A theoretical investigation on the Cd doping of Cu-depleted CuInSe2 materials

Because of their outstanding characteristics and affordable price, polycrystalline thin film solar cells based on CuIn_1-xGa_xSe₂ (CIGS) absorber layer have emerged to be one of the most promising materials for photovoltaic applications [1-2]. To further enhance the efficiency of these solar cells much effort is spent on the in-depth investigation of the production methods. However, the effect of structural defects and dopants upon the macroscopic properties and morphology of epitaxially grown CIGS films is not yet fully understood [3]. More importantly, it is well established that the best cells can be prepared by growing the CIGS absorber layer under Cu-poor conditions [4]. Thus, it is particularly important to understand the microstructure of these Cu-poor materials on the atomic level and to shed new light on the doping of such Cu-depleted phases. To describe these phases in our calculations we have used bulk CuIn₅Se₈ as model system. We have employed ab-initio calculations and moleculer dynamics simulations based on density functional theory (DFT) to investigate the insertion energy of Cd atoms and their diffusion in CuIn₅Se₈. The results of our investigations show, that CuIn₅Se₈ can be doped with small amounts of Cd up to an upper concentration limit of 0.59 atomic percent. Furthermore, we found that in bulk CuIn₅Se₈ the Cu antisite positions and Cu vacancy sites are the most probable binding sites for Cd atoms, and In sites are very unlikely to be occupied by Cd. Regarding the special distribution of the atoms, our data indicates, that energetically it is favorable if Cd atoms sitting in Cu antisites are located far away from the replaced Cu atoms. Hence, the inclusion of Cd dopants might be the cause of the Cu depleted regions present at the interface between the CIGS absorber and the CdS buffer layer in solar cells. Moreover, the molecular- dynamics simulations suggest, that at the synthesis temperature of CIGS materials the free energy barriers along the diffusion path of Cd atoms in CuIn₅Se₈ are rather low.