A Solar Material That Holds Its Charge Longer
Semiconductors that absorb sunlight well usually lose the energy they capture quickly. An Oxford-led study in Nature finds that bismuth sulfide partly escapes that trade-off, because of how its energy levels are arranged. It is a physical mechanism, not a product.
Materials that absorb sunlight strongly tend to lose the electrical charges that light creates just as quickly, which limits how much energy a solar cell can collect. Physicists at the University of Oxford, working with Wuhan University, report in Nature that bismuth sulfide sidesteps this trade-off. Just below the energy level through which it absorbs light sits a second, weaker route through which charges are lost slowly. Charges spend much of their time there, in what the researchers call a temporary "reservoir." The team fired extremely short laser pulses at thin films of the material and found that, unlike in most semiconductors, charges were lost more slowly as it cooled. It estimates this arrangement cuts the rate of charge loss by about 75% at room temperature compared with materials that absorb light as strongly. The researchers call bismuth sulfide the first candidate for a "pseudo-direct" semiconductor. The study describes a mechanism in thin films, not a working solar cell.
The trade-off
In a semiconductor, light knocks electrons loose, leaving positive "holes" behind; a solar cell produces power by collecting those charges before they recombine. The paper notes that a good absorber of light is, by physical principle, also a good emitter, so strong absorbers usually lose charges fast. The Oxford physics department and Nanowerk describe the same trade-off between absorbing sunlight and holding on to the energy it captures.
What the team measured
The researchers fired extremely short laser pulses at thin films of bismuth sulfide and measured how fast the resulting charges disappeared, using ultrafast terahertz spectroscopy, according to the paper, Oxford Physics and Nanowerk. They took measurements from room temperature down to near absolute zero.
In most semiconductors, charges are lost faster as the material cools. In bismuth sulfide they were lost more slowly, according to the paper, Oxford Physics and Nanowerk.
The mechanism
Calculations by Professor Marina Filip's group explain why, according to Oxford Physics and Nanowerk. Bismuth sulfide absorbs light mainly through one strong "direct" route between energy levels. Just below it lies a weaker "indirect" route, through which charges are lost only slowly. The paper puts the gap between the two at about 40 thousandths of an electronvolt, comparable to the energy of heat at room temperature. As the material cools, more charges settle into the lower, slower state, which is why loss slows down, according to the paper and Oxford Physics.
The team's model estimates that this arrangement suppresses charge loss by about 75% at room temperature compared with a purely direct semiconductor, according to the paper, Oxford Physics and Nanowerk.
What it is and isn't
The study describes a physical mechanism in thin films; it does not report a solar cell built from the material. The paper says the effect offers "far-reaching potential for solar energy conversion" and proposes searching materials databases for other pseudo-direct semiconductors and tuning the energy gap in layered materials. The paper describes bismuth sulfide as non-toxic and stable.