Pseudohalide anion engineering for highly efficient and stable perovskite solar cells

In recent works in Science, Nature and Nature Energy, pseudohalide anions, such as SCN−, CH3COO−, and HCOO− have demonstrated the unique advantages in aspects of tuning the growth, properties, and stability of perovskite films in connection with solar cells. The FAPbI3 perovskite solar cell devices with HCOO− engineering achieved a power conversion efficiency up to 25.6% (certified 25.2%) with long-term operational stability. In recent works in Science, Nature and Nature Energy, pseudohalide anions, such as SCN−, CH3COO−, and HCOO− have demonstrated the unique advantages in aspects of tuning the growth, properties, and stability of perovskite films in connection with solar cells. The FAPbI3 perovskite solar cell devices with HCOO− engineering achieved a power conversion efficiency up to 25.6% (certified 25.2%) with long-term operational stability. Perovskite solar cells (PSCs) have become the hottest topic in photovoltaic field and attracted tremendous attention from both academia and industry.1Zhang X. Strain control for halide perovskites.Matter. 2020; 2: 294-296Google Scholar In 2009, metal halide perovskites were first substituted as sensitizers for dyes in liquid electrolyte-based dye-sensitized solar cells, and the devices achieved a power conversion efficiency (PCE) of 3.8%.2Kojima A. Teshima K. Shirai Y. Miyasaka T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells.J. Am. Chem. Soc. 2009; 131: 6050-6051Google Scholar In 2012, an all-solid-state PSC showed a PCE of 9.7%, which aroused “perovskite fervour.” To date, an unprecedented certified PCE of 25.5% (https://www.nrel.gov/pv/cell-effi-ciency.html) has been achieved for PSCs, right up there with crystalline silicon solar cells. The rocket-up performance is mainly attributed to the lead halide perovskite materials with excellent optoelectronic properties, including a suitable direct bandgap, high absorbance coefficient, long carrier mobility, high defect tolerance and intrinsic low trap density. In general, lead halide perovskites have a formula of APbX3, where A site is monovalent cation (such as Cs+, CH3NH3+ and (CH(NH2)2+) and X site is halide (such as Cl−, Br− and I−). The APbX3 perovskite materials are usually prepared by the reaction of a PbX2 with an AX salt, where the A-site cations occupy the void spaces between the connected [PbX6]4- octahedrons.3Xie Y. Yin J. Zheng J. Fan Y. Wu J. Zhang X. Facile RbBr interface modification improves perovskite solar cell efficiency.Mater. Today Chem. 2019; 14: 100179Google Scholar As for the halides in perovskite, the chemical activity of I− ions is relatively high, and those vacancy defects form easily. In addition, I− vacancies are the dominant narrative of the unwanted ionic conductivity in perovskite films, which has a negative effect on their device operational stability. Significantly, I− ions can be partially replaced by Br− ions, which can mainly tune the bandgap and enhance the stability of the resulting material. While Cl− ions are too small to directly substitute I− in the APbX3 lattices, but they can tune the film growth process and present in the grain boundaries to improve the crystallinity and stability. Therefore, the halide anion design can affect the photovoltaic performance and stability of fabricated devices. Pseudohalide anions, such as BF4−, PF6−, SCN−, CH3COO− (Ac−) and HCOO−, have the similar chemical behaviors and properties to the true halide anions in connection with PSCs.4Chen J. Kim S.G. Park N.G. FA0.88Cs0.12PbI3-x(PF6)x interlayer formed by ion exchange reaction between perovskite and hole transporting layer for improving photovoltaic performance and stability.Adv. Mater. 2018; 30: 1801948Google Scholar, 5Zhang J. Wu S. Liu T. Zhu Z. Jen A.K.Y. Boosting photovoltaic performance for lead halide perovskites solar cells with BF4− anion substitutions.Adv. Funct. Mater. 2019; 29: 1808833Google Scholar, 6Tai Q. You P. Sang H. Liu Z. Hu C. Chan H.L. Yan F. Efficient and stable perovskite solar cells prepared in ambient air irrespective of the humidity.Nat. Commun. 2016; 7: 11105Google Scholar, 7Kim D. Jung H.J. Park I.J. Larson B.W. Dunfield S.P. Xiao C. Kim J. Tong J. Boonmongkolras P. Ji S.G. et al.Efficient, stable silicon tandem cells enabled by anion-engineered wide-bandgap perovskites.Science. 