Compositional Design of Spontaneous Heterointerface Modulators for Perovskite Solar Cells to Mitigate Concentration Sensitivity

Alkyl-primary-ammonium bis(trifluoromethanesulfonyl)imides (RA-TFSIs), whose archetype is n-octylammonium TFSI (OA-TFSI), have recently emerged as spontaneous heterointerface modulators (SHMs) for perovskite solar cells (PSCs). Hole-transport material (HTM) solutions containing RA-TFSIs enable spontaneous perovskite passivation via HTM deposition and enhance the photovoltaic performance. Moreover, this perovskite passivation improves the fabrication efficiency of PSCs by obviating conventional postpassivation processes. A key aspect of developing PSC fabrication processes using SHMs is the concentration sensitivity of RA-TFSIs; excessive OA-TFSI in the HTM solution leads to some OA cations failing to attach to the perovskite surface, hindering carrier collection. Herein, we address this issue by employing a newly synthesized ethylammonium TFSI (EA-TFSI). EA-TFSI enhances the photovoltaic properties while mitigating the concentration sensitivity, owing to its small cation size, reducing the risk of poor carrier collection. For OA-TFSI, increasing its concentration to twice the optimal amount decreases the power conversion efficiency (PCE) by 14%, accompanied by drops in the fill factor (FF). However, upon EA-TFSI addition, PCE decreases by only 4%, with the FF values remaining unchanged. This study offers insights into designing SHMs for PSCs to achieve broad process windows, which is a crucial, yet rarely discussed aspect.