Reduced-Dimensional α-CsPbX3 Perovskites for Efficient and Stable Photovoltaics

Abstract

•CsAc and HPbX3 were adopted in CsPbX3 preparation, leading to large film thickness•The addition of PEAX can induce reduced-dimensional quasi-2D perovskite formation•The quasi-2D perovskite α-CsPbI3 shows PCE of 12.4% with improved stability Insufficient film thickness and undesirable phase transition are the two major obstacles that limit the performance of inorganic CsPbX3 perovskite devices. We found that, by adopting a new precursor pair, cesium acetate (CsAc) and hydrogen lead trihalide (HPbX3), we were able to overcome the notorious solubility limitation for Cs precursors to fabricate high-quality CsPbX3 perovskite films with large film thickness (∼500 nm). Moreover, further introduction of a judicious amount of PEAI into the new precursor system can induce reduced-dimensional, inorganic quasi-2D perovskite formation, and the resulting inorganic quasi-2D perovskites significantly suppressed the undesirable phase transition. Following this approach, we reported a state-of-the-art power conversion efficiency to date, 12.4%, for reduced-dimensional inorganic α-CsPbI3 perovskite solar cells with greatly improved performance longevity. Inorganic CsPbX3 perovskites have gained great attention owing to their excellent thermal stability and carrier transport properties. However, the power conversion efficiency (PCE) of solution-processed CsPbX3 perovskite solar cells is still far inferior to that of their hybrid analogues. Insufficient film thickness and undesirable phase transition are the two major obstacles limiting their device performance. Here, we show that by adopting a new precursor pair, cesium acetate (CsAc) and hydrogen lead trihalide (HPbX3), we were able to overcome the notorious solubility limitation for Cs precursors to fabricate high-quality CsPbX3 perovskite films with large film thickness (∼500 nm). We further introduced a judicious amount of phenylethylammonium iodide (PEAI) into the system to induce reduced-dimensional perovskite formation. Unprecedentedly, the resulting quasi-2D perovskites significantly suppressed undesirable phase transition and thus reduced the film's trap density. Following this approach, we reported a state-of-the-art PCE to date, 12.4%, for reduced-dimensional α-CsPbI3 perovskite photovoltaics with greatly improved performance longevity. Inorganic CsPbX3 perovskites have gained great attention owing to their excellent thermal stability and carrier transport properties. However, the power conversion efficiency (PCE) of solution-processed CsPbX3 perovskite solar cells is still far inferior to that of their hybrid analogues. Insufficient film thickness and undesirable phase transition are the two major obstacles limiting their device performance. Here, we show that by adopting a new precursor pair, cesium acetate (CsAc) and hydrogen lead trihalide (HPbX3), we were able to overcome the notorious solubility limitation for Cs precursors to fabricate high-quality CsPbX3 perovskite films with large film thickness (∼500 nm). We further introduced a judicious amount of phenylethylammonium iodide (PEAI) into the system to induce reduced-dimensional perovskite formation. Unprecedentedly, the resulting quasi-2D perovskites significantly suppressed undesirable phase transition and thus reduced the film's trap density. Following this approach, we reported a state-of-the-art PCE to date, 12.4%, for reduced-dimensional α-CsPbI3 perovskite photovoltaics with greatly improved performance longevity. Organic-inorganic hybrid perovskites (CH3NH3PbX3) have emerged as the most promising next-generation photovoltaic materials recently due to their marvelous photophysical properties.1Stranks S. Eperon G. Grancini G. Menelaou C. Alcocer M. 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Compositional engineering of perovskite materials for high-performance solar cells.Nature. 2015; 517: 476-480Crossref PubMed Scopus (4901) Google Scholar, 8Tan H. Jain A. Voznyy O. Lan X. de Arquer F. Fan J. Quintero-Bermudez R. Yuan M. Zhang B. Zhao Y. et al.Efficient and stable solution-processed planar perovskite solar cells via contact passivation.Science. 2017; 355: 722-726Crossref PubMed Scopus (1759) Google Scholar, 9Kim H. Lee C. Im J. Lee K. Moehl T. Marchioro A. Moon S. Humphry-Baker R. Yum J. Moser J. et al.Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%.Sci. Rep. 2012; 2: 591Crossref PubMed Scopus (6261) Google Scholar Hybrid perovskites demonstrating improved thermal stabilities have been documented,10Quan L. Yuan M. Comin R. Voznyy O. Beauregard E. Hoogland S. Buin A. Kirmani A. Zhao K. Amassian A. et al.Ligand-stabilized reduced-dimensionality perovskites.J. Am. Chem. Soc. 2016; 138: 2649-2655Crossref PubMed Scopus (950) Google Scholar, 11Li X. Dar M. Yi C. Luo J. Tschumi M. Zakeeruddin S. Nazeerudin M. Han H. Grätzel M. Improved performance and stability of perovskite solar cells by crystal crosslinking with alkylphosphonic acid ω-ammonium chlorides.Nat. Chem. 2015; 7: 703-711Crossref PubMed Scopus (944) Google Scholar, 12Mei A. Li X. Liu L. Ku Z. Liu T. Rong Y. Xu M. Hu M. Chen J. Yang Y. et al.A hole-conductor−free, fully printable mesoscopic perovskite solar cell with high stability.Science. 2014; 345: 295-298Crossref PubMed Scopus (2484) Google Scholar however, the ultimate solution to solve this thermal instability problem is to replace the organic cation entirely with an inorganic cation, such as cesium (Cs).13Green M. Ho-Baillie A. Snaith H. The emergence of perovskite solar.Nat. Photonics. 2014; 8: 506-514Crossref Scopus (4969) Google Scholar, 14Kulbak 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: 167-172Crossref PubMed Scopus (737) Google Scholar Inorganic cesium lead halide perovskites (CsPbX3) thus have gained widespread attention due to their great thermal stabilities and outstanding carrier transport properties. Stoumpos et al. demonstrated that CsPbBr3 perovskite possessed ultra-high mobility for both electrons and holes, indicating potential application in efficient optoelectronics with excellent thermal stability.15Stoumpos C. Malliakas C. Peters J. Liu Z. Sebastian M. Im J. Chasapis T. Wibowo A. Chung D. Freeman A. Crystal growth of the perovskite semiconductor CsPbBr3: a new material for high-energy radiation detection.Cryst. Growth Des. 2013; 13: 2722-2727Crossref Scopus (990) Google Scholar Here, inorganic CsPbX3 perovskites would be expected to make greater impact in real applications compared with their hybrid analogs. Several solution-processed approaches to fabricate inorganic CsPbX3 perovskite solar cells have been reported.16Eperson G. Patern G. Sutton R. Zampetti A. Haghighiad A. Cacialli F. Snaith H. Inorganic caesium lead iodide perovskite solar cells.J. Mater. Chem. A. 2015; 3: 19688-19695Crossref Google Scholar, 17Wang Q. Zheng X. Deng Y. Zhao J. Chen Z. Huang J. Stabilizing the α-phase of CsPbI3 perovskite by sulfobetaine zwitterionsin one-step spin-coating films.Joule. 2017; 1: 371-382Abstract Full Text Full Text PDF Scopus (358) Google Scholar Unfortunately, photovoltaic device performance is still far inferior to that of their hybrid analog, CH3NH3PbX3, to date.18Zhou H. Chen Q. Li G. Luo S. Song T. Duan H. Hong Z. You J. Liu Y. Yang Y. Interface engineering of highly efficient perovskite solar cells.Science. 2014; 345: 542-546Crossref PubMed Scopus (5546) Google Scholar, 19Liang P. Liao C. Chueh C. Zuo F. Williams S. Xin X. Lin J. Jen A. Additive enhanced crystallization of solution-processed perovskite for highly efficient planar-heterojunction solar cells.Adv. Mater. 2014; 26: 3748-3754Crossref PubMed Scopus (1300) Google Scholar, 20Zhu Z. Bai Y. Zhang T. Liu Z. Long X. Wie Z. Wang Z. Zhang L. Wang J. Yan F. et al.High-performance hole-extraction layer of sol–gel-processed NiO nanocrystals for inverted planar perovskite solar cells.Angew. Chem. Int. Ed. 2014; 53: 12571-12575PubMed Google Scholar The relatively low device performance for solution-processed inorganic CsPbX3 solar cells can be ascribed to the following: (1) a solution processing method to fabricate high-quality CsPbX3 perovskite films with large film thickness (>400 nm) remains elusive. CsPbX3 perovskites exhibit a similar absorption coefficient to their hybrid CH3NH3PbX3 analog.21Liang 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 Accordingly, in principle, the optimal film thickness should be at least 400 nm to allow CsPbX3 absorbers to fully absorb the incident light. In practice, the best-performing vacuum-deposited CsPbX3 solar cells possess a film thickness of around 500 nm.22Chen 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: 1605290-1605297Crossref Scopus (300) Google Scholar Unfortunately, the established single-step deposition methods solely relied on mixed stoichiometric amounts of cesium halide (CsX) and lead halide (PbX2) as precursors. This precursor solution suffered from low molar concentration, caused by the poor solubility of CsX in polar aprotic solvents. Consequently, the resulting CsPbX3 perovskite film thickness has been severely limited.23Beal R. Slotcavage D. Leijtens T. Bowring A. Belisle R. Nguyen W. Burkhard G. Hoke E. McGehee M. Cesium lead halide perovskites with improved stability for tandem solar cells.J. Phys. Chem. Lett. 2016; 7: 746-751Crossref PubMed Scopus (855) Google Scholar Film thickness generated through this method is normally around 200–300 nm, which is much less than the optimal values (400–600 nm) required for high-efficiency inorganic perovskite solar cells.22Chen 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: 1605290-1605297Crossref Scopus (300) Google Scholar (2) The black cube phase of iodide-rich narrower-band-gap CsPb(I1-xBrx)3 (x < 0.5) perovskites are thermodynamically unstable at room temperature.24Sutton R. Eperon G. Miranda L. Parrott E. Kamino B. Patel J. Horantner M. Johnston M. Haghighirad A. Moore D. 