Overcoming the intrinsic difference between hydrophilic CH₃NH₃PbI₃ and hydrophobic C₆₀ thin films to improve the photovoltaic performance

Dimethylformamide/dimethyl sulfoxide solvent mixtures were used as the CH₃NH₃PbI₃ (MAPbI₃) precursor solvent in a one-step spin coating method to fabricate smooth and hydrophilic crystalline MAPbI₃ thin films on top of hydrophobic carbon-60 (C₆₀) thin film for highly efficient photovoltaics. The structural, optical, and excitonic characteristics of the resultant MAPbI₃ thin films were analyzed using X-ray diffraction (XRD), atomic-force microscopy, absorbance spectroscopy, photoluminescence (PL) spectrometry, and nanosecond time-resolved PL. There was a trade-off between the crystallinity and surface roughness of the MAPbI₃ thin films, which strongly influenced the device performance of MAPbI₃-based photovoltaics. The high power conversion efficiency (PCE) of 17.55% was achieved by improving the wettability of MAPbI₃ precursor solutions on top of the C₆₀ thin films. In addition, it was predicted that the fill factor and PCE could be further improved by increasing the crystallinity of the MAPbI₃ thin film while keeping it smooth.