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• W2967236839 abstract "•Fluid dynamics and phase transition act as a key for scalable coated CsPbI2Br film•Controlling drying dynamics can overcome the fluid instability and moisture attack•The ideal sequential crystallization with changing halide composition was observed•High photovoltaic performance was obtained for blade-coated CsPbI2Br PSCs All-inorganic halide perovskites hold promise for improving the thermal stability of perovskite solar cells (PSCs), but their moisture sensitivity significantly limits scalable fabrication of high-quality thin films over large areas under ambient conditions. Upscaling of uniform and pinhole-free coatings is further complicated by the fluid dynamics of the ink and its solidification mechanisms. For the first time, we demonstrate the control of film formation during ambient-air scalable fabrication of CsPbI2Br perovskite films using blade coating and investigate the coupling between the fluid dynamics and the structural evolution during film formation. As a result, we achieve power conversion efficiencies of 14.7% (aperture, 0.03 cm2) and 12.5% (aperture, 1.0 cm2), which is the highest performance for 1.0 cm2 all-inorganic PSCs. These results present important lessons on controlling the solidification of inks for the practical fabrication of perovskite photovoltaics. All-inorganic halide perovskites hold promise for emerging thin-film photovoltaics due to their excellent thermal stability. Unfortunately, it has been challenging to achieve high-quality films over large areas using scalable methods under realistic ambient conditions. Herein, we investigated the coupling between the fluid dynamics and the structural evolution during controlled film formation for ambient scalable fabrication of CsPbI2Br perovskite films using blade coating. We simultaneously overcame the negative influences of moisture attack and the Bénard-Marangoni instability in the drying ink and achieved an ideal sequential crystallization with changing halide composition during the film formation. As a result, we produced highly crystalline, uniform, and pinhole-free CsPbI2Br films with superior photophysical and transport properties. High-performance solar cells are fabricated to achieve power conversion efficiencies (PCEs) of 14.7% for small-aperture-area (0.03 cm2) devices and 12.5% for the large-aperture-area (1.0 cm2) ones, the highest PCE reported to date for large-area all-inorganic perovskite solar cells. All-inorganic halide perovskites hold promise for emerging thin-film photovoltaics due to their excellent thermal stability. Unfortunately, it has been challenging to achieve high-quality films over large areas using scalable methods under realistic ambient conditions. Herein, we investigated the coupling between the fluid dynamics and the structural evolution during controlled film formation for ambient scalable fabrication of CsPbI2Br perovskite films using blade coating. We simultaneously overcame the negative influences of moisture attack and the Bénard-Marangoni instability in the drying ink and achieved an ideal sequential crystallization with changing halide composition during the film formation. As a result, we produced highly crystalline, uniform, and pinhole-free CsPbI2Br films with superior photophysical and transport properties. High-performance solar cells are fabricated to achieve power conversion efficiencies (PCEs) of 14.7% for small-aperture-area (0.03 cm2) devices and 12.5% for the large-aperture-area (1.0 cm2) ones, the highest PCE reported to date for large-area all-inorganic perovskite solar cells. Halide perovskite solar cells (PSCs) have attracted immense attention in the past several years,1Kojima A. Teshima K. Shirai Y. Miyasaka T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells.J. Am. Chem. Soc. 2009; 131: 6050-6051Crossref PubMed Scopus (15167) Google Scholar, 2Park N.