Solution-processed nickel oxide hole transport layer for highly efficient perovskite-based photovoltaics

Solution processed NiOx is one of the promising hole transport layer (HTL) for planar perovskite solar cells, which can replace hygroscopic poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) HTL. In this study, we investigated effects of ethylenediamine (EDA) additive in NiOx precursor solution (nickel nitrate hexahydrate dissolved in ethyleneglycol) on optoelectronic and surface morphological properties of resultant solution processed NiOx films. By varying EDA content (0-10.0 v/v %) in the precursor, we could find out that adequate EDA additive (similar to 5.0%) provide much reduced electrical resistivity and enhanced optical transmission compared with control NiOx film (No EDA) by suppressing formation of byproducts (i.e. nickel hydroxide). In addition, AFM surface topography showed much compact and dense deposition of NiOx film on ITO electrode. This contributed to improve charge transport properties and suppress charge recombination loss at ITO/perovskite interface, which provided strong enhancement in fill factor from 0.599 to 0.714 in the perovskite solar cells. As a result, a power conversion efficiency (PCE) was strongly increased from 13.9 (No EDA) to 16.7% (EDA 5.0%). This also outperformed the performance (14.3%) of device using PEDOT: PSS, which indicates that the adequate control of EDA additive for NiOx HTL could offer much promising photovoltaic performance.