Substitutional Cd Dopant as Photohole Transfer Mediator Boosting Photoelectrochemical Solar Energy Conversion of 2D Cd-ZnIn2 S4 Photoanode.

Fast recombination dynamics of photocarriers competing with sluggish surface photohole oxidation kinetics severely restricts the photoelectrochemical (PEC) conversion efficiency of photoanode. Here, a defect engineering strategy is developed to regulate photohole transfer and interfacial injection dynamics of 2D ZnIn2 S4 (ZIS). Via selectively introducing substitutional Cd dopant at Zn sites of the ZIS basal plane, energy band structure and surface electrochemical activity are successfully modulated in the Cd-doped ZIS (Cd-ZIS) nanosheet array photoanode. Comprehensive characterizations manifest that a shallow acceptor level induced by Cd doping and superior electrochemical activity make surface Cd dopants simultaneously act as capture centers and active sites to mediate photohole dynamics at the reaction interface. In depth photocarrier dynamics analysis demonstrates that highly efficient photohole capture of Cd dopants brings about effective space separation of photocarriers and acceleration of surface reaction kinetics. Therefore, the optimum 2D Cd-ZIS achieves excellent PEC solar energy conversion efficiency with a photocurrent density of 5.1 mA cm-2 at 1.23 VRHE and a record of applied bias photon-to-current efficiency (ABPE) of 3.0%. This work sheds light on a microstructure design strategy to effectively regulate photohole dynamics for the next-generation semiconducting PEC photoanodes.

Elimination of Intragrain Defect to Enhance the Performance of FAPbI3 Perovskite Solar Cells by Ionic Liquid.

Ionic liquids have been widely used to improve the efficiency and stability of perovskite solar cells (PSCs), and are generally believed to passivate defects on the grain boundaries of perovskites. However, few studies have focused on the relevant effects of ionic liquids on intragrain defects in perovskites which have been shown to be critical for the performance of PSCs. In this work, the effect of ionic liquid 1-hexyl-3-methylimidazolium iodide (HMII) on intragrain defects of formamidinium lead iodide (FAPbI3) perovskite is investigated. Abundant {111}c intragrain planar defects in pure FAPbI3 grains are found to be significantly reduced by the addition of the ionic liquid HMII, shown by using ultra-low-dose selected area electron diffraction. As a result, longer charge carrier lifetimes, higher photoluminescence quantum yield, better charge carrier transport properties, lower Urbach energy, and current-voltage hysteresis are achieved, and the champion power conversion efficiency of 24.09% is demonstrated. These observations suggest that ionic liquids significantly improve device performance resulting from the elimination of {111}c intragrain planar defects.

Symmetry-breaking charge separation in a null-excitonic 3-dimensional rigid nonconjugated trimer.

Photoinduced symmetry-breaking charge separation (SB-CS) has been extensively observed in various oligomers and aggregates, which holds great potential for robust artificial solar energy conversion systems. It attaches great importance to the precise manipulation of interchromophore electronic coupling in realizing efficient SB-CS. The emerging studies on SB-CS suggested that it could be realized in null-excitonic aggregates, and a long-lived SB-CS state was observed, which offers an advanced platform and has gathered immense attention in the SB-CS field. Here, we unveiled the null-exciton coupling induced ultrafast SB-CS in a rigid polycyclic aromatic hydrocarbon framework, triperyleno[3,3,3]propellane triimides (TPPTI), in which three chromophores were attached through a nonconjugated bridge. Through a combination of theoretical calculations and steady-state absorption results, we demonstrated that this nonconjugated TPPTI possesses negligible exciton coupling. Increased solvent polarity was found to significantly enhance state mixing between local excited and charge transfer states. Using transient absorption spectroscopy, ultrafast SB-CS was observed in highly polar dimethylformamide, facilitated by a selective hole-transfer coupling and a favorable charge separation free energy (ΔGCS). Additionally, the rate ratio between SB-CS and charge recombination was at least high to 1800 in dimethylformamide. This investigation provides profound insights into the role of null-exciton coupling in dominating ultrafast SB-CS in multichromophoric systems.