-G. Grätzel M. Miyasaka T. Organic-Inorganic Halide Perovskite Photovoltaics. Springer International Publishing, ISBN 978-3-319-35112-4, 2016Crossref Scopus (74) Google Scholar with their power conversion efficiency (PCE) increasing rapidly to 24.2%.3National Renewable Energy Laboratory (NREL). (2019). https://www.nrel.gov/pv/assets/pdfs/best-reserch-cell-efficiencies.20190411.Google Scholar Unfortunately, traditional organic-inorganic halide perovskites suffer from poor thermal stability due to the ease of evaporation of the small organic components methylammonium (MA) and formamidinium (FA),4Berhe T.A. Su W.-N. Chen C.-H. Pan C.-J. Cheng J.-H. Chen H.-M. Tsai M.-C. Chen L.-Y. Dubale A.A. Hwang B.-J. Organometal halide perovskite solar cells: degradation and stability.Energy Environ. Sci. 2016; 9: 323-356Crossref Google Scholar, 5Conings B. Drijkoningen J. Gauquelin N. Babayigit A. D’Haen J. D’Olieslaeger L. Ethirajan A. Verbeeck J. Manca J. Mosconi E. et al.Intrinsic thermal instability of methylammonium lead trihalide perovskite.Adv. Energy Mater. 2015; 5: 1500477Crossref Scopus (1493) Google Scholar hindering progress toward large-scale commercialization. Inorganic halide perovskites (CsPbX3) (X = Cl, Br, and I), which can be compositionally stable up to 400°C, have been developing rapidly in recent years,6Liang J. Zhao P. Wang C. Wang Y. Hu Y. Zhu G. Ma L. Liu J. Jin Z. CsPb0.9Sn0.1IBr2 based all-inorganic perovskite solar cells with exceptional efficiency and stability.J. Am. Chem. Soc. 2017; 139: 14009-14012Crossref PubMed Scopus (394) Google Scholar, 7Luo P. Zhou Y. Zhou S. Lu Y. Xu C. Xia W. Sun L. Fast anion-exchange from CsPbI3 to CsPbBr3 via Br2-vapor-assisted deposition for air-stable all-inorganic perovskite solar cells.Chem. Eng. J. 2018; 343: 146-154Crossref Scopus (61) Google Scholar, 8Xiang W. Wang Z. Kubicki D.J. Tress W. Luo J. Prochowicz D. Akin S. Emsley L. Zhou J. Dietler G. et al.Europium-doped CsPbI2Br for stable and highly efficient inorganic perovskite solar cells.Joule. 2019; 3: 205-214Abstract Full Text Full Text PDF Scopus (327) Google Scholar, 9Ho-Baillie A. Zhang M. Lau C.F.J. Ma F.-J. Huang S. Untapped potentials of inorganic metal halide perovskite solar cells.Joule. 2019; 3: 938-955Abstract Full Text Full Text PDF Scopus (150) Google Scholar and they have an optical band gap suitable to their implementation as front cell in silicon-perovskite hybrid tandem solar cells.10Zhao D. Yu Y. Wang C. Liao W. Shrestha N. Grice C.R. Cimaroli A.J. Guan L. Ellingson R.J. Zhu K. et al.Low-bandgap mixed tin–lead iodide perovskite absorbers with long carrier lifetimes for all-perovskite tandem solar cells.Nat. Energy. 2017; 2: 17018Crossref Scopus (522) Google Scholar, 11McMeekin D.P. Sadoughi G. Rehman W. Eperon G.E. Saliba M. Hörantner M.T. Haghighirad A. Sakai N. Korte L. Rech B. et al.A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells.Science. 2016; 351: 151-155Crossref PubMed Scopus (2167) Google Scholar, 12Bush K.A. Palmstrom A.F. Yu Z.J. Boccard M. Cheacharoen R. Mailoa J.P. McMeekin D.P. Hoye R.L.Z. Bailie C.D. Leijtens T. et al..23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability.Nat. Energy. 2017; 2: 17009Crossref Scopus (1028) Google Scholar, 13Giles E.E. Leijtens T. Bush K.A. Prasanna R. Green T. Wang J.T.-W. McMeekin D.P. Volonakis G. Milot R.L. May R. et al.Perovskite-perovskite tandem photovoltaics with optimized band gaps.Science. 