an inorganic cation such as cesium (Cs). Stoumpos et al. reported that Cs-containing perovskite has high mobility of holes (up to 320 cm2V s−1) and electrons (up to 2300 cm2V s−1) and estimated that CsPbBr3 has an electron mobility of ∼1000 cm2V s−1 and an electron lifetime of 2.5 μs.7,8 Kulbak et al. have demonstrated a CsPbBr3 perovskite device with a PCE higher than 5%,9,10 and Sutton et al. have reported a CsPbI2Br cell with a maximum PCE at 9.8% and a stabilized PCE at 5.6%.11 Ma et al. have also reported a hole-transporting material (HTM)-free CsPbIBr2 cell with a PCE at 4.7%.12 The bestreported Cs perovskite device has a PCE of 6.5%.13 Perovskite films can be fabricated via a solution process or physical deposition either in one step or sequentially. Various deposition techniques include spin coating,14,15 spraying,16,17 and thermal evaporation in ambient18,19 or in vacuum.12,20 Spraying, a widely used technique in industrial processes, has advantages over spin coating due to its ability to scale up for a © 2016 American Chemical Society
Received: August 8, 2016 Accepted: August 17, 2016 Published: August 17, 2016 573
DOI: 10.1021/acsenergylett.6b00341 ACS Energy Lett. 2016, 1, 573−577
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ACS Energy Letters
Figure 1. (a) XRD patterns of the CsPbIBr2 films on an mp-TiO2/bl-TiO2/FTO glass substrate with varying substrate temperature during spraying. EDS (17 μm line scan) spectra of the CsPbIBr2 film deposited when the substrate temperature is at (b) room temperature and (c) 160 and (d) 350 °C during spraying.
under reverse scan, the short-circuit current density is JSC = 7.94 mA cm−2, the open-circuit voltage is VOC = 1121 mV, the fill factor is FF = 70%, and PCE = 6.3%. When measured under forward scan, JSC = 7.84 mA cm−2, VOC = 1127 mV, FF = 71.7%, and PCE = 6.2%. To investigate the effect of the substrate temperature during the spraying of CsI, the temperature of the PbBr2/mp-TiO2/blTiO2/FTO/glass substrates was set at room temperature and 160 and 350 °C, during spraying in air. The samples were then annealed in air at 325 °C for 10 min. Figure 1a shows the X-ray diffraction (XRD) patterns of perovskite films prepared using different substrate temperatures. The main peaks at 14.82, 21.05, and 29.94°correspond to the (100), (110), and (220) planes of the CsPbIBr2 perovskite orthorhombic phase.8,12 However, as the substrate temperature increases (≥160 °C), CsI (27.5°)21 becomes prominent. An energy dispersive X-ray spectroscopy (EDS) measurement at 15 kV was carried out by a 17 μm line scan of the CsPbIBr2 films. Results are shown in Figure 1b−d, and the atomic ratios as a function substrate temperature are also summarized in Table 1. The results confirm that the perovskite film composition is of CsPbIBr2 when the film is deposited on a substrate kept at room temperature. As the substrate temperature increases to 350 °C, the atomic ratios of Pb/Cs and Br/I decrease. This is consistent
with the XRD results, suggesting that higher substrate temperature results in segregation of CsI. The effect of annealing temperature on the quality of the perovskite film was also examined. After the spraying of CsI on PbBr2/mp-TiO2/bl-TiO2/FTO glass substrates at room temperature in air, the samples were then annealed at 275 or 300 °C or 325 or 350 °C for 10 min in air. Good crystallinity could be achieved for all films annealed at temperatures ranging from 275 to 350 °C, as shown Figure 2a. The absorption coefficients (α) of the samples were calculated from their measured optical transmission (T) and reflection (R) spectra using α = t−1 ln(1 − R)/T). All of the absorption coefficients (see Figure S1 in the Supporting Information) are above 5 × 104 cm−1 in the wavelength range of 575 nm. The onset of the optical band gap edge transition is at around 600−615 nm (2.02−2.06 eV), which is similar to that for CsPbIBr2 fabricated by dual-source evaporation (2.05 eV).12 Figure 2b shows the photoluminescence (PL) decay traces of a sample with CsPbIBr2 annealed at different temperatures. Using a biexponential decay function, the PL decay traces were fitted to determine the decay times of the fast (τ1) and slow (τ2) components, which are summarized in Table 2. The presence of the fast component (τ1) in the PL decay indicates the presence of defect trapping, and the slow component (τ2) corresponds to the radiative recombination time. A typical lifetime (τ2) above 11 ns is found when the CsPbIBr2 film is annealed at temperatures in the range of 275−325 °C. This lifetime is comparable with that of the CsPbIBr2 film (9.35− 17.7 ns) reported in ref 12. Note, the lifetime dramatically drops when the CsPbIBr2 film is annealed at 350 °C, while the lifetime is longest when the CsPbIBr2 film is annealed at 300 °C.
