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.