A Bilayered Carbon with High Sunlight-Absorption Coefficient Designed by Flat-Band-Guided Strategy

The electronic band structure of a material determines its photovoltaic performance. In this work, we developed a methodology to explore desired semiconducting materials with a specialized designed fitness function for characterizing flat electronic bands via the generalization of our in-house-developed CALYPSO method. An intriguing bilayered carbon with sp 3 - and sp 2 - hybridized bonds, related to a bilayered reconstruction of (111) diamond or twisted graphene, was proposed and termed as two-dimensional diaphite. Two-dimensional diaphite has a direct electronic bandgap of 1.20 eV, with a high density of states near the band edges caused by the nearby flat bands, affording an extremely high sunlight-absorption coefficient. The type-II van der Waals heterojunctions composed of 2D diaphite and InSe monolayers yielded extraordinary performance for solar-to-electricity conversion with a theoretical power conversion efficiency of up to a record-breaking value of 24.1%, making it a potential candidate for high-efficiency and ultrathin photovoltaic devices. Our results show that the approach proposed herein is a feasible way of searching for functional semiconducting materials with specialized characteristic energy-band structures.