2020; 368: 155-160Google Scholar, 8Zhang W. Saliba M. Moore D.T. Pathak S.K. Hörantner M.T. Stergiopoulos T. Stranks S.D. Eperon G.E. Alexander-Webber J.A. Abate A. et al.Ultrasmooth organic-inorganic perovskite thin-film formation and crystallization for efficient planar heterojunction solar cells.Nat. Commun. 2015; 6: 6142Google Scholar, 9Liang C. Gu H. Xia Y. Wang Z. Liu X. Xia J. Zuo S. Hu Y. Gao X. Hui W. et al.Two-dimensional Ruddlesden–Popper layered perovskite solar cells based on phase-pure thin films.Nat. Energy. 2021; 6: 38-45Google Scholar, 10Jeong J. Kim M. Seo J. Lu H. Ahlawat P. Mishra A. Yang Y. Hope M.A. Eickemeyer F.T. Kim M. et al.Pseudo-halide anion engineering for α-FAPbI3 perovskite solar cells.Nature. 2021; 592: 381-385Google Scholar Among them, BF4−, PF6− and SCN− anions with the similar ionic radius can (partially) substitute I− because they meet the Goldschmidt’s tolerance criteria and obtain the ability to control the characteristics of the resulting perovskite materials, which include stability, bandgap, morphology, and charge transport properties. For instance, on the basis of FA0.88Cs0.12PbI3 perovskite film, the iodides were partly substituted with PF6− to form FA0.88Cs0.12PbI3−x(PF6)x interlayer, which could suppress the trap density and improved PCE of PSCs.4Chen J. Kim S.G. Park N.G. FA0.88Cs0.12PbI3-x(PF6)x interlayer formed by ion exchange reaction between perovskite and hole transporting layer for improving photovoltaic performance and stability.Adv. Mater. 2018; 30: 1801948Google Scholar Similarly, BF4− could be partially substituted the halides to form FA0.83MA0.17Pb(IBr)3−x(BF4)x perovskite, which resulted in slight lattice expansion and relaxation. The BF4− not only suppressed nonradiative recombination, but also reduced the charge transport loss, resulting in significantly improved PCE of PSCs.5Zhang J. Wu S. Liu T. Zhu Z. Jen A.K.Y. Boosting photovoltaic performance for lead halide perovskites solar cells with BF4− anion substitutions.Adv. Funct. Mater. 2019; 29: 1808833Google Scholar In 2016, Pb(SCN)2 precursor was used to prepare high-quality CH3NH3PbI3−x(SCN)x films in ambient air irrespective of the humidity, which were applied for efficient and stable PSCs.6Tai Q. You P. Sang H. Liu Z. Hu C. Chan H.L. Yan F. Efficient and stable perovskite solar cells prepared in ambient air irrespective of the humidity.Nat. Commun. 2016; 7: 11105Google Scholar In 2020, it was found that SCN− anion engineering of two-dimensional additives was critical for controlling the structural and electrical properties of the 2D passivation layers based on a lead iodide framework.7Kim D. Jung H.J. Park I.J. Larson B.W. Dunfield S.P. Xiao C. Kim J. Tong J. Boonmongkolras P. Ji S.G. et al.Efficient, stable silicon tandem cells enabled by anion-engineered wide-bandgap perovskites.Science. 2020; 368: 155-160Google Scholar The 2D PEA(I0.25SCN0.75) layered structure is located at the grain boundary of the 3D perovskite host as a passivation agent as shown in the bright-field and high-resolution TEM images (Figures 1A and 1B ). The J-V curve of the champion cell with the PEA(I0.25SCN0.75) additive led to a stable PSC of 20.7% with a wide bandgap of ∼1.7 eV (Figure 1C). And the device with the mixed anion additive retained more than 80% stability after 1000 h of continuous illumination (Figure 1D). Moreover, a monolithic two-terminal wide-bandgap perovskite/silicon tandem solar cell showed a high PCE of 26.7%. As for Ac− and HCOO−, they are difficult to substitute the halides in perovskite, but they can improve the quality of perovskite films by controlling the nucleation and growth of the perovskite crystals.8Zhang W. Saliba M. Moore D.T. Pathak S.K. Hörantner M.T. Stergiopoulos T. Stranks S.D. Eperon G.E. Alexander-Webber J.A. Abate A. et al.Ultrasmooth organic-inorganic perovskite thin-film formation and crystallization for efficient planar heterojunction solar cells.Nat. Commun. 2015; 6: 6142Google Scholar, 9Liang C. Gu H. Xia Y. Wang Z. Liu X. Xia J. Zuo S. Hu Y. Gao X. Hui W. et al.Two-dimensional Ruddlesden–Popper layered perovskite solar cells based on phase-pure thin films.Nat. Energy. 2021; 6: 38-45Google Scholar, 10Jeong J. Kim M. Seo J. Lu H. Ahlawat P. Mishra A. Yang Y. Hope M.A. Eickemeyer F.T. Kim M. et al.Pseudo-halide anion engineering for α-FAPbI3 perovskite solar cells.Nature. 