Improvement of red light harvesting ability and open circuit voltage of Cu:NiOx based p-i-n planar perovskite solar cells boosted by cysteine enhanced interface contact

Inorganic NiO is a promising hole extracting material for p-i-n planar perovskite solar cells due to its low cost, ultra-high photo and thermal-stability, and high energy level of conduction band blocking photoelectron from perovskite to cathode to prevent charge recombination at the interface. However, the power conversion efficiency (PCE) of NiO-based p-i-n devices is still lower than TiO2 based n-i-p devices because of relatively low light harvesting ability in the red light region and the low work function of NiO limited the improvement of open circuit voltage. In this study, a novel surface modification method was developed for NiO-based p-i-n planar devices, which enhanced red light harvesting ability and device open voltage. It proved that all three functional groups of surface modifier played key role to boost the device performance. Detailed investigations show an increased interface contact between perovskite and hole transport layer improving charge extraction, preferential orientation of perovskite crystal that insures external quantum efficiency higher than 90% in the whole visible light range and even up to 95% in short wavelength region. The surface treatment improves the short circuit current density from 20 to 23.6 mA cm-2 and open voltage from 1.06 to 1.12 V.