An intermediate phase stability for high performance of perovskite solar cells

Nan et al. reported an interesting point of view on the phase stabilization mechanism of formamidinium lead halide (FAPbX3) perovskites in the September 2021 issue of Chem. A critical intermediate phase bearing the structure characteristics from the 2H phase to the 3C phase is successfully explained using a molecular-level description of the phase transition process and mechanism. In this study, we investigate how the intermediate stabilization with cationic or anionic additives affects the representative efficiency enhancement of perovskite solar cells. Nan et al. reported an interesting point of view on the phase stabilization mechanism of formamidinium lead halide (FAPbX3) perovskites in the September 2021 issue of Chem. A critical intermediate phase bearing the structure characteristics from the 2H phase to the 3C phase is successfully explained using a molecular-level description of the phase transition process and mechanism. In this study, we investigate how the intermediate stabilization with cationic or anionic additives affects the representative efficiency enhancement of perovskite solar cells. Main textFA+ cationic-based perovskites (FAPbX3) have emerged as one of the most promising candidate materials for high-efficiency and stable perovskite solar cells (PSCs). Owing to their remarkable optoelectronic properties with narrow bandgap and higher stability than methylammonium ion (MA+) based perovskites, all high-efficiency PSCs exceeding 23% efficiency have used FAPbX3 based compositions. However, a black perovskite phase of FAPbX3 (i.e., cubic α-phase) spontaneously transitions to a yellow deactivated non-perovskite phase (i.e., hexagonal δ-phase) at room temperature. Therefore, a good understanding of perovskite’s intermediate phase is crucial for producing high-performance PSCs.According to the report from Zi-Ang Nan et al., “Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase,”1Nan Z.-A. Chen L. Liu Q. Wang S.-H. Chen Z.-X. Kang S.-Y. Ji J.-B. Tan Y.-Y. Hui Y. Yan J.-W. et al.Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase.Chem. 2021; 7: 2513-2526Abstract Full Text Full Text PDF Scopus (17) Google Scholar several intermediate phase compositions exist, including where the Pb 2+ cation is shared with an A-site organic +1 charged cation and −1 charged halide anion from basic perovskite structure [PbX6] octahedrons depending on the stacking pattern. Using density functional theory calculations with single-crystal X-ray diffraction measurements for a detailed understanding of the crystal structure, they discovered that there are five distinct intermediate phases (3C, 8H, 6H, 4H, and 2H) for FAPbX3 single crystals. They demonstrated that corner-sharing sites with smaller halide and cubic cages, which are energetically preferred with smaller cations, are essential for the cubic perovskite phase’s stabilization. Therefore, high-quality FAPbI3 based perovskite films can be produced by optimizing factors such as perovskite compositions, precursor solution, stoichiometry, and various types of additives.Based on the above guidelines for the fabrication of high-quality perovskite film, we would like to explain how researchers were able to achieve world records for the perovskite composition, solution condition, and additives. Table 1 shows the world record efficiency of PSCs and their perovskite compositions. Numerous world records for PSCs efficiency have been established with FAPbI3-based perovskite compositions, ranging from 17.9% to 25.2%. Prof. Seok’s group published the first successful fabrication of a high-crystalline FAPbI3 based α-phase perovskite structure using a mixture of FAPbI3 (85%) and MAPbBr3 (15%) with γ-butyrolactone (GBL) and dimethylsulfoxide (DMSO) followed by toluene drop-casting method. The 15% addition of MAPbBr3 can produce a high purity of 3C α-phase crystal structure. This approach set a world record of 17.9% in 2017 and had a significant impact on the development of perovskite solar cells. A smaller amount of MAPbBr3 (5%) can also produce a pure α-phase crystal structure with a large amount of MACl (35%) mixing in the FAPbI3 solution. 