2021; 592: 381-385Google Scholar For instance, a compact perovskite film with full coverage on the substrate was obtained by one-step solution coating using Pb(Ac)2 as the lead source without anti-solution.8Zhang W. Saliba M. Moore D.T. Pathak S.K. Hörantner M.T. Stergiopoulos T. Stranks S.D. Eperon G.E. Alexander-Webber J.A. Abate A. et al.Ultrasmooth organic-inorganic perovskite thin-film formation and crystallization for efficient planar heterojunction solar cells.Nat. Commun. 2015; 6: 6142Google Scholar In 2021, Huang group reported phase-pure 2D perovskite quantum wells with a single well width by introducing molten salt spacer n-butylamine acetate (BAAc), instead of the traditional halide spacer n-butylamine iodide (BAI).9Liang C. Gu H. Xia Y. Wang Z. Liu X. Xia J. Zuo S. Hu Y. Gao X. Hui W. et al.Two-dimensional Ruddlesden–Popper layered perovskite solar cells based on phase-pure thin films.Nat. Energy. 2021; 6: 38-45Google Scholar Due to the strong ionic coordination between n-butylamine acetate and the perovskite framework, a uniformly distributed intermediate phase can be formed from the precursor solution with near-monodisperse unit cell particles during the initial stage of solution processing. The intermediate phases subsequently crystallized to uniform phase-pure quantum well films with microscale vertically aligned grains (Figures 2A and 2B ). The high phase purity and vertically aligned structure could potentially facilitate charge transport and stabilize the perovskite framework. The 2D perovskite solar cells achieved an optimized PCE of 16.25% with open voltage of 1.31 V. After keeping them in (65 ± 10%) relative humidity for 4,680 h, under operational test at 85°C for 558 h, or continuous illumination for 1,100 h, the cells showed extraordinary stability with < 10% efficiency degradation. In 2021, HCOO− anions were used to suppress the predominant lattice defects of the anion vacancies in FAPbI3 perovskite films that are present at the surface or the grain boundaries, and to augment the crystallinity of the films.10Jeong J. Kim M. Seo J. Lu H. Ahlawat P. Mishra A. Yang Y. Hope M.A. Eickemeyer F.T. Kim M. et al.Pseudo-halide anion engineering for α-FAPbI3 perovskite solar cells.Nature. 2021; 592: 381-385Google Scholar By introducing 2% FAHCOO into the FAPbI3 precursor solution (2% Fo-FAPbI3), the crystallinity of the FAPbI3 films was obviously improved because of the strong coordination of HCOO− anions with Pb2+ cations. Compared to the reference film, the 2% Fo-FAPbI3 film had a slightly larger grain size of up to 2 μm. A calculated structure can illustrate an HCOO− anion passivating an I− vacancy at the FAPbI3 surface (Figure 2C). Importantly, the HCOO− anions can passivate I− vacancies at the FAPbI3 surface and the HCOO− anions have higher binding energy to I− vacant sites than those of the Cl−, Br−, I− and BF4− (Figure 2D). Furthermore, FA+ cations at the interface form stronger bonds with HCOO− than with the other anions. A configuration of the fabricated FAPbI3 PSCs is illustrated in Figure 2E. The improved crystallinity and the defect passivation of the FAPbI3 films conducive to the efficiency and stability of the PSCs. At last, the FAPbI3 solar cell devices achieved a power conversion efficiency up to 25.6% (certified 25.2%) with long-term operational stability (Figure 1 h). In summary, the pseudohalide anions of BF4−, PF6−, SCN−, Ac−, and HCOO− have the unique advantages to tune the quality, stability, and properties of perovskite films in connection with PSCs. As for BF4−, PF6−, and SCN− with the similar ionic radius to that of I−, they can (partially) substitute I− to control the stability, bandgap, morphology, and charge transport properties of the perovskite films to improve the efficiency and stability of mixed-ion PSCs. While the Ac− and HCOO− anions are too active to directly substitute the true halide anions in perovskite lattices, they can control the nucleation and growth of the perovskite crystals and present in the grain boundaries and surfaces as passivating agents to improve the crystallinity and stability for enhanced PSCs. Therefore, the pseudohalide anion engineering could pave a universal way to achieve highly efficient and stable PSCs. This work was supported by the National Natural Science Foundation of China ( 11804166 ), and the China Postdoctoral Science Foundation ( 2018M630587 ).