Bismuth Sulfide Holds Solar Charges Longer

Physicists at the University of Oxford say they have found a semiconductor that gets round one of the basic trade-offs in solar energy: materials that soak up light well usually lose the energy they capture just as quickly. In bismuth sulfide, they report, charges last far longer than that rule would predict.

The work, led by Professor Laura Herz and published in Nature on 28 October, describes a physical mechanism rather than a better solar panel, and the university says so plainly. But the team estimates that, at room temperature, the effect cuts the rate at which charges are lost by about 75% compared with conventional materials that absorb light as strongly.

In brief

Strong light absorbers normally lose their charges fast; bismuth sulfide appears to break that link.
The cause is an unusual pair of energy levels that lets charges sit in a slow-loss state.
The researchers say the study shows a mechanism, not a finished solar cell.

The trade-off solar materials usually face

Solar cells work because a semiconductor absorbs sunlight and uses the energy to free electrons, leaving behind positively charged gaps called holes. To make electricity, those charges must be collected before an electron and a hole meet and cancel each other out, wasting the energy.

The problem, as the Oxford release puts it, is that semiconductors that absorb light more strongly also tend to lose their charges more quickly. The paper traces this back to a principle set out by Gustav Kirchhoff in 1860, that a good absorber must also be a good emitter. Applied to semiconductors, that reciprocity “results in substantial limitations to solar energy conversion”, the authors write.

“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,” Herz said in the release. “Our work shows that this trade-off is not inevitable.”

How the ‘reservoir’ works

Calculations by Professor Marina Filip’s group, also in Oxford’s Department of Physics, showed what makes bismuth sulfide different. It absorbs light mainly through one strong route between energy levels, but just below that 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 the lower states, which act as a temporary “reservoir”, the university says.

The paper calls this a pseudo-direct bandgap. To test it, the team fired extremely short laser pulses at thin films of bismuth sulfide and tracked how fast the charges disappeared, using ultrafast terahertz spectroscopy, at temperatures from room temperature down to just above absolute zero.

The results ran against the usual pattern. In most semiconductors, charges are lost faster as the material is cooled. In bismuth sulfide they were lost more slowly, as more charges settled into the reservoir, matching the team’s model. The paper says the feature “substantially retards” electron–hole recombination compared with classic direct semiconductors.

“We are not simply improving an existing semiconductor property but identifying a different way in which semiconductors can behave,” Herz said. “Understanding this mechanism gives us a roadmap for discovering and potentially engineering other materials with similarly advantageous properties.”

A design rule, not yet a better panel

The useful way to read this result is as a rule for where to look, not as a material about to appear on rooftops. The team has shown, in thin films and with laser measurements, that a particular arrangement of energy levels lets a strong absorber hold on to its charges for longer. The study does not include a solar cell built around that effect, and the release gives no efficiency figure for one.

That still matters. Much materials work pushes a known trade-off a little further. A mechanism that sidesteps the trade-off gives researchers a new property to search for, and the team’s calculations suggest other materials with it could be found by screening materials databases, or engineered. The 75% figure is also an estimate from the team’s model and measurements, not a device result.

The second use the university mentions may come sooner in the lab than in industry: longer-lived charges could give more time for the chemical reactions that turn sunlight into fuels. Either way, the next evidence to watch for is a working device that shows the slower charge loss turning into more collected current.

UK-led, with a Chinese partner

The research was a collaboration between Oxford and Wuhan University in China. It was funded by the UK’s Engineering and Physical Sciences Research Council (EPSRC), with additional support from the Italian Ministry of University and Research and Oxford Photovoltaics, according to the release.

It arrives in a busy week for solar materials on TSN’s Tech for Good beat. Chinese researchers recently reported flexible perovskite modules reaching 20.2% at commercial size, and Ricoh has announced plans to mass-produce inkjet-printed perovskite cells. Those are steps along a known road to factories; the Oxford result is earlier and more basic, closer to lab work such as the recyclable flow battery that kept 98.1% of its capacity over 2,200 cycles.

For now, bismuth sulfide’s slow-leaking reservoir is a promising piece of physics. Whether it becomes a better solar cell depends on the device work the researchers say comes next.