Atomic-scale analyses represent oxygen-containing layers near grain boundaries in Cu2ZnSnSe4 films. In article number 1501902, Sung-Yoon Chung and co-workers identify the presence of nanoscale layers in which oxygen is substantially substituted for Se. Theoretical calculations demonstrate that oxygen substitution lowers the valence band maximum and thus makes a key contribution to the formation of a barrier blocking the holes from bulk grains.