Investigation of Cu(In,Ga)Se2 using Monte Carlo and the cluster expansion technique

CIGS based solar cells are among the most promising thin-film techniques for cheap, yet efficient modules. They have been investigated for many years, but the full potential of CIGS cells has not yet been exhausted and many effects are not understood. For instance, the band gap of the absorber material Cu(In,Ga)Se2 varies with Ga content. The question why solar cells with high Ga content have low efficiencies, despite the fact that the band gap should have the optimum value, is still unanswered. We are using Monte Carlo simulations in combination with a cluster expansion to investigate the homogeneity of the In-Ga distribution as a possible cause of the low efficiency of cells with high Ga content. The cluster expansion is created by a fit to ab initio electronic structure energies. The results we found are crucial for the processing of solar cells, shed light on structural properties and give hints on how to significantly improve solar cell performance. Above the transition temperature from the separated to the mixed phase, we observe different sizes of the In and Ga domains for a given temperature. The In domains in the Ga-rich compound are smaller and less abundant than the Ga domains in the In-rich compound. This translates into the Ga-rich material being less homogeneous.