Long aliphatic chain-based spontaneous perovskite passivator: trade-off between the passivation effect and hole-transport material compatibility

Abstract

Perovskite passivation plays a crucial role in achieving efficient perovskite solar cells (PSCs). A drawback of conventional perovskite passivation is the requirement of additional processes. However, spontaneous perovskite passivation using emerging alkyl-primary-ammonium bis(trifluoromethylsulfonyl)imides (RA-TFSIs) skips the additional processes, increasing the efficiency of PSC fabrication. During the deposition of the RA-TFSI additive-containing hole-transport material (HTM) solution, the RA cations spontaneously passivate the perovskite, exploiting the high adsorption energy of the RA cations on the perovskite surface. Moreover, RA-TFSIs replace the commonly used Li-TFSI, circumventing the use of detrimental Li species. However, RA-TFSIs are nascent; hence, further exploration of their composition and functions is imperative. In particular, the compatibility of HTMs with perovskite passivators comprising long aliphatic chains—not limited to RA-TFSIs—has been scarcely investigated. In this study, a newly synthesized dodecylammonium-TFSI (DDA-TFSI) spontaneous perovskite passivator was validated. The DDA-TFSI HTM additive enhanced the photovoltaic (PV) performance by its spontaneous perovskite passivation effects, yielding a power-conversion efficiency of 21.9% with an open-circuit voltage of 1.14 V, which are relatively high for Li-free FAPbI₃-based PSCs without post-passivation treatment. Nevertheless, the DDA-TFSI HTM additive—even at the optimal amount—degraded the uniformity of spiro-OMeTAD HTM layers, presumably owing to excess reduction of the dipole of the perovskite surface. This work highlights the importance of compatibility between HTMs and perovskite passivator materials. Moreover, although discussions on such negative features regarding non-uniform HTM layers are prone to be avoided, this knowledge fills the gap in the PSC research field and will eventually advance PSCs further.