Grain-boundary physics in polycrystalline photovoltaic materials

We use the combination of high-resolution electron microscopy and density-functional theory to study the atomic structure and electronic effects of grain boundaries in polycrystalline photovoltaic materials such as Si, CdTe, CuInSe2, and CuGaSe2. We find that grain boundaries containing dislocation cores create deep levels in Si, CdTe, and CuGaSe2. Surprisingly, however, they do not create deep levels in CuInSe2. We further find that the presence of Ga in grain boundaries in CuInSe2 generates deep levels. These results may explain the fact that Si and CdTe solar cells usually require special passivation, whereas CuInSe2 solar cells do not. The passivation of grain boundaries in Si and CdTe is also studied. We find that grain boundaries in CdTe can be passivated very well by Cl, Br, and I.