Solar energy conversion, particularly solar-driven chemical fuel formation, has been intensely studied in the past decades as a potential approach for renewable energy generation. Efficient ...solar-to-fuel conversion requires artificial photosynthetic systems with strong light absorption, long-lived charge separation and efficient catalysis. Colloidal quantum confined nanoheterostructures have emerged as promising materials for this application because of the ability to tailor their properties through size, shape and composition. In particular, colloidal one-dimensional (1D) semiconductor nanorods (NRs) offer the opportunity to simultaneously maintain quantum confinement in radial dimensions for tunable light absorptions and bulk like carrier transport in the axial direction for long-distance charge separations. In addition, the versatile chemistry of colloidal NRs enables the formation of semiconductor heterojunctions (such as CdSe/CdS dot-in-rod NRs) to separate photogenerated electron-hole pairs and deposition of metallic domains to accept charges and catalyze redox reactions. In this review, we summarize research progress on colloidal NR heterostructures and their applications for solar energy conversion, emphasizing mechanistic insights into the working principle of these systems gained from spectroscopic studies. Following a brief overview of synthesis of various NRs and heterostructures, we introduce their electronic structures and dynamics of exciton and carrier transport and interfacial transfer. We discuss how these exciton and carrier dynamics are controlled by their structures and provide key mechanistic understanding on their photocatalytic performance, including the photo-reduction of a redox mediator (methyl viologen) and light driven H
2
generation. We discuss the solar-driven H
2
generation mechanism, key efficiency limiting steps, and potential approaches for rational improvement in semiconductor NR/metal heterostructures (such as Pt tipped CdSe@CdS dot-in-rod NRs). Finally, we conclude by pointing out challenges to be addressed in future research.
Colloidal one-dimensional (1D) semiconductor nanorods (NRs) offer the opportunity to simultaneously maintain quantum confinement in radial dimensions for tunable light absorptions and bulk like carrier transport in the axial direction for long-distance charge separations.
Organic semiconductors offer a tunable platform for photocatalysis, yet the more difficult exciton dissociation, compared to that in inorganic semiconductors, lowers their photocatalytic activities. ...In this work, we report that the charge carrier lifetime is dramatically prolonged by incorporating a suitable donor-acceptor (β-ketene-cyano) pair into a covalent organic framework nanosheet. These nanosheets show an apparent quantum efficiency up to 82.6% at 450 nm using platinum as co-catalyst for photocatalytic H
evolution. Charge carrier kinetic analysis and femtosecond transient absorption spectroscopy characterizations verify that these modified covalent organic framework nanosheets have intrinsically lower exciton binding energies and longer-lived charge carriers than the corresponding nanosheets without the donor-acceptor unit. This work provides a model for gaining insight into the nature of short-lived active species in polymeric organic photocatalysts.
Abstract
In the field of perovskite light-emitting diodes (PeLEDs), the performance of blue emissive electroluminescence devices lags behind the other counterparts due to the lack of fabrication ...methodology. Herein, we demonstrate the in situ fabrication of CsPbClBr
2
nanocrystal films by using mixed ligands of 2-phenylethanamine bromide (PEABr) and 3,3-diphenylpropylamine bromide (DPPABr). PEABr dominates the formation of quasi-two-dimensional perovskites with small-
n
domains, while DPPABr induces the formation of large-
n
domains. Strong blue emission at 470 nm with a photoluminescence quantum yield up to 60% was obtained by mixing the two ligands due to the formation of a narrower quantum-well width distribution. Based on such films, efficient blue PeLEDs with a maximum external quantum efficiency of 8.8% were achieved at 473 nm. Furthermore, we illustrate that the use of dual-ligand with respective tendency of forming small-
n
and large-
n
domains is a versatile strategy to achieve narrow quantum-well width distribution for photoluminescence enhancement.
The mechanisms of triplet energy transfer across the inorganic nanocrystal/organic molecule interface remain poorly understood. Many seemingly contradictory results have been reported, mainly because ...of the complicated trap states characteristic of inorganic semiconductors and the ill-defined relative energetics between semiconductors and molecules used in these studies. Here we clarify the transfer mechanisms by performing combined transient absorption and photoluminescence measurements, both with sub-picosecond time resolution, on model systems comprising lead halide perovskite nanocrystals with very low surface trap densities as the triplet donor and polyacenes which either favour or prohibit charge transfer as the triplet acceptors. Hole transfer from nanocrystals to tetracene is energetically favoured, and hence triplet transfer proceeds via a charge separated state. In contrast, charge transfer to naphthalene is energetically unfavourable and spectroscopy shows direct triplet transfer from nanocrystals to naphthalene; nonetheless, this "direct" process could also be mediated by a high-energy, virtual charge-transfer state.
