Optimization of Hydrogenated Amorphous Si Layer for Si Heterojunction Solar Cells

Abstract

Crystalline silicon (c-Si) solar cells that are well-established technologies have already achieved a power conversion efficiency (PCE) as high as 26.7% by using the interdigitated-back-contacted (IBC) configuration. However, this efficiency is still limited by the spectral mismatch between t¬he absorption characteristics of c-Si and the AM 1.5 spectrum. To better utilize especially the low wavelength irradiance and to exceed the single-junction silicon solar cell theoretical PCE limit (29.43%), the perovskite/c-Si tandem solar cell concept was proposed.
This thesis project aims to develop a high efficiency single-side textured front-back contacted (FBC) silicon heterojunction (SHJ) solar cell as the bottom cells for 2-terminal (2-T) perovskite/c-Si tandem solar cells. Therefore, we firstly focus on the optimizations of the deposition parameters (power and precursor gases) of hydrogenated intrinsic amorphous silicon ((i)a-Si:H) passivation layer and the subsequent application of hydrogen plasma treatment (HPT) on the (i)a-Si:H layer. An optimized 8.8 nm-thick single (i)a-Si:H together with hydrogen plasma treatment (HPT) achieve an effective lifetime (τeff) of 1.3 ms, implied open-circuit voltage (iVOC) of 704.3 mV on symmetrical passivated double-side-flat <100> oriented crystalline silicon (c-Si) wafer. To further improve the passivation, a bilayer structure of (i)a-Si:H with a total thickness of 10 nm is optimized to boost further the τeff to 8.3 ms and iVOC to 733.5 mV.
Afterwards, we implement the optimized (i)a-Si:H layers into the front/back-contacted (FBC) rear junction solar cells with single-side textured morphology, which is designed to be used as the substrate for future integration of solution-processed perovskite top cell. For solar cells investigated, we keep the textured rear side always the same with optimized contact stacks consisting of p-type hydrogenated nanocrystalline silicon oxide ((p)-nc-SiOX:H) and p-type hydrogenated nanocrystalline silicon ((p)-nc-Si:H, while varying the layer stacks on the flat front side. Specifically cell performances are studied with either n-type hydrogenated nanocrystalline silicon ((n)-nc-Si:H) or n-type hydrogenated amorphous silicon ((n)-a-Si:H) with varied optimized (i)a-Si:H layers. We observe enhanced passivation qualities of solar cell precusors when adding the (n)-nc-Si:H/(n)-a-Si:H layer on top of the (i)a-Si:H, which enables the potential for realizing high-efficiency solar cells.
During the fabrication of FBC-SHJ solar cells, we also find that a thinner (i)a-Si:H on the front side is critical to improve the device efficiency thanks to the effective reduction of both parasitic absorption and carrier transport losses. Accordingly the gain in JSC and FF dominate the improvement of PCE. Lastly, we present rear junction FBC-SHJ solar cell with n optimized stack of (n)nc-Si:H and (i)a-Si:H, with VOC of 711 mV, JSC of 34.82 mA/cm2, FF of 79.64% and PCE of 19.72%. This optimized solar cell is also considered to be ready for its application in tandem device fabrication.