Impact of the hole transport layer on the space charge distribution and hysteresis in perovskite solar cells analysed by capacitance–voltage profiling†

Abstract

This study explores the influence of the hole transport layer (HTL) on space charge distribution and hysteresis in perovskite solar cells (PSCs) using capacitance–voltage (C–V) profiling. Drift-diffusion simulations and experimental C–V measurements were employed to analyse devices incorporating Spiro-OMeTAD and CuSCN as HTLs. The simulations revealed that ionic charge accumulation predominantly at the perovskite/HTL interface affects the internal electric field distribution, with mobile cation density playing a crucial role in screening the built-in electric field within the perovskite layer. The density of mobile cations in the perovskite can increase by the diffusion of Li⁺ and Co³⁺ ions from the Spiro-OMeTAD layer, resulting in a more steep and narrow doping profile compared to the CuSCN-based device. Simulations and experiments demonstrate that mobile ions, despite not directly responding to the high-frequency AC signals used in C–V characterisation, influence capacitance by affecting electronic carrier distribution. Analysis of doping profiles reveals that bias-modulated ionic accumulation at interfaces contributes to both U-shaped and distinct W-shaped doping profiles observed in Spiro-OMeTAD devices. Devices with Spiro-OMeTAD exhibited higher capacitance and more pronounced hysteresis due to intensified charge accumulation, while CuSCN-based devices displayed a faster capacitance response and reduced hysteresis, attributed to a more uniform charge distribution. Despite the increased hysteresis in Spiro-OMeTAD devices, they achieved higher power conversion efficiencies (PCE), highlighting a complex relationship between hysteresis and performance and emphasising the importance of HTL selection and ion management for PSC optimisation.