Ni-Co-O hole transport materials: gap state assisted hole extraction with superior electrical conductivity

Organic-inorganic hybrid perovskite solar cells (PSCs) have been rapidly evolving as a promising candidate for next-generation photovoltaic technologies. P-type organics or polymers are generally used as hole transport materials (HTMs), which are expensive and non-sustainable for long-term applications. Herein, we demonstrate an inorganic Ni-Co-O hole conductor that displayed fast hole extraction and transport by the presence of gap states and relative high hole conductivity. Detailed structural inspection reveals that Co3+ and Ni2+ ions would reform into Co2+ and Ni3+ sites with the incorporation of Co, which contributes to greatly enhanced hole concentration. Inverted heterojunction devices based on NiCoOx hole transport layers yielded a maximum power conversion efficiency (PCE) of 20.03%, with 16.9% improvement compared with those based on NiOx layers (17.14%). This novel HTM with a facile synthesis process provides a new strategy for designing efficient carrier transport materials such that efficient charge collection and transport are achieved.