Semiconductors group

Abstract:

Kirchhoff demonstrated the reciprocity between absorptivity and emissivity for blackbody radiation in 1860 (ref. 1), thereby establishing the general principle that a good absorber must also be a good emitter. This principle underpins the generalized Planck’s law2 and, when applied to semiconductors—in the form of the van Roosbroeck–Shockley equation3—results in substantial limitations to solar energy conversion. Here we demonstrate how this reciprocity can be circumvented in semiconductors, such as bismuth sulfide, showing a pseudo-direct bandgap character—that is, featuring a lower-energy indirect transition paired with a direct transition placed a few kBT above. After determining the pseudo-direct bandgap nature of bismuth sulfide, we reveal how this feature substantially retards bimolecular electron–hole recombination in this material with respect to classic direct semiconductors4. The resulting combination of a high absorption coefficient and delayed recombination offers the unprecedented possibility of overcoming limitations associated with Planck’s law, with far-reaching potential for solar energy conversion.

Abstract:

Dataset supporting research article" "Delayed Electron-Hole Recombination in a Pseudo-Direct Semiconductor"

Abstract:

Abstract Controlling excitation transport is central to the operation of light-harvesting and light-emitting devices based on nanocrystal films. Exciton diffusion is typically described as hopping through a homogeneous medium; however, even highly monodisperse nanocrystal ensembles exhibit energetic disorder arising from size variation. We demonstrate that such disorder couples exciton diffusivity in real space to energetic relaxation within the density of states. Using spectrally and temporally resolved photoluminescence reabsorption spectroscopy of CsPbBr3 nanocrystal films, we separately track exciton diffusion and energetic relaxation across a range of temperatures and nanocrystal sizes. We find that at low temperature, excitons preferentially migrate downhill toward larger, lower-energy nanocrystals, causing a transient collapse of exciton diffusivity that is particularly pronounced for smaller nanocrystals exhibiting higher energetic disorder. At room temperature, these effects are effectively countered by thermally activated uphill transfer, restoring normal diffusion. Nanocrystal solids thus offer tunable transport properties governed by the interplay between disorder and thermal energy.

Abstract:

Reliable predictions of long-term stability are essential as halide perovskite solar cells approach widespread commercial deployment. However, extrapolation from aging tests requires knowledge of material degradation dynamics, which remain unclear for halide perovskites. Here, we reveal the structural and optoelectronic evolution of formamidinium-caesium lead-halide perovskite films under heat and/or light using in situ X-ray diffraction and photoluminescence. Heat and light combined induce much faster degradation than either stressor alone, with 1 sun illumination degrading material 53 times faster at 85 °C than room temperature. For all compositions investigated the degradation rate is highly nonlinear in time and follows a sigmoidal decay, suggestive of interface-driven dynamics. Degradation proceeds in three steps: halide segregation (for mixed-halide perovskites), A-site cation diffusion triggered by heterogeneity and strain, and finally iodine and formamidinium loss. Our findings provide a quantitative framework for lifetime predictions and highlight the necessity of combined heat and light ageing tests.

Abstract:

We investigate the impact of employing fullerene self-assembled monolayers (SAMs) in conjunction with SnO2 electron-transport layers (ETLs) in negative–intrinsic–positive (n–i–p) perovskite solar cells. We compare the efficacy of the fullerene-SAM surface-binding group—carboxylic or phosphonic acid—on the passivation, charge transport, and energetics of the modified-SnO2 surface. Planar n–i–p perovskite solar cells with these SAM-modified SnO2 exhibit significantly reduced hysteresis, and the steady-state maximum power point tracked efficiencies (η MPP) of the devices improved from an average of 18.6–20.0% comparing devices with neat SnO2 to the C60-PA SAM inclusion, respectively. While C60-PA SAM improved the initial solar cell efficiency, this benefit was not maintained during light exposure at elevated temperatures as the fullerene SAM weakened adhesion at the perovskite/metal oxide interface in the aged devices. These results highlight the importance of improving the mechanical robustness of perovskite/charge-transport layer interfaces under operational aging conditions.