Charge Accumulation Effects in Hole-Transporting Materials on Current–Voltage Performance in Solar Cells

Hole-transporting material (HTM) layer often plays a crucial buffer role in efficient solar cells and is normally known to serve as a preferential extractor of photogenerated holes from the adjacent absorber film. Here, the CuI film is controllably prepared and the current density–voltage (J–V) properties in the Sb2S3 solar cells with CuI and spiro-OMeTAD as the HTMs that possess remarkably different hole mobilities (μh) are comparatively investigated. While two solar cells exhibit a comparable efficiency of 4–5%, the CuI-based solar cell of a much lower μh in the HTM layer exhibits an abnormal J–V property, quite different from the spiro-OMeTAD-based one. The μh-defined contribution of the HTM layer to photocurrent generation is found to cause the abnormal J–V property and the principles behind the μh-dependent overall device performance get elucidated. An intensity-dependent hole accumulation barrier model is proposed to understand the big discrepancy between the short-circuit current density values measured under AM 1.5G illumination and expected from external quantum efficiency, and a two-source equivalent circuit model is developed to understand the μh-defined contribution of the HTM layer to a high photocurrent generation, offering mechanistic insights into the importance of the charge mobility in the HTM layer to solar cell performance.