Ternary I-III-VI 2 semiconductor nanocrystals (NCs), such as CuInS 2 , are receiving growing attention as they offer the possibility to overcome the toxicity concerns related to heavy metals for numerous technologies spanning from solar cells, luminescent solar concentrators (LSCs) and artificial lighting to bioimaging. Despite the intense research activity, the fundamental mechanisms underpinning the optical properties of CuInS 2 NCs are still not fully understood. Studies suggest that the characteristic Stokes-shifted and long-lived luminescence arises from radiative decay of conduction band electrons to copperrelated defects that are particularly abundant in non-stoichiometric NCs or into a strongly localized HOMO based on Cu(3d) states. However, a recent theoretical model points to a further phenomenon; namely the detailed structure and odd-even parity states of the valence band. Crucially, this model, which has not been experimentally validated, predicts a distinctive optical behaviour in defect-free NCs: the quadratic dependence of both the radiative decay rate and the Stokes shift on the NC radius. If this origin was confirmed, this would have crucial implications for LSC devices as the large solar spectral coverage ensured by low bandgap (large size) NCs would come with a cost in terms of increased reabsorption of the guided near-IR luminescence. Here, we test this hypothesis by studying stoichiometric CuInS 2 NCs of varying sizes. Data reveal, for the first time, the spectroscopic signatures theoretically predicted for the free band edge exciton of I-III-VI 2 NCs, thus providing experimental support to the valence-band structure model. At very low temperatures the same NCs also show dynamic signatures of dark-state emission likely originating from enhanced electron-hole spin interaction. We then evaluated the trade-off between the enhanced solar harvesting of large NCs and their progressively smaller Δ SS on the efficiency of LSCs by performing Monte Carlo ray tracing simulations based on the experimental data that provided useful guidelines for the design of efficient LSCs based on stoichiometric CuInS 2 NCs. Finally, based on such theoretical insights, we fabricated largearea plastic LSC devices showing optical grade quality and an optical power efficiency as high as 6.8%, corresponding to the highest value reported to date for large-area LSC devices.
Low‐power photon upconversion (UC) based on sensitized triplet–triplet annihilation (sTTA) is considered as the most promising upward wavelength‐shifting technique to enhance the light‐harvesting capability of solar devices. Colloidal nanocrystals (NCs) with conjugated organic ligands have been recently proposed to extend the limited light‐harvesting capability of molecular absorbers. Key to their functioning is efficient energy transfer (ET) from the NC to the triplet state of the ligands that sensitize free annihilator moieties responsible for the upconverted luminescence. The ET efficiency is typically limited by parasitic processes, above all nonradiative hole‐transfer to the ligand highest occupied molecular orbital (HOMO). Here, a new exciton‐manipulation approach is demonstrated that enables loss‐free ET by electronically doping CdSe NCs with gold impurities that introduce a hole‐accepting intragap state above the HOMO energy of 9‐anthracene acid ligands. Upon photoexcitation, the NC photoholes are rapidly routed to the Au‐level, producing a long‐lived bound exciton in perfect resonance with the ligand triplet. This hinders hole‐transfer leading to ≈100% efficient ET that translates into an upconversion quantum yield as high as ≈12% (≈24% in the normalized definition), which is the highest performance for NC‐based upconverters based on sTTA to date and approaches the record efficiency of optimized organic systems.
† These authors contributed equally to this work. 'Charge engineering' of semiconductor nanocrystals (NCs) through so-called electronic impurity doping is a long-lasting challenge in colloidal chemistry and holds promise for groundbreaking advancements in many optoelectronic, photonic and spin-based nanotechnologies. To date, our knowledge is limited to a few paradigmatic studies on a small number of model compounds and doping conditions, with important electronic dopants still unexplored in nanoscale systems. Equally importantly, fine tuning of charge engineered NCs is hampered by the statistical limitations of traditional approaches. The resulting intrinsic doping inhomogeneity restricts fundamental studies to statistically averaged behaviours and complicates the realization of advanced device concepts based on their advantageous functionalities. Here we aim to address these issues by realizing the first example of II-VI NCs electronically doped with an exact number of heterovalent gold atoms, a known p-type acceptor impurity in bulk chalcogenides. Single-dopant accuracy across entire NC ensembles is obtained through a novel non-injection synthesis employing ligand-exchanged gold clusters as 'quantized' dopant sources to seed the nucleation of CdSe NCs in organic media. Structural, spectroscopic and magneto-optical investigations trace a comprehensive picture of the physical processes resulting from the exact doping level of the NCs. Gold atoms, doped here for the first time into II-VI NCs, are found to incorporate as nonmagnetic Au + species activating intense size-tuneable intragap photoluminescence and artificially offsetting the hole occupancy of valence band states. Fundamentally, the transient conversion of Au + to paramagnetic Au 2+ (5d 9 configuration) under optical excitation results in strong photoinduced magnetism and diluted magnetic semiconductor behaviour revealing the contribution of individual paramagnetic impurities to the macroscopic magnetism of the NCs. Altogether, our results demonstrate a new chemical approach towards NCs with physical functionalities tailored to the single impurity level and offer a versatile platform for future investigations and device exploitation of individual and collective impurity processes in quantum confined structures.
Ternary I-III-VI2 nanocrystals (NCs), such as AgInS2 and CuInS2, are garnering interest as heavy-metal-free materials for photovoltaics, luminescent solar concentrators, LEDs, and bioimaging. The origin of the emission and absorption properties in this class of NCs is still a subject of debate. Recent theoretical and experimental studies revealed that the characteristic Stokes-shifted and long-lived luminescence of stoichiometric CuInS2 NCs arises from the detailed structure of the valence band featuring two sublevels with different parity. The same valence band substructure is predicted to occur in AgInS2 NCs, yet no experimental confirmation is available to date. Here, we use complementary spectroscopic, spectro-electrochemical, and magneto-optical investigations as a function of temperature to investigate the band structure and the excitonic recombination mechanisms in stoichiometric AgInS2 NCs. Transient transmission measurements reveal the signatures of two subbands with opposite parity, and photoluminescence studies at cryogenic temperatures evidence a dark state emission due to enhanced exchange interaction, consistent with the behavior of stoichiometric CuInS2 NCs. Lowering the temperature as well as applying reducing electrochemical potentials further suppress electron trapping, which represents the main nonradiative channel for exciton decay, leading to nearly 100% emission efficiency.
Metal clusters with appropriate molecular ligands have been shown to be suitable subnanometer building blocks for supramolecular architectures with controlled secondary interactions, providing access to physical regimes not achievable with conventional intermolecular motifs. An example is the excimer photophysics exhibited by individual cluster-based superstructures produced by top-down etching of gold nanoparticles. Now, a supramolecular architecture of copper clusters is presented with controlled optical properties and efficient non-resonant luminescence produced via a novel bottom-up synthesis using mild green reductants followed by a ligand exchange reaction and spontaneous supramolecular assembly. Spectroscopic experiments confirm the formation of the intercluster network and reveal the permanent nature of their excimer-like behavior, thus extending the potential impact and applicability of metal cluster superstructures as efficient and stable non-resonant single-particle emitters.
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