22.7% efficiency with the world record of PSCs in 2019 was performed with the above mixing conditions. Reducing the bromide content from 15% to 5% reduces the potential for halide segregation and forms a stabilized intermediate phase. Interestingly, after adding a high amount of MACl (40%) into the FAPbI3 perovskite without MAPbBr3 addition, a strong α-phase perovskite structure was observed even without annealing process.2Kim M. Kim G.-H. Lee T.K. Choi I.W. Choi H.W. Jo Y. Yoon Y.J. Kim J.W. Lee J. Huh D. et al.Methylammonium Chloride Induces Intermediate Phase Stabilization for Efficient Perovskite Solar Cells.Joule. 2019; 3: 2179-2192Abstract Full Text Full Text PDF Scopus (851) Google Scholar Such a MACl addition exerts synergistic effects on 3C phase stabilization due to the partial substitution of FA+ with MA+ and the considerable effects of small residual Cl− anion. Only stabilized 3C-phase intermediate forms are created before the annealing process, and the annealing process can induce increased crystallinity of the pure 3C crystal phase.Table 1The world record of FAPbI3-based PSCs and their perovskite compositions and parametersWorld record efficiencyVoc (V)Jsc (mA/cm2)FF (%)Perovskite compositionsMethod17.9%4Jeon N.J. Noh J.H. Yang W.S. Kim Y.C. Ryu S. Seo J. Seok S.I. Compositional engineering of perovskite materials for high-performance solar cells.Nature. 2015; 517: 476-480Crossref PubMed Scopus (4801) Google Scholar1.11421.8073.6FAPbI3(85%) + MAPbBr3(15%)One step with antisolvent22.7%5Jung E.H. Jeon N.J. Park E.Y. Moon C.S. Shin T.J. Yang T.-Y. Noh J.H. Seo J. Efficient, stable and scalable perovskite solar cells using poly(3-hexylthiophene).Nature. 2019; 567: 511-515Crossref PubMed Scopus (1429) Google Scholar1.14424.9279.6FAPbI3(95%) + MAPbBr3(5%) + MACl(35%)One step with antisolvent23.3%6Jiang Q. Zhao Y. Zhang X. Yang X. Chen Y. Chu Z. Ye Q. Li X. Yin Z. You J. Surface passivation of perovskite film for efficient solar cells.Nat. Photonics. 2019; 13: 460-466Crossref Scopus (2460) Google Scholar1.16925.4079.8PbI2+FAI(90mg/mL) + MAI(6.39mg/ mL) + MACl(9mg/mL)Two step24.2%7Yoo J.J. Seo G. Chua M.R. Park T.G. Lu Y. Rotermund F. Kim Y.-K. Moon C.S. Jeon N.J. Correa-Baena J.-P. et al.Efficient perovskite solar cells via improved carrier management.Nature. 2021; 590: 587-593Crossref PubMed Scopus (962) Google Scholar1.19424.1684.0FAPbI3(99.2%) + MAPbBr3(0.8%) + PbI2(9%) + MACl(35%)One step with antisolvent25.2%7Yoo J.J. Seo G. Chua M.R. Park T.G. Lu Y. Rotermund F. Kim Y.-K. Moon C.S. Jeon N.J. Correa-Baena J.-P. et al.Efficient perovskite solar cells via improved carrier management.Nature. 2021; 590: 587-593Crossref PubMed Scopus (962) Google Scholar1.18025.1484.8FAPbI3(99.2%) + MAPbBr3(0.8%) + PbI2(9%) + MACl(35%)One step with antisolvent Open table in a new tab The current world record perovskite composition consists of FAPbI3 with a small amount of 0.8% MAPbBr3 and excess additions of 35% MACl and 9% PbI2. This method resulted in lower bromide contents of 5% to 0.8% and higher efficiency of 24.2% and 25.2%. The FF value, in particular, was considerably increased from above 80% to 84%. Such high values can be explained by the precursor solution, perovskite compositions, stoichiometry, and various type additives, as reported in Zi-Ang Nan et al. In the precursor solution, 8:1 of dimethylformamide (DMF): DMSO was used (typically, a 4:1 DMF: DMSO ratio has been used) which can reduce the transformation of the 3C phase to 2H phase because excess DMSO precursor solution can show 6H or 4H intermediated phase rather than pure 3C phase. Smaller additions of 0.8% Br content can also reduce halide segregation and provide a long-term stabilization in perovskite compositions. In the case of additive, a considerable amount of MACl additive was used, which also resulted in a good perovskite crystal phase, as previously mentioned. From the perspective of stoichiometry, 9% excess PbI2 is interesting. Zi-Ang Nan et