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

The potential of solar cells to address global energy and environmental challenges by replacing fossil fuels is increasingly recognized and promoted worldwide. However, the current photoelectric conversion efficiency of solar cells remains relatively low. Three main factors limit this efficiency: light absorption, separation and transport of photogenerated electron-hole pairs, and charge collection. Photovoltaic materials play a central role in determining the performance of solar cells. Therefore, improving the efficiency of solar cells largely depends on enhancing light absorption and suppressing carrier recombination. Research in these areas focuses heavily on manipulating energy band structures. Developing new photovoltaic materials with optimized band alignment remains a major 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, in collaboration with Peking University, made significant progress by incorporating Sn into the In/Ga sites of chalcopyrite-type materials such as CuInS₂ and CuGaS₂. This approach introduced a half-filled intermediate band within the bandgap, effectively broadening the material’s optical absorption range. For example, Sn-doped CuGaS₂ reduced its bandgap to 1.8 eV, extending absorption into the near-infrared region (down to 1.0 eV), while Sn-doped CuInS₂ thin films achieved a bandgap as low as 1.0 eV. This intermediate band acts as a bridge for low-energy photon transitions, overcoming the limitations imposed by the material’s original bandgap. It enables three distinct electron transition channels—valence band to conduction band, valence band to intermediate band, and intermediate band to conduction band—which collectively cover most of the solar spectrum, significantly increasing the photocurrent and potentially boosting the overall conversion efficiency of solar cells. [Image: Energy Band Structure and Broad Spectral Absorption of Sn-doped CuGaS₂ Nanoparticles and Sn-doped CuInS₂ Thin Films] [Image: KBiFe₂O₅ crystal structure, polarization temperature response, room-temperature magnetic response, spectral absorption, and photoelectric response diagram] In another breakthrough, the research team successfully developed a novel narrow-bandgap ferroelectric photovoltaic material called KBiFe₂O₅. By regulating the ion coordination field, they were able to reduce the bandgap width. The material features a three-dimensional framework structure with FeO₄ tetrahedra connected by Bi₂O₂ chains. Its bandgap was measured at 1.59 eV, making it the narrowest among known high-temperature multiferroic materials. Due to its intrinsic polarization field, the recombination of photogenerated carriers is significantly suppressed, resulting in a strong photovoltaic response. The sample generated a voltage exceeding the material’s bandgap, reaching up to 8.8 V with a photocurrent of 15 mA/cm²—outperforming existing ferroelectric photovoltaic materials. This research marks a significant step forward. On one hand, it demonstrates the successful development of an intermediate-band solar cell material with enhanced spectral response and photocurrent. On the other hand, it shows how bandgap engineering can be controlled through structural design in ferroelectric photovoltaic materials. These findings open new possibilities for designing next-generation solar cells with precise microstructures and high conversion efficiency. The study was supported by several key programs, including the National Natural Science Foundation of China, the National 863 Program, the Innovation Project of the Chinese Academy of Sciences, and the Class B Pilot Project of the Chinese Academy of Sciences. The results were published in *Nature Scientific Reports* in 2013 (vol. 3, pp. 1265; vol. 3, pp. 1286).

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