Compositional Design of Spontaneous Heterointerface Modulators for Perovskite Solar Cells Allowing a Broad Process Window

Spontaneous heterointerface modulation techniques have significantly contributed to the rapid development of perovskite solar cells (PSCs), and alkyl-primary-ammonium bis(trifluoromethanesulfonyl)imides (RA–TFSIs), whose archetype is n-octylammonium bis(trifluoromethanesulfonyl)imides (OA–TFSI), have recently emerged as functional additives for hole transport materials (HTMs). RA–TFSIs are designed to allow spontaneous perovskite passivation via the HTM deposition process; leveraging the high reactivity of RA cations toward the perovskite surface, these additives spontaneously and effectively suppress the defects over the perovskite surface and thereby enhance photovoltaic (PV) performance. Moreover, this perovskite passivation negates the need for conventional post-passivation processes, thereby improving the fabrication efficiency of PSCs. Although the advantages of these PSC fabrication processes have been less discussed than methods aimed at enhancing PV performance, they are crucial for further advancement of PSCs, especially in the context of spontaneous heterointerface modulation techniques. A key aspect is the concentration sensitivity of RA–TFSI; excessive OA–TFSI in the HTM solution leads to some OA cations failing to attach to the perovskite surface during spontaneous passivation, remaining in the HTM core and hindering carrier collection. To address this issue, we herein developed RA–TFSIs and synthesized ethylammonium bis(trifluoromethanesulfonyl)imide (EA–TFSI) for the first time. EA–TFSI not only enhanced the PV properties of PSCs but also significantly mitigated the concentration sensitivity owing to its small cation size, reducing the risk of poor carrier collection. In the case of OA–TFSI, increasing its concentration to twice the optimal amount decreased power conversion efficiency (PCE) by 14%, accompanied by drops in fill factor (FF). However, upon EA–TFSI addition, PCE decreased by only 4%, with FF values remaining unchanged (i.e., nearly 100% retention). This research offers insights into designing nascent yet potent spontaneous heterointerface modulators for PSCs, including RA–TFSI, to facilitate a broad process window, which is critical yet rarely discussed aspect. Therefore, this study will contribute to the further development of PSCs.