Emerging Van der Waals Heterostructures for Ultrathin Solar Cells: Implication of Type-II Band Edge Alignment on Photo-Conversion Efficiency

The emerging two-dimensional (2D) materials, ZrSe₂ and HfSe₂, have been envisaged to construct the possible van der Waals heterostructures (vdW), for enhancement of solar cell efficiency. We have performed systematic first principles electronic structure calculations to explore the electronic band structures and optical properties of the heterostructures in order to determine the photo-conversion efficiency (PCE). We have considered both H and T phases of ZrSe₂ and HfSe₂ while constructing four different possibilities of heterostructures. From the analysis of band edge alignment, it has been found that the heterostructures are having Type-II band alignment, which helps in boosting the effective separation of electron-hole pairs, paving the way towards its application in materials for ultra-thin excitonic heterojunction solar cells. The electrostatic potential shows a huge difference in the dip which signifies the inbuilt electric field that is generated between two different surfaces therefore enhancing the transfer of electrons and holes between different layers. The optical absorption spectra of the heterostructures cover the wider range of the incident solar energy therefore better utilization of the incident radiation. The results show that the heterostructure formed by the H phase of ZrSe₂/HfSe₂ has attained a maximum power conversion efficiency of over 21%.