Researchers from Oxford University, together with colleagues from Wuhan University (China), have discovered an unusual semiconductor - bismuth sulfide - which, according to their data, makes it possible to get around a long-standing limitation of solar cells. The work was published in the journal Nature.
The limitation is due to the fact that materials that absorb light well usually lose charges quickly. In a solar cell, a semiconductor absorbs light and releases electrons (negatively charged particles), leaving positively charged "holes". To obtain current, these charges must be collected before the electron and the hole meet and annihilate. The more strongly a material absorbs light, the faster, as a rule, the charges disappear, which reduces the energy conversion efficiency.
Bismuth sulfide, as it turned out, combines strong light absorption with slow charge loss. Scientists explain this by an unusual arrangement of energy levels: electrons can move to a lower level, which serves as a temporary "reservoir" and delays their recombination (the process by which charges disappear).
In experiments, thin films of bismuth sulfide were irradiated with ultrashort laser pulses and the rate at which charges disappear was measured using ultrafast terahertz spectroscopy. The measurements were carried out at temperatures from room temperature to just above absolute zero. Contrary to usual behavior, on cooling, charges in bismuth sulfide were lost more slowly rather than faster, which confirmed the theoretical model.
According to the authors' estimates, at room temperature such a structure slows charge loss by about 75% compared with conventional materials that absorb light equally strongly. "Our work shows that this dilemma is not inevitable," Oxford University's press service quoted the head of the study, Professor Laura Herz, as saying.
According to the scientists, the longer lifetime of charges could increase the efficiency of solar cells and make chemical reactions for producing fuel from sunlight easier. However, the study demonstrates a physical mechanism, not a finished device. The authors emphasize that further development is required to create improved solar panels.