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

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.