Effect of molecular configuration of additives on perovskite crystallization and hot carriers behavior in perovskite solar cells

The development of additive materials is of great significance for achieving high performance perovskite solar cells (PSCs). Current studies on additive agents mainly focus on the development of new structures, while the effect of molecular configuration on perovskite crystallization and hot carriers behavior in device is rare to investigate, which will need to take into consideration when we start designing new additives for PSCs application. In this work, two similar functional molecules, tris[4-(2-thienyl)phenyl]amine (TTPA) and tri(2-thienyl)benzene (TTPB), are applied as additives in perovskite crystallization process. Due to a large steric hindrance, the molecular configuration of TTPA shows very distorted molecular configuration, while TTPB is much more planar. We found that TTPB can effectively reorient perovskite crystallization with the desired facet orientation in comparison to TTPA, which is propitious to reduce the trap density of perovskite films and increase perovskite surface potential. As a result, power conversion efficiency (PCE) is significantly improved from 21.08% for TTPA to 23.67% for TTPB. More importantly, detailed transient absorption characterization reveals that additive TTPB with relative to TTPA is favorable for slowing down hot-carrier cooling process, and thus accelerating the extraction of hot carriers by carrier extraction layers as well as suppressing charge-carrier recombination in device. The findings of the present work can provide some new and important insights for designing high-performance additives for photovoltaic devices and in-depth understanding the hot-carrier dynamics in device.