Table 1. Atomic Ratios of Pb/Cs and Br/I Calculated by Averaging the Atomic Ratios of the 100 Line-Scanning Points atomic ratios
room temperature
160 °C
350 °C
Pb/Cs Br/I
1 2
1 2
0.4 1 574
DOI: 10.1021/acsenergylett.6b00341 ACS Energy Lett. 2016, 1, 573−577
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Figure 2. (a) XRD patterns and (b) PL decay traces and top-view SEM of the CsPbIBr2 films on mp-TiO2/bl-TiO2/FTO glass annealed at different temperatures of (c) 275, (d) 300, (e) 325, and (f) 350 °C after spray-assisted deposition.
and PCE due to nonoptimum composition of the perovskite film, as shown in Figure 1 (see also Table 3 for the photovoltaic device parameters). Figure 4 shows the electrical performance of CsPbIBr2 cells when the substrate temperature is kept at room temperature during spray-assisted deposition while the annealing temperature is allowed to vary from 275 to 350 °C. Respectful performance (PCE > 4.7%) can be achieved by cells when the CsPbIBr2 is annealed at temperatures in the range of 275−325 °C (see Table 4 for the photovoltaic device parameters). Planar as well as HTM-free devices were also fabricated at the optimal substrate (room temperature) and annealing (300 °C) temperatures. Results are shown in Figure S2 in the Supporting Information. Although the voltage of the planar device remains above 1 V, the FF and current suffer possibly due to the existence of pinholes in the CsPbIBr2. The HTM-free mesoporous device in this work is less efficient than that in the HTM-free planar device reported in ref 12, which could be due to the nonuniformity of the CsPbIBr2 film in this work. These findings suggest room for improvement for future devices using cesium perovskite films deposited using sprayassisted technique.
Table 2. Typical Lifetimes Extracted from TCSPC for CsPbIBr2 with Different Annealing Temperatures lifetime
275 °C
300 °C
325 °C
350 °C
τ1 τ2
2.39 11.1
4.05 17.5
3.2 14.8
2.36 11.1
The morphology of the CsPbIBr2 films is also studied by topview scanning electron microscopy (SEM), as shown in Figure 2c−f. Higher annealing temperature appears to result in a denser film as the size of pinholes reduces. Perovskite grains 500−1000 nm in size are generally observed. To investigate the effects of substrate temperatures during spraying and the annealing temperature on solar cell performance, Au/spiro-OMeTAD/CsPbIBr2/mp-TiO2/bl-TiO2/FTO/ glass devices were fabricated under different conditions. Figure 3a shows a cross-sectional SEM image of a typical complete device where the bl-TiO2 is 45 nm, the mp-TiO2/ CsPbIBr2 layer is ∼200 nm thick, the CsPbIBr2 overlayer is ∼160 nm thick, and the spiro-OMeTAD is ∼100 nm thick. The rest of Figure 3 shows that increasing the substrate temperature causes drops in density−voltage (J−V) characteristics of the CsPbIBr2 device including JSC, VOC, and also FF 575
DOI: 10.1021/acsenergylett.6b00341 ACS Energy Lett. 2016, 1, 573−577
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ACS Energy Letters
Figure 4. (a) Distribution (error bars are standard deviations) of PCE and electrical characteristics of the Au/spiro-OMeTAD/ CsPbIBr2/mp-TiO2/bl-TiO2/FTO/glass devices measured under reverse scan when the CsPbIBr2 film is annealed at different temperatures, (b) current density−voltage (J−V) curves under reverse and forward scans, (c) EQE, and (d) stabilized PCE and maximum power point current density (JMPP) of the bestperforming CsPbIBr2 device.