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 As we know, the band gap for bromide-rich CsPb(I1-xBrx)3 (x > 0.5) perovskites is too large to sufficiently harvest photon energy. Hence, in order to achieve higher PCE, narrower-band-gap iodide-rich CsPb(I1-xBrx)3 (x < 0.5) perovskites have to be utilized as absorbers. Regrettably, iodide-rich perovskites are thermally unstable in black cube phase (α-CsPbX3), and transformed spontaneously into non-perovskite yellow orthorhombic phase (δ-CsPbX3) under ambient conditions.25Zhou W. Zhao Y. Zhu X. Fu R. Li Q. Zhao Y. Liu K. Yu D. Light-independent ionic transport in inorganic perovskite and ultrastable Cs-based perovskite solar cells.J. Phys. Chem. Lett. 2017; 8: 4122-4128Crossref PubMed Scopus (194) Google Scholar Phase instability problems for iodide-rich perovskites have thus drawn wide research attention. Luther et al. demonstrated that nanocrystal surface engineering is a promising way to stabilize black phase α-CsPbI3 at room temperature, resulting in stabilized α-CsPbI3 perovskite quantum dot solar cells with PCE up to 10%.26Swarnkar A. Marshall A. Sanehira E. Chernomordik B. Moore D. Christians J. Chakrabarti T. Luther J. Quantum dot-induced phase stabilization of á-CsPbI3 perovskite for high-efficiency photovoltaics.Science. 2016; 354: 92-95Crossref PubMed Scopus (1893) Google Scholar Zhao et al. recently claimed that a bication two-dimensional (2D) ethylenediamine perovskite component can stabilize α-CsPbI3 perovskite through efficient surface passivation.27Zhang T. Dar M.I. Li G. Xu F. Guo N. Gratzel M. Zhao Y. Bication lead iodide 2D perovskite component to stabilize inorganic α-CsPbI3 perovskite phase for high-efficiency solar cells.Sci. Adv. 2017; 3: e1700841Crossref PubMed Scopus (503) Google Scholar Moreover, the bication 2D perovskite components enabled efficient charge carrier transfer between different grains, and thus led to a record PCE of 11.8% for all-inorganic perovskite solar cells. The strategy represents a milestone for the development of high-efficiency α-CsPbI3 photovoltaics. However, strategies to stabilize bulk α-CsPbI3 perovskite nanocomposites remain limited, hindering further PCE evolution for inorganic perovskite solar cells. Here, we report a facial single-step deposition method to fabricate high-efficiency α-CsPbX3 perovskite solar cells. We used highly soluble cesium acetate (CsAc) and hydrogen lead trihalide (HPbX3)28Wang F. Yu H. Xu H. Zhao N. HPbI3: a new precursor compound for highly efficient solution-processed perovskite solar cells.Adv. Funct. Mater. 2015; 25: 1120-1126Crossref Scopus (265) Google Scholar as a new precursor pair for the first time to overcome the aforementioned Cs-precursor solubility limitation. Strong interaction between CsAc and HPbX3 induced precursors transforming to ultra-smooth, high-quality CsPbX3 perovskite films at relatively low temperature. We further introduced a larger organic cation, phenylethylammonium (PEA, C8H9NH), into the system to induce reduced-dimensional (quasi-2D) perovskite formation.29Smith I. Hoke E. Solis-Ibarra D. McGehee M.D. Karunadasa H.I. A layered hybrid perovskite solar-cell absorber with enhanced moisture stability.Angew. Chem. Int. Ed. 2014; 53: 1-5Crossref Scopus (1452) Google Scholar Unprecedentedly, the resulting quasi-2D perovskites significantly suppressed the undesirable phase transition of the films, and thus greatly reduced the film's trap-state density. The inorganic α-CsPbI3 solar cells produced through this approach exhibit a highly reproducible PCE of 12.4%, together with greatly improved performance longevity. Previous studies found lead acetate (PbAc2) to be a promising precursor for high-quality CH3NH3PbI3 film fabrication.30Xu J. Buin A. Ip A. Li W. Voznyy O. Comin R. Yuan M. Jeon S. Ning Z. McDowell J. et al.Perovskite–fullerene hybrid materials suppress hysteresis in planar diodes.Nat. Commun. 