Tuning Band Alignment at Grain Boundaries for Efficiency Enhancement in Cu2ZnSnS4 Solar Cells

Conducting atomic force microscopy has been performedfor a fundamentalunderstanding of the mechanism responsible for the lower power conversionefficiency (PCE) of Cu2ZnSnS4 (CZTS) solar cellsthan that of CuIn1-x Ga x Se2 (CIGS) solar cells. The differencein efficiency is partly attributed to the distinctly different bandalignment between the grain boundaries (GBs) and grain interior (GI)for the two materials. While CIGS shows type-II band alignment, CZTSwas discovered to demonstrate type-I band alignment with the conductionband shifting downward while the valence band shifting upward at theGBs. The type-I band alignment in CZTS leads to both electron andhole trapping, enhancing their recombination, and lowers the PEC.Band engineering was realized by moderate oxidative annealing of CZTS.The preferential GB oxidation changes the band alignment into inversetype-I (i.e., the conduction band upward bending and valence banddownward bending at GBs). The blocking of carrier recombination atGBs leads to 30% enhancement in PCE. Our work reveals the criticalrole that band alignment between the grain boundary and interior playsin polycrystalline thin film solar cells and suggests band alignmentengineering as a practical approach to enhance PCE. Furthermore, conductingAFM has been shown to be a powerful tool for qualitative and semiquantitativecharacterization of band alignment in polycrystalline films.