Researchers find strong absorption and slower charge loss in bismuth sulfide
A 28 October study measures the combination in thin films and proposes an electronic structure that could guide the search for solar-energy materials.
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A 28 October study measures the combination in thin films and proposes an electronic structure that could guide the search for solar-energy materials.
Researchers have measured an unusual combination in bismuth-sulfide (Bi2S3) thin films: strong light absorption alongside slower intrinsic electron–hole recombination than in conventional direct semiconductors. Their study, published in Nature on 28 October 2026, proposes that closely spaced electronic states explain the result. The researchers describe a physical mechanism with potential for solar energy, rather than an improved working solar cell.
Sunlight gives electrons energy to move through a semiconductor, leaving positively charged vacancies called holes. Solar cells must collect these mobile charges before electrons recombine with holes. Recombination therefore competes with electricity generation. The challenge is that the electronic properties that make many materials absorb light strongly also make this intrinsic charge-loss process fast.
Earlier studies disagreed about whether bismuth sulfide has a direct or indirect bandgap—the energy gap an electron must cross to become mobile. In a direct transition, as in gallium arsenide, the relevant electron and hole states have matching crystal momentum, allowing light absorption and emission without an extra momentum-transfer step. An indirect transition generally needs help from a crystal vibration called a phonon. That extra step weakens absorption and slows recombination that emits light.
The authors’ calculations put bismuth sulfide’s lowest-energy indirect transition less than 40 millielectronvolts (meV) below its lowest direct transition. They describe this separation as only a few times kBT, the thermal-energy scale set by temperature. The proposed arrangement lets direct transitions absorb light strongly while charges redistribute into nearby, lower-energy indirect states. Those states act as a temporary reservoir from which recombination is less likely.
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The authors propose bismuth sulfide as the first candidate for a new category they call “pseudo-direct semiconductors”. Their calculations retain this mixed direct–indirect character across several computational approaches. The category and the claim of a first candidate are the authors’ interpretation of the evidence.
The underlying trade-off traces back to Gustav Kirchhoff’s 1860 absorption–emission principle. In semiconductors, the van Roosbroeck–Shockley relation connects absorption with radiative recombination, in which an electron and hole recombine and emit light. The paper says the strict link applies when both processes involve the same electronic state. Its proposed mechanism separates their dominant states, which is why the authors describe circumventing the usual constraint rather than violating Planck’s law.
The researchers made the films by heating bismuth-sulfide powder until it evaporated and deposited onto quartz. They used ultrashort laser pulses to create mobile charges, then followed their disappearance through terahertz and millimetre-wave conductivity measurements and the light the films emitted. Temperature-dependent measurements extended from room temperature down to 4 kelvin, just above absolute zero.
The collaboration involved the University of Oxford and Wuhan University, Tech Xplore said. Marina R. Filip’s Oxford physics group performed the electronic-structure calculations.
At room temperature, the films’ visible-light absorption coefficient exceeded 100,000 cm−1—a measure of how strongly light is absorbed over a given distance. Fitting the charge-decay measurements gave a bimolecular recombination coefficient, k2, of approximately 5 × 10−11 cm3 s−1. This coefficient describes electron–hole losses that depend on the densities of both charges; it is not itself a carrier lifetime. The paper compares it with values from earlier literature.
| Material | Approximate k2 (cm3 s−1) | Basis |
|---|---|---|
| Bismuth sulfide | 5 × 10−11 | Room-temperature fit in this study |
| Gallium arsenide | 10−10 | Literature value cited by the authors |
| Lead-halide perovskites | 10−9 | Literature value cited by the authors |
To separate this intrinsic process from defect-mediated losses, the researchers varied the laser energy delivered per unit area. Stronger excitation produced faster decay on short timescales, the signature of bimolecular recombination. Their model fitted this density-dependent component alongside a defect-related component whose decay slowed over time.
Cooling slowed recombination, opposite to the usual intrinsic bimolecular trend in both direct and indirect semiconductors. The team interprets this as support for charges settling into the lower-energy reservoir. Cooling also slowed defect-mediated losses, however, so the temperature trend alone does not isolate the proposed mechanism. Separately, the dominant absorption remained nearly unchanged at very low temperatures, supporting the authors’ conclusion that strong direct transitions drive it.
Analysis
The slower intrinsic process did not remove the films’ other losses. In the measured longer-timescale decay, almost all mobile charges recombined within about 100 nanoseconds. The authors found this behaviour compatible with defect-mediated trapping. Those losses remain a practical obstacle to making use of the electronic structure.
Laura M. Herz, an Oxford physics professor and the study’s corresponding author, says the mechanism offers a principle for finding and potentially engineering other solar-energy materials. The team says longer-lived charges could have more time to be collected as electricity or to drive reactions that turn sunlight into fuels. Its proposed next steps include screening materials databases and engineering similar electronic structures, followed by work to translate the mechanism into improved solar-cell technology.
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