Photovoltaic devices are renewable energy sources that are expected to play a vital role in reducing carbon footprint. Among these devices, III-V solar cells have drawn significant research attention for potential applications in vehicles owing to their high efficiency and high-power generation capacity. Recently, car manufacturers like Toyota have demonstrated that hybrid cars can travel up to 45 km a day using solar energy. This shows that the widespread adoption of solar-powered vehicles could significantly reduce carbon dioxide emissions. Nevertheless, the widespread adoption of this technology is challenging due to the high cost of manufacturing III-V solar cells, an issue that requires urgent solutions.
Hydride vapor phase epitaxy (HVPE) is deemed a convenient and promising method for reducing the production cost of III-V solar cells. For a long time, III-V solar cells fabricated via HVPE have not achieved the desirable high performance attributed to various issues associated with low yielding conversion efficiency and the quality of the passivation layers and the interfaces. Interestingly, most of these issues can be addressed by using a multi-chamber HVPE system that allows chamber switching, as demonstrated in recent research findings. Unfortunately, the technical limitations of HVPE are associated with the growth of materials containing aluminum.
Metal monochlorides like InCl and GaCl are often used as precursors to grow III-V semiconductors. The violent reaction between AlCl and HVPE reactor is another limitation of the HVPE system. Based on recent breakthroughs leading to the solution to these limitations, the photocurrent production and conversion efficiency of InGaP/GaAs have been improved. Further improvements require the introduction of AlInGaP back-surface field (BSF) in the cell structure. The inclusion of both window and the BSF layers, it is necessary to grow Al-containing materials multiple times and in a way that does not influence the growth of other layers.
Herein, Dr. Yasushi Shoji, Dr. Ryuji Oshima, Dr. Kikuo Makita and Dr. Takeyoshi Sugaya from the National Institute of Advanced Industrial Science and Technology (AIST) together with Dr. Akinori Ubukata from Taiyo Nippon Sanso Corporation presented III-V tandem solar cells based on AlInGaP BSF layers. The solar cells were grown via a triple-chamber HVPE system using an aluminum trichloride precursor for depositing Al. In particular, the authors investigated the underlying phenomenon occurring in the layers beyond the AlInGaP layers fabricated via HVPE. Finally, the performance of the resulting InGaP/GaAs tandem solar cells was examined. The work is currently published in the journal, Solar RRL.
The authors reported that HVPE-grown AlInGaP layers exhibited n-type conductivity due to the Si contamination In contrast, p-type conductivity could be realized by doping with Zn concentration higher than Si contamination concentration. The introduction of the AlInGaP BSF layer contributed to improving the open-circuit voltage and short-circuit current density of the InGaP single-junction solar cells. Consequently, the efficiency of the InGaP solar cells was 17.1%, while that of InGaP/GaAs tandem cells was 28.3% – the highest value ever reported for HVPE-grown solar cells.
In summary, the researchers reported the performance improvement of InGaP single-junction solar cells fabricated in a triple-chamber HVPE system by introducing p-type AlInGaP BSF layers. Results showed that this fabrication approach allowed low-cost fabrication of III-V solar cells with record efficiency and improved performance. In a statement to Advances in Engineering, Dr. Yasushi Shoji said that their study would contribute to developing a – HVPE system for mass production of high performance III-V devices for use in hybrid cars and other applications.
Shoji, Y., Oshima, R., Makita, K., Ubukata, A., & Sugaya, T. (2021). 28.3% Efficient III–V Tandem Solar Cells Fabricated Using a Triple‐Chamber Hydride Vapor Phase Epitaxy System. Solar RRL, 6(4), 2100948.