Surface defect passivation by hydrogen sulfide (H2S) reaction and stability under various stress conditions for different Si surfaces
Date
2025
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
The objective of this research was to develop a novel Si surface passivation method using sulfur (S) as a passivating element to withstand industry-standard high temperature contacting and metallization schemes for p-type passivated emitter and rear contact (p-PERC) solar cells. With the successful application of an aluminum oxide (Al2O3) passivation layer, the PERC cell’s back surface passivation has been improved drastically. However, the front n+ diffused junction surface is still poorly passivated by the standard amorphous silicon nitride (a-SiNX:H) anti-reflection coating (ARC) layer. This project addressed the passivation challenges of both the front n+ emitter and the undiffused p-Si back surface. ☐ To pursue these objectives, systematic investigation of process-structure-properties-performance relationships of this novel advanced defect passivation approach was performed. S-passivation was carried out by reacting industrial Czochralski (Cz) Si wafers in H2S using an atmospheric pressure thermal chemical vapor reactor (APTCVR) at temperatures up to 700°C. After systematic optimization of reaction processes (temperature, time, and gas concentration), extremely low surface recombination velocities (SRVs) of 1.5 cm/s and 8 cm/s on n-type and p-type Si, respectively, by S-passivation were demonstrated. In-depth surface and interface characterization using Fourier transform infrared spectroscopy (FTIR), time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and x-ray photoelectron spectroscopy (XPS) revealed sulfur bonded with Si as silicon sulfide (SiS2) after reacting the Si surface in H2S. Furthermore, application of the optimized S-passivation to the n+ diffused emitter surface led to a low surface recombination current density, J0n+ ≈ 40 fA/cm2 (~ 1/4 of the industry standard a-SiNX:H-passivation), and high implied VOC (686 mV) in p-PERC solar cell structures. The S-passivation was further shown to preserve the bulk quality of the p-type and n-type Si, better than the silicon dioxide (SiO2) or Al2O3 passivation processes. ☐ After successful demonstration of effective S-passivation of the Si surface, the air, thermal, and illumination stability of the passivation structure were characterized. S-passivation itself degrades in air due to competing reactions with moisture and oxygen to form oxides. This can be eliminated by an a-SiNX:H capping layer (also acting as an anti-reflective coating). After a-SiNX:H process optimization, illumination and thermally stable S-passivation with SRV < 5 cm/s and J0 < 80 fA/cm2 were demonstrated. Incorporation of S/a-SiNx:H passivation stack of n+ diffused emitter surface in p-PERC cells yielded an efficiency ≈ 20% with VOC ≈ 650 mV, using manufacturing metallization and contacting schemes. Loss of VOC and efficiency in completed p-PERC cell have resulted from the diffusion of sulfur and/or modification of the a-SiNX:H layer after high-temperature exposure in the contact firing process. This was corroborated from detailed surface analysis using XPS, ToF-SIMS and SEM. Nonetheless, promising results of S-passivation, being incorporated into p-PERC cell, inspired the application of S-passivation into Si-heterojunction cells that do not require high temperature metal firing step and showed an encouraging iVOC = 684 mV. However, after evaporated metallization a low cell VOC of 606 mV was recorded, which was caused by an increase in defect density (Dit) and decrease in fixed charge density (Qfix) estimated by numerical software at the SiS2/Si interface. ☐ While the S-passivation of Si surfaces show significant promise with excellent passivation quality of non-metallized cell structures, further works are needed to develop high-temperature-tolerant capping layer and/or engineering of advanced device structures.
Description
Keywords
Hydrogen sulfide reaction, Silicon solar cell, Stability, Surface defect passivation