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

Area of Science:

• Materials Science
• Semiconductor Physics
• Solar Energy

Background:

• Defect identification and mitigation in monocrystalline silicon (c-Si) solar cells are critical for high photoconversion efficiency.
• Surface defect passivation is essential for spectroscopically detecting bulk defects in c-Si.
• Conventional dielectric passivation methods require high temperatures, potentially affecting bulk defect characteristics.

Purpose of the Study:

• To investigate Nafion, an organic copolymer, as a novel room-temperature surface passivation technique for Czochralski (Cz) silicon wafers.
• To evaluate the efficacy of Nafion passivation across a wide temperature range (6–473 K) for bulk defect detection using electron paramagnetic resonance (EPR) spectroscopy.
• To compare Nafion's passivation performance against established methods like Al2O3 and liquid HF/HCl.

Main Methods:

• Surface passivation of n-type and p-type Cz Si wafers using Nafion.
• Electron paramagnetic resonance (EPR) spectroscopy for defect detection.
• Measurement of implied open-circuit voltage (iVoc) and recombination current prefactor (J0).
• Scanning electron microscopy (SEM) and photoluminescence (PL) for surface analysis.
• Testing passivation stability over time.

Main Results:

• Nafion provided comparable passivation to state-of-the-art methods at room temperature, achieving iVoc of 713 mV and J0 of 5 fA/cm² for n-type Cz Si.
• For p-type Cz Si, Nafion yielded iVoc of 682 mV and J0 of 22.4 fA/cm².
• Nafion effectively passivated laser-scribed c-Si solar cell fragments, confirmed by SEM and PL.
• EPR data confirmed the necessity of H-terminated surfaces and field-effect passivation due to fixed negative charge in Nafion.
• Passivation quality was maintained for ~24 hours but degraded long-term due to SiO2 growth.

Conclusions:

• Nafion is a promising room-temperature surface passivation material for c-Si solar cells.
• It enables effective defect detection via EPR spectroscopy, comparable to high-temperature methods.
• Further research into long-term stability and degradation mechanisms is warranted.