Bismuth sulfide solar material holds charges longer

A little-studied solar material has shown it can break a rule that has limited solar cells for years, according to researchers at the University of Oxford. Bismuth sulfide absorbs sunlight strongly yet holds on to the electrical charges that light creates for far longer than comparable materials, the team reported in a study published in the journal Nature on October 28.

Solar cells depend on semiconductors, materials that absorb sunlight and use its energy to free electrons, leaving behind positively charged gaps known as holes. Electricity flows only when those charges are collected before an electron and a hole meet again and cancel each other out, wasting the energy they carry. That race between collection and reunion shapes how efficient any solar cell can be.

The difficulty is that the two qualities a solar cell wants most tend to work against each other. Materials that soak up light strongly also tend to lose their charges quickly, because the same properties that make it easy for light to create charges also make their return journey efficient. Designers have long had to accept that trade-off, sacrificing absorption to keep charges alive longer or accepting faster losses to capture more light.

Bismuth sulfide appears to sidestep the problem. Electronic structure calculations by Professor Marina Filip's group in Oxford's Department of Physics showed that the compound absorbs light mainly through a strong route between energy levels, while a second, much weaker route sits just below it in energy. Charges that settle into those lower states are lost only slowly, so the lower states act as a temporary reservoir where charges can wait instead of recombining at once. Because the energy levels sit close together, charges spend much of their time in the reservoir, and the compound keeps behaving as a strong absorber at the same time.

"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," said corresponding author Professor Laura Herz of Oxford's Department of Physics, in a statement from the university.

How the team tested the effect

To test the picture, the researchers excited thin films of bismuth sulfide with extremely short laser pulses and tracked how quickly the resulting charges disappeared, using a technique called ultrafast terahertz spectroscopy. They repeated the measurements across a wide temperature range, from room temperature down to just above absolute zero, watching how the behaviour changed as the films cooled.

The cooling test gave the clearest confirmation. In most semiconductors, charges are lost faster as the material is cooled. In bismuth sulfide the opposite happened: charge loss slowed as temperatures fell, consistent with more charges settling into the reservoir states. The result matched the team's model of what physicists call a pseudo-direct semiconductor, a compound that absorbs like a direct semiconductor but loses charges like an indirect semiconductor.

At room temperature, the team estimated that the arrangement cuts the rate at which charges are lost by about 75% compared with conventional materials that absorb light as strongly. That figure describes the charge-loss rate, not panel output, and the researchers were careful to draw the distinction: the finding is not a finished solar cell with a higher efficiency record, but a physical mechanism demonstrated in thin films.

What longer-lived charges could mean

For solar energy, longer-lived charges matter because charges that survive longer have a greater chance of reaching the electrical contacts of a cell before they are wasted. That could reduce a fundamental energy loss in solar cells without giving up strong absorption. The same extra lifetime could also help technologies that use sunlight to drive chemical reactions, such as producing fuels, where charges need time to take part in the reaction before they disappear.

The researchers said the next step is to turn the mechanism into practical devices and to look for other compounds with the same property, either by screening materials databases or by engineering them deliberately. The study was an international collaboration between the University of Oxford and Wuhan University in China, and its full title is "Delayed electron-hole recombination in a pseudo-direct semiconductor."

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Reported by TechXplore and based on the University of Oxford announcement, the work points to a fresh feature for materials scientists to search for: energy levels arranged so that a compound absorbs light readily while giving the resulting charges a better chance of doing useful work. If other materials with the same arrangement can be found or built, the trade-off that has constrained solar cell design may turn out to be less rigid than it long appeared. Further details are available in the Nature paper and in the TechXplore report on the discovery.