Thermally Stable Phenylethylammonium-based Perovskite Passivation: Spontaneous Passivation with Phenylethylammonium Bis(trifluoromethylsulfonyl)imide under Deposition of PTAA for Enhancing Photovoltaic Performances of Perovskite Solar Cells

Perovskite passivation has been vital for obtaining highly efficient and stable perovskite solar cells (PSCs). Phenylethylammnonium (PEA)-based passivator is one of the most major categories as it is effective in improving PSC performances taking advantages of its large absorption energy over perovskite surface. However, this conventional passivator suffers from thermal stability issues; under thermal stress even at moderate temperature (e.g., 50℃) for several minutes, the overlayer of the perovskite passivated with PEA iodide salt (PEAI) transforms to a two-dimensional perovskite of (PEA)2PbI4, which hampers carrier transfer, thus negating the passivation effects and/or degrading the photovoltaic (PV) performances. Herein, we propose a novel yet simple strategy to address the thermal stability issue using a newly synthesized PEA bis(trifluoromethylsulfonyl)imide (PEA-TFSI) additive for poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) hole transport materials (HTMs). Upon the deposition of the PTAA HTM solution with a PEA-TFSI additive over the perovskite layers, the PEA cations spontaneously passivated the perovskite, forming a monolayer-like passivation overlayer. The resulting PEA-based passivation did not cause crystallization at the elevated temperature of 85℃; hence, it did not cause PV performance drop due to the thermal stress and retained a higher PV performance compared to the samples with the conventional Li-TFSI additive for over 800 h. Besides, compared to PSCs with Li-TFSI, PSCs with PEA-TFSI exhibited an improvement in stability under humid conditions (30℃, 50% relative humidity) for over 600 h, taking advantage of the resulting hydrophobicity. The optimal PEA-TFSI additive enhanced the PV performance, reaching a power conversion efficiency of up to 22.1% (21.2% in the quasi-steady state). These values are relatively high in the PTAA-based PSCs with an n-i-p structure particularly without using lithium species, which is considered to be detrimental for PSCs. These high PV performances were most likely attributed to improved affinity at the interface between PTAA and the perovskite, which is hardly attainable by an aliphatic ammonium passivator, and the PEA passivation effects. These results provide novel insights into the commonly used PEA-based perovskite passivation particularly in combining thermally stable PTAA HTMs and the promising alkylammonium spontaneous passivators, which spurs the further development of PSCs.