Crystalline Si Surface Passivation with Nafion for Bulk Defects Detection with Electron Paramagnetic Resonance

Here, in monocrystalline Si (c-Si) solar cells, identification and mitigation of bulk defects are crucial to achieving a high photoconversion efficiency. To spectroscopically detect defects in the c-Si bulk, it is desirable to passivate the surface defects. Passivation of the c-Si surface with dielectrics such as Al₂O₃ and SiNx requires deposition at elevated temperatures, which can influence defects in the bulk. Herein, we report on the passivation of different Czochralski (Cz) Si wafer surfaces by an organic copolymer, Nafion. We test the efficacy of the surface passivation at temperatures ranging from 6 to 473 K to detect bulk defects using electron paramagnetic resonance (EPR) spectroscopy. By comparing with state-of-the-art passivation layers, including Al₂O₃ and liquid HF/HCl, we found that at room temperature, Nafion can provide comparable passivation of n-type Cz Si with an implied open-circuit voltage (iV_oc) of 713 mV and a recombination current prefactor J_o of 5 fA/cm². For p-type Cz Si, we obtained an iV_oc of 682 mV with a J_o of 22.4 fA/cm². Scanning electron microscopy and photoluminescence reveal that Nafion can also be used to passivate the surface of c-Si solar cell fragments scribed from a solar cell module by using a laser. Consistent with previous studies, analysis of the EPR spectroscopy data confirms that the H-terminated surface is necessary, and fixed negative charge in Nafion is responsible for the field-effect passivation. While the surface passivation quality was maintained for almost 24 h, which is sufficient for spectroscopic measurements, the passivation degraded over longer durations, which can be attributed to surface SiO_x growth. These results show that Nafion is a promising room-temperature surface passivation technique to study bulk defects in c-Si.. Authors: Chen, Kejun [Colorado School of Mines, Golden, CO (United States); National Renewable Energy Laboratory (NREL), Golden, CO (United States)] (ORCID:0000000323433127); Johnston, Steve W. [National Renewable Energy Laboratory (NREL), Golden, CO (United States)]; Taylor, P. Craig [Colorado School of Mines, Golden, CO (United States)]; Mulder, David W. [National Renewable Energy Laboratory (NREL), Golden, CO (United States)] (ORCID:0000000305590145); Guthrey, Harvey L. [National Renewable Energy Laboratory (NREL), Golden, CO (United States)] (ORCID:0000000315743379). DOE Contract: AC36-08GO28308. Subjects: 14 SOLAR ENERGY; 36 MATERIALS SCIENCE; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Al2O3 dielectric passivation; crystalline silicon; electron paramagnetic resonance; nafion; photoluminescence; surface passivation