Boosted Photoreforming of Plastic Waste via Defect-Rich NiPS3 Nanosheets.

Sustainable conversion of plastic waste to mitigate environmental threats and reclaim waste value is important. Ambient-condition photoreforming is practically attractive to convert waste to hydrogen (H2); however, it has poor performance because of mutual constraint between proton reduction and substrate oxidation. Here, we realize a cooperative photoredox using defect-rich chalcogenide nanosheet-coupled photocatalysts, e.g., d-NiPS3/CdS, to give an ultrahigh H2 evolution of ∼40 mmol gcat-1 h-1 and organic acid yield up to 78 µmol within 9 h, together with excellent stability beyond 100 h in photoreforming of commercial waste plastic poly(lactic acid) and poly(ethylene terephthalate). Significantly, these metrics represent one of the most efficient plastic photoreforming reported. In situ ultrafast spectroscopic studies confirm a charge transfer-mediated reaction mechanism in which d-NiPS3 rapidly extracts electrons from CdS to boost H2 evolution, favoring hole-dominated substrate oxidation to improve overall efficiency. This work opens practical avenues for converting plastic waste into fuels and chemicals.

Protic Amine Carboxylic Acid Ionic Liquids Additives Regulate α-FAPbI3 Phase Transition for High Efficiency Perovskite Solar Cells.

The α-phase formamidinium lead tri-iodide (α-FAPbI3 ) has become the most promising photovoltaic absorber for perovskite solar cells (PSCs) due to its outstanding semiconductor properties and astonishing high efficiency. However, the incomplete crystallization and phase transition of α-FAPbI3 substantially undermine the performance and stability of PSCs. In this work, a series of the protic amine carboxylic acid ion liquids are introduced as the precursor additives to efficiently regulate the crystal growth and phase transition processes of α-FAPbI3 . The MA2 Pb3 I8 ·2DMSO phase is inhibited in annealing process, which remarkably optimizes the phase transition process of α-FAPbI3 . It is noted that the functional groups of carboxyl and ammonium passivate the undercoordinated lead ions, halide vacancies, and organic vacancies, eliminating the deleterious nonradiative recombination. Consequently, the small-area devices incorporated with 2% methylammonium butyrate (MAB) and 1.5% n-butylammonium formate (BAFa) in perovskite show champion efficiencies of 25.10% and 24.52%, respectively. Furthermore, the large-area modules (5 cm × 5 cm) achieve PCEs of 21.26% and 19.27% for MAB and BAFa additives, indicating the great potential for commercializing large-area PSCs.

Thermally activated delayed fluorescence (TADF) organic molecules for efficient X-ray scintillation and imaging.

X-ray detection, which plays an important role in medical and industrial fields, usually relies on inorganic scintillators to convert X-rays to visible photons; although several high-quantum-yield fluorescent molecules have been tested as scintillators, they are generally less efficient. High-energy radiation can ionize molecules and create secondary electrons and ions. As a result, a high fraction of triplet states is generated, which act as scintillation loss channels. Here we found that X-ray-induced triplet excitons can be exploited for emission through very rapid, thermally activated up-conversion. We report scintillators based on three thermally activated delayed fluorescence molecules with different emission bands, which showed significantly higher efficiency than conventional anthracene-based scintillators. X-ray imaging with 16.6 line pairs mm-1 resolution was also demonstrated. These results highlight the importance of efficient and prompt harvesting of triplet excitons for efficient X-ray scintillation and radiation detection.

Verifying the Charge-Transfer Mechanism in S-Scheme Heterojunctions Using Femtosecond Transient Absorption Spectroscopy.

