Flexible molecules dedicate to release strain of inverted inorganic perovskite solar cell

Abstract

The tensile strain in inorganic perovskite films induced by thermal annealing is one of the primary factors contributing to the inefficiency and instability of inorganic perovskite solar cells (IPSCs), which reduces the defect formation energy. Here, a flexible molecule 5-maleimidovaleric acid (5-MVA) was introduced as a strain buffer to release the residual strain of CsPbI_2.85Br_0.15 perovskite. Maleic anhydride and carboxyl groups in 5-MVA interact strongly with the uncoordinated Pb²⁺ through Lewis acid-base reaction, thus tightly “pull” the perovskite lattice. The in-between soft carbon chain increased the structural flexibility of CsPbI_2.85Br_0.15 perovskite materials, which effectively relieved the intrinsic internal strain of CsPbI_2.85Br_0.15, resisted the corrosion of external strain, and also reduced the formation of defects such as V_I and Pb⁰. In addition, the introduction of 5-MVA improved crystal quality, passivated residual defects, and narrowed energy level barriers. Eventually, power conversion efficiency (PCE) of NiO_x-based inverted IPSCs increased from 19.25% to 20.82% with the open-circuit voltage enhanced from 1.164 V to 1.230 V. The release of strain also improved the stability of CsPbI_2.85Br_0.15 perovskite films and devices.