Effect of tantalum doping in a TiO2 compact layer on the performance of planar spiro-OMeTAD free perovskite solar cells

Perovskite solar cells (PSCs) are currently the most exciting solar photovoltaic technologies for future deployment. A conventional PSC device structure typically employs a titanium dioxide (TiO2) electron transport layer. However, the low electrical conductivity of TiO2 is an obstacle to PSC efficiency enhancement. In this paper, we report on the conductivity enhancement of TiO2 by tantalum (Ta) doping and its effect on improving the device performance. In contrast to commonly used mesoporous TiO2, our work used a planar PSC device structure based on a compact TiO2 layer with the device structure being: FTO/compact-TiO2/CH3NH3PbI3/P3HT/Ag. Photovoltaic measurements show that Ta doping of compact TiO2 results in an improvement in the fill factor (FF) of the devices due to a decrease in the series resistance (R-s), attributed to improved charge transport and an increase in the shunt resistance (R-sh), due to reduced leakage paths. These changes were examined using Kelvin probe force microscopy (KPFM) which indicated that the Fermi level of TiO2 shifts downward upon Ta doping providing driving force for the electron transfer from the perovskite LUMO to the TiO2 conduction band resulting in higher current density. Furthermore, impedance spectroscopy analysis of the devices suggests a decrease in the charge transfer resistance with Ta-doping and an increase in R-sh due to the higher recombination resistance of doped films. PSC devices with Ta doping of 3.0% led to a 40% improvement in the overall efficiency as compared to un-doped TiO2 with the best device showing a power conversion efficiency of ca. 9.94%.