Light illumination intensity dependence of photovoltaic parameter in polymer solar cells with ammonium heptamolybdate as hole extraction layer.

A low-temperature, solution-processed molybdenum oxide (MoO) layer and a facile method for polymer solar cells (PSCs) is developed. The PSCs based on a MoO 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 (NH)MoO molecule decomposes and forms the molybdenum oxide (MoO) molecule when undergoing thermal annealing treatment. In this study, PSCs with the MoO layer as the HEL exhibited better photovoltaic performance, especially under weak light illumination intensity (from 100 to 10mWcm) 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 and PEDOT:PSS layer as the HEL. That the slopes of the open-circuit voltage (V) 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) versus light illumination intensity plots are close to 1 reveals that the effective charge carrier transport and collection mechanism of the MoO/indium tin oxide (ITO) anode is the weaker bimolecular recombination in short-circuit conditions. Our results indicate that MoO is an alternative candidate for high-performance PSCs, especially under weak light illumination intensity.