In2S3 as alternative for CdS buffer layer in Cu(In, Ga) (S, Se)2 (CIGS)-base thin film solar cells has gained a lot of scientifc attention in recent years. When indium sulfide is deposited on CIGS, it forms an intermixed absorber-buffer interface involving a high concentration of Cu and Na. So the question is whether the CIGS-In2S3 interface is stable or we would expect to have Na containing and Cu containing secondary phases at the interface region. There are two theoretical ways of addressing this question: Defect thermodynamics calculations and direct interface calculations. To unveil the driving force of Na and Cu formation in In2S3, we compared the formation energy of these dopants in buffer and absorber layers.
For studying the defect thermodynamics, we have used an exchange potential with 25% contribution of Hartree-Fock exchange. For direct interface calculations, however, with systems containing around one thousand atoms a hybrid functional cannot be used. Therefore, the interface calculations were performed using the PBE functional, in which the atoms were allowed to relax until the Hellmann-Feynman forces acting on them became less than 0.05 eV/Å. To construct the interface we have used a periodic slab model, in which the slab is repeated in the direction perpendicular to the interface (z-axis), and each slab is separated from its periodic images by a vacuum gap of 16 Å. Since constituents of both chalcopyrite and spinel-like structure at this orientation relationship form layered crystal structure consisting of alternative anion and cationplanes, four different interface termination layers have been considered across the interface. Note that these termination layers are fundamentally different, representing differently electronically compensated interfaces.
According to our calculations both, Na and Cu, would preferably occupy the interstitial vacancy in In2S3. The very high solubility of Na and Cu in In2S3 represents their massive driving force for the occurrence of side reactions leading to the formation of interfacial phases. To get a first impression from direct interface calculations, we used CuInSe2(112)-In2S3(103) interface orientation. The results obtained from geometry optimization indicate structural distortions and massive structural relaxations as shown in the Figure. As the next step, we have modified the interface to get a junction between the CuInSe2 absorber layer and Cu-containing and Na-containing buffer layers. These results reveal that both Na and Cu containing indium sulfides form more stable interfaces with the absorber layer than pure In2S3.