The S-scheme heterojunction is flourishing in photocatalysis because it concurrently realizes separated charge carriers and sufficient redox ability. Steady-state charge transfer has been confirmed by other methods. However, an essential part, the transfer dynamics in S-scheme heterojunctions, is still missing. To compensate, a series of cadmium sulfide/pyrene-alt-difluorinated benzothiadiazole heterojunctions were constructed and the photophysical processes were investigated with femtosecond transient absorption spectroscopy. Encouragingly, an interfacial charge-transfer signal was detected in the spectra of the heterojunction, which provides solid evidence for S-scheme charge transfer to complement the results from well-established methods. Furthermore, the lifetime for interfacial charge transfer was calculated to be ca. 78.6 ps. Moreover, the S-scheme heterojunction photocatalysts exhibit higher photocatalytic conversion of 1,2-diols and H2 production rates than bare cadmium sulfide.

Thermally Activated Delayed Near-Infrared Photoluminescence from Functionalized Lead-Free Nanocrystals.

As an analogue to thermally activated delayed fluorescence (TADF) of organic molecules, thermally activated delayed photoluminescence (TADPL) observed in molecule-functionalized semiconductor nanocrystals represents an exotic mechanism to harvest energy from dark molecular triplets and to obtain controllable, long-lived PL from nanocrystals. The reported TADPL systems have successfully covered the visible spectrum. However, TADF molecules already emit very efficiently in the visible, diminishing the technological impact of the less-efficient nanocrystal-molecule TADPL. Here we report bright, near-infrared TADPL in lead-free CuInSe2 nanocrystals functionalized with carboxylated tetracene ligands, which results from efficient triplet energy transfer from photoexcited nanocrystals to ligands, followed with thermally activated reverse energy transfer from ligand triplets back to nanocrystals. This strategy prolonged the nanocrystal exciton lifetime from 100 ns to 60 µs at room temperature.

Highly Effective Hybrid Copper(I) Iodide Cluster Emitter with Negative Thermal Quenched Phosphorescence for X-Ray Imaging.

The low efficiency triplet emission of hybrid copper(I) iodide clusters is a critical obstacle to their further practical optoelectronic application. Herein, we present an efficient hybrid copper(I) iodide cluster emitter (DBA)4 Cu4 I4 , where the cooperation of excited state structure reorganization and the metallophilicity interaction enables ultra-bright triplet yellow-orange emission with a photoluminescence quantum yield over 94.9 %, and the phonon-assisted de-trapping process of exciton induces the negative thermal quenching effect at 80-300 K. We also investigate the potential of this emitter for X-ray imaging. The (DBA)4 Cu4 I4 wafer demonstrates a light yield higher than 104  photons MeV-1 and a high spatial resolution of ≈5.0 lp mm-1 , showing great potential in practical X-ray imaging applications. Our new copper(I) iodide cluster emitter can serve as a model for investigating the thermodynamic mechanism of photoluminescence in hybrid copper(I) halide phosphorescence materials.

Spectral analysis on the acceptor concentration-dependent fluorescence resonance energy transfer process in CuInS2@ZnS-SQ complexes.

Owing to the broad spectral response and flexible choices of donors and acceptors, fluorescence resonance energy transfer (FRET) system based on quantum dots (QDs) is a potential candidate for enhancing performance of solar cells and other optoelectronic devices. Thus it is necessary to develop such FRET systems with high efficiency and understand the involved photophysical dynamics. Here, with type I CuInS2@ZnS core-shell quantum dots as the energy donor, series of CuInS2@ZnS-SQ complexes are synthesized by adjusting the acceptor (squaric acid, SQ) concentration. The FRET dynamics of the samples is systematically investigated by virtue of steady-state emission, time-resolved fluorescence decay, and transient absorption measurements. The experimental results display a positive correlation between the energy transfer efficient (η). The best energy transfer efficient achieved from experimental data is 52%. This work provides better understanding of the photophysical dynamics in similar complexes and facilitates further development of new photoelectronic devices based on relevant FRET systems.

Unambiguous spectral characterization on triplet energy transfer from quantum dots mediated by hole transfer competing with other carrier dynamics.

