Non-contact optical probes of semiconductor performance.
Physicists at the University of Oxford, working with colleagues at Wuhan University, have identified a material that sidesteps a long-standing trade-off between absorbing sunlight and holding on to the energy it captures. The material, bismuth sulfide, has an unusual arrangement of energy levels that allows it to absorb light strongly while losing its electrical charges more slowly, pointing to a new way to design more efficient solar cells. The study has been published in Nature.
Solar cells rely on materials called semiconductors, which absorb sunlight and use its energy to free tiny negatively charged particles called electrons, leaving behind positively charged gaps known as holes. To produce electricity, these charges must be collected before an electron and a hole meet and cancel each other out, wasting the energy they carry. This process is called electron–hole recombination.
This creates a fundamental challenge. In 1860 Gustav Kirchhoff showed that a good absorber of light must also be a good emitter, and in semiconductors this means that the strongest absorbers also tend to lose their charges most quickly. Perovskites, a promising newer class of solar materials, absorb light strongly but lose their charges rapidly, while silicon, used in most of today’s solar panels, holds on to its charges for longer but absorbs light far less strongly. This trade-off limits how efficiently solar cells can turn sunlight into electricity.
‘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,’ commented Professor Laura Herz, Professor of Physics in the Department of Physics, who led the study. ‘Our work shows that this trade-off is not inevitable. 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.’
The team excited thin films of bismuth sulfide with extremely short laser pulses and then measured how quickly the resulting charges disappeared, using a technique called ultrafast terahertz spectroscopy. They carried out the measurements at temperatures ranging from room temperature to just above absolute zero.
Calculations by Professor Marina Filip’s group in the Department of Physics revealed what makes the material unusual. It absorbs light mainly through one strong route between energy levels, but just below this in energy lies a second, much weaker route, through which charges are lost only slowly. The two are so close in energy that charges spend much of their time in these lower-energy states, which act as a temporary ‘reservoir’ and slow their loss.
The experiments confirmed this picture. In most semiconductors, charges are lost faster as the material is cooled, but in bismuth sulfide they were lost more slowly, as more charges settled into the reservoir. This matched the team’s model of what physicists call a pseudo-direct semiconductor. The team estimates that, at room temperature, this arrangement cuts the rate at which charges are lost by about 75% compared with conventional materials that absorb light as strongly.
‘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. ‘Bismuth sulfide combines characteristics that are usually associated with two distinct classes of materials. Understanding this mechanism gives us a roadmap for discovering and potentially engineering other materials with similarly advantageous properties.’
For solar energy, this could matter in two ways. Charges that last longer have more chance of being collected, which could reduce one of the fundamental energy losses in solar cells without sacrificing strong light absorption. Longer-lived charges could also allow more time for the chemical reactions needed to turn sunlight into fuels.
The team’s calculations suggest that other materials with the same property could be found by screening materials databases or engineered by stretching layered crystals such as bismuth sulfide. The researchers stress that the study demonstrates a physical mechanism rather than a finished solar cell.
The research was an international collaboration between the University of Oxford and Wuhan University in China. An Oxford team led by Professor Herz carried out the experimental investigation of charge recombination, using ultrafast terahertz spectroscopy alongside optical absorption and other spectroscopic measurements, while Professor Filip’s group carried out the electronic-structure calculations that helped explain the behaviour. The bismuth sulfide thin films were fabricated by a team at Wuhan University led by Professor Qianqian Lin.
The research was funded by the UK Engineering and Physical Sciences Research Council (EPSRC), with additional support from the Italian Ministry of University and Research and Oxford Photovoltaics.
Delayed electron–hole recombination in a pseudo-direct semiconductor, M Righetto et al, Nature, 28 October 2026.