al. also mentioned the effect of PbI2 excess, which can increase the transformation into the 3C-perovskite phase compared with FAI excess intermediate phase. Experiments have shown that a PbI2 excess can result in higher Voc and FF values, such as 1.18V and 84.8% of 25.2% efficiency.In order to achieve high crystallinity for high-performance perovskite devices, an intermediate phase engineering method is a crucial point. Currently, the efficiency of perovskite solar cells is reportedly very close to the commercialization standard of 25.5%.3www.nrel.gov/pv/cell-efficiency.htmlGoogle Scholar However, compared to commercialized solar cells such as silicon solar cells, perovskite solar cells still have stability issues. The most fundamental aspect of this stability issue is understanding how to develop a material that is structurally stable and has high crystallinity. A more advanced intermediate phase engineering approach, we believe, is a crucial insight that can fundamentally solve this challenge. We look forward to the day we can commercialize stable and high-efficiency perovskite solar cells using an intermediate phase engineering approach. Main textFA+ cationic-based perovskites (FAPbX3) have emerged as one of the most promising candidate materials for high-efficiency and stable perovskite solar cells (PSCs). Owing to their remarkable optoelectronic properties with narrow bandgap and higher stability than methylammonium ion (MA+) based perovskites, all high-efficiency PSCs exceeding 23% efficiency have used FAPbX3 based compositions. However, a black perovskite phase of FAPbX3 (i.e., cubic α-phase) spontaneously transitions to a yellow deactivated non-perovskite phase (i.e., hexagonal δ-phase) at room temperature. Therefore, a good understanding of perovskite’s intermediate phase is crucial for producing high-performance PSCs.According to the report from Zi-Ang Nan et al., “Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase,”1Nan Z.-A. Chen L. Liu Q. Wang S.-H. Chen Z.-X. Kang S.-Y. Ji J.-B. Tan Y.-Y. Hui Y. Yan J.-W. et al.Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase.Chem. 2021; 7: 2513-2526Abstract Full Text Full Text PDF Scopus (17) Google Scholar several intermediate phase compositions exist, including where the Pb 2+ cation is shared with an A-site organic +1 charged cation and −1 charged halide anion from basic perovskite structure [PbX6] octahedrons depending on the stacking pattern. Using density functional theory calculations with single-crystal X-ray diffraction measurements for a detailed understanding of the crystal structure, they discovered that there are five distinct intermediate phases (3C, 8H, 6H, 4H, and 2H) for FAPbX3 single crystals. They demonstrated that corner-sharing sites with smaller halide and cubic cages, which are energetically preferred with smaller cations, are essential for the cubic perovskite phase’s stabilization. Therefore, high-quality FAPbI3 based perovskite films can be produced by optimizing factors such as perovskite compositions, precursor solution, stoichiometry, and various types of additives.Based on the above guidelines for the fabrication of high-quality perovskite film, we would like to explain how researchers were able to achieve world records for the perovskite composition, solution condition, and additives. Table 1 shows the world record efficiency of PSCs and their perovskite compositions. Numerous world records for PSCs efficiency have been established with FAPbI3-based perovskite compositions, ranging from 17.9% to 25.2%. Prof. Seok’s group published the first successful fabrication of a high-crystalline FAPbI3 based α-phase perovskite structure using a mixture of FAPbI3 (85%) and MAPbBr3 (15%) with γ-butyrolactone (GBL) and dimethylsulfoxide (DMSO) followed by toluene drop-casting method. The 15% addition of MAPbBr3 can produce a high purity of 3C α-phase crystal structure. This approach set a world record of 17.9% in 2017 and had a significant impact on the development of perovskite solar cells. A smaller amount of MAPbBr3 (5%) can also produce a pure α-phase crystal structure with a large amount of MACl (35%) mixing in the FAPbI3 solution. 