Emerging chalcogenide based van der Waals heterostructures for ultrathin excitonic solar cells with enhanced photo-conversion efficiency

Abstract

The emerging chalcogenide based two-dimensional materials, zirconium-diselenide (ZrSe₂) and hafnium-diselenide (HfSe₂), have been reported to construct promising van der Waals (vdW) heterostructures, which could improve the efficiency of ultrathin excitonic solar cells. We perform systematic first-principles electronic structure calculations to explore the electronic band structures and optical properties and hence determine the power conversion efficiency (PCE) of the heterostructures. We have considered both the 2H and 1T phases of ZrSe₂ and HfSe₂ which give rise to four different possible heterostructures. From the analysis of band edge alignment, three of the heterostructure configurations have been identified to exhibit type II band alignment, which helps in boosting the effective separation of electron–hole pairs, paving the way towards their application in ultrathin excitonic heterojunction solar cells. The electrostatic potential shows a huge difference in the dip, which signifies the in-built electric field that is generated between two different surfaces, thus enhancing the transfer of electrons and holes between different layers. A greater range of incoming solar energy can be absorbed by the heterostructure, therefore providing better utilization of the incident radiation. The results show that the heterostructure formed by the 2H phase of ZrSe₂/HfSe₂ has reached a maximum power conversion efficiency of more than 21%.