Charge Transfer from Quantum-Confined 0D, 1D, and 2D Nanocrystals.

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D-2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.

Transient Photoinduced Pb2+ Disproportionation for Exciton Self-Trapping and Broadband Emission in Low-Dimensional Lead Halide Perovskites.

Low-dimensional lead halide perovskites with broadband emission hold great promise for single-component white-light-emitting (WLE) devices. The origin of their broadband emission has been commonly attributed to self-trapped excitons (STEs) composed of localized electronic polarization with a distorted lattice. Unfortunately, the exact electronic and structural nature of the STE species in these WLE materials remains elusive, hindering the rational design of high-efficiency WLE materials. In this study, by combining ultrafast transient absorption spectroscopy and ab initio calculations, we uncover surprisingly similar STE features in two prototypical low dimensional WLE perovskite single crystals: 1D (DMEDA)PbBr4 and 2D (EDBE)PbBr4, despite of their different dimensionalities. Photoexcited excitons rapidly localize to intrinsic STEs within ∼250 fs, contributing to the white light emission. Crucially, STEs in both systems exhibit characteristic absorption features akin to those of Pb+ and Pb3+. Further atomic level theoretical simulations confirm photoexcited electrons and holes are localized on the Pb2+ site to form Pb+- and Pb3+-like species, resembling transient photoinduced Pb2+ disproportionation. This study provides conclusive evidence on the key excited state species for exciton self-trapping and broadband emission in low dimensional lead halide WLE perovskites and paves the way for the rational design of high-efficiency WLE materials.

A New Metal-Free Molecular Perovskite-Related Single Crystal with Quantum Wire Structure for High-Performance X-Ray Detection.

The family of metal-free molecular perovskites, an emerging novel class of eco-friendly semiconductor, welcomes a new member with a unique 1D hexagonal perovskite structure. Lowering dimensionality at molecular level is a facile strategy for crystal structure conversion, optoelectronic property regulation, and device performance optimization. Herein, the study reports the design, synthesis, packing structure, and photophysical properties of the 1D metal-free molecular perovskite-related single crystal, rac-3APD-NH4I3(rac-3APD= racemic-3-Aminopiperidinium), that features a quantum wire structure formed by infinite chains of face-sharing NH4I6 octahedra, enabling strong quantum confinement with strongly self-trapped excited (STE) states to give efficient warm orange emission with a photoluminescence quantum yield (PLQY) as high as ≈41.6%. The study accordingly unveils its photoexcited carrier dynamics: rac-3APD-NH4I3 relaxes to STE state with a short lifetime of 10 ps but decays to ground state by emitting photons with a relatively longer lifetime of 560 ps. Additionally, strong quantum confinement effect is conducive to charge transport along the octahedral channels that enables the co-planar single-crystal X-ray detectors to achieve a sensitivity as high as 1556 µC Gyair -1 cm-2. This work demonstrates the first case of photoluminescence mechanism and photophysical dynamics of 1D metal-free perovskite-related semiconductor, as well as the promise for high-performance X-ray detector.

Floating-gate memristor based on a MoS2/h-BN/AuNPs mixed-dimensional heterostructure.

Memristors have recently received substantial attention because of its promising and unique application scenes emerging in neuromorphic computing which can achieve gains in computation speed by mimicking the topology of brains in electronic circuits. Traditional memristors made of bulk MoO3 and HfO2, etc. suffer from low switching ratio, short durability and poor stability. In this work, a floating-gate memristor is developed based on a mixed-dimensional heterostructure which is comprised of two-dimensional (2D) molybdenum disulfide (MoS2) and 0-dimensional (0D) Au nanoparticles (AuNPs) separated by an insulating hexagonal boron nitride (h-BN) layer, hereafter, MoS2/h-BN/AuNPs. We find that under the modulation of back-gate voltages, the MoS2/h-BN/AuNPs device operates reliably between a high resistance state (HRS) and a low resistance state (LRS) and that it shows multiple stable LRS states, demonstrating high potential of our memristor in application of multibit storage. The modulation effect can be attributed to the electron quantum tunneling between the AuNPs charge-trapping layer and MoS2 channel. Our memristor exhibits excellent durability and stability: the HRS and LRS remain more than 104 s without obvious degradation and the on/off ratio retains > 104 after more than 3000 switching cycles. We also demonstrate frequency-dependent memory properties upon electrical and optical pulse stimuli.

Regulating Crystallinity Mismatch Between Donor and Acceptor to Improve Exciton/Charge Transport in Efficient Organic Solar Cells.

Achieving a more balanced charge transport by morphological control is crucial in reducing bimolecular and trap-assisted recombination and enhancing the critical parameters for efficient organic solar cells (OSCs). Hence, a facile strategy is proposed to reduce the crystallinity difference between donor and acceptor by incorporating a novel multifunctional liquid crystal small molecule (LCSM) BDTPF4-C6 into the binary blend. BDTPF4-C6 is the first LCSM based on a tetrafluorobenzene unit and features a low liquid crystal phase transition temperature and strong self-assembly ability, conducive to regulating the active layer morphology. When BDTPF4-C6 is introduced as a guest molecule into the PM6 : Y6 binary, it exhibits better compatibility with the donor PM6 and primarily resides within the PM6 phase because of the similarity-intermiscibility principle. Moreover, systematic studies revealed that BDTPF4-C6 could be used as a seeding agent for PM6 to enhance its crystallinity, thereby forming a more balanced and favourable charge transport with suppressed charge recombination. Intriguingly, dual Förster resonance energy transfer was observed between the guest molecule and the host donor and acceptor, resulting in an improved current density. This study demonstrates a facile approach to balance the charge mobilities and offers new insights into boosting the efficiency of single-junction OSCs beyond 20 %.

