Ternary I‐III‐VI2nanocrystals (NCs), such as CuInS2, are receiving attention as heavy‐metals‐free materials for solar cells, luminescent solar concentrators (LSCs), LEDs, and bio‐imaging. The origin of the optical properties of CuInS2NCs are however not fully understood. A recent theoretical model suggests that their characteristic Stokes‐shifted and long‐lived luminescence arises from the structure of the valence band (VB) and predicts distinctive optical behaviours in defect‐free NCs: the quadratic dependence of the radiative decay rate and the Stokes shift on the NC radius. If confirmed, this would have crucial implications for LSCs as the solar spectral coverage ensured by low‐bandgap NCs would be accompanied by increased re‐absorption losses. Here, by studying stoichiometric CuInS2NCs, it is revealed for the first time the spectroscopic signatures predicted for the free band‐edge exciton, thus supporting the VB‐structure model. At very low temperatures, the NCs also show dark‐state emission likely originating from enhanced electron‐hole spin interaction. The impact of the observed optical behaviours on LSCs is evaluated by Monte Carlo ray‐tracing simulations. Based on the emerging device design guidelines, optical‐grade large‐area (30×30 cm2) LSCs with optical power efficiency (OPE) as high as 6.8% are fabricated, corresponding to the highest value reported to date for large‐area devices.
Ternary metal‐chalcogenide semiconductor nanocrystals are an attractive class of materials due to their tunable optoelectronic properties that result from a wide range of compositional flexibility and structural diversity. Here, the phase‐controlled synthesis of colloidal silver iron sulfide (AgFeS2) nanocrystals is reported and their resonant light–matter interactions are investigated. The product composition can be shifted selectively from tetragonal to orthorhombic by simply adjusting the coordinating ligand concentration, while keeping the other reaction parameters unchanged. The results show that excess ligands impact precursor reactivity, and consequently the nanocrystal growth rate, thus deterministically dictating the resulting crystal structure. Moreover, it is demonstrated that the strong ultraviolet‐visible extinction peak exhibited by AgFeS2nanocrystals is a consequence of a quasi‐static dielectric resonance (DR), analogous to the optical response observed in CuFeS2nanocrystals. Spectroscopic studies and computational calculations confirm that a negative permittivity at ultraviolet/visible frequencies arises due to the electronic structure of these intermediate‐band (IB) semiconductor nanocrystals, resulting in a DR consisting of resonant valence‐band‐to‐intermediate‐band excitations, as opposed to the well‐known localized surface plasmon resonance response typically observed in metallic nanostructures. Overall, these results expand the current library of an underexplored class of IB semiconductors with unique optical properties, and also enrich the understanding of DRs in ternary metal‐iron‐sulfide nanomaterials.
more » « less- Award ID(s):
- 1719797
- NSF-PAR ID:
- 10367318
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Small
- Volume:
- 18
- Issue:
- 9
- ISSN:
- 1613-6810
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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