First-principles Study of Defect Control in Solar Cell Materials

Abstract

One of the most important issues in improving solar cell efficiency and reducing the cost is to introduce the charge carriers through defect control and reduce the recombination of photo-generated carriers. For example, CdTe is one of the leading materials for low cost, high efficiency thin-film solar cell absorbers. However, CdTe solar cell currently has only achieved a efficiency of about 20%, which is relatively low compared with its theoretical limit 30%. One of the main reasons is because of the low majority carrier concentration of CdTe and low carrier life time caused by defect-induced carrier recombination. Similar issues also affects ∼ the application of other leading thin-film solar cells such as Cu(In,Ga)Se2 and Cu2ZnSn(S,Se)4, where doping difficulty has led to low open circuit voltage. Using first-principles band structure methods we systematically studied the defect properties in thin-film solar cells, including calculation of the defect formation energies and transition energy levels of intrinsic and extrinsic point defects and defect complexes, grain boundaries, defect diffusion barriers, and carrier concentration as function of atomic chemical potentials and temperature. From the calculated results, we investigate the limiting factors for p-type and n-type doping and the potential non-radiative recombination centers in thin-film solar cell materials. Possible approaches to control the doping and reduce the recombination centers, as well as passivation of grain boundary defect levels are discussed. General understanding of the chemical trends of defect properties in thin-film solar cell will also be discussed.