The efficiency of silicon solar cells could be improved by specific defects that boost carrier collection out of the cell or improve passivation of the absorber layer. Scientists at the U.S. energy department's National Renewable Energy Laboratory (NREL) ran simulations to add impurities to layers adjacent to the silicon wafer in a solar cell. They introduced defects within a thin tunneling silicon dioxide (SiO2) layer, which forms part of the passivated contact for carrier collection, and within the aluminum oxide (Al2O3) surface passivation layer next to the silicon (Si) cell wafer. The simulations were accomplished using NREL's supercomputer. In both cases, specific defects were identified to be beneficial. The researchers said finding the right defect was key; to promote carrier collection through the tunneling SiO2 layer, the defects needed to have energy levels outside the Si bandgap but close to one of the band edges in order to selectively collect one type of photocarrier and block the other. In contrast, for surface passivation of Si by Al2O3, without carrier collection, a beneficial defect was deep below the valence band of silicon and held a permanent negative charge. The simulations removed certain atoms from the oxide layers adjacent to the Si wafer and replaced them with an atom from a different element to create the defect. For example, when an oxygen atom was replaced by a fluorine atom it resulted in a defect that could possibly promote electron collection while blocking holes. The defects were sorted according to their energy level and charge state. More research is needed in order to determine which defects would produce the best results, the team said, though the principles used in the study are applicable to other materials and devices, such as photoanodes and 2D semiconductors. A recent study by the same team showed that the addition of oxygen could improve the performance of those semiconductors. For solar cells and photoanodes, engineered defects could possibly allow thicker, more robust carrier-selective tunneling transport layers or corrosion protection layers that might be easier to fabricate. The research was published in Applied Physics Letters (doi: http://dx.doi.org/10.1063/1.4939628).