Bismuth sulfide absorbs light strongly while slowing charge loss
Bismuth sulfide combines strong light absorption with slower electron–hole recombination through closely spaced direct and indirect electronic transitions. A peer-reviewed experiment and modelling study used thin films and ultrafast measurements to examine this arrangement. Lower-energy states provide a temporary reservoir for charges, suggesting a material-design route for solar energy conversion while completed solar-cell performance remains to be demonstrated.
Science··Evening
Nearby transitions preserve absorption and delay recombination
Researchers at Oxford and Wuhan universities identified an electronic structure in bismuth sulfide that combines strong light absorption with slower loss of photoexcited charges. Their peer-reviewed study examined thin semiconductor films through experiments and electronic-structure calculations. When light is absorbed, electrons gain energy and leave positively charged holes behind. Solar cells must collect those charges before the electrons and holes recombine. Strong absorbers generally also emit strongly, linking desirable absorption with rapid recombination. Bismuth sulfide has a lower-energy indirect transition close to a strong direct transition. The researchers describe this mixed arrangement as a pseudo-direct bandgap, bringing the absorption and charge-retention properties into a different balance.[1], [2]
Cooling shifts charges into a lower-energy reservoir
The team excited the films with very short laser pulses and followed charge loss using ultrafast terahertz spectroscopy. Measurements ranged from room temperature to just above absolute zero. Calculations placed the strong absorption route slightly above a weaker, indirect route. Charges can move into those lower-energy states, creating a temporary reservoir that delays their recombination while absorption through nearby direct states remains strong. Cooling increased the share of charges in that reservoir, and measured recombination slowed. Oxford's experimental work was led by Laura Herz; Marina Filip's group calculated the electronic structure, while Qianqian Lin's team at Wuhan University fabricated the films. The temperature dependence helped connect the observed charge dynamics with the proposed band structure.[1], [2]
The model quantifies a route to longer-lived charges
For a separation of approximately 40 millielectronvolts between relevant transitions, the model calculated about 75% suppression of bimolecular recombination at room temperature. That percentage concerns a charge-loss rate, rather than an efficiency increase measured in a finished solar cell. Longer-lived charges could provide additional collection time in photovoltaics and additional time for reactions converting sunlight into fuels. Accessing lower indirect states also costs a small amount of energy, so band separation remains part of the design trade-off. Proposed searches include screening materials databases and stretching layered crystals to tune their electronic structure. Practical devices would still depend on defects, charge transport and architecture; the demonstrated result is a material mechanism supported by thin-film measurements and modelling.[1], [2]
