Characterising the Passivation Mechanism of Lithium Fluoride at the Perovskite/C₆₀ Interface

Abstract

Buckminsterfullerene (C₆₀) is often used as the electron transport layer (ETL) in perovskite solar cells (PSCs). At the perovskite/C₆₀ interface, significant non-radiative recombination occurs, causing a mismatch between the open-circuit voltage (V_OC) and the quasi-Fermi level splitting (QFLS). It has been reported that an ultra-thin (1 nm) lithium fluoride (LiF) layer at the interface improves the V_OC of PSCs, but the exact passivation mechanism of the LiF layer is still up to debate. In this master thesis, the effect of LiF on the properties of the perovskite (PVK) layer and the PVK/C₆₀ interface is investigated. The growth mechanism of LiF is studied with atomic force microscopy (AFM). It was found that LiF grows on the perovskite surface via an island growth mechanism. It was also shown that lowering the evaporation rate significantly improves the coverage. From steady-state microwave conductivity (SSMC) and time-resolved microwave conductivity (TRMC) measurements, it was demonstrated that LiF did not have a beneficial effect on the opto-electronic properties of the perovskite film alone. Further SSMC and TRMC measurements showed that a thin LiF interlayer passivates electron traps at the PVK/C₆₀ interface, thereby reducing non-radiative recombination. This passivation of interfacial defects leads to both an improved QFLS for the PVK/C₆₀ bilayer system and an increased V_OC in the fabricated PSCs.