2016; 354: 861-865Crossref PubMed Scopus (945) Google Scholar, 14Ahmad W. Khan J. Niu G. Tang J. Inorganic CsPbI3 perovskite-based solar cells: a choice for a tandem device.Sol. RRL. 2017; 1: 1-9Crossref Scopus (214) Google Scholar Recent advances have been reported in terms of novel surface passivation,15Wang Y. Zhang T. Kan M. Li Y. Wang T. Zhao Y. Efficient α-CsPbI3 photovoltaics with surface terminated organic cations.Joule. 2018; 2: 2065-2075Abstract Full Text Full Text PDF Scopus (232) Google Scholar, 16Li B. Zhang Y. Fu L. Yu T. Zhou S. Zhang L. Yin L. Surface passivation engineering strategy to fully-inorganic cubic CsPbI3 perovskites for high-performance solar cells.Nat. Commun. 2018; 9: 1076Crossref PubMed Scopus (453) Google Scholar, 17Zeng Q. Zhang X. Feng X. Lu S. Chen Z. Yong X. Redfern S. Wei A.T. Wang H. Shen H. et al.Polymer-passivated inorganic cesium lead mixed-halide perovskites for stable and efficient solar cells with high open-circuit voltage over 1.3 V.Adv. Mater. 2018; 30: 1705393Crossref Scopus (359) Google Scholar, 18Wang Y. Zhang T. Kan M. Zhao Y. Bifunctional stabilization of all-Inorganic α-CsPbI3 perovskite for 17% efficiency photovoltaics.J. Am. Chem. Soc. 2018; 140: 12345-12348Crossref PubMed Scopus (472) Google Scholar reduced dimensional structures and their mixture with three-dimensional (3D) perovskites,19Jiang Y. Yuan J. Ni Y. Yang J. Wang Y. Jiu T. Yuan M. Chen J. Reduced-dimensional α-CsPbX3 perovskites for efficient and stable photovoltaics.Joule. 2018; 2: 1356-1368Abstract Full Text Full Text PDF Scopus (285) Google Scholar partial substitution of iodide for bromide,20Zhang J. Bai D. Jin Z. Bian H. Wang K. Sun J. Wang Q. Liu (F).S. 3D-2D-0D Interface profiling for record efficiency all-inorganic CsPbBrI2 perovskite solar cells with superior stability.Adv. Energy Mater. 2018; 8: 1703246Crossref Scopus (295) Google Scholar, 21Sutton 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: 1502458Crossref Scopus (1071) Google Scholar, 22Lau 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: 573-577Crossref Scopus (212) Google Scholar, 23Li Z. Xu J. Zhou S. Zhang B. Liu X. Dai S. Yao J. CsBr-Induced stable CsPbI3- xBr x ( x < 1) perovskite films at low temperature for highly efficient planar heterojunction solar cells.ACS Appl. Mater. Interfaces. 2018; 10: 38183-38192Crossref PubMed Scopus (52) Google Scholar solvent engineering,24Wang P. Zhang X. Zhou Y. Jiang Q. Ye Q. Chu Z. Li X. Yang X. Yin Z. You J. Solvent-controlled growth of inorganic perovskite films in dry environment for efficient and stable solar cells.Nat. Commun. 2018; 9: 2225Crossref PubMed Scopus (470) Google Scholar and changes to the device architecture,25Bian H. Bai D. Jin Z. Wang K. Liang L. Wang H. Zhang J. Wang Q. Liu (F).S. Graded bandgap CsPbI2+x Br1−x perovskite solar cells with a stabilized efficiency of 14.4%.Joule. 2018; 2: 1500Abstract Full Text Full Text PDF Scopus (253) Google Scholar all of which have enabled considerable progress in phase stability and efficiency of inorganic PSCs.18Wang Y. Zhang T. Kan M. Zhao Y. Bifunctional stabilization of all-Inorganic α-CsPbI3 perovskite for 17% efficiency photovoltaics.J. Am. Chem. Soc. 2018; 140: 12345-12348Crossref PubMed Scopus (472) Google Scholar, 26Chen W. Chen H. Xu G. Xue R. Wang S. Li Y. Precise control of crystal growth for highly efficient CsPbI2Br perovskite solar cells.Joule. 