Understanding the role of grain boundaries in sulfide thin film solar cells with scanning probe microscopy

Grain boundaries (GBs) in Cu(In,Ga)Se₂ (CIGS) and CdTe thin film solar cells are known to be beneficial for achieving high conversion efficiencies. In this work, we will show that Cu₂ZnSnS₄ (CZTS) and Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells have the same GB electronic properties as CIGS and CdTe. Scanning Kelvin Probe Microscopy (SKPM) measurements reveal a higher positive surface potential at the GBs as compared to the grain while Conductive Atomic Force Microscopy (C-AFM) measurements show higher current flow in the vicinity of the GBs. These two measurement results are similar to those obtained for CIGS and CdTe and together they demonstrate the enhanced minority carrier collection taking place at the GBs of CZTS and CZTSSe. This implies that CZTS and CZTSSe solar cells have the potential to achieve the high efficiencies that CIGS and CdTe solar cells enjoy.