Origin of temperature-dependent performance of hole-transport-layer-free perovskite solar cells doped with CuSCN

Recently, a hole-transport-layer (HTL)-free structure was proposed to decrease the cost of organic lead halide perovskite solar cell (PSC) fabrication. In HTL-free PSCs, instead of using an HTL insertion, the HTL material can be added directly into a perovskite precursor solution to improve hole transport. For example, copper thiocyanate (CuSCN) is used for p-type doping of methylammonium lead triiodide (MAPI) via spin coating from a mixed solution. However, the optimum annealing temperature for CuSCN-doped MAPI (CuSCN:MAPI) PSCs is lower than the 100 °C that is typical for undoped MAPI PSCs. In this study, the origin of such lower annealing temperatures of CuSCN:MAPI PSCs is investigated. The highest power conversion efficiency (PCE) and enhanced electron transport in CuSCN:MAPI are obtained with annealing at 60 °C. Using transmission electron microscopy-energy-dispersive X-ray spectroscopy, it is revealed that annealing at 60 °C results in the uniform distribution of CuSCN, while the annealing at 100 °C induces the aggregation of CuSCN with a diameter of ~20 nm. A large energy barrier formed by the shallow-lying conduction band minimum of these CuSCN clusters hinders electron transport. The lower PCE of CuSCN:MAPI PSCs with annealing at 100 °C is attributed to this deterioration in the electron transport.