2018; 3: 191-204Abstract Full Text Full Text PDF Scopus (336) Google Scholar Despite the significant progress made in the phase stabilization and efficiency improvement, the high-performance inorganic PSCs based on CsPbI3 and CsPbI2Br—without exception—have been fabricated in inert (N2) atmosphere, owing largely to the moisture sensitivity of these perovskite and their processing.27Zhou Y. Zhao Y. Chemical stability and instability of inorganic halide perovskites.Energy Environ. Sci. 2019; 12: 1495-1511Crossref Google Scholar However, these conditions are far from being suitable for industrial-scale high-throughput manufacturing and implementation of very low-cost solar cell fabrication in ambient air.28Kirmani A.R. Sheikh A.D. Niazi M. Haque R. Liu M. de Arquer F.P.G. Xu P. Sun B. Voznyy O. Gasparini N. et al.Overcoming the ambient manufacturability-scalability-performance bottleneck in colloidal quantum dot photovoltaics.Adv. Mater. 2018; 30: e1801661Crossref PubMed Scopus (65) Google Scholar Consequently, inorganic PSCs have been produced into small-area devices using lab-scale processes, such as spin-coating. Meniscus-guided methods for scalable coating under ambient conditions, such as blade coating, were first introduced for traditional organic-inorganic halide (MAPbI3) PSCs.29Deng Y. Peng E. Shao Y. Xiao Z. Dong Q. Huang J. Scalable fabrication of efficient organolead trihalide perovskite solar cells with doctor-bladed active layers.Energy Environ. Sci. 2015; 8: 1544-1550Crossref Google Scholar, 30Yang Z. Chueh C.-C. Zuo F. Kim J.H. Liang P.-W. Jen A.K.-Y. High-performance fully printable perovskite solar cells via blade-coating technique under the ambient condition.Adv. Energy Mater. 2015; 5: 1500328Crossref Scopus (260) Google Scholar The phase transition pathways of the disordered precursor to the solid-state perovskite phase were investigated for both scalable and spin-coating processes and gleaned into the role of fluid dynamics in meniscus-guided methods.3National Renewable Energy Laboratory (NREL). (2019). https://www.nrel.gov/pv/assets/pdfs/best-reserch-cell-efficiencies.20190411.Google Scholar, 31Li J. Munir R. Fan Y. Niu T. Liu Y. Zhong Y. Yang Z. Tian Y. Liu B. Sun J. et al.Phase transition control for high-performance blade-coated perovskite solar cells.Joule. 2018; 2: 1313-1330Abstract Full Text Full Text PDF Scopus (138) Google Scholar, 32He M. Li B. Cui X. Jiang B. He Y. Chen Y. O’Neil D. Szymanski P. Ei-Sayed M.A. Huang J. et al.Meniscus-assisted solution printing of large-grained perovskite films for high-efficiency solar cells.Nat. Commun. 2017; 8: 16045Crossref PubMed Scopus (316) Google Scholar, 33Zhang Y. Wang P. Tang M.C. Barrit D. Ke W. Liu J. Luo T. Liu Y. Niu T. Smilgies D.M. et al.Dynamical transformation of 2D perovskites with alternating cations in the interlayer space for high-performance photovoltaics.J. Am. Chem. Soc. 2019; 141: 2684-2694Crossref PubMed Scopus (140) Google Scholar, 34Niu T. Lu J. Tang M.-C. Barrit D. Smilgies D.-M. Yang Z. Li J. Fan Y. Luo T. McCulloch I. et al.High performance ambient-air-stable FAPbI 3 perovskite solar cells with molecule-passivated Ruddlesden–Popper/3D heterostructured film.Energy Environ. Sci. 2018; 11: 3358-3366Crossref Google Scholar, 35Barrit D. Cheng P. Tang M.-C. Wang K. Dang H. Smilgies D.-M. Liu S.F. Anthopoulos T.D. Zhao K. Amassian A. Impact of the solvation state of lead iodide on its two-step conversion to MAPbI3: an in situ investigation.Adv. Funct. Mater. 2019; 28: 1803269Google Scholar The direct crystallization of the perovskite by circumventing intermediate phases was found to be a key requirement for improving morphology, microstructure, and photophysical and transport properties in solid-state films.31Li J. Munir R. Fan Y. Niu T. Liu Y. Zhong Y. Yang Z. Tian Y. Liu B. Sun J. et al.Phase transition control for high-performance blade-coated perovskite solar cells.Joule. 2018; 2: 1313-1330Abstract Full Text Full Text PDF Scopus (138) Google Scholar Very small amounts of surfactants could alter the fluid dynamics of perovskite during drying.36Deng Y. Zheng X. Bai Y. Wang Q. Zhao J. Huang J. Surfactant-controlled ink drying enables high-speed deposition of perovskite films for efficient photovoltaic modules.Nat. Energy. 