2015; 6: 7081Crossref PubMed Scopus (901) Google Scholar In addition, most of the acetates exhibit very good solubility in polar solvents, including polar aprotic solvents, such as dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF). These discoveries motivated us to consider using CsAc as a new Cs precursor for inorganic CsPbX3 perovskite preparation. After inspection of the stoichiometry of CsPbX3, HPbX3 was adopted as the counterpart of the new precursor pair, providing the source of lead and halides.31Zhou Z. Wang Z. Zhou Y. Pang S. Wang D. Xu H. Liu Z. Padture N. Cui G. Methylamine-gas-induced defect-healing behavior of CH3NH3PbI3 thin films for perovskite solar cells.Angew. Chem. Int. Ed. 2015; 54: 9705-9709Crossref PubMed Scopus (355) Google Scholar The new precursor pair was then confirmed to be readily soluble in DMF and DMSO, enabling the solution concentration to go up to 1.3 M. Briefly, DMSO solution containing stoichiometric amounts of CsAc and HPbX3 was prepared and filtered. Subsequent spin coating followed by annealing at 100°C yielded uniform and ultra-smooth films. The mirror-like film exhibits an excellent reflective feature and inorganic CsPbX3 cubic phase perovskite with a deep black color (Figure 1D). Band-gap tuning of inorganic perovskites, CsPb(I1−xBrx)3 (0 ≤ x ≤ 1), was achieved by adjusting the molar ratio between HPbBr3 and HPbI3 of the precursors. It is surprising that the strong color change was observed for the whole series of CsPb(I1−xBrx)3 perovskite films at such a relatively low temperature (<100°C) (Figure S1). As documented, yellow-to-black phase transformation for CsPbI3 and CsPbI2Br perovskites should take place at around 350°C and 230°C, respectively.32Frolova L. Anokhin D. Piryazev A. Luchkin S. Dremova N. Stevenson K. Troshin P. Highly efficient all-inorganic planar heterojunction perovskite solar cells produced by thermal coevaporation of CsI and PbI2.J. Phys. Chem. Lett. 2017; 8: 67-72Crossref PubMed Scopus (240) Google Scholar However, by using the new precursor pair, a black cube α-CsPbI3 perovskite film emerged at 100°C. The film exhibited an absorbance onset at around 718 nm, identifying a band gap of 1.73 eV, in good agreement with a previous report.16Eperson G. Patern G. Sutton R. Zampetti A. Haghighiad A. Cacialli F. Snaith H. Inorganic caesium lead iodide perovskite solar cells.J. Mater. Chem. A. 2015; 3: 19688-19695Crossref Google Scholar Furthermore, the X-ray diffraction (XRD) pattern confirmed that the pure cubic α-CsPbI3 phase perovskite structure formed was not associated with any impurities. We speculated that HPbX3 crystals significantly reduce the crystallization energy barrier required for the α-CsPbI3 phase formation,33Pang S. Zhou Y. Wang Z. Yang M. Krause A. Zhou Z. Zhu K. Padture N. Cui G. Transformative evolution of organolead triiodide perovskite thin films from strong room-temperature solid–gas interaction between HPbI3-CH3NH2 precursor pair.J. Am. Chem. Soc. 2016; 138: 750-753Crossref PubMed Scopus (143) Google Scholar leading to low-temperature phase transition taking place; unfortunately, we are yet to fully understand the impetus for this low-temperature phase transition. Absorption spectra are acquired for the entire range of CsPb(I1−xBrx)3 perovskites, as shown in Figure 1A. The absorbance onset of CsPb(I1−xBrx)3 nanocomposites can be tuned from 718 nm (1.73 eV) to 540 nm (2.3 eV), in agreement with previous reports.34Liang J. Wang C. Wang Y. Xu Z. Lu Z. Ma Y. Hu Y. Xiao C. Yi X. Zhu G. et al.All-inorganic perovskite solar cells.J. Am. Chem. Soc. 2016; 138: 15829-15832Crossref PubMed Scopus (764) Google Scholar We plotted the absorbance onset energy as a function of the bromide content of CsPb(I1-xBrx)3 in Figure 1B. The linear trend of the absorbance plot followed Vegard's law, indicating CsPbI3 and CsPbBr3 have good miscibility, and thus no inhomogeneous landscape exists in the whole series of nanocomposites.35Noh J. Im S. Heo J. Mandal T. Sang S. Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.Nano Lett. 2013; 13: 1764-1769Crossref PubMed Scopus (3752) Google Scholar Detailed crystalline structures of the resulting films were then investigated through XRD, as shown in Figure 1C. For CsPbI3 perovskite film, all the XRD peaks can be indexed to the cubic α-CsPbI3 perovskite. The main peaks at 14.62°, 20.65°, and 28.94° correspond to the standard (100), (110), and (200) planes of α-CsPbI3 perovskite. No peaks are indexable to any impurity phase in the XRD pattern. Lattice contraction of the unit cells takes place when the Br component increases; therefore, according to Bragg's law, the corresponding XRD peaks should shift to higher angles. We indeed observed peaks characteristic of the CsPbI2Br, CsPbIBr2 and CsPbBr3 gradually shifting to higher angles (Figure 1C), in good agreement with expectation. All the films appear to be very smooth, uniform, and pinhole free. Scanning electron microscopy (SEM) analysis was performed to investigate the film morphology (Figures 1E and S2). As shown in Figure 1E, smooth pinhole-free CsPbIBr2 films with uniform and large grains were formed, and crystalline domains grew as large as ∼2 μm, indicating good carrier transport behavior. Notably, following