Scientists at NREL are studying the somewhat paradoxical possibility that certain defects in silicon solar cells could actually improve their performance, something that, according to principal scientist Pauls Stradins (who is also project leader of the silicon photovoltaics group at NREL) runs counter to conventional wisdom.
Solar cell efficiency is frequently hampered by deep-level defects, but theoretical research at NREL suggests that defects with properly engineered energy levels can improve carrier collection out of the cell, or improve surface passivation of the absorber layer. To investigate this, researchers ran simulations involving the addition of layers adjacent to the silicon wafer in a solar cell. In doing so, they introduced defects within a thin tunnelling silicon dioxide (SiO2) layer that forms part of "passivated contact" for carrier collection, and within the aluminum oxide (Al2O3) surface passivation layer next to the silicon (Si) cell wafer. In both cases, specific defects were identified to be beneficial.
The simulations were carried out using the NREL supercomputer and the National Energy Research Scientific Computing Centre. The research by Stradins, Yuanyue Liu, Su-Huai Wei, Hui-Xiong Deng, and Junwei Luo, “Suppress carrier recombination by introducing defects: The case of Si solar cell,” appears in Applied Physics Letters.
Key to the process was finding the right defect. They need 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 is deep below the valence band of silicon and holds 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, thereby creating a “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 then sorted according to their energy level and charge state.
More research is needed in order to determine which defects would produce the best results. However, the principles used in the study are applicable to other materials and devices, such as photoanodes and two-dimensional semiconductors. A recent study by the same authors has shown 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 funded by the U.S. Department of Energy SunShot Initiative as part of a joint project of Georgia Institute of Technology, Fraunhofer ISE, and NREL, with a goal to develop a record efficiency silicon solar cell.
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