Atomic-scale defects in solar cell material CuInSe₂ from hybrid-functional calculations

The operation and properties of any semiconductor device rely on its defect microstructure. In thin-film solar cells, point defects are a determining factor for the conversion efficiency of the device by controlling doping but also by degrading device performance. Additionally, point defects play a role in material growth and processing by mediating diffusion. Knowledge of point defects and defect-related processes is therefore essential in optimizing solar cell performance. In this thesis, point defects in the solar cell absorber material CuInSe₂ (CIS) have been investigated with computational methods. Starting from the thermodynamics of individual point defects and extending to diffusion kinetics and clustering, the aim of the thesis is to gain a comprehensive understanding of the defect microstructural features of the material and their effect on its electronic properties. The calculations have been performed with density-functional theory (DFT) employing a hybrid exchange-correlation functional, which has been demonstrated to describe semiconductor properties better than previously used (semi)local-density functionals. The calculations presented in this thesis show that point defects in CIS participate in a variety of competing atomic-scale processes, which affect their distribution within the material. By taking into account defect interactions, it is demonstrated that there exists a thermodynamic driving force towards the creation of defect complexes such as InCu-2VCu and VSe-VCu. Interaction of intrinsic defects with impurities can also lead to surprising effects: it is found that by introducing sodium atoms into CIS, copper mass transport is reduced due to the capture of copper vacancies. The effect of the prevalent point defects and defect complexes on the electronic properties is found to essentially depend on whether the defect is of cationic or anionic type. Only selenium-related anionic defects are observed to induce deep defect levels within the CIS band gap, implying that they may act as recombination centers. The results presented in this thesis help to explain and interpret experimental observations of atomic-scale phenomena occurring in CIS. Further, they provide computational insight on defect-related mechanisms that may sometimes remain out of reach in experiments. The findings can be employed to gain better control over film quality and device operation in CIS-based solar cells.