Lattice Structures And Electronic Properties Of CIGS/CDS Interface From First-principles Calculations

With the continuous development of the technology, CIGS thin-film solar have rapidly developed and become a hot topic because of its high conversion efficiency, low cost, long-term stability and resistance to radiation characteristics. The general structure of CIGS thin film solar cell is front electrode|ZnO window layer|CdS buffer layer|CIGS light absorbing layer|Mo back electrode|glass. Its core layer is an p-n heterojunction, which is formed by p-CIGS and n-CdS material. Due to the existence of the mutant composition and structure of the adjacent layers at the interface, and some defects at the interface, there will be the interface states at the band gap region of the interface, which will prevent the photo-generated carriers through the Fermi level, resulting in a reduced bandgap, thereby reducing the open circuit voltage. Domestic and foreign experts have studied interface morphology, chemical composition and band offset and their impact on battery performance of the heterojunction interface in experimental and theoretical aspects. However, there are little theoretical research at the atomic level and the electron microscopic level on CIGS/CdS heterojunction interface. Using first-principles calculations within density functional theory, we studied the atomic structures and electronic properties of CuInSe2bulk, CdS bulk, ideal CuInSe2(112) surface, the perfect and defective (2Vcu+Incu) CuInGaSe2/CdS interfaces theoretically. We found that the band gap (0.4eV) of CuInSe2bulk by GGA+U is larger than that (0.04eV) by GGA method. Secondly, for the ideal CuInSe2(112) surface model, we found several surface states near the Fermi level in the band gap of CuInSe2bulk, and these additional states are mainly formed by Se-4/p and In-5s orbital. Furthermore, we found that the local lattice structure of (2Vcu+Incu) CuInGaSe2/CdS interface is more somewhat disorganized than the perfect interface. By analyzing the local density of states projected on several atomic layers of the two interfaces models, we found that for the (2Vcu+Incu) interface, the interface states near the Fermi level in CuInGaSe2and CdS band gap regions are mainly composed of interfacial Se-4p, Cu-3d and S-3p orbitals. While for the perfect interface, there are no clear interface states in the CuInGaSe2region, and only some interface states which are mainly composed of S-3p orbitals in the valance band of CdS region.