Thermally stable phenylethylammonium-based perovskite passivation: spontaneous passivation with phenylethylammonium bis(trifluoromethylsulfonyl)imide during deposition of PTAA for enhancing photovoltaic performance of perovskite solar cells

Abstract

Perovskite passivation has become instrumental for obtaining highly efficient and stable perovskite solar cells (PSCs). A class of phenylethylammonium (PEA)-based passivators is one of the most major categories as it is effective in improving PSC performances exploiting its large adsorption energy over the perovskite surface. However, the conventional PEA-based passivation suffers from thermal stability issues; under thermal stress even at moderate temperatures (e.g., 50 °C) for several minutes, the overlayer of the perovskite passivated with PEA iodide transforms into a two-dimensional (2D) 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 and simple strategy to address the thermal stability problems using a newly synthesized PEA bis(trifluoromethylsulfonyl)imide (PEA-TFSI) additive for the poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) hole transport material (HTM). During HTM deposition with the PEA-TFSI additive over perovskite layers, the PEA cations spontaneously passivated the perovskite, forming a monolayer-like passivation overlayer. The resulting PEA-based passivation did not cause crystallization to the 2D perovskite of (PEA)₂PbI₄ at 85 °C; hence, it did not cause a PV performance drop due to thermal stress. The PSCs with optimal PEA-TFSI addition exhibited effectively enhanced PV performances, achieving a 22.1% power conversion efficiency. This enhancement can be attributed to both the improved affinity at the PTAA/perovskite interface, which is crucial in combining PTAA HTMs yet is hardly attainable by aliphatic-ammonium-based passivators, and the PEA passivation effects. This study provides novel insights into commonly used PEA-based passivators and paves the way for perovskite passivation effectively combined with thermally stable PTAA HTMs.