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

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.