Thin-film silicon solar cells

Abstract

The world is in need for an energy transition from the usage of fossil fuels to renewable energy source to reduce the harmful effects climate and health. Focusing on accelerating the research and development of photovoltaic technologies is an important step towards energy transition. The thin-film silicon solar modules has shown a great potential and a high market prospect. It is flexible, light-weight and also has a low manufacturing and installation cost. It is used for solar pumps and building integration. However, the highest initial conversion efficiency for a micromorph is 14.8%. Thus, we focus on further optimizing the multi-junction solar cells to reach higher efficiency by focusing on the improving the p-layer, texturing and silicon oxide intermediate reflector(SOIR). Further, standardized and error-free external quantum efficiency (EQE) measurement was done to obtain reliable Jsc values.

This work introduces deposition of single, micromorph and triple-junction thin-film solar cells on glass and wafers. The experiments were designed to study the effects of varying the material, thickness of the p-layer and n-SiOx reflective layer and textures of the solar cells. The Jsc from the EQE, Voc, FF and efficiency were the main parameters used to analyze the results.

The Jsc from the EQE measurement was not reliable as the EQE had an artifact while measuring micromorph devices. It was solved by increasing the bias light intensity and decreasing probe light intensity. The forward voltage bias was also found to be important for a cell with low shunt resistance and to measure the target cell in short circuit condition. The standard bias lights to be used are 1-3 to bias the top cell and 7-8 to bias the bottom cell combined with 50% reduction in the probe light intensity.

After correcting the EQE, the optimization of the top cell p-layer was performed on a-Si single junction solar cells on Asahi substrates. It was found that both the contact layer and the window layer of the p-layer have a high influence on the performance of the solar cell. A maximum conversion efficiency of 10% and Voc of 910mV were achieved for the a-Si single junction solar cells. Further, it was also found that there is more than one way to reach high efficiency with an optimal p layer by changing the material composition and thickness of both the contact and window layer. Additionally, p-SiOx contact layer resulted in good performing solar cells for larger range of F_B2H6 of the window layer than p-nc-Si contact layer and the insertion of the buffer layer in the i/p interface showed no significant improvement in the performance of the solar cell.

Furthermore, comparison of the performance of textures of different feature sizes was done on solar cells in p-i-n and n-i-p architecture. It was found that the best optoelectronic performance was obtained from asahi and smooth pyramidal textured solar cell in p-i-n and n-i-p configuration respectively in both double and triple junction solar cell. Further, the honeycomb textured substrate with R_rms around 270nm is not suitable to deposit on double and triple junction solar cell as it resulted in shunted cells.

Finally, the SOIR can be used in a-Si/nc-Si/nc-Si triple junction solar cell to divide current within middle and bottom junctions. The reflection of light in the infrared region increases with the thickness of the SOIR layer. A thick 90nm n-SiOx layer with F_B2H6=3.2sccm gives the best electrical performance and current distribution property for a micromorph.