this single-step spinning approach, we were able to fabricate mirror-like CsPbI3 films with an area as high as 9 × 9 cm2 (Figure 1D). The as-prepared perovskite films were separately utilized in planar perovskite solar cells as the active layer. Optimized device performance for CsPbBr3, CsPbIBr2, CsPbI2Br, and CsPbI3 perovskites are shown in Figure 2A. The corresponding external quantum efficiency (EQE) spectra are shown in Figure 2B. Typically, the CsPbBr3-based all-inorganic photovoltaics delivered a representative PCE of 5.96%, comparable with the best results reported yet.21Liang 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 As expected, owing to the substitution of iodide, the narrower-band-gap CsPbIBr2-based device exhibits a higher short-circuit current density (Jsc), with very promising PCE of 8.54%. The value represents the best PCE to date for CsPbIBr2 perovskite-based solar cells.36Ma Q. Huang S. Wen X. Green M. Ho-Baillie A. Hole transport layer free inorganic CsPbIBr2 perovskite solar cell by dual source thermal evaporation.Adv. Energy Mater. 2016; 6: 1502202Crossref Scopus (337) Google Scholar Furthermore, with a band gap of approximately 1.9 eV, CsPbI2Br perovskite-based devices display a clear further enhancement in Jsc. The best-performing PCE based on CsPbI2Br perovskite reached 10.99%, which is superior to the best vacuum-deposited CsPbI2Br solar cells to date.22Chen 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: 1605290-1605297Crossref Scopus (300) Google Scholar Unfortunately, in spite of possessing the most promising band gap in the whole series, α-CsPbI3 perovskite delivered the lowest device performance, with a best-performing PCE of 4.38%. The device exhibits a fairly low fill factor (FF) and Jsc. EQE spectra revealed inferior carrier collection efficiency for α-CsPbI3 perovskite, suggesting high trap-state densities should exist in the films. We speculate this is owing to the thermal dynamical instability of black phase α-CsPbI3 perovskite and means portions of α-CsPbI3 perovskite have already transformed into δ-CsPbI3 non-perovskite phase during the device aging and testing process. We then decided to track the device performance evolution in time for each perovskite nanocomposites under ambient atmosphere (Figure 2C). The results clearly revealed both CsPbBr3 and CsPbIBr2 perovskites exhibit good device stability under the period of testing. On the contrary, all the iodide-rich nanocomposites, both CsPbI2Br and CsPbI3 perovskites, are unstable and suffer from undesirable phase transition. Black α-CsPbI3 perovskite films converted to completely yellow δ-CsPbI3 within only 4 hr (Figure 4E), resulting in degradation of device performance to almost zero. Phase transition temperature for orthorhombic to cubic reduced with increasing bromide content.23Beal R. Slotcavage D. Leijtens T. Bowring A. Belisle R. Nguyen W. Burkhard G. Hoke E. McGehee M. Cesium lead halide perovskites with improved stability for tandem solar cells.J. Phys. Chem. Lett. 2016; 7: 746-751Crossref PubMed Scopus (855) Google Scholar Hence, CsPbI2Br devices demonstrated relatively better stability compared with CsPbI3, but were still very unstable. The PCE degraded from 10.9% to less than 1% within 1 week. In any event, it became clear that stabilization of the black cubic phase of narrower-band-gap iodide-rich CsPb(I1-xBrx)3 (x < 0.5) perovskites is a prerequisite to achieve more efficient inorganic perovskite solar cells. It is reported that reduced-dimensional (quasi-2D) perovskites, PEA2(CH3NH3)n-1PbnI3n+1 (Figure 3A), exhibit improved moisture stability while retaining the high performance of conventional CH3NH3PbI3 perovskite.10Quan L. Yuan M. Comin R. Voznyy O. Beauregard E. Hoogland S. Buin A. Kirmani A. Zhao K. Amassian A. et al.Ligand-stabilized reduced-dimensionality perovskites.J. Am. Chem. Soc. 2016; 138: 2649-2655Crossref PubMed Scopus (950) Google Scholar, 37Stoumpos C. Cao D. Clark D. Young J. Rondinelli J. Jang J. Hupp J. Kanatzidis M. Ruddlesden–Popper hybrid lead iodide perovskite 2D homologous semiconductors.Chem. Mater. 