Molecular triplet sensitization using colloidal semiconductor nanocrystals or quantum dots (QDs) is important for many photochemical and photonic applications. Current QD sensitizers often contain ...toxic elements such as Cd and Pb. In order to “go green” with these sensitizers, we investigate triplet energy transfer from InP-based QDs. Time-resolved spectroscopy studies revealed picosecond hole trapping in core-only QDs, which could complicate and/or inhibit energy transfer. We therefore developed InP/ZnSe/ZnS core/shell QDs that effectively suppressed hole trapping and meanwhile allowed for triplet energy transfer to surface-anchored anthracene acceptors with an efficiency of 84%. The sensitized molecular triplets participated in triplet–triplet-annihilation, enabling photon upconversion with a normalized quantum yield reaching 10.0% ± 0.1%. This study demonstrates that nontoxic InP-based QDs can be engineered to be on par with state-of-the-art Cd- or Pb-containing QDs for use as triplet sensitizers.
Recently reported colloidal lead halide perovskite quantum dots (QDs) with tunable photoluminescence (PL) wavelengths covering the whole visible spectrum and exceptionally high PL quantum yields ...(QYs, 50–90%) constitute a new family of functional materials with potential applications in light-harvesting and -emitting devices. By transient absorption spectroscopy, we show that the high PL QYs (∼79%) can be attributed to negligible electron or hole trapping pathways in CsPbBr3 QDs: ∼94% of lowest excitonic states decayed with a single-exponential time constant of 4.5 ± 0.2 ns. Furthermore, excitons in CsPbBr3 QDs can be efficiently dissociated in the presence of electron or hole acceptors. The half-lives of electron transfer (ET) to benzoquinone and subsequent charge recombination are 65 ± 5 ps and 2.6 ± 0.4 ns, respectively. The half-lives for hole transfer (HT) to phenothiazine and the subsequent charge recombination are 49 ± 6 ps and 1.0 ± 0.2 ns, respectively. The lack of electron and hole traps and fast interfacial ET and HT rates are key properties that may enable the development of efficient lead halide perovskite QDs-based light-harvesting and -emitting devices.
Lead halide perovskite nanocrystals (NCs) hold strong promise for a variety of light‐harvesting, emitting, and detecting applications, all of which, however, could be complicated by multicarrier ...Auger recombination. Therefore, complete documentation of the size‐ and composition‐dependent Auger recombination rates of these NCs is highly desirable, as it can guide system design in many applications. Herein we report the synthesis and Auger measurements of monodisperse APbX3 (A=Cs and FA; X=Cl, Br, and I) NCs in an extensive size range (ca. 3–9 nm). The biexciton Auger lifetime of all the NCs scales linearly with the NC volume. The scaling coefficient is virtually independent of the cation but rather depends sensitively on the anion, and is 0.035, 0.085, and 0.142 ps nm−3 for Cl, Br, and I, respectively. In all of these nanocrystals the Auger recombination is much faster than in standard CdSe and PbSe NCs (ca. 1 ps nm−3).
Lawful behavior: A wide range of monodisperse lead perovskite nanocrystals with different cation and anion compositions and varying sizes were synthesized and their biexciton Auger recombination lifetimes measured by ultrafast spectroscopy (see picture). Volume scaling laws for the Auger lifetime of the nanocrystals were determined, thus enabling facile estimation of Auger rates, which are key parameters for perovskite‐nanocrystal‐based devices.
Photon upconversion (UC) based on sensitized triplet–triplet annihilation (TTA), TTA-UC, can potentially alleviate the transmission loss of below-band-gap photons in solar energy conversion. TTA-UC ...across various spectral windows has been demonstrated, but efficient visible-to-ultraviolet (UV) UC remains a big challenge primarily due to the lack of suitable triplet sensitizers. Here we report a TTA-UC system sensitized by quantum-confined CsPbBr3 perovskite nanocrystals (NCs) that simultaneously achieves a high photon energy gain of up to 0.7 eV (443–355 nm) and a high UC efficiency up to 10.2%. Time-resolved spectroscopy studies reveal that the performance is mainly enabled by ultrafast and efficient triplet energy transfer from the strongly confined NC sensitizers to triplet acceptors.
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