Shanghai Silicate Institute made progress in the research of new photovoltaic materials

The growing importance of solar cells in addressing global energy and environmental challenges is widely acknowledged. As a promising alternative to fossil fuels, solar technology has the potential to revolutionize energy production. However, the current photoelectric conversion efficiency of solar cells remains relatively low. Three key factors limit this efficiency: light absorption, charge carrier separation and transport, and charge collection. Improving these aspects is crucial for enhancing the performance of solar cells. Photovoltaic materials play a central role in determining the efficiency of solar cells. To boost efficiency, researchers focus on maximizing light absorption and minimizing the recombination of photogenerated charge carriers. These efforts are largely centered on manipulating the material's energy band structure. Developing new photovoltaic materials with optimal band alignment remains a significant challenge and an active area of research. Recently, a research team led by Professor Huang Fuqiang from the Shanghai Institute of Ceramics, Chinese Academy of Sciences, and Peking University’s Department of Chemistry made a breakthrough. They introduced tin (Sn) into the In/Ga sites of chalcopyrite-type materials such as CuInS₂ and CuGaS₂. This innovative approach created a half-filled intermediate band within the bandgap, significantly broadening the material's optical response range. For example, Sn-doped CuGaS₂ reduced its bandgap to 1.8 eV, extending its absorption into the near-infrared region. Similarly, Sn-doped CuInS₂ thin films achieved a bandgap as low as 1.0 eV. This allows for three distinct electron transition channels—valence band to conduction band, valence band to intermediate band, and intermediate band to conduction band—which cover most of the solar spectrum. This advancement leads to a substantial increase in photocurrent and holds great promise for improving solar cell efficiency.  Another notable achievement from the same team was the development of a novel narrow-bandgap ferroelectric photovoltaic material, KBiFe₂O₅. By adjusting the ion coordination field, they successfully reduced the bandgap to 1.59 eV—the smallest among known high-temperature multiferroic materials. The material features a three-dimensional framework structure with FeO₄ tetrahedra connected by Bi₂O₂ chains. Its intrinsic polarization field helps suppress charge recombination, resulting in a strong photovoltaic response. The device produced a voltage of up to 8.8 V and a photocurrent of 15 mA/cm², surpassing the performance of existing ferroelectric photovoltaic materials. This research marks a significant step forward in the development of next-generation solar cells. It not only introduces a new class of intermediate-band materials with enhanced spectral response and photocurrent but also demonstrates how bandgap engineering can be used to improve the performance of ferroelectric photovoltaics. These findings open up new possibilities for designing solar cells with controlled microstructures and higher efficiency. The study was supported by several key funding programs, including the National Natural Science Foundation of China, the National 863 Program, and the Chinese Academy of Sciences' Innovation and Pilot Projects. The results were published in *Nature Scientific Reports* in 2013 (Volume 3, Pages 1265 and 1286).

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