2016; 28: 2852-2867Crossref Scopus (1272) Google Scholar In addition, Jin et al. demonstrated surface ligand functionalization to be a promising strategy to stabilize metastable FAPbI3 perovskite phases.38Fu Y. Wu T. Wang J. Zhao J. Shearer M. Zhao Y. Hamers R. Kan E. Deng K. Zhu X. et al.Stabilization of the metastable lead iodide perovskite phase via surface functionalization.Nano Lett. 2017; 17: 4405-4414Crossref PubMed Scopus (166) Google Scholar Reduced surface energy was claimed to be a dominant reason for phase stability enhancement.39Peng W. Yin J. Ho K. Ouellette O. Bastiani M. Murali B. Tall O. Shen C. Miao X. Pan J. et al.Ultralow self-doping in two-dimensional hybrid perovskite single crystals.Nano Lett. 2017; 17: 4759-4767Crossref PubMed Scopus (208) Google Scholar These pioneer works inspired us to explore the diversity of the quasi-2D perovskites, aiming for stabilization of the black cubic phase of iodide-rich inorganic perovskites, since the all-iodide CsPbX3 (CsPbI3), which possesses the most suitable band gap, should be bearing the brunt. Inorganic quasi-2D perovskites can be systematically synthesized by introducing phenylethylammonium (PEA) at a judiciously chosen stoichiometry.10Quan L. Yuan M. Comin R. Voznyy O. Beauregard E. Hoogland S. Buin A. Kirmani A. Zhao K. Amassian A. et al.Ligand-stabilized reduced-dimensionality perovskites.J. Am. Chem. Soc. 2016; 138: 2649-2655Crossref PubMed Scopus (950) Google Scholar In principle, we could achieve dimensionality modulation of quasi-2D perovskites by adjusting the molar ratio of PEA to Cs, yielding compounds with different 〈n〉 values with the formula PEA2Csn-1PbnX3n+1 (Figure 3A). We successfully obtained the expected inorganic quasi-2D perovskite films through a stoichiometric reaction between HPbI3, CsAc, and PEAI, followed by single-step spin casting. The dimensional tuning of the electronic band gap, Eg, can be monitored through the absorption spectra, which revealed onset of higher-energy absorption for lower 〈n〉 values (Figure 3B). Strikingly, perovskite films with lower 〈n〉 value (n ≤ 5) exhibit multiple absorption peaks that can be ascribed to mixed grain comprising a variety of 〈n〉 values, consistent with previous reports.40Liao Y. Liu H. Zhou W. Yang D. Shang Y. Shi Z. Li B. Jiang X. Zhang L. Quan L. et al.Highly oriented low-dimensional tin halide perovskites with enhanced stability and photovoltaic performance.J. Am. Chem. Soc. 2017; 139: 6693-6699Crossref PubMed Scopus (576) Google Scholar For example, absorption spectra of 〈n〉 = 3 perovskite film exhibited four distinct peaks that can be attributed to characteristic 〈n〉 = 1, 2, 3, and 4 perovskite phases, respectively. Similar phenomena were also observed in 〈n〉 = 5 perovskite film. These results suggested this approach creates a distribution of domains with varying numbers of perovskite layers centered on an average value of〈n〉 rather than a pure phase.10Quan L. Yuan M. Comin R. Voznyy O. Beauregard E. Hoogland S. Buin A. Kirmani A. Zhao K. Amassian A. et al.Ligand-stabilized reduced-dimensionality perovskites.J. Am. Chem. Soc. 2016; 138: 2649-2655Crossref PubMed Scopus (950) Google Scholar Remarkably, the dielectric quantum confinement effects become negligible for large 〈n〉 values, and then the band gap approaches 〈n〉 independence. Consequently, the energy landscape becomes flat in perovskite films with large 〈n〉 values (〈n〉 > 10); the photo-generated charge carriers can then travel as freely as in traditional 3D CsPbI3 perovskite without encountering any energy barriers.41Quan L. Zhao Y. de Arquer F. Sabatini R. Walters G. Voznyy O. Comin R. Li Y. Fan J. Tan H. et al.Tailoring the energy landscape in quasi-2D halide perovskites enables efficient green-light emission.Nano Lett. 2017; 17: 3701-3709Crossref PubMed Scopus (324) Google Scholar As shown in Figure 3B, the 〈n〉 = 10, 40 and 60 quasi-2D perovskites indeed exhibit a very similar optical band gap compared with 3D cubic α-CsPbI3 perovskite, indicating the homogeneous flat energy landscape.41Quan L. Zhao Y. de Arquer F. Sabatini R. Walters G. Voznyy O. Comin R. Li Y. Fan J. Tan H. et al.Tailoring the energy landscape in quasi-2D halide perovskites enables efficient green-light emission.Nano Lett. 2017; 17: 3701-3709Crossref PubMed Scopus (324) Google Scholar, 42Yuan M. Quan L. Comin R. Walters G. Sabatini R. Voznyy O. Hoogland S. Zhao Y. Beauregard E. Kanjanaboos P. et al.Perovskite energy funnels for efficient light-emitting diodes.Nat. Nanotechnol. 2016; 11: 872-877Crossref PubMed Scopus (1580) Google Scholar To gain more insight into the crystalline structure, we present the low-angle XRD pattern of quasi-2D perovskite powder materials together with α-CsPbI3 powder in Figure 3C. In principle, d spacing of the layered quasi-2D perovskites would increase with increasing 〈n〉 values due to the insertion of the additional PbI6 sheet (Figure 3A).43Yao K. Wang X. Xu Y. Li F. Zhou L. Multilayered perovskite materials based on polymeric-ammonium cations for stable large-area solar cell.Chem. Mater. 2016; 28: 3131-3138Crossref Scopus (155) Google Scholar Consequently, several low-diffraction-angle peaks should emerge in the inorganic quasi-2D perovskite films. As shown, the XRD pattern did exhibit a series of low-angle diffraction peaks in all cases except for α-CsPbI3 perovskite powder (Figure 3C). These low-diffraction-angle peaks can be ascribed to the typical (0k0) reflections from the layered structure,43Yao K. Wang X. Xu Y. Li F. Zhou L. Multilayered perovskite materials based on polymeric-ammonium cations for stable large-area solar cell.Chem. Mater. 