2018; 3: 560-566Crossref Scopus (458) Google Scholar The resulting insights helped improve the understanding of, and control over, the structural evolution, crystal growth behavior, and fluid dynamics, leading to a rational transfer from spin-coating to scalable fabrication processes.31Li J. Munir R. Fan Y. Niu T. Liu Y. Zhong Y. Yang Z. Tian Y. Liu B. Sun J. et al.Phase transition control for high-performance blade-coated perovskite solar cells.Joule. 2018; 2: 1313-1330Abstract Full Text Full Text PDF Scopus (138) Google Scholar, 36Deng Y. Zheng X. Bai Y. Wang Q. Zhao J. Huang J. Surfactant-controlled ink drying enables high-speed deposition of perovskite films for efficient photovoltaic modules.Nat. Energy. 2018; 3: 560-566Crossref Scopus (458) Google Scholar As a result, the solar cell efficiency based on blade-coated inorganic-organic perovskite films increased rapidly to over 20% for a small-area cell (~0.1 cm2), 17.2%–20.0% for a 1.0 cm2 cell,32He M. Li B. Cui X. Jiang B. He Y. Chen Y. O’Neil D. Szymanski P. Ei-Sayed M.A. Huang J. et al.Meniscus-assisted solution printing of large-grained perovskite films for high-efficiency solar cells.Nat. Commun. 2017; 8: 16045Crossref PubMed Scopus (316) Google Scholar, 37Wu W.Q. Yang Z. Rudd P.N. Shao Y. Dai X. Wei H. Zhao J. Fang Y. Wang Q. Liu Y. et al.Bilateral alkylamine for suppressing charge recombination and improving stability in blade-coated perovskite solar cells.Sci. Adv. 2019; 5: eaav8925Crossref PubMed Scopus (311) Google Scholar, 38Ding J. Han Q. Ge Q.-Q. Xue D.-J. Ma J.-Y. Zhao B.-Y. Chen Y.-X. Liu J. Mitzi D.B. Hu J.-S. Fully air-bladed high-efficiency perovskite photovoltaics.Joule. 2019; 3: 402-416Abstract Full Text Full Text PDF Scopus (97) Google Scholar and 14.6% for a large module (57.2 cm2).32He M. Li B. Cui X. Jiang B. He Y. Chen Y. O’Neil D. Szymanski P. Ei-Sayed M.A. Huang J. et al.Meniscus-assisted solution printing of large-grained perovskite films for high-efficiency solar cells.Nat. Commun. 2017; 8: 16045Crossref PubMed Scopus (316) Google Scholar By contrast, the efficiency of large-area all-inorganic PSCs is only 11.61% (aperture of 1.0 cm2) based on CsPbI2Br film spin-coated in inert atmosphere.39Yin G. Zhao H. Jiang H. Yuan S. Niu T. Zhao K. Liu Z. Liu (F).S Precursor engineering for all-Inorganic CsPbI2Br perovskite solar cells with 14.78% efficiency.Adv. Funct. Mater. 2018; 28: 1803269Crossref Scopus (238) Google Scholar Therefore, breakthroughs are needed for inorganic perovskites to be upscaled and amenable to ambient manufacturing, requiring understanding and control over the phase transformation behavior, fluid dynamics, and moisture sensitivity. Herein, we demonstrate that good controlled film formation dynamics via leveraging processing temperature during ambient scalable blading not only helps in constructing highly crystalline uniform pinhole-free all-inorganic perovskite (CsPbI2Br) films but also has strong capability to enhance the ambient phase stability in contrast to conventional spin-coated films. The fluid dynamics and structural evolution during ambient scalable blading were thoroughly investigated. The results reveal that both moisture attack and the Bénard-Marangoni instability during ink drying can be eliminated at the intermediate processing temperature, leading to the formation of a high-quality film. Sequential crystallization with changing halide composition during ink drying was observed at the intermediate processing temperature, where the enhanced mass transport is in favor of precursor crystallization. The