First-principles study of dislocations in Cu(In,Ga)Se2 solar cell absorbers

Abstract

Among the thin-film solar cells, the maximum efficiencies are achieved by devices that use Cu(In,Ga)Se2 as absorber. However, this fact should not mask that there is room for improvement, if we could mitigate the main sources of efficiency loss in this solar cell type, which are induced by lattice defects. Therefore, a more complete picture of the nature of defects in Cu(In,Ga)Se2-based solar cells would help to improve the growth process in such way that detrimental defects are avoided and the efficiency increased. In order to achieve this goal, first-principles calculations provide valuable insights that complement experimental studies and can also be used as predictive tools. These calculations have been and continue to be successfully used for the case of point and planar defects in Cu(In,Ga)Se2-based solar cells. However, a defect type that has been studied to a lesser extent are lattice dislocations. The aim of this thesis is to carry out a complete study of the structural and electronic properties of Frank partials and perfect dislocations in CuInSe2 and CuGaSe2 . Results from this study allow us to solve, at least partially, the puzzle of Cu(In,Ga)Se2-based solar cells which exhibit decent efficiencies and at the same time have a very high dislocation density. Specifically, in the case of Frank partials our results suggest that these cores prefer to be non-stoichiometric and, as a consequence, are expected to be highly detrimental. Therefore, this defect type should not be present in a fully grown and highly efficient device. Furthermore, we relate the beneficial effect of the Cu-rich stage of the three-stage co-evaporation process used to deposit the absorber in high-efficiency devices with the disappearance of these loops. In the case of stoichiometric perfect dislocations, our results show that their electrical activity is related to the presence of cation-cation or anion-anion "wrong" bonds in the cores. Moreover, we found that cation-rich α-cores are active in the Cu(In,Ga)Se2 semiconductor alloy, whereas the anion-rich β-cores are not. These results, along with the study of sodium segregation tendency into the electrically active cores, are put in perspective with respect to the experimental findings and structural models available in literature