First-principles Study Of Doping Property In Solar Cell Semiconductors CuInS₂and CuI

CuInS2(ClS) thin film solar cell has become one of the promising photovoltaic devices, due to its low manufacture cost and high conversion efficiency. Although the preparing technique under a vacuum condition is popular, the non-vacuum technique has been attractive due to its low investment on equipments. A variety of defects could be formed under the non-vacuum condition, such as intrinsic impurities caused by the deviation from stoichiometry, and the extrinsic impurities from the environment. Therefore, evaluating the impact of these defects on the performance of the semiconductor is helpful to control the harmful defects in the preparation process.In this study, properties of CIS perfect crystal, such as lattice parameters, bulk modulus, band gaps, band structures and density of states were obtained by density functional theory (DFT), GGA (LDA)+U and hybrid DFT. A detailed first-principles study on the formation energies and transition levels of point defects Vcu, VIn, Vs, Incu, CuIn and Os in CIS were investigated using hybrid DFT. Three different possible preparation conditions have been considered:(a) Cu-poor, In-rich, S-rich and O-rich;(b) Cu-rich, In-poor, S-rich and O-rich; and (c) Cu-rich, In-rich, S-poor and O-rich. In order to be n-type, the dominant donor defect of the materials must have low formation energy and shallow transition energy (close to the conduction band minimum); in order to be p-type, the dominant acceptor defect in materials must have low formation energy and shallow transition energy (close to the valence band maximum). Evaluating the formation energies and transition energies of the different defects, the mechanism of self-doping n-/p-type CIS was estimated. In addition, the influence of formation energies and transition energies of defects (Vcu, VI, ICu, CuI, NI, OI) on the semiconductor type of CuI was studied with the same method.The results showed that hybrid functional HSE06accurately described the structural and electronic properties of CIS and gave a band gap of1.40eV, agreed well with the experimental value. P-type semiconductor CIS was obtained under S-rich condition with a certain copper and indium content, while n-type semiconductor CIS was easily obtained under the Cu-rich, In-rich, S-poor and non-oxygen conditions. The hybrid functional PBEO accurately described the band gap of CuI (3.24eV). For p-type acceptor impurities, it was found VCu had the shallowest transition energy level and the lowest formation energy among all considered defects under I-rich condition. As a result, p-type semiconductor CuI was obtained under the conditions of Cu-poor and I-rich.According to theoretical evaluation, the different semiconductor type of CIS and Cul could be obtained by the control of the preparing condition. This would save money and time for the semiconductor or solar cell industry.