Revealing the Role of Bifunctional Molecules in Crystallizing Methylammonium Lead Iodide through Geometric Isomers

Abstract

Multifunctional additives are widely utilized in enhancing the performance of perovskite solar cells, and the role of the additives is highly related to the contained various functional groups. Here, we introduce a pair of geometric isomers, which act as bifunctional additives with a terminal −NH3+ group and a terminal −COOH group, into methylammonium lead iodide perovskite. Surprisingly, although the two additives share exactly the same functional groups, we find that they play opposite roles in the perovskite crystallization process. The systematical investigation indicates that only the trans structure can cross-link the perovskite crystals through specific interactions. Thus, the trans isomer facilitates the improved perovskite crystallization and reduced defects, resulting in the efficiency increasing to 13.30 from 6.52% compared with the cis isomer. This work helps to better understand the structure–property relationship and provides new inspiration for designing novel multifunctional additives.