Deep-level defects frequently hamper efficiency of solar cells, but NREL research suggests that defects with properly engineered energy levels can improve carrier collection out of cell, or improve surface passivation of absorber layer. Researchers introduced defects within thin tunneling SiO2 layer that forms part of "passivated contact" for carrier collection, and within Al2O3 surface passivation layer next to Si cell wafer. In both cases, specific defects were identified to be beneficial.