film formation control enables the fabrication of high-quality large-area CsPbI2Br film with superior optoelectronic properties. The bladed device efficiencies reach 14.7% for a 0.03 cm2 cell and a record of 12.5% for a 1.0 cm2 cell. Based on the ambient scalable fabrication of high-quality all-inorganic perovskite films, further promotion of device stability for large-area solar cells is predicted. The blade coating of CsPbI2Br perovskite films was performed in ambient air (relative humidity ~30%–40% RH, temperature ~25°C) as shown in Figure 1A. Briefly, the perovskite precursors PbI2- dimethyl sulfoxide (DMSO) adduct, PbBr2-DMSO adduct, and CsI in a solvent mixture DMSO: γ-butyrolactone: N,N-dimethylformamide (DMSO: GBL: DMF) were cast on the substrate (see Experimental Procedures for details). The blade spreads the solution over the substrate (glass-FTO-TiO2) at a high speed of 2 m min−1, which is compatible with realistic industrial-scale manufacturing. The high speed yields a dynamic meniscus during the blade coating process that creates Landau-Levich flow.36Deng Y. Zheng X. Bai Y. Wang Q. Zhao J. Huang J. Surfactant-controlled ink drying enables high-speed deposition of perovskite films for efficient photovoltaic modules.Nat. Energy. 2018; 3: 560-566Crossref Scopus (458) Google Scholar This in turn drives ions or molecules close to the blade to follow its motion and entrains the surrounding material, yielding a flat ink layer after the passage of the blade. This Landau-Levich flow-assisted fabrication of high-quality films was demonstrated recently for MAPbI3-based perovskite films40Ro H.W. Downing J.M. Engmann S. Herzing A.A. DeLongchamp D.M. Richter L.J. Mukherjee S. Ade H. Abdelsamie M. Jagadamma L.K. et al.Morphology changes upon scaling a high-efficiency, solution-processed solar cell.Energy Environ. Sci. 2016; 9: 2835-2846Crossref Google Scholar, 41Maleki M. Reyssat M. Restagno F. Quéré D. Clanet C. Landau-Levich menisci.J. Colloid Interface Sci. 2011; 354: 359-363Crossref PubMed Scopus (70) Google Scholar and had previously been used for colloidal quantum dot solar cells, polymer-fullerene bulk heterojunction layers and molecule-polymer blend field-effect transistors,40Ro H.W. Downing J.M. Engmann S. Herzing A.A. DeLongchamp D.M. Richter L.J. Mukherjee S. Ade H. Abdelsamie M. Jagadamma L.K. et al.Morphology changes upon scaling a high-efficiency, solution-processed solar cell.Energy Environ. Sci. 2016; 9: 2835-2846Crossref Google Scholar, 42Zhao K. Hu H. Spada E. Jagadamma L.K. Yan B. Abdelsamie M. Yang Y. Yu L. Munir R. Li R. et al.Highly efficient polymer solar cells with printed photoactive layer: rational process transfer from spin-coating.J. Mater. Chem. A. 2016; 4: 16036-16046Crossref Google Scholar, 43Niazi M.R. Li R. Qiang Li E. Kirmani A.R. Abdelsamie M. Wang Q. Pan W. Payne M.M. Anthony J.E. Smilgies D.M. et al.Solution-printed organic semiconductor blends exhibiting transport properties on par with single crystals.Nat. Commun. 2015; 6: 8598Crossref PubMed Scopus (199) Google Scholar, 44Richter L.J. DeLongchamp D.M. Amassian A. Morphology development in solution-processed functional organic blend films: an in situ viewpoint.Chem. Rev. 2017; 117: 6332-6366Crossref PubMed Scopus (133) Google Scholar in all cases delivering device performance on par with spin-cast films. Based on this idea, we fabricated high-quality CsPbI2Br perovskite films in ambient conditions via blade coating, a scalable meniscus-guided technique. All blade-coated samples were thermally annealed in air at 150°C for 10 min after deposition. Figure 1B compares top-view scanning electron microscopy (SEM) images for the blade-coated CsPbI2Br film fabricated at a stage