Methylammonium Compensation Effects in MAPbI 3 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 3 ) perovskite layer. The CuSCN dissolved in diethyl sulfide (DES) was spin-coated on the MAPbI 3 layer. For high-quality and dense CuSCN layers, post-annealing was carried out at various temperatures and times. However, the unwanted dissociation of MAPbI 3 to PbI 2 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 3 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 2 in the MAPbI 3 precursor solution was investigated. The excess MAI molar ratio in the MAPbI 3 precursor solution reduced MAPbI 3 surface damage despite using DES polar solvent for CuSCN solution. In addition, dissociation of MAPbI 3 to PbI 2 following an adequate post-annealing process was well suppressed. The excess MAI molar ratio in the MAPbI 3 precursor could be compensated for the MA loss and effectively suppress phase separation from MAPbI 3 to MAI + PbI 2 during post-annealing. The efficiency based on the normal planar structure of CuSCN/MAPbI 3 (using excess MAI)/TiO 2 was approximately 17%. The CuSCN-based MAPbI 3 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.