Solar photovoltaics has long been constrained by an awkward trade-off: semiconductors that absorb sunlight strongly also tend to lose their photogenerated charges quickly, before they can be collected as electricity. A study led by the University of Oxford, published in Nature, reports that bismuth sulfide appears to escape this compromise, absorbing light strongly while surrendering its charges far more slowly than conventional materials.
The team, led from Oxford's Department of Physics, excited thin films of bismuth sulfide with ultrashort laser pulses and tracked the fate of the resulting charges using ultrafast terahertz spectroscopy, at temperatures ranging from ambient down to just above absolute zero. Electronic structure calculations revealed the mechanism: a strong absorbing transition sits just above a much weaker one, and the lower states act as a reservoir where charges linger instead of recombining.
The behaviour matches that of a so-called pseudo-direct semiconductor, and the researchers estimate that at room temperature the arrangement cuts the rate of charge loss by about 75 per cent compared with conventional expectations. Rather than incrementally improving an existing material, the work identifies a different way semiconductors can behave, offering a roadmap for discovering or engineering other compounds that combine strong absorption with long-lived charges for higher-output solar panels.