Origins of the s-shape characteristic in J-V curve of inverted-type perovskite solar cells

Fullerene derivative thin films have been widely used in inverted-type perovskite solar cells as the electron transport layer (ETL) and hole blocking layer. However, the smooth contact at the interface between the hydrophobic [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) and hydrophilic CH₃NH₃PbI₃ (MAPbI₃) thin film has not yet been completely understood. The contact at the PCBM/MAPbI₃ interface strongly influences the photovoltaic performance. The photovoltaic devices were characterized by measuring the light intensity-dependent current density-voltage (J-V) curves and impedance spectra, which show that the contact at the PCBM/MAPbI₃ interface simultaneously influences the shunt resistance (carrier recombination) and series resistance (interfacial contact). In addition, x-ray diffraction patterns, atomic force microscopic images, absorbance spectra and photoluminescence spectra were used to explore the contact at the PCBM/MAPbI₃ interface. The experimental results show that the flat MAPbI₃ thin film cannot be completely covered by a PCBM thin film and thereby results in the s-shape characteristic in the J-V curve of the resultant solar cells. The s-shaped J-V curve can be suppressed by increasing the crystallinity and surface roughness of the MAPbI₃ thin film. With the use of an interface modification layer in between the PCBM thin film and Ag cathode, the power conversion efficiency of MAPbI₃ solar cells can be increased from 10.50% to 13.71%.