Long Aliphatic Chain-Based Spontaneous Perovskite Passivator Combined with Spiro-OMeTAD Hole Transport Material in Perovskite Solar Cells

Perovskite passivation plays a crucial role in achieving efficient perovskite solar cells (PSCs). A drawback of conventional perovskite passivation is the requirement of an additional process, whereas spontaneous perovskite passivation using alkyl-primary-ammonium- bis(trifluoromethylsulfonyl)imides (RA-TFSIs) as additives for hole-transport materials (HTMs), which recently emerged, skips the additional process, increasing the efficiency of PSC fabrication. During the deposition of the RA-TFSI additive-containing HTM solution, the RA cations spontaneously passivate the perovskite, exploiting the high adsorption energy of the RA cations over the perovskite surface. Moreover, RA-TFSI replaces the commonly used Li-TFSI, circumventing the use of Li species, which are detrimental to PSCs. 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-TFSI—has been scarcely investigated. In this study, a newly synthesized dodecylammonium-TFSI (DDA-TFSI) spontaneous perovskite passivator was validated. The DDA-TFSI suppressed the defects on the perovskite surfaces via spontaneous perovskite passivation. Consequently, the DDA-TFSI HTM additive enhanced the photovoltaic (PV) performance by increasing the open-circuit voltage, 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 spio-OMeTAD HTM layers, presumably owing to excess reduction of the dipole of the perovskite surface. This trend differs from the use of RA-TFSIs reported thus far for spiro-OMeTAD HTMs and may limit the enhancement of PV performance. Although discussions on such negative features regarding non-uniform spiro-OMeTAD layers in this work are prone to be avoided, this knowledge will eventually help enhance PV performance. Therefore, the findings of this study, including the expanding variety of RA-TFSI materials, provide valuable guidance for the advancement of PSCs.