We have studied the atomic and electronic structure of Cu 2ZnSnSe4 and CuInSe2 grain boundaries using first-principles calculations. We find that the constituent atoms at the grain boundary in Cu2ZnSnSe4 create localized defect states that promote the recombination of photon-excited electron and hole carriers. In distinct contrast, significantly lower density of defect states is found at the grain boundaries in CuInSe2, which is consistent with the experimental observation that CuInSe2 solar cells exhibit high conversion efficiency without the need for deliberate passivation. Our investigations suggest that it is essential to effectively remove these defect states in order to improve the conversion efficiency of solar cells with Cu 2ZnSnSe4 as photovoltaic absorber materials. © 2011 American Chemical Society.