2016; 28: 3131-3138Crossref Scopus (155) Google Scholar associated with the aforementioned d-spacing enhancement. These low-diffraction-angle features represent the unambiguous signatures of the layered structure, corresponding here to the quasi-2D perovskites with the formula PEA2Csn-1PbnI3n+1.44Wang Z. Lin Q. Chmiel F. Sakai N. Herz L. Snaith H. Efficient ambient-air-stable solar cells with 2D–3D heterostructured butylammonium-caesium-formamidinium lead halide perovskites.Nat. Energy. 2017; 2: 17135Crossref Scopus (989) Google Scholar Photoluminescence (PL) spectra of quasi-2D perovskites clearly demonstrated blue-shifted PL emission energy with decreasing 〈n〉 values (Figure 3E). This distinct feature is fully consistent with the absorption spectra, further indicating that successful dimensional modulation was achieved. To understand the kinetics of excitons and free carriers in various perovskite films, time-resolved PL spectra were measured (Figure 3F). Significantly enhanced PL lifetime was observed for both 〈n〉 = 40 and 60 quasi-2D perovskites compared with the as-prepared α-CsPbI3 films. Specifically, the carrier average lifetimes (τave) were 51.0, 317.1, and 189.0 ns for 〈n〉 = ∞, 40, and 60, respectively (Table S2). The significantly improved carrier lifetime indicated the reduced trap-state density in quasi-2D perovskite films, which would be beneficial for optoelectronic applications. We speculate that the reduced trap-state density is owing to the suppression of undesirable cubic to orthorhombic phase transition in quasi-2D perovskites.45Luo P. Xia W. Zhou S. Sun L. Cheng J. Xu C. Liu Y. Solvent engineering for ambient-air-processed, phase-stable CsPbI3 in perovskite solar cells.J. Phys. Chem. Lett. 2016; 7: 3603-3608Crossref PubMed Scopus (306) Google Scholar We then turned to evaluate the phase stability of 〈n〉 = 10, 40, and 60 perovskites through film XRD (Figure 3D). Strong (100) diffraction peaks (2θ = 14°) characteristic of the perovskite structure are observed for all fresh film samples. A new peak at around 2θ = 10° emerged in the degraded films, corresponding to the characteristic diffraction peak of undesirable orthorhombic phase.46Fai C. Lau J. Deng X. Ma Q. Zheng J. Yun J. Green M. Huang S. Ho-Baillie A. CsPbIBr2 perovskite solar cell by spray-assisted deposition ACS.Energy Lett. 2016; 1: 573-577Crossref Scopus (212) Google Scholar For 3D CsPbI3 perovskite film, a barely single distinct peak at 2θ = 9.8° was observed in the spectrum, indicating that the black cube phase had completely degraded to yellow orthorhombic phase. In contrast, only a very weak orthorhombic phase reflection peak at 2θ = 9.8° appeared in 〈n〉 = 10, 40, and 60 perovskite films after aging, demonstrating significant improvements in stability compared with 3D CsPbI3 perovskite. Furthermore, quasi-2D perovskites with lower 〈n〉 values (〈n〉 = 10) were found to exhibit better phase stability than those with higher 〈n〉 values (〈n〉 = 40, 60). First-principles density functional theory (DFT) simulation was used to calculate the decomposition energy for different perovskites. The result shows that the degraded reaction for CsPbI3 was exothermic (thermodynamically favorable) and the reaction occurs spontaneously, which means a serious phase instability problem for CsPbI3 (Table S1). Quasi-2D perovskites (〈n〉 = 1, 2, and 3) possess large decomposition energy; with increasing 〈n〉 values, the calculated decomposition energy decreased. From the thermodynamic point of view, large decomposition energy indicating a phase-separation reaction can be avoided, which is in good agreement with observed experimental data. We therefore fabricated planar solar cells using 〈n〉 = 10, 40, and 60 quasi-2D perovskites as absorbers with the device architecture of Figure 4A. Cross-sectional SEM characterization of〈n〉 = 60 quasi-2D perovskite-based solar cell depicted the uniform stacking of different functional layers, as shown in Figure 4B. The thickness of 〈n〉 = 60 perovskite films was determined to be ∼500 nm, which is able to sufficiently absorb the incident light. A top-view SEM image of 〈n〉 = 60 perovskite films revealed a dense-grained uniform morphology with grain sizes in the range of 200–500 nm (Figure 4C). SEM images for other 〈n〉 value such as 10, 40, and larger are shown in Figure S3. The focused ion beam-high-resolution transmission electron microscopy measurements and single-area electron diffraction patterns of 〈n〉 = 60 perovskite films are also shown in Figures S4 and S5. Device performance measured after optimization is shown in Figure 4D. The best-performing 