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

Abstract

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%.