Figure 3. (a) Cross-sectional SEM image of a typical Au/spiroOMeTAD/CsPbIBr2/mp-TiO2/bl-TiO2/FTO device. (b) Current density−voltage (J−V) curves and (c) distribution (error bars are standard deviations) of PCE of Au/spiro-OMeTAD/CsPbIBr2/mpTiO2/bl-TiO2/FTO/glass cells fabricated using different substrate temperatures during spray-assisted solution processing of CsPbIBr2. Electrical characteristics of the best cell and an average cell in each category measured under reverse scan (open circuit to short circuit) are summarized in Table 3.
Table 4. Photovoltaic Device Parameters of Au/SpiroOMeTAD/CsPbIBr2/mp-TiO2/bl-TiO2/FTO Device Fabricated Scan When the CsPbIBr2 Film Is Annealed at Different Temperatures
Table 3. Photovoltaic Device Parameters of the Au/SpiroOMeTAD/CsPbIBr2/mp-TiO2/bl-TiO2/FTO Device Fabricated Using Different Substrate Temperatures during Spray-Assisted Solution Processing of CsPbIBr2 substrate temp. [°C]
JSC [mA cm−2]
VOC [mV]
FF [%]
max. PCE [%]
average PCE [%]
standard deviation of PCE
RT 160 350
7.0 5.3 3.9
1079 931 965
72 58 58
5.4 2.9 2.2
4.8 2.6 1.0
0.56 0.32 0.77
annealing temp. [°C]
JSC [mA cm−2]
VOC [mV]
FF [%]
max. PCE [%]
average PCE [%]
standard deviation of PCE
275 300 325 350
7.0 7.9 7.0 3.3
1100 1121 1079 1166
71 70 71 70
5.5 6.3 5.4 2.7
4.7 5.7 4.8 2.3
0.69 0.47 0.56 0.54
with the VOC of the CsPbIBr2 cell fabricated via dual-source evaporation, which is 959 mV.12 The short-circuit current of our cell can be further improved by increasing the thickness of the CsPbIBr2 absorber to be beyond the current 160 nm. In summary, we have demonstrated an inorganic halide perovskite solar cell using a spray-assisted solution-processed CsPbIBr2 film. This allows sequential solution processing of the CsPbIBr2 film, overcoming the solubility problem of the bromide ion in the precursor solution that would otherwise occur in a single-step solution process. The spraying of CsI in air is demonstrated to be successful, and the annealing of the CsPbIBr2 film in air is also successful in producing the CsPbIBr2 film, thermally stable at 300 °C, with an optical band gap of 2.05 eV. The effects of substrate temperature during spraying and the annealing temperature on film quality and device performance are studied. The substrate temperature during spraying is the most critical parameter as a high substrate temperature will result in excess CsI in the film, which has a detrimental effect on JSC, VOC, and also the FF. The optimal annealing condition of CsPbIBr2 is found to be 300 °C for 10 min. The best-performing device fabricated using these conditions achieves a stabilized conversion efficiency of 6.3% with negligible hysteresis. The grain size and morphology of the
A preliminary thermal stability test was performed on CsPbIBr2 films, which involves heating at 300 °C on a hot plate in a glovebox for 1.5 h. Results shown in Figure S3 in the Supporting Information indicate no detectable phase change or the emergence of new impurity peaks or color change after the heat treatment, indicating promising thermal stability of the CsPbIBr2 film. Finally, the PCE of the best-performing cell measured under reverse scan at 30 mV s −1 is 6.3% with a VOC of 1127 mV, a JSC of 7.9 mA cm−2, and a FF of 72%. Under forward scan, PCE = 6.3%, JSC = 7.9 mA cm−2, VOC = 1121 mV, and FF = 71%. The external quantum efficiency (EQE) of the best device is presented in Figure 4c, together with the integrated current density as a function of wavelength. The integrated JSC is 7.03 mA cm−1. The slightly lower integrated JSC from the EQE is due to the lack of light soaking during EQE measurement. Light soaking that stabilizes the current density of the cell typically takes a few seconds. Stabilized PCEs were measured to be 6.3%. This is the highest stabilized efficiency achieved for an inorganic lead halide perovskite solar cell due to the lower level of hysteresis. Our CsPbIBr2 cells have higher VOC compared 576
DOI: 10.1021/acsenergylett.6b00341 ACS Energy Lett. 2016, 1, 573−577
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ACS Energy Letters perovskite films are expected to be further improved for planarfriendly or HTM-free devices. The low hysteresis and the high stabilized power of the demonstrated spray-assisted deposited CsPbIBr2 cells mean that cesium metal halide perovskites remain viable alternatives to organic metal halide perovskites as the cesium-containing ones can withstand higher temperature.
