Compositional\nDesign of Spontaneous Heterointerface\nModulators for Perovskite Solar Cells to Mitigate Concentration Sensitivity

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