First-principles study of carbon impurities in CuInSe_2 and CuGaSe_2, present in non-vacuum synthesis methods

摘要

A first-principles study of the structural and electronic properties of carbon impurities in CuInSe_2 and CuGaSe_2 is presented. Carbon is present in organic molecules in the precursor solutions used in non-vacuum growth methods for CuInSe_2 and CuGaSe_2 based photovoltaic cells. These growth methods make more efficient use of material, time, and energy than traditional vacuum methods. The formation energies of several carbon impurities are calculated using the hybrid HSE06 functional. C_(Cu) acts as a shallow donor, C_(In) and interstitial C yield deep donor levels in CuInSe_2, while in CuGaSe_2 C_(Ga) and interstitial C act as deep amphoteric defects. So, these defects reduce the majority carrier (hole) concentration in p-type CuInSe_2 and CuGaSe_2 by compensating the acceptor levels. The deep defects are likely to act as recombination centers for the photogenerated charge carriers and are thus detrimental for the performance of the photovoltaic cells. On the other hand, the formation energies of the carbon impurities are high, even under C-rich growth conditions. Thus, few C impurities will form in CuInSe_2 and CuGaSe_2 in thermodynamic equilibrium. However, the deposition of the precursor solution in non-vacuum growth methods presents conditions far from thermodynamic equilibrium. In this case, our calculations show that C impurities formed in non-equilibrium tend to segregate from CuInSe_2 and CuGaSe_2 by approaching thermodynamic equilibrium, e.g., via thorough annealing.