22.7% efficiency with the world record of PSCs in 2019 was performed with the above mixing conditions. Reducing the bromide content from 15% to 5% reduces the potential for halide segregation and forms a stabilized intermediate phase. Interestingly, after adding a high amount of MACl (40%) into the FAPbI3 perovskite without MAPbBr3 addition, a strong α-phase perovskite structure was observed even without annealing process.2Kim M. Kim G.-H. Lee T.K. Choi I.W. Choi H.W. Jo Y. Yoon Y.J. Kim J.W. Lee J. Huh D. et al.Methylammonium Chloride Induces Intermediate Phase Stabilization for Efficient Perovskite Solar Cells.Joule. 2019; 3: 2179-2192Abstract Full Text Full Text PDF Scopus (851) Google Scholar Such a MACl addition exerts synergistic effects on 3C phase stabilization due to the partial substitution of FA+ with MA+ and the considerable effects of small residual Cl− anion. Only stabilized 3C-phase intermediate forms are created before the annealing process, and the annealing process can induce increased crystallinity of the pure 3C crystal phase.Table 1The world record of FAPbI3-based PSCs and their perovskite compositions and parametersWorld record efficiencyVoc (V)Jsc (mA/cm2)FF (%)Perovskite compositionsMethod17.9%4Jeon N.J. Noh J.H. Yang W.S. Kim Y.C. Ryu S. Seo J. Seok S.I. Compositional engineering of perovskite materials for high-performance solar cells.Nature. 2015; 517: 476-480Crossref PubMed Scopus (4801) Google Scholar1.11421.8073.6FAPbI3(85%) + MAPbBr3(15%)One step with antisolvent22.7%5Jung E.H. Jeon N.J. Park E.Y. Moon C.S. Shin T.J. Yang T.-Y. Noh J.H. Seo J. Efficient, stable and scalable perovskite solar cells using poly(3-hexylthiophene).Nature. 2019; 567: 511-515Crossref PubMed Scopus (1429) Google Scholar1.14424.9279.6FAPbI3(95%) + MAPbBr3(5%) + MACl(35%)One step with antisolvent23.3%6Jiang Q. Zhao Y. Zhang X. Yang X. Chen Y. Chu Z. Ye Q. Li X. Yin Z. You J. Surface passivation of perovskite film for efficient solar cells.Nat. Photonics. 2019; 13: 460-466Crossref Scopus (2460) Google Scholar1.16925.4079.8PbI2+FAI(90mg/mL) + MAI(6.39mg/ mL) + MACl(9mg/mL)Two step24.2%7Yoo J.J. Seo G. Chua M.R. Park T.G. Lu Y. Rotermund F. Kim Y.-K. Moon C.S. Jeon N.J. Correa-Baena J.-P. et al.Efficient perovskite solar cells via improved carrier management.Nature. 2021; 590: 587-593Crossref PubMed Scopus (962) Google Scholar1.19424.1684.0FAPbI3(99.2%) + MAPbBr3(0.8%) + PbI2(9%) + MACl(35%)One step with antisolvent25.2%7Yoo J.J. Seo G. Chua M.R. Park T.G. Lu Y. Rotermund F. Kim Y.-K. Moon C.S. Jeon N.J. Correa-Baena J.-P. et al.Efficient perovskite solar cells via improved carrier management.Nature. 2021; 590: 587-593Crossref PubMed Scopus (962) Google Scholar1.18025.1484.8FAPbI3(99.2%) + MAPbBr3(0.8%) + PbI2(9%) + MACl(35%)One step with antisolvent Open table in a new tab The current world record perovskite composition consists of FAPbI3 with a small amount of 0.8% MAPbBr3 and excess additions of 35% MACl and 9% PbI2. This method resulted in lower bromide contents of 5% to 0.8% and higher efficiency of 24.2% and 25.2%. The FF value, in particular, was considerably increased from above 80% to 84%. Such high values can be explained by the precursor solution, perovskite compositions, stoichiometry, and various type additives, as reported in Zi-Ang Nan et al. In the precursor solution, 8:1 of dimethylformamide (DMF): DMSO was used (typically, a 4:1 DMF: DMSO ratio has been used) which can reduce the transformation of the 3C phase to 2H phase because excess DMSO precursor solution can show 6H or 4H intermediated phase rather than pure 3C phase. Smaller additions of 0.8% Br content can also reduce halide segregation and provide a long-term stabilization in perovskite compositions. In the case of additive, a considerable amount of MACl additive was used, which also resulted in a good perovskite crystal phase, as previously mentioned. From the perspective of stoichiometry, 9% excess PbI2 is interesting. Zi-Ang Nan et al. also mentioned the effect of PbI2 