Methylammonium Compensation Effects in MAPbI₃ Perovskite Solar Cells for High-Quality Inorganic CuSCN Hole Transport Layers

Recent studies have demonstrated that copper (I) thiocyanate (CuSCN) has huge potential as a hole extraction material (HEM) for perovskite solar cells. Here, we used CuSCN as a HEM and analyzed its relationships with a methylammonium lead iodide (MAPbI3) perovskite layer. The CuSCN dissolved in diethyl sulfide (DES) was spin-coated on the MAPbI3 layer. For high-quality and dense CuSCN layers, post-annealing was carried out at various temperatures and times. However, the unwanted dissociation of MAPbI3 to PbI2 was observed due to the post-annealing for a long time at elevated temperatures. In addition, DES, which is used as a CuSCN solvent, is a polar solvent that damages the surface of MAPbI3 perovskites and causes poor interfacial properties between the perovskite layer and HEM. To solve this problem, the effect of the molar ratio of methylammonium iodide (MAI) and PbI2 in the MAPbI3 precursor solution was investigated. The excess MAI molar ratio in the MAPbI3 precursor solution reduced MAPbI3 surface damage despite using DES polar solvent for CuSCN solution. In addition, dissociation of MAPbI3 to PbI2 following an adequate post-annealing process was well suppressed. The excess MAI molar ratio in the MAPbI3 precursor could be compensated for the MA loss and effectively suppress phase separation from MAPbI3 to MAI + PbI2 during post-annealing. The efficiency based on the normal planar structure of CuSCN/MAPbI3 (using excess MAI)/TiO2 was approximately 17%. The CuSCN-based MAPbI3 device shows more optimized stability than the conventional spiro-OMeTAD under damp heat (85 °C and 85% relative humidity) conditions because of the robust inorganic HEM.