Triplet generation by quantum dots (QDs)-sensitized molecules emerges great potential in many applications. However, the mechanism of triplet energy transfer (TET) is still fuzzy especially due to the complicated energy level alignment of QDs and molecules or trap states in QDs. Here, CdSe QDs and 5-tetracene carboxylic acid (TCA) molecules are selected as the triplet donor and acceptor, respectively, to form a TET system. By tuning the band gap of CdSe, the CdSe-TCA complex is exactly designed to present a Type-II like alignment of relative energetics. Coupling the transient absorption and time-resolved fluorescence spectra, all carrier dynamics is distinctly elucidated. Quantitative analysis demonstrates that hole transfer persisting for ∼ 2 ps outcompetes all other carrier dynamics such as electron trapping (∼100 ps level), charge recombination (∼ 5 ns) and the so-called "back transfer charge recombination" (∼50 ns), and thus leads to a hole-transfer-mediated TET process. The low TET yield (∼34.0%) ascribed to electron behavior can be further improved if electron trapping and charge recombination are efficiently suppressed. The observation on distinguishable carrier dynamics attributed to legitimate design of energy level alignment facilitates a better understanding of the TET mechanism from QDs to molecules as well as further development of photoelectronic devices based on such TET systems.

Auger-Assisted Electron Transfer between Adjacent Quantum Wells in Two-Dimensional Layered Perovskites.

Two-dimensional (2D) layered perovskites are naturally formed multiple quantum-well (QW) materials, holding great promise for applications in many optoelectronic devices. However, the further use of 2D layered perovskites in some devices is limited by the lack of QW-to-QW carrier transport/transfer due to the energy barrier formed by the insulating ligands between QWs. Herein, we report an Auger-assisted electron transfer between adjacent QWs in (CmH2m+1NH3)2PbI4 2D perovskites particularly with m = 12 and 18, where the electron energy barrier (Eb) is similar to the QW band gap energy (Eg). This Auger-assisted QW-to-QW electron transfer mechanism is established by the observation of a long-lived and derivative-like transient absorption feature, which is a signature of the quantum confined Stark effect induced by the electron-hole separation (thus an internal electric field) between different QW layers. Our finding provides a new guideline to design 2D perovskites with an optically tunable QW-to-QW charge transport property, advancing their applications in optoelectronics and optical modulations.

Molecular Gold Nanocluster Au156 Showing Metallic Electron Dynamics.

The boundary between molecular and metallic gold nanoclusters is of special interest. The difficulty in obtaining atomically precise nanoclusters larger than 2 nm limits the determination of such a boundary. The synthesis and total structural determination of the largest all-alkynyl-protected gold nanocluster (Ph4P)6[Au156(C≡CR)60] (R = 4-CF3C6H4-) (Au156) are reported. It presents an ideal platform for studying the relationship between the structure and the metallic nature. Au156 has a rod shape with the length and width of the kernel being 2.38 and 2.04 nm, respectively. The cluster contains a concentric Au126 core structure (Au46@Au50@Au30) protected by 30 linear RC≡C-Au-C≡CR staple motifs. It is interesting that Au156 displays multiple excitonic peaks in the steady-state absorption spectrum (molecular) and pump-power-dependent excited-state dynamics as revealed in the transient absorption spectrum (metallic), which indicates that Au156 is a critical crossover cluster for the transition from molecular to metallic state. Au156 is the smallest-sized gold nanocluster showing metal-like electron dynamics, and it is recognized that the cluster shape is one of the important factors determining the molecular or metallic nature of a gold nanocluster.

Ultrafast photophysical process of bi-exciton Auger recombination in CuInS2 quantum dots studied by transient-absorption spectroscopy.

