Nickel oxide carrier selective contacts for silicon solar cells

Highly selective contacts which allow one type of carrier (e.g. holes) to conduct and block the other type (e.g. electrons) are the key to achieving high efficiency in thin-film solar cells. Metal-insulator-semiconductor (MIS) carrier selective contacts can make a solar cell possible without a diffused p-n junction. In this work, we introduce nickel oxide (NiO) as a potential hole selective (electron blocking) contact on silicon. Using a metal/NiO/n-Si, structure we made a proof-of-concept device which showed effective dark current suppression and photovoltaic power conversion when illuminated. We found that in the forward bias regime the injection current is suppressed due to the high conduction band offset between Si and NiO. This results in an improvement of Voc by 20 mV. The Si/NiO interface is limited by defects but can be improved. The origin of these defects is the non-stoichiometry of NiO confirmed by X-ray photoemission spectroscopy. However, NiO is p-doped through non-stoichiometry, which is important for hole selectivity. Therefore, NiO stoichiometry is proposed to be the design optimization parameter in metal/NiO/Si selective contacts.