Silicon defect passivation by H2S reaction and patterning process of interdigitated back contact silicon heterojunction (IBC-SHJ) solar cell
Date
2018
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
The interdigitated back contact Si heterojunction (IBC-SHJ) solar cell has achieved record efficiency of 26.7% for Si based devices, because of excellent surface passivation provided by thin a-Si:H layers deposited at low temperature as the heterojunction partner and elimination of shading loss on the front surface of IBC structure, since both base and emitter contacts are at the rear surface. ☐ The back contact patterning approach was evaluated to enhance the IBC-SHJ solar cell performance and improve its stability in this work. A single i. a-Si:H layer passivation at the gap between contacts and a potential contamination arising from presence of photoresist in PECVD a-Si:H growth lead to low and unstable cell efficiency, which was only 15% at the beginning and degraded to 14.3% within a day. Therefore, boosting the cell performance and stability with a robust back contact patterning method for a clean deposition process and sufficient gap passivation is the main goal of this work. The highest cell performance of 20.2% with a perfect stability over time was achieved with improved patterning approach. ☐ To further improve the IBC-SHJ solar cell efficiency, front surface field (FSF) using more conductive doped layer for additional field-effect passivation and its effect on device series resistance was investigated. To evaluate the effect of FSF, four different structures of front surface stack layers were studied, including 1) i. a-Si:H / ARC (control sample); 2) n+ FSF / i. a-Si:H / ARC; 3) i. a-Si:H / n. a-Si:H / ARC; and 4) n. a-Si:H / ARC. The FSF can be provided by either n+ phosphorous diffusion or n. a-Si:H via PECVD. Slightly better surface passivation was observed using n+ FSF. However, no improvement on device performance by FSF was found among these four different structures. ☐ Since both the base and emitter contacts are at the back surface of IBC-SHJ solar cell, the optical and electrical properties can be optimized separately, and therefore high passivation level and minimum absorption loss are the two goals for the front surface of IBC-SHJ solar cell. Based on this idea, the i. a-Si:H with high absorption coefficient and bandgap of ~1.7 eV mostly used for the front surface passivation can be replaced by alternative passivation layers with higher bandgap and reduced thickness to minimize the absorption loss and enhance its short circuit current (JSC). To realize this, we propose a new c-Si surface passivation scheme using H2S reaction. X-ray photoelectron spectroscopy (XPS) identified the bonding state S-Si-S contributing to the passivation of c-Si surface defects. Monolayer thickness of sulfur forming on c-Si surface is capable of providing the same passivation level as ~8 nm thick i. a-Si:H layer often used in IBC-SHJ solar cell. Compared to i. a-Si:H, SiS2 passivation layer possessing higher bandgap and reduced thickness shows a great potential for improving the JSC of IBC-SHJ solar cell especially in shorter wavelength (< 700 nm). Although the surface passivation by H2S reaction is unstable in air due to hydrolysis and oxidation of SiS2, the degradation of τeff can be eliminated by a capping layer of a-SiNX:H deposited right after H2S reaction, which also acts as an anti-reflection layer for IBC-SHJ solar cell. ☐ It is found from XPS analysis that the sulfur not only terminated c-Si surface dangling bonds, but also can diffuse into Si bulk during H2S reaction. This opened up another interesting concept of using H2S reaction to passivate mc-Si wafer bulk defects. This work shows that the reduction of both trap and recombination states can be achieved by H2S reaction of commercial mc-Si wafers, which is demonstrated from the injection level dependent τeff characterization. Further wet chemistry process confirmed both the surface and bulk of mc-Si wafer can be passivated in H2S reaction, and the unstable H2S-passivated surface can be removed and re-passivated by other materials without affecting the passivated bulk quality. The preservation of H2S passivated bulk quality in multiple wet chemistry processes will allow fabrication of mc-Si / a-Si:H heterojunction solar cells.
Description
Keywords
Applied sciences, Semiconductor, Silicon defect, Silicon surface passivation, X-ray photoelectron spectroscopy