Researchers at the University of Oxford have discovered that bismuth sulfide material can overcome a major limitation in solar energy. The problem was that materials that absorb sunlight strongly usually lose electrical charge too quickly to generate efficient power. The identified semiconductor material has an unusual internal energy setup that acts like a temporary waiting room. Specifically, it holds onto captured energy longer and reducing charge loss by about 75% at room temperature. “One of the longstanding challenges in developing efficient solar-energy materials is that the properties which make a semiconductor very good at absorbing sunlight also tend to make the resulting electrical charges disappear quickly. Our work shows that this trade-off is not inevitable,” said Professor Laura Herz, corresponding author from the Department of Physics, University of Oxford.The key technology Solar panels generate power by capturing freed electrons before these charges recombine with positively charged holes and dissipate energy as heat.Materials designed to absorb maximum sunlight naturally create a crowded environment where these charges collide almost instantly. This rapid recombination creates an efficiency barrier for standard light-harvesting semiconductors.Bismuth sulfide-based semiconductors could help change this. The Oxford team used ultrafast terahertz spectroscopy, which basically fires laser pulses lasting mere fractions of a second. This technique helped to probe thin films of the material.What they found surprised them. Bismuth sulfide features a rare quantum arrangement known as a pseudo-direct semiconductor.Light energy enters through a primary, high-energy pathway where sunlight is strongly absorbed. Immediately below this entrance lies a lower-energy state that captures the excited charges. Acting as a temporary holding tank, this secondary state stores the charges safely before dissipating them.This secondary route dissipates charges poorly, allowing electrons to rest safely in storage until harvest time. At room temperature, this microscopic waiting room slashes charge loss by a 75% compared to conventional materials.“By uncovering how an unusual electronic structure can combine strong light absorption with much slower recombination, we have identified a new principle for designing semiconductors that could help us harvest solar energy more efficiently,” Herz added. Non-toxic bismuth sulfideThe material is not yet a market-ready rooftop panel. The Oxford team noted that the study proves a fundamental physical mechanism rather than a commercial product.However, knowing that this electronic shortcut exists is what matters the most. Researchers can now scan materials databases and design custom semiconductors with the same trapdoor property. “What makes this discovery particularly exciting is that we are not simply improving an existing semiconductor property but identifying a different way in which semiconductors can behave,” said Professor Herz. “Understanding this mechanism gives us a roadmap for discovering and potentially engineering other materials with similarly advantageous properties.”It could unlock not only solar panels but also longer-lived charges for solar-to-chemical fuel conversion.Bismuth sulfide provides environmental and manufacturing advantages as an Earth-abundant and non-toxic alternative to materials containing lead or cadmium. Researchers can produce these thin films efficiently using accessible methods such as low-cost solution processing, thermal evaporation, and chemical bath deposition.The study was published in the journal Nature on October 28.