〈n〉 = 60 values deliver PCE of 12.4%, with an open-circuit voltage of 1.07 V, a short-circuit current density of 16.59 mA cm−2, and FF of 70%. The EQE spectra shown in Figure 4F exhibit a broad plateau over the range of 350–710 nm due to the strong photon harvesting ability of such thick films. Integration of the EQE curve under AM 1.5 illumination conditions produces a photocurrent of 16.6 mA cm−2, in good agreement with the value extracted from J-V characterization. Maximum power point tracking of the 〈n〉 = 60-based device for 100 s resulted in a stabilized efficiency of ∼12.2% (Figure S6). Intriguingly, 〈n〉 = 40 perovskite also exhibited much improved performance compared with standard α-CsPbI3 perovskite, yielding a champion PCE of 11.3%. We have also carried out voltage-transient measurements and we were able to extract the carrier lifetime and recombination rates as shown in Figure S7. The 〈n〉 = 60 quasi-2D perovskites exhibited nearly 6- to 10-fold lower recombination rates compared with control CsPbI3 perovskite, indicating much lower trap density exists in quasi-2D perovskites. The trend is in good agreement with the PL lifetime measurements. All these results suggest that the quasi-2D structure significantly suppresses the undesirable phase transition, leading to the reduced film trap density and much improved device performance. The J-V curves obtained from different scanning directions as well as scanning rates of 〈n〉 = 40 and 60 all show negligible hysteresis, as shown in Figures S8 and S9. Moreover, the performance of 〈n〉 = 60 quasi-2D CsPbX3 with other halogen rates were also examined (Figures S10 and S11). Due to suitable band gap and the capacity of sufficiently harvested photon energy, all-iodide quasi-2D CsPbI3 shows obvious superiority. In order to highlight the importance of the perovskite absorption thickness, the device performance with different absorption thicknesses controlled by the precursor concentration are shown in Figure S12. Finally, we examined the solar cell performance and its evolution in time (Figures 4G and S13). The high efficiency 〈n〉 = 60 device declined to 10.3% PCE after 40 days. Meanwhile, the 〈n〉 = 40 device exhibited promising stability, retaining 93% of the initial performance after 40 days storage. Moreover, even if the 〈n〉 = 80 and 100 devices possess comparable performance although slightly lower than〈n〉 = 40 and 60 devices, the stability has been significantly degraded probably due to the decreased thermally stability of perovskite caused by the reduced content of PEAI. Moderate 〈n〉 value perovskites (〈n〉 = 10) showed improved performance together with good long-term stability, however, the absolute device performance is rather low compared with that of the state-of-the-art perovskite device due to poor carrier transportation, hindering their further application. In an analogous study, α-CsPbI3 solar cells totally lost their PCE, degrading to below 1% within 3 hr. We also fabricated quasi-2D PEA2Csn-1Pbn(I2/3Br1/3)3n+1 perovskites with different 〈n〉 values and provided the J-V curves; the device performance is shown in Figure S14. The quasi-2D PEA2Csn-1Pbn(I2/3Br1/3)3n+1 perovskites with 〈n〉 = 40 and 〈n〉 = 60 do exhibit improved device performance compared the control CsPbI2Br perovskite. However, due to the inferior band gap and absorbing range compared with quasi-2D CsPbI3, the best-performing performance for 〈n〉 = 60 PEA2Csn-1Pbn(I2/3Br1/3)3n+1 perovskites was 11.9%. Based on these findings, 〈n〉 = 60 quasi-2D perovskite solar cell of PEA2Csn-1PbnBr3n+1 and PEA2Csn-1Pbn(I1/3Br2/3)3n+1 was fabricated, and improved device performance and stability were observed. The findings confirmed the quasi-2D strategy to be a versatile method to enhance device stability and performance of CsPbX3 perovskites. In summary, we report highly robust ligand-stabilized α-CsPbX3 perovskites fabricated through a facial single-step deposition technique for high-efficiency inorganic perovskite solar cell applications. By adopting a new precursor pair, CsAc and HPbX3, we were able to overcome the notorious solubility limitation for Cs precursors to fabricate high-quality CsPbX3 perovskite films with large film thickness. We further introduced a judicious amount of PEAI into the system to induce inorganic quasi-2D perovskites, PEA2Csn-1PbnI3n+1. Unprecedentedly, the quasi-2D perovskites greatly suppressed the undesirable phase transition, reduced the film's trap-state density, and resulting in a champion 12.4% PCE with greatly improved performance longevity. The approach is highly robust and compatible with a large-scale, low-temperature roll-to-roll process. The innovation described here paves the way for the manufacture of large-scale flexible inorganic perovskite solar cells.