excess, which can increase the transformation into the 3C-perovskite phase compared with FAI excess intermediate phase. Experiments have shown that a PbI2 excess can result in higher Voc and FF values, such as 1.18V and 84.8% of 25.2% efficiency.In order to achieve high crystallinity for high-performance perovskite devices, an intermediate phase engineering method is a crucial point. Currently, the efficiency of perovskite solar cells is reportedly very close to the commercialization standard of 25.5%.3www.nrel.gov/pv/cell-efficiency.htmlGoogle Scholar However, compared to commercialized solar cells such as silicon solar cells, perovskite solar cells still have stability issues. The most fundamental aspect of this stability issue is understanding how to develop a material that is structurally stable and has high crystallinity. A more advanced intermediate phase engineering approach, we believe, is a crucial insight that can fundamentally solve this challenge. We look forward to the day we can commercialize stable and high-efficiency perovskite solar cells using an intermediate phase engineering approach. FA+ cationic-based perovskites (FAPbX3) have emerged as one of the most promising candidate materials for high-efficiency and stable perovskite solar cells (PSCs). Owing to their remarkable optoelectronic properties with narrow bandgap and higher stability than methylammonium ion (MA+) based perovskites, all high-efficiency PSCs exceeding 23% efficiency have used FAPbX3 based compositions. However, a black perovskite phase of FAPbX3 (i.e., cubic α-phase) spontaneously transitions to a yellow deactivated non-perovskite phase (i.e., hexagonal δ-phase) at room temperature. Therefore, a good understanding of perovskite’s intermediate phase is crucial for producing high-performance PSCs. According to the report from Zi-Ang Nan et al., “Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase,”1Nan Z.-A. Chen L. Liu Q. Wang S.-H. Chen Z.-X. Kang S.-Y. Ji J.-B. Tan Y.-Y. Hui Y. Yan J.-W. et al.Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase.Chem. 2021; 7: 2513-2526Abstract Full Text Full Text PDF Scopus (17) Google Scholar several intermediate phase compositions exist, including where the Pb 2+ cation is shared with an A-site organic +1 charged cation and −1 charged halide anion from basic perovskite structure [PbX6] octahedrons depending on the stacking pattern. Using density functional theory calculations with single-crystal X-ray diffraction measurements for a detailed understanding of the crystal structure, they discovered that there are five distinct intermediate phases (3C, 8H, 6H, 4H, and 2H) for FAPbX3 single crystals. They demonstrated that corner-sharing sites with smaller halide and cubic cages, which are energetically preferred with smaller cations, are essential for the cubic perovskite phase’s stabilization. Therefore, high-quality FAPbI3 based perovskite films can be produced by optimizing factors such as perovskite compositions, precursor solution, stoichiometry, and various types of additives. Based on the above guidelines for the fabrication of high-quality perovskite film, we would like to explain how researchers were able to achieve world records for the perovskite composition, solution condition, and additives. Table 1 shows the world record efficiency of PSCs and their perovskite compositions. Numerous world records for PSCs efficiency have been established with FAPbI3-based perovskite compositions, ranging from 17.9% to 25.2%. Prof. Seok’s group published the first successful fabrication of a high-crystalline FAPbI3 based α-phase perovskite structure using a mixture of FAPbI3 (85%) and MAPbBr3 (15%) with γ-butyrolactone (GBL) and dimethylsulfoxide (DMSO) followed by toluene drop-casting method. The 15% addition of MAPbBr3 can produce a high purity of 3C α-phase crystal structure. This approach set a world record of 17.9% in 2017 and had a significant impact on the development of perovskite solar cells. A smaller amount of MAPbBr3 (5%) can also produce a pure α-phase crystal structure with a large amount of MACl (35%) mixing in the FAPbI3 solution. 