Light illumination intensity dependence of photovoltaic parameter in polymer solar cells with ammonium heptamolybdate...

A low-temperature, solution-processed molybdenum oxide (MoO_X) layer and a facile method for polymer solar cells (PSCs) is developed. The PSCs based on a MoO_X layer as the hole extraction layer (HEL) is a significant advance for achieving higher photovoltaic performance, especially under weaker light illumination intensity. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) measurements show that the (NH₄)₆Mo₇O₂₄ molecule decomposes and forms the molybdenum oxide (MoO_X) molecule when undergoing thermal annealing treatment. In this study, PSCs with the MoO_X layer as the HEL exhibited better photovoltaic performance, especially under weak light illumination intensity (from 100 to 10mWcm^-2) compared to poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)-based PSCs. Analysis of the current density-voltage (J-V) characteristics at various light intensities provides information on the different recombination mechanisms in the PSCs with a MoO_X and PEDOT:PSS layer as the HEL. That the slopes of the open-circuit voltage (V_OC) versus light illumination intensity plots are close to 1 unity (kT/q) reveals that bimolecular recombination is the dominant and weaker monomolecular recombination mechanism in open-circuit conditions. That the slopes of the short-circuit current density (J_SC) versus light illumination intensity plots are close to 1 reveals that the effective charge carrier transport and collection mechanism of the MoO_X/indium tin oxide (ITO) anode is the weaker bimolecular recombination in short-circuit conditions. Our results indicate that MoO_X is an alternative candidate for high-performance PSCs, especially under weak light illumination intensity.