by Solar technologies have become instrumental in reducing carbon emissions and combating climate change. Among the various solar technologies, silicon heterojunction (SHJ) solar cells have shown great promise for future photovoltaic applications due to their high power conversion efficiencies, low-temperature processing, and low temperature coefficients. However, the fabrication processes of most high-efficiency SHJ solar cells are expensive and not scalable, which limits their large-scale deployment.
To address this challenge, a team of researchers from Suzhou Maxwell Technologies Co. Ltd., Soochow University, New South Wales University, and Dalian University of Technology has developed a new method for fabricating SHJ solar cells using more affordable and scalable techniques. Their findings, published in the journal Nature Energy, demonstrate SHJ solar cells with power-conversion efficiencies of up to 26.4%.
Professor Xinbo Yang, a co-author of the paper, explained that the commercial success of high-efficiency SHJ technology has been hindered by the cost-effective translation of several process steps into a production environment. This includes the deposition of a high transparent window-layer and low-cost metallization.
The team addressed the first challenge by replacing conventional doped a-Si:H with doped hydrogenated nanocrystalline silicon (nc-Si:H) or its alloys with oxygen (nc-SiOx:H) and carbon (nc-SiC:H). This helped reduce parasitic absorption and series resistance, resulting in higher efficiency.
Previously, researchers employed very high frequency (VHF) plasma-enhanced chemical vapor deposition (PECVD) systems instead of standard radiofrequency systems to fabricate SHJ structures. The use of VHF increased the deposition rate and reduced ion bombardment, leading to improved performance. However, VHF PECVD systems require a reactor chamber size that is less than a quarter wavelength than the plasma frequency to ensure uniform deposition.
The second challenge involves the high-cost metallization process for SHJ solar cells. By replacing the screen-printed silver electrodes with high-quality and low-cost plated copper electrodes, the team achieved not only a lower cost but also significantly reduced shading losses and improved contact resistivity.
In their fabrication process, the researchers used wet-chemical processes on commercial n-type silicon wafers. They applied intrinsic a-Si:H passivation layers on both sides of the wafer and deposited nc-SiOx:H(n) layers on the front as the front window layer. The rear deposition involved a nc-Si:H(p) layer. To enhance performance, a newly-developed transition metal doped indium oxide (IMO) layer was sputtered on both sides. Finally, seed-free plated-copper electrodes were used instead of screen-printed silver electrodes.
Through a series of tests, the researchers achieved remarkable results. Under a PECVD excitation frequency of 27 MHz, the SHJ cells achieved a high efficiency of 25.98%. The large-area nc-SiOx:H(n) thickness and device-PCE distributions were superior due to minimized standing wave effects achieved by reducing the plasma frequency. By replacing screen-printed silver with plated-copper electrodes, the current density and fill factor of the devices significantly improved, resulting in a world-record power-conversion efficiency of 26.41% for full-size SHJ solar cells with plated copper electrodes.
The scalability and compatibility of the deposition techniques used in this study make it possible to fabricate SHJ solar cells on a large scale and deploy them in real-world applications. The researchers anticipate that other research teams can utilize these techniques to develop SHJ cells and other photovoltaic technologies.
However, further studies are needed to understand the effect of PECVD excitation frequency on the microstructure of nc-SiOx:H(n) films and device stability. Additionally, optimization of the copper plating system for mass-production is necessary for reducing process complexity and improving yield.
In conclusion, the development of silicon heterojunction solar cells with record-breaking efficiency demonstrates the potential for affordable and scalable fabrication processes in the solar industry. These advancements bring us one step closer to achieving widespread adoption of solar technologies and mitigating the environmental impacts of traditional energy sources.