temperature of 80°C and the conventional spin-coated film fabricated at room temperature in N2-filled glove box with anti-solvent (chlorobenzene) drip. We observed a uniform and compact film surface with morphologically tight grain boundaries in the blade-coated film (Figure 1B, top image), while the spin-coated film shows ~200–300 nm diameter distinct features and evidence of more exposed grain boundaries (Figure 1B, bottom image). The cross-sectional SEM image confirms the compactness of the CsPbI2Br layer (ca. 500 nm thickness) (Figure 1C). The X-ray diffraction (XRD) patterns exhibit a lower value of full-width-at-half-maximum (FWHM) of the α-phase (100) peak (2θ = 14.6°) for the blade-coated film in contrast to the spin-coated one (0.10° versus 0.17°), indicating increased grain size in the blade-coated film (Figures 1D and S1A). The blade-coated film also exhibits significantly improved ambient phase stability in contrast to the spin-coated one, as the former does not change color and continues to feature the α-phase upon ambient air exposure for 3 h (relative humidity and temperature of ~50% and 25°C, respectively) (Figures 1D and S1B), while the latter’s color fades to yellowish white during the same time (Figure S1B), with complete phase degradation from the α-phase to the δ-phase (Figure 1D). These results clearly illustrate the advantage of blade coating for the fabrication of high-quality CsPbI2Br perovskite film over the conventional spin-coating strategy. Phase transformation behavior in blade coating is favorable for producing a stabilized version of the perovskite α-phase even when fabricated in ambient conditions. The processing temperature during blade coating appears to be of critical importance for film formation and phase stabilization. We varied the processing temperatures from 25°C to 130°C and, in doing so, changed the film formation dynamics and solid-state microstructural outcome of thin films, despite applying the same annealing conditions to all samples. For the 25°C case, we observed yellowish blemishes in the film after thermal annealing (Figure S2A). The film cast at 80°C is uniform, shiny, and dark in appearance without observable pinholes or blemishes in the large area (7 × 10 cm2) (Figure 2A). However, further elevating the temperature to 100°C and 130°C yields films with a poor appearance in terms of reduced uniformity and lighter color. The processing-temperature-dependent morphology was evaluated using SEM and atomic force microscopy (AFM). The 25°C film exhibits distinct grain-like features on the order of ~200–300 nm in size interspersed within a film with an undefined microstructure (Figures 2B and S2B for lower magnification). The film morphology is considerably more uniform and compact at 80°C (Figure 1B, top image), but further increasing temperature to 100°C and 130°C yields coarser film morphology with appearance of voids and very large domain-like features (~10 μm) connected by heterogeneous boundaries (Figures 2B and S2B for lower magnification). AFM images indicate a root mean square (RMS) roughness value of 5.5 nm for the 80°C film (Figure S2C), which is significantly lower than other cases (~29–132 nm). We further confirmed from XRD patterns that the observed yellowish color for the 25°C film is due to the formation of the CsPbI2Br δ-phase with the peak locating at 2θ = 10.0° (Figure 2C).24Wang P. Zhang X. Zhou Y. Jiang Q. Ye Q. Chu Z. Li X. Yang X. Yin Z. You J. Solvent-controlled growth of inorganic perovskite films in dry environment for efficient and stable solar cells.Nat. Commun. 2018; 9: 2225Crossref PubMed Scopus (470) Google Scholar, 31Li J. Munir R. Fan Y. Niu T. Liu Y. Zhong Y. Yang Z. Tian Y. Liu B. Sun J. et al.Phase transition control for high-performance blade-coated perovskite solar cells.Joule. 