According to the research team, deep-level defects frequently hamper the efficiency of solar cells, but theoretical research suggests that defects with properly engineered energy levels can improve carrier collection out of the cell, or improve surface passivation of the absorber layer. The team ran simulations to add impurities to layers adjacent to the silicon wafer in a solar cell, NREL said in a Jan. 12 statement. In their research, the team introduced defects within a thin tunneling silicon dioxide (SiO2) layer that forms part of “passivated contact” for carrier collection, and within the aluminum oxide (AI2O3) surface passivation layer next to the silicon (Si) cell wafer. In both cases, NREL said, specific defects were identified to be beneficial.
Schematic of a ‘good’ defect (red cross), which helps collection of electrons from photo-absorber (n-Si), and blocks the holes, hence suppresses carriers recombination. Credit: NREL
The research paper “Suppress carrier recombination by introducing defects: The case of Si solar cell” by principal scientist Pauls Stradins, along with Yuanyue Liu, Su-Huai Wei, Hui-Xiong Deng, and Junwei Luo, appears in Applied Physics Letters.
NREL said that finding the right defect was key to the process. The team found that in order to promote carrier collection through the tunneling SiO2 layer, the defects 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, NREL said, for surface passivation of Si by AI2O3, 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.”
According to NREL, more research is needed in order to determine which defects would produce the best results.
The research was funded by DOE’s SunShot Initiative as part of a joint project of Georgia Institute of Technology, Fraunhofer ISE, and NREL.
Lead image: Solar cells. Credit: Shutterstock.