The Solar Material That Holds On to Its Charges for Longer

The Solar Material That Holds On to Its Charges for Longer

A solar cell has a difficult job. It must soak up sunlight strongly, and it must also hold on to the electrical charges that sunlight creates long enough to collect them as electricity. Usually, the better a material is at the first job, the worse it is at the second: strong absorbers tend to lose their charges quickly.

A study led by the University of Oxford, published in Nature on 28 October 2026, reports a material that appears to sidestep that trade-off. It is bismuth sulfide, a familiar semiconductor. The researchers are clear about what they have — and have not — shown: they have demonstrated a physical mechanism in thin films of the material, not a finished, higher-efficiency solar panel.

A waiting room for charges

When light hits a semiconductor, it frees negatively charged electrons and leaves behind positively charged gaps called holes. Electricity comes from collecting those charges before they meet again and disappear — a process called recombination.

Professor Laura Herz of Oxford's Department of Physics, the study's corresponding author, put the problem plainly: the properties that make a semiconductor very good at absorbing sunlight also tend to make its charges disappear quickly.

Bismuth sulfide behaves differently because of the arrangement of its energy levels. Calculations by Professor Marina Filip's group at Oxford showed that the material absorbs light mainly through one strong route between energy levels. Just below it in energy — about 40 thousandths of an electronvolt lower — sits a second, much weaker route, through which charges are lost only slowly. Charges spend much of their time in those lower-energy states, which act as a temporary reservoir, or waiting room.

The team tested this picture by shining extremely short laser pulses onto thin films of bismuth sulfide and tracking how quickly the charges disappeared, using ultrafast terahertz spectroscopy. In most semiconductors, charges are lost faster as the material is cooled. In bismuth sulfide, the opposite happened: charge loss slowed as the material cooled, because more charges settled into the reservoir — just as the reservoir picture predicts.

The researchers call bismuth sulfide a "pseudo-direct" semiconductor. Their model 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. That is an estimate of a charge-loss rate in thin films. It is not a 75% increase in solar-panel efficiency, which depends on many other factors, and the study does not report a new efficiency record for a working device.

Why it matters for learners

The value of the finding is as a design idea. If researchers can find or engineer other materials with the same trick — strong absorption through one route, slow loss through a nearby reservoir — they may be able to build solar cells that waste less of the sunlight they capture. The Oxford team worked with Wuhan University in China on the study.

For students, it is a neat example of how physics is often about trade-offs, and how progress comes from finding a clever way around one. Energy levels, electrons and holes, and the idea of recombination all sit squarely in GCSE and A-level physics — and here they are deciding how well a future solar panel might work.

If your child is curious about how sunlight becomes electricity, or why some materials make better solar cells than others, a first lesson in physics can connect energy levels and electric charge to the technology on rooftops today. Book your first lesson

Sources:

• Righetto, M. et al., "Delayed electron–hole recombination in a pseudo-direct semiconductor", Nature (2026), published 28 October 2026. DOI: 10.1038/s41586-026-11223-x: https://www.nature.com/articles/s41586-026-11223-x
• University of Oxford study reported via TechXplore, "Semiconductors could sidestep a key solar-energy trade-off through a newly identified mechanism", 28 October 2026: https://techxplore.com/news/2026-10-semiconductors-sidestep-key-solar-energy.html

London Tutors — post your tutoring request and we will match you with a verified tutor.