Charting the Irreversible Degradation Modes of Low Bandgap Pb‐Sn Perovskite Compositions for De‐Risking Practical Industrial Development

Abstract

The commercialization of a solar technology necessitates the fulfillment of specific requirements both regarding efficiency and stability to enter and gain space in the photovoltaic market. These aims are heavily dependent on the selection of suitable materials, which is critical for suppressing any reliability risks arising from inherent instabilities. Focusing on the absorber material, herein the most suitable low bandgap lead‐tin composition candidate for all‐perovskite tandem applications is investigated by studying their degradation mechanisms with both widely available and advanced characterization techniques. Three irreversible degradation processes are identified in narrow bandgap Pb‐Sn perovskite absorbers: 1) Tin (Sn) oxidation upon air exposure, 2) methylammonium (MA) loss upon heat exposure, and 3) formamidinium (FA) and cesium (Cs) segregation leading to impurity phase formation. From an industrial perspective, it is proposed to refocus attention on FASn0.5Pb0.5I3 which minimizes all three effects while maintaining a suitable bandgap for a bottom cell and good performance. Moreover, a practical and highly sensitive characterization method is proposed to monitor the oxidation, which can be deployed both in laboratory and industrial environments and provide useful information for the technological development process, including, the effectiveness of encapsulation methods, and the acceptable time windows for air exposure.

Natural Antioxidant Vitamin C Improves Photovoltaic Performance of Tin–Lead Mixed Perovskite Solar Cells

“…Since a perovskite solar cell (PSC) was first reported to achieve power conversion efficiency (PCE) of 3.8% in 2009, the highest certified efficiency of lead (Pb)-based PSCs has exceeded 26% after 15 years of rapid development, which is comparable to commercial monocrystalline silicon solar cells. − However, the energy band gap ( E g ) of this high-efficiency Pb-based perovskite semiconductor absorbent layer is restricted to 1.4–1.6 eV. − According to the Shockley–Queisser (S–Q) theory of single-junction solar cells, the theoretical maximum PCE could be achieved with a E g value close to 1.2 eV. , Introducing 50% tin (Sn) to replace Pb could tune the E g to 1.2 eV, making Sn–Pb mixed perovskite a promising candidate for fabricating highly efficient PSCs. − However, the highest PCE ever reported for Sn–Pb mixed PSCs is only 23.7%, which is not only far from the theoretical limit, but also significantly lower than that of pure Pb-based counterpart. − The easy oxidation of Sn 2+ at B site to Sn 4+ in perovskite crystal even under a low-oxygen condition, which causes severe p-type self-doping and formation of Sn vacancy defects, is recognized as one of main reasons for the inferior PCE. , Incorporating antioxidant additives (such as Sn powder, SnF 2 , or hydroxyl benzenesulfonic acid) in perovskite precursor solution is reported to be an effective way to suppress the oxidation of Sn 2+ in Sn–Pb mixed PSCs and to fabricate high-quality perovskite film with lower defects. ,− Although additives can well restrict Sn 4+ in the precursor preparation and film crystallization, the oxidation of Sn 2+ still exists after crystallization. , Therefore, it is highly necessary to introduce additional antioxidant additives through post-treatment methods to restrict the oxidation of Sn 2+ and improve the photovoltaic performance of Sn–Pb mixed PSCs.…”