2018; 2: 1313-1330Abstract Full Text Full Text PDF Scopus (138) Google Scholar, 45Zhang X. Munir R. Xu Z. Liu Y. Tsai H. Nie W. Li J. Niu T. Smilgies D.M. Kanatzidis M.G. et al.Phase transition control for high performance ruddlesden-popper perovskite solar cells.Adv. Mater. 2018; 30: e1707166Crossref PubMed Scopus (205) Google Scholar, 46Koh T.M. Shanmugam V. Schlipf J. Oesinghaus L. Müller-Buschbaum P. Ramakrishnan N. Swamy V. Mathews N. Boix P.P. Mhaisalkar S.G. Nanostructuring mixed-dimensional perovskites: a route toward tunable, efficient photovoltaics.Adv. Mater. 2016; 28: 3653-3661Crossref PubMed Scopus (217) Google Scholar By contrast, the films cast at elevated temperatures exhibit pure α-phase with three main peaks at 14.6°, 20.7°, and 29.5° assigned to the (100), (110), and (200) planes of the cubic phase, respectively.24Wang P. Zhang X. Zhou Y. Jiang Q. Ye Q. Chu Z. Li X. Yang X. Yin Z. You J. Solvent-controlled growth of inorganic perovskite films in dry environment for efficient and stable solar cells.Nat. Commun. 2018; 9: 2225Crossref PubMed Scopus (470) Google Scholar, 31Li J. Munir R. Fan Y. Niu T. Liu Y. Zhong Y. Yang Z. Tian Y. Liu B. Sun J. et al.Phase transition control for high-performance blade-coated perovskite solar cells.Joule. 2018; 2: 1313-1330Abstract Full Text Full Text PDF Scopus (138) Google Scholar, 45Zhang X. Munir R. Xu Z. Liu Y. Tsai H. Nie W. Li J. Niu T. Smilgies D.M. Kanatzidis M.G. et al.Phase transition control for high performance ruddlesden-popper perovskite solar cells.Adv. Mater. 2018; 30: e1707166Crossref PubMed Scopus (205) Google Scholar, 46Koh T.M. Shanmugam V. Schlipf J. Oesinghaus L. Müller-Buschbaum P. Ramakrishnan N. Swamy V. Mathews N. Boix P.P. Mhaisalkar S.G. Nanostructuring mixed-dimensional perovskites: a route toward tunable, efficient photovoltaics.Adv. Mater. 2016; 28: 3653-3661Crossref PubMed Scopus (217) Google Scholar By carefully checking the α-phase (100) peak at 14.6°, the FWHM values were found to vary from 0.22° to 0.10°, 0.16°, and 0.21° with increasing temperatures from 25°C to 80°C, 100°C, and 130°C, respectively, with the lowest FWHM value for the 80°C film, indicative of the largest crystallite size. The qualitative changes in the solid-state microstructural outcome point to the critical role of the film formation dynamics. Superior film quality can be achieved at the intermediate temperature of 80°C. While lowering the processing temperature leads to longer duration of solvent evaporation under ambient condition and provides more opportunity for moisture attack. This in turn results in poor crystallinity of the CsPbI2Br α-phase and the formation of the CsPbI2Br δ-phase despite applying thermal annealing to film. Elevating the processing temperature can be an effective way to counterbalance moisture attack on the drying ink. However, the fluid could be affected by the Bénard-Marangoni convection, which is driven by temperature-gradient-induced surface tension variations. The strength of the Bénard-Marangoni convection is usually characterized by the Marangoni number Ma, a dimensionless number comparing the surface tension forces to the viscous forces as follows:47Pearson J.R.A. On convection cells induced by surface tension.J. Fluid Mech. 1958; 4: 489-500Crossref Scopus (1378) Google ScholarMa=−dγdTΔTLμα,(Equation 1) where γ is the surface tension, ΔT is the temperature difference acr" @default.
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• W2967236839 title "Scalable Ambient Fabrication of High-Performance CsPbI2Br Solar Cells" @default.
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