Understanding the Defect 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 structural defects, such as lamellar twins, stacking faults, and double-positioning twin boundaries in polycrystalline photovoltaic materials such as Si, CdTe, and CuInSe₂. We find that individual lamellar twins and stacking faults do not create deep levels in all these materials. However, areas with high density of these defects can form buried wurtzite layers that introduce a barrier to the majority carriers. Double-positioning twin boundaries, which contain dislocation cores, create deep levels in Si and CdTe. Surprisingly, however, they do not create deep levels in CuInSe₂. These results may explain the fact that Si and CdTe solar cells usually require special passivation, whereas CuInSe₂ solar cells do not. Our further study on the passivation effects indicates that grain boundaries in Si cannot be passivated completely by H alone. On the other hand, grain boundaries in CdTe can be passivated well by Cl and I.