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.

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Polydopamine (PDA) has been increasingly exploited as an advanced functional material, and its emergent light absorption property plays a crucial role in determining various utilizations. However, the rational design and efficient regulation of PDA absorption property remain a challenge due to the complex structure within PDA. In this work, we propose a facile method to regulate the light absorption behaviors of PDA by constructing donor-acceptor pairs within the microstructures through the chemical connections between indoledihydroxy/indolequinone and their oligomers with 2,2,6,6-tetramethylpiperidine-1-oxyl moiety. The detailed structural and spectral analysis, as well as the density functional theory simulation, further confirms the existence of donor-acceptor molecular pair structures, which could decrease the energy bandgap and increase the electron delocalization for enhancing light absorption across a broad spectrum. These rationally designed PDA nanoparticles with tunable absorption properties also show improved total photothermal effect and demonstrate excellent performances in solar desalination.

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.

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The present work reports highly efficient flexible and reabsorption-free scintillators based on two zero-dimensional (0D) organic copper halides (TBA)­CuX2 (TBA = tetrabutylammonium cation; X = Cl, Br). The (TBA)­CuX2 exhibit highly luminescent green and sky-blue emissions peaked at 510 and 498 nm, with large Stokes shifts of 224 and 209 nm and high photoluminescence quantum yields (PLQYs) of 92.8% and 80.5% at room temperature for (TBA)­CuCl2 and (TBA)­CuBr2 single crystals (SCs), respectively. Interestingly, above room temperature, their PLQYs increase with temperature and reach near unity at 320 and 345 K for (TBA)­CuCl2 and (TBA)­CuBr2, respectively. The excellent properties originate from self-trapped excitons (STEs) in individual [CuX2]− quantum rods, which is demonstrated by the temperature-dependent PL, ultrafast transient absorption (TA) combined with density functional theory (DFT) calculations. The (TBA)­CuX2 scintillators show bright radioluminescence (RL), impressive linear response to dose rate in a broad range, and high light yields. Their potential application in X-ray imaging is demonstrated by using (TBA)­CuX2 composite scintillation screens. Importantly, flexible scintillators are demonstrated to be superior than flat ones for imaging nonplanar objects by conformally coating, which produce accurate images with negligible distortion.

Triplet energy transfer from colloidal nanocrystals is a novel approach to sensitizing molecular triplets that are important for many applications. Recent studies suggest that this triplet transfer can be mediated by a hole transfer process when it is energetically allowed. In contrast, electron-transfer-mediated triplet transfer has not been observed yet, which is likely due to hole-trapping in typical II–VI group nanocrystals inhibiting the hole transfer step following initial electron transfer and hence disrupting a complete triplet exciton transfer. Here we report electron-transfer-mediated triplet energy transfer from CsPbCl3 and CsPbBr3 perovskite nanocrystals to surface-anchored rhodamine molecules. The mechanism was unambiguously established by ultrafast spectroscopy; control experiments using CdS nanocrystals also confirmed the role of hole-trapping in inhibiting this mechanism. The sensitized rhodamine triplets engaged in a variety of applications such as photon upconversion and singlet oxygen generation. Compared to conventional one-step triplet transfer, the electron-transfer-mediated mechanism is less demanding in terms of interfacial electronic coupling and hence is more generally implementable. Overall, this study not only establishes a complete framework of triplet energy transfer across nanocrystal/molecule interfaces but also greatly expands the scope of molecular triplet sensitization using nanocrystals.

Infrared solar cells that utilize low-bandgap colloidal quantum dots (QDs) are promising devices to enhance the utilization of solar energy by expanding the harvested photons of common photovoltaics into the infrared region. However, the present technology for synthesis of PbS QDs cannot produce highly efficient infrared solar cells. Here, we develop a general synthesis framework for low-bandgap PbS QDs (0.65-1 eV) via cation-exchange from ZnS nanorods (NRs). First, ZnS NRs are converted to superlattices with segregated PbS domains within each rod. Then, sulfur precursors are released via dissolution of the ZnS NRs during the cation-exchange, which promotes size-focusing of PbS QDs. PbS QDs synthesized through this new method have the advantages of high monodispersity, ease of size control, in-situ passivation of chloride, high stability, and "clean" surface. We fabricated infrared solar cells based on these PbS QDs with different bandgaps, using conventional ligand exchange and device structure. All of our devices produced in this manner show excellent performance, showcasing the high quality of our PbS QDs. The highest performance of infrared solar cells was achieved using ~0.95 eV PbS QDs, exhibiting an efficiency of 10.0% under AM 1.5 solar illumination, a perovskitefiltered efficiency of 4.2% and a silicon-filtered efficiency of 1.1%.