Methylammonium Compensation Effects in MAPbI 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 (MAPbI) perovskite layer. The CuSCN dissolved in diethyl sulfide (DES) was spin-coated on the MAPbI layer. For high-quality and dense CuSCN layers, post-annealing was carried out at various temperatures and times. However, the unwanted dissociation of MAPbI to PbI 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 MAPbI 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 PbI in the MAPbI precursor solution was investigated. The excess MAI molar ratio in the MAPbI precursor solution reduced MAPbI surface damage despite using DES polar solvent for CuSCN solution. In addition, dissociation of MAPbI to PbI following an adequate post-annealing process was well suppressed. The excess MAI molar ratio in the MAPbI precursor could be compensated for the MA loss and effectively suppress phase separation from MAPbI to MAI + PbI during post-annealing. The efficiency based on the normal planar structure of CuSCN/MAPbI (using excess MAI)/TiO was approximately 17%. The CuSCN-based MAPbI 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.