Lattice Structures And Electronic Properties Of WZ-CdS/WZ-CuInS₂/MoS₂ Interface From First-principles Calculations

The wurtzite Cu In S2 has been researched in a solar cell device with its unique optical properties, and shows flexibility in stoichiometry during the process of preparations. Nowadays, this promising photoelectricity material can be hopefully used as a light absorption material in solar cells. The general structure of CIGS thin film solar cell is: front electrode | Zn O window layer | Cd S buffer layer | CIGS light absorbing layer | Mo back electrode | glass. In the CIS deposition preparation process, portion of S diffuses into Mo back contact and CIS absorption layer, during the S diffusion, combining part of Mo, to form an intermediate Mo S2 layer between CIS and Mo layer. The formation of a thin Mo S2 layer is the facilitation of a quasi ohmic electrical contact across Mo-CIS interface in CIS thin film solar cells. It is helpful to improve the efficiency of the CIS solar cells and be beneficial for device performance. In this work, we used first-principles plane-wave calculations within density functional theory to theoretically study WZ-Cu In S2 absorption layer with intermediate Mo S2 layer and WZ-Cd S buffer layer at atomic scale, respectively. We studied the atomic structure, bonding energy, electrons’ redistribution and electronic properties. Our study found that, WZ-Cu In S2(100) with Mo S2(100), WZ-Cd S(100) faces can make good surface lattice matching, and the degree of lattice mismatch is about 3.5%, 5.6% respectively. For WZ-CIS(100)/Mo S2(-100) interface, after relaxation the atomic positions and the bond lengths change slightly on the interface. The interface bonding energy is about-0.65 J/m2. Via analysis density of states(DOS), difference charge density and Bader charges we find that the electrons are largely redistributed on the interface, and there are some interface states near the Fermi level, which are mainly caused by In-5s orbital in the WZ-CIS region and S-3p orbital in the Mo S2 region. On the interface the orbital hybridizations of different interfacial atoms highly enhance the bonding ability of the atoms. Electron transformation and orbital hybridization together promote bonding between atoms and increase the adhesion energy of the interface. For WZ-CIS(100)/WZ-Cd S(100) interface, we investigated the perfect bi-layer and monolayer terminated WZ-CIS(100)/WZ-Cd S(100) interfaces. After relaxation the atomic positions and the bond lengths change slightly on the two interfaces. The WZ-CIS/WZ-Cd S interfaces can exist stably, with the interface bonding energies are-0.481 J/m2(bi-layer terminated interface) and-0.677 J/m2(monolayer terminated interface). Via analysis of density of states, difference charge density and Bader charges of the WZ-CIS/WZ-Cd S interfaces, no interface state is found near the Fermi level. The stronger adhesion of monolayer terminated interface is attributed to more electron transformations and orbital hybridizations, promoting more stable interfacial bonds than those on bi-layer terminated interface.