Carrier transportation in polycrystalline CuInSe₂ thin films with Cu-deficient grain boundaries

The interface energies and electronic structures of (112) grain boundaries of CuInSe₂ thin films were investigated by first-principle calculations. It is found that the grain boundary with a Cu vacancy has low interface energy and exists widely in the films. The Cu deficiency may cause the charge imbalance and result in an upward band bending at the grain boundary. It also weakens the repulsion between Cu-3d orbital and Se-4p orbital and leads to the downward shift of valence band maximum. The two mechanisms, namely the band bending from the charge imbalance and the depression of the valence band maximum, have effects on the (112) grain boundaries with different defects. The change of band structure forms a potential barrier to prevent electrons or holes from approaching the grain boundary and reduces their recombination. This might be used to explain the effects of the grain boundary on carrier transportation and why polycrystalline Cu(In,Ga)Se₂ thin film solar cells have better performance than single-crystal cells.