Low temperature processed NiOx hole transport layers for efficient polymer solar cells

Abstract

We here demonstrate the use of solution processed NiOx thin films as the hole transport layer (HTL) in a thiophene–quinoxaline copolymer:fullerene solar cell. The NiOx films, which are prepared by UV-ozone treating a nickel formate precursor, outperform the solar cells prepared in this study that use PEDOT:PSS as HTL. The power conversion efficiency improves from 5.3% to 6.1% when replacing PEDOT:PSS with NiOx. Unlike most conventional ways of fabricating solution processed NiOx HTLs, our method does not require high temperature (>300 °C). In fact, we were able to produce high performing NiOx HTLs without the use of any thermal annealing. X-ray photoelectron spectroscopy revealed that a mixture of oxides and hydroxides is formed as a result of the UV-ozone treatment, which differs in composition from those formed by high temperature annealing; UV-ozone treatment produces NiOOH, while only the high temperature annealing produces any significant amount of NiO. Contact potential difference (CPD) measurements reveal an increased work function for all UV-ozone treated NiOx films, consistent with the presence of NiOOH at the surface. The high work function of the UV-ozone treated NiOx films leads to an improved energy level matching between the donor and the HTL, resulting in higher fill factor and hole injection current.