Dehydration of molybdenum oxide hole extraction layers via microwave annealing for the improvement of efficiency and lifetime in organic solar cells

Abstract

A significant contribution to the improvement of efficiency and lifetime of organic solar cells is due to the successful engineering of the metal contact/organic interface by introducing appropriate interlayers. In the current work we show that a short microwave post-annealing treatment in air of an under-stoichiometric molybdenum oxide (MoO_x) hole transport layer significantly enhanced the performance and lifetime of an organic solar cell based on a poly(3-hexylthiophene):[6,6]-phenyl-C₇₁-butyric acid methyl ester (P3HT:PC₇₁BM) blend. The enhanced performance is mainly driven by improvement in the short circuit current (J_sc) and the fill factor (FF), caused by, except for an increase of the anode work function, reduced series resistance, and increased shunt resistance and also higher charge generation efficiency, reduced recombination losses and improved hole transport towards the anode contact. In addition, the lifetime of the devices with microwave annealed MoO_x interlayers was also significantly improved compared to those with as-deposited MoO_x and, especially, those with the PEDOT-PSS interlayer. The above were attributed to effective dehydration which was also followed by structural transformation and crystallization of the MoO_x layer during microwave annealing. The removal of absorbed water molecules led to alterations of the structure and microstructure of the MoO_x films, visible in the X-ray diffraction patterns, infrared and Raman spectra and atomic force microscopy images recorded on their surface without influencing the oxide's chemical composition as evidenced by X-ray photoelectron spectroscopy. During microwave annealing the substrate remains practically at room temperature, so the method is applicable for films deposited on plastics or other temperature-sensitive substrates.