22.7% efficiency with the world record of PSCs in 2019 was performed with the above mixing conditions. Reducing the bromide content from 15% to 5% reduces the potential for halide segregation and forms a stabilized intermediate phase. Interestingly, after adding a high amount of MACl (40%) into the FAPbI3 perovskite without MAPbBr3 addition, a strong α-phase perovskite structure was observed even without annealing process.2Kim M. Kim G.-H. Lee T.K. Choi I.W. Choi H.W. Jo Y. Yoon Y.J. Kim J.W. Lee J. Huh D. et al.Methylammonium Chloride Induces Intermediate Phase Stabilization for Efficient Perovskite Solar Cells.Joule. 2019; 3: 2179-2192Abstract Full Text Full Text PDF Scopus (851) Google Scholar Such a MACl addition exerts synergistic effects on 3C phase stabilization due to the partial substitution of FA+ with MA+ and the considerable effects of small residual Cl− anion. Only stabilized 3C-phase intermediate forms are created before the annealing process, and the annealing process can induce increased crystallinity of the pure 3C crystal phase. The current world record perovskite composition consists of FAPbI3 with a small amount of 0.8% MAPbBr3 and excess additions of 35% MACl and 9% PbI2. This method resulted in lower bromide contents of 5% to 0.8% and higher efficiency of 24.2% and 25.2%. The FF value, in particular, was considerably increased from above 80% to 84%. Such high values can be explained by the precursor solution, perovskite compositions, stoichiometry, and various type additives, as reported in Zi-Ang Nan et al. In the precursor solution, 8:1 of dimethylformamide (DMF): DMSO was used (typically, a 4:1 DMF: DMSO ratio has been used) which can reduce the transformation of the 3C phase to 2H phase because excess DMSO precursor solution can show 6H or 4H intermediated phase rather than pure 3C phase. Smaller additions of 0.8% Br content can also reduce halide segregation and provide a long-term stabilization in perovskite compositions. In the case of additive, a considerable amount of MACl additive was used, which also resulted in a good perovskite crystal phase, as previously mentioned. From the perspective of stoichiometry, 9% excess PbI2 is interesting. Zi-Ang Nan et al. also mentioned the effect of PbI2 excess, which can increase the transformation into the 3C-perovskite phase compared with FAI excess intermediate phase. Experiments have shown that a PbI2 excess can result in higher Voc and FF values, such as 1.18V and 84.8% of 25.2% efficiency. In order to achieve high crystallinity for high-performance perovskite devices, an intermediate phase engineering method is a crucial point. Currently, the efficiency of perovskite solar cells is reportedly very close to the commercialization standard of 25.5%.3www.nrel.gov/pv/cell-efficiency.htmlGoogle Scholar However, compared to commercialized solar cells such as silicon solar cells, perovskite solar cells still have stability issues. The most fundamental aspect of this stability issue is understanding how to develop a material that is structurally stable and has high crystallinity. A more advanced intermediate phase engineering approach, we believe, is a crucial insight that can fundamentally solve this challenge. We look forward to the day we can commercialize stable and high-efficiency perovskite solar cells using an intermediate phase engineering approach. Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phaseNan et al.ChemAugust 13, 2021In BriefMolecular-level understanding of the phase stabilization mechanism of formamidinium lead halide (FAPbX3) perovskites is critical to realizing highly efficient and stable perovskite solar cells. In this work, a key phase transition intermediate single crystal (8H phase) with unique structure was isolated and identified, which provides the complete phase evolution map of FAPbX3 from the viewpoint of structural chemistry and reveals the intrinsic doping effect of cations and anions on the phase stabilization of FAPbX3 based on Pauling’s rule. 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