Auger recombination is an ultrafast and unnegligible photophysical process in colloidal semiconductor quantum dots (QDs) due to competition with charge separation or radiative recombination processes, pivotal for their applications ranging from bio-labeling, light-emitting diodes, QD lasing to solar energy conversion. Among diverse QDs, ternary chalcopyrite is recently receiving significant attention for its heavy-metal free property and remarkable optical performance. Given deficient understanding of the Auger process for ternary chalcopyrite QDs, CuInS2 QDs with various sizes are synthesized as a representative and the bi-exciton lifetime (τBX) is derived by virtue of ultrafast time resolved absorption spectrum. The trend of τBX varying with size is consistent with the universal scaling of τBX versus QD volume (V): τBX = γV. The scaling factor γ is 6.6 ± 0.5 ps·nm-3 for CuInS2 QDs, and the bi-exciton Auger lifetime is 4-5 times slower than typical CdSe QDs with the same volume, suggesting reduced Auger recombination rate in ternary chalcopyrite. This work facilitates clearer understanding of Auger process and provides further insight for rational design of light-harvesting and emitting devices based on ternary chalcopyrite QDs.

Triplet Energy Transfer from Perovskite Nanocrystals Mediated by Electron Transfer.

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.

Total Structure Determination of the Largest Alkynyl-Protected fcc Gold Nanocluster Au110 and the Study on Its Ultrafast Excited-State Dynamics.

Great attention has been paid to nanoclusters having face-centered-cubic (fcc) metal kernels, because of the similarity of metal packing to that of bulk gold. So far, there is no precedent example of an all-alkynyl-protected fcc gold nanocluster with more than 100 gold atoms. We report the synthesis and total structure determination of an alkynyl-protected gold nanocluster [NEt3H]2[Au110(p-CF3C6H4C≡C)48] (Au110). It has an fcc Au86 kernel with 24 peripheral Au(C≡CR)2 staples. The Au86 kernel consists of six close packing layers in the pattern of Au6:Au16:Au21:Au21:Au16:Au6. Electronic absorption spectroscopy shows Au110 has a molecular-like discrete electronic structure, and transient absorption experiments reveal its nonmetallic nature.

Triplet Energy Transfer from CsPbBr3 Nanocrystals Enabled by Quantum Confinement.

The spectral properties of lead halide perovskite nanocrystals (NCs) can be engineered by tuning either their sizes via the quantum confinement effect or their compositions using anion and/or cation exchange. To date, the latter is more frequently adopted, primarily because of the ease of ion exchange for lead halide perovskites, making the quantum confinement effect seemingly redundant for perovskite NCs. Here we report that quantum confinement is required for triplet energy transfer (TET) from perovskite NCs to polycyclic aromatic hydrocarbons (PAHs). Static and transient spectroscopy measurements on CsPbBr3 NC-pyrene hybrids showed that efficient TET occurred only for small-sized, quantum-confined CsPbBr3 NCs. The influences of the size-dependent driving force and spectral overlap on the TET rate were found to be negligible. Instead, the TET rate scaled linearly with carrier probability density at the NC surface, consistent with a Dexter-type TET mechanism requiring wave function exchange between the NC donors and pyrene acceptors. Efficient TET funnels the excitation energy generated in strongly light-absorbing perovskite NCs into long-lived triplets in PAHs, which may find broad applications such as photon upconversion and photoredox catalysis.

Reaction-based probe for hydrogen sulfite: dual-channel and good ratiometric response.

We designed and synthesized a new series of intramolecular charge transfer (ICT) molecules (compounds T1, T2 and T3) by attaching various electron-donating thiophene groups to the triphenylamine backbone with aldehyde group as the electron acceptor. Based on the nucleophilic addition reaction between hydrogen sulfite and aldehyde, all compounds could act as ratiometric optical probe for hydrogen sulfite and displayed efficient chromogenic and fluorogenic signaling. Upon the addition of hydrogen sulfite anions, probe T3 displayed apparent fluorescent color changes from yellowish-green to blue, with a large emission wavelength shift (Δλ = 120 nm). T3 responded to hydrogen sulfite with high sensitivity and the detection limit was determined to be as low as 0.9 µM. At the same time, apparent changes in UV-vis spectra could also be observed. By virtue of the special nucleophilic addition reaction with aldehyde, T3 displayed high selectivity over other anions. Copyright © 2016 John Wiley & Sons, Ltd.

Ultrafast Interfacial Electron and Hole Transfer from CsPbBr3 Perovskite Quantum Dots.

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.