| May 12, 2022 |
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(Nanowerk News) While efficiency is a primary concern for solar cells, researchers have also focused on developing solar cells that are lightweight, low-cost, and flexible. However, the fabrication process itself has posed a serious environmental concern: the use of toxic materials and generation of industrial waste.
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For instance, copper indium gallium selenide (CIGSe) is a thin-film solar cell that offers several advantages over traditional silicon solar cells. Thin-film solar cells are about 100 times thinner, cheaper to make, and are easier to install on rooftops and vehicles.
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Moreover, compared to other photovoltaic materials used in thin-film solar cells like amorphous silicon, cadmium-telluride, and organic materials, CIGSe absorbs light more strongly and can be prepared into thinner films.
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However, they contain a buffer layer of cadmium sulfide, which is highly toxic and carcinogenic. This makes finding alternative, non-toxic materials essential for large-scale production and installation of CIGSe panels.
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For Professors Jakapan Chantana and Takashi Minemoto at Ritsumeikan University, Japan, removing cadmium from solar cells was as important as developing an eco-friendly manufacturing process that is both efficient and affordable. Addressing these issues in a new study (Solar RRL, “Formation of Native Inx(O,S)y Buffer through Surface Oxidation of Cu(In,Ga)(S,Se)2 Absorber for Significantly Enhanced Conversion Efficiency of Flexible and Cd-Free Solar Cell by All-Dry Process”), a research team led by them developed a strategy in which the traditional cadmium sulfide buffer layer was replaced with a native buffer layer formed by oxidizing the surface of the Cu(In,Ga)(S,Se)2 CIGSSe layer with an air-annealing process.
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| The roll-to-roll process with accelerated oxidation for the fabrication of CIGGSe solar cells. (Image: Ritsumeikan University)
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While attempts to oxidize the CIGSSe layer have been made before, the surface usually takes months to oxidize. With the new method, however, the team reduced the oxidation time to a few hours, allowing for a faster manufacturing by a “roll-to-roll” process.
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In this process, a CIGSSe layer is initially deposited on a flexible stainless-steel substrate. The deposition is then followed by an air-annealing process where the surface of the CIGSSe layer is oxidized to form native buffer layers of Inx(O,S)y.
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By experimenting with different oxidation conditions, the researchers fabricated a CIGSSe solar cell with a maximum energy conversion efficiency of 16.7% after 6 hours of oxidation at 130 °C.
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“We have disclosed for the first time that the CIGSSe surface oxidized through an optimized air-annealing process leads to a strong enhancement in energy conversion efficiency,” says Prof. Minemoto.
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Although the reported efficiency is lower than that of conventional solar cells (which typically exceed 20%), the developed method manages to do away with cadmium, making the solar cells eco-friendly.
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“In the conventional process, cadmium is deposited on the CIGSSe layer via a chemical bath deposition process. By eliminating this step, we have created a completely dry manufacturing process that generates less waste,” explains Prof. Chantana. Moreover, the process is also cost-effective.
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In order to make solar energy a viable source of clean energy, solar panels must become more efficient, economical, and eco-friendly.
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“The method developed in our study can be scaled to large-scale manufacturing applications, which is what we need to make solar cells a clean energy resource not only in Japan but all over the world,” concludes Prof. Minemoto.
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