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(8) Stoumpos, C. C.; Malliakas, C. D.; Peters, J. a.; Liu, Z.; Sebastian, M.; Im, J.; Chasapis, T. C.; Wibowo, A. C.; Chung, D. Y.; Freeman, A. J.; et al. Crystal Growth of the Perovskite Semiconductor CsPbBr3: A New Material for High-Energy Radiation Detection. Cryst. Growth Des. 2013, 13, 2722−2727. (9) Kulbak, M.; Cahen, D.; Hodes, G. How Important Is the Organic Part of Lead Halide Perovskite Photovoltaic Cells? Efficient CsPbBr3 Cells. J. Phys. Chem. Lett. 2015, 6, 2452−2456. (10) Kulbak, M.; Gupta, S.; Kedem, N.; Levine, I.; Bendikov, T.; Hodes, G.; Cahen, D. Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells. J. Phys. Chem. Lett. 2016, 7, 167−172. (11) 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, 1502458/1−1502458/6. (12) 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, 1502202/1−1502202/6. (13) Beal, R. E.; Slotcavage, D. J.; Leijtens, T.; Bowring, A. R.; Belisle, R. a; Nguyen, W. H.; Burkhard, G.; 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. (14) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316−319. (15) Kim, J.; Yun, J. S.; Wen, X.; Soufiani, A. M.; Lau, C. F. J.; Wilkinson, B.; Seidel, J.; Green, M. A.; Huang, S.; Ho-Baillie, A. W. Y. Nucleation and Growth Control of HC(NH2)2PbI3 for Planar Perovskite Solar Cell. J. Phys. Chem. C 2016, 120, 11262−11267. (16) Barrows, A.; Pearson, A.; Kwak, C.; Dunbar, A.; Buckley, A.; Lidzey, D. Efficient Planar Heterojunction Mixed-Halide Perovskite Solar Cells Deposited via Spray-Deposition. Energy Environ. Sci. 2014, 7, 2944/1−2944/7. (17) Xia, X.; Wu, W.; Li, H.; Zheng, B.; Xue, Y.; Xu, J.; Zhang, D.; Gao, C.; Liu, X. Spray Reaction Prepared FA1‑x CsxPbI3 Solid Solution as a Light Harvester for Perovskite Solar Cells with Improved Humidity Stability †. RSC Adv. 2016, 6, 14792−14798. (18) Chen, Q.; Zhou, H.; Hong, Z.; Luo, S.; Duan, H.-S.; Wang, H.H.; Liu, Y.; Li, G.; Yang, Y. Planar Heterojunction Perovskite Solar Cells via Vapor-Assisted Solution Process. J. Am. Chem. Soc. 2014, 136, 622−625. (19) Sheng, R.; Ho-Baillie, A.; Huang, S.; Chen, S.; Wen, X.; Hao, X.; Green, M. A. Methylammonium Lead Bromide Perovskite-Based Solar Cells by Vapor-Assisted Deposition. J. Phys. Chem. C 2015, 119, 3545−3549. (20) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395−398. (21) Triloki; Garg, P.; Rai, R.; Singh, B. K. Structural Characterization of as-Deposited Cesium Iodide Films Studied by X-Ray Diffraction and Transmission Electron Microscopy Techniques. Nucl. Instrum. Methods Phys. Res., Sect. A 2014, 736, 128−134.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00341. Detailed experimental procedures, optical characterization, device performance of mesoporous vs planar devices, and preliminary thermal stability results (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS 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). C.F.J.L. is supported by the Australian Government and UNSW via an Australian Postgraduate Award scholarship and Engineering Research Award, respectively. The authors would like to thank Rui Sheng of UNSW for the cross-sectional SEM imaging. We thank the Electron Microscopy Unit and the BioMedical Imaging Facility at UNSW for SEM and fluorescence imaging support.
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REFERENCES
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DOI: 10.1021/acsenergylett.6b00341 ACS Energy Lett. 2016, 1, 573−577