Long-Range Hot Charge Transfer Exciton Dissociation in an Organic/2D Semiconductor Hybrid Excitonic Heterostructure.

Whether and how an electron-hole pair at the donor-acceptor interface separates from their mutual Coulombic interaction has been a long-standing question for both fundamental interests and optoelectronic applications. This question is particularly interesting but yet to be unraveled in the emerging mixed-dimensional organic/2D semiconductor excitonic heterostructures where the Coulomb interaction is poorly screened. Here, by tracking the characteristic electroabsorption (Stark effect) signal from separated charges using transient absorption spectroscopy, we directly follow the electron-hole pair separation process in a model organic/2D heterostructure, vanadium oxide phthalocyanine/monolayer MoS2. After sub-100 fs photoinduced interfacial electron transfer, we observe a barrier-less long-range electron-hole pair separation to free carriers within 1 ps by hot charge transfer exciton dissociation. Further experiment reveals the key role of the charge delocalization in organic layers sustained by the local crystallinity, while the inherent in-plane delocalization of the 2D semiconductor has a negligible contribution to charge pair separation. This study reconciles the seemingly contradicting charge transfer exciton emission and dissociation process and is important to the future development of efficient organic/2D semiconductor optoelectronic devices.

Above 100% Efficiency Photocharge Generation in Monolayer Semiconductors by Singlet Fission Sensitization.

Sensitizing inorganic semiconductors using singlet fission (SF) materials, which produce two excitons from one absorbed photon, can potentially boost their light-to-electricity conversion efficiency. The SF sensitization is particularly exciting for two-dimensional (2D) layered semiconductors with atomically flat surface and high carrier mobility but limited light absorption. However, efficiently harnessing triplet excitons from SF by charge transfer at organic/inorganic interface has been challenging, and the intricate interplay among competing processes remains unresolved. Here, we investigate SF sensitization in high-quality organic/2D bilayer heterostructures featuring TIPS-Pc single crystals. Through transient magneto-optical spectroscopy, we demonstrate that despite an ultrafast SF process in sub-100 fs, a significant fraction of singlet excitons in TIPS-Pc dissociate at the interface before fission, while triplet excitons from SF undergo diffusion-limited charge transfer at the interface in ∼10 ps to ns. Remarkably, the photocharge generation efficiency reaches 126% in heterostructures with optimal thickness, resulting from the competitive interplay between singlet exciton fission, dissociation, and triplet exciton transport. This presents a promising strategy for advancing SF-enhanced 2D optoelectronics beyond the conventional limits.

Single-junction organic solar cells with over 19% efficiency enabled by a refined double-fibril network morphology.

In organic photovoltaics, morphological control of donor and acceptor domains on the nanoscale is the key for enabling efficient exciton diffusion and dissociation, carrier transport and suppression of recombination losses. To realize this, here, we demonstrated a double-fibril network based on a ternary donor-acceptor morphology with multi-length scales constructed by combining ancillary conjugated polymer crystallizers and a non-fullerene acceptor filament assembly. Using this approach, we achieved an average power conversion efficiency of 19.3% (certified 19.2%). The success lies in the good match between the photoelectric parameters and the morphological characteristic lengths, which utilizes the excitons and free charges efficiently. This strategy leads to an enhanced exciton diffusion length and a reduced recombination rate, hence minimizing photon-to-electron losses in the ternary devices as compared to their binary counterparts. The double-fibril network morphology strategy minimizes losses and maximizes the power output, offering the possibility of 20% power conversion efficiencies in single-junction organic photovoltaics.

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.

Dynamic Exciton Polaron in Two-Dimensional Lead Halide Perovskites and Implications for Optoelectronic Applications.

ConspectusThe past few years have witnessed an exciting revival of the research interest in two-dimensional (2D) lead halide perovskites. The renaissance is strongly motivated by the great success of their three-dimensional (3D) counterparts in optoelectronic applications. Different from 3D lead halide perovskites where free carriers are generated upon photoexcitation, 2D lead halide perovskites experience weaker dielectric screening and stronger quantum confinement effects. Therefore, strongly bound excitons with binding energy of up to a few hundreds of meV are considered to be the main excited-state species responsible for optoelectronic processes in 2D perovskites. In addition to strong excitonic effects, polaronic effects are also inherent in the soft and anharmonic lattice of lead halide perovskites, and polaronic structural relaxation is found to strongly renormalize carrier excited-state behaviors. For example, ferroelectric large polaron formation and liquid-like solvation of band edge carriers are proposed to account for the exceptional properties of 3D lead halide perovskites. As for 2D lead halide perovskites, polaronic characteristics have also been observed in exciton spectral characters, but how the interplay between excitonic effect and polaronic effect redefines the nature of exciton polarons and their excited-state behaviors still remains largely unexplored.In this Account, we discuss our recent experimental findings about the excited-state properties of exciton polarons in 2D lead halide perovskites. We begin our discussion by introducing a conventional view of strongly bound excitons in 2D lead halide perovskites with large exciton binding energy, which is typically estimated from steady-state absorption spectra. However, owing to the soft and anharmonic lattice, excitons in 2D lead halide perovskites exhibit significant polaronic characters and exist as exciton polarons. It is still unclear how polaronic effects would affect the exciton properties in 2D lead halide perovskites, especially in their excited-state dynamics. By probing exchange interaction, we found that both intra- and inter-exciton Coulomb interaction strengths are substantially weakened by the polaronic screening effect, which is manifested as (1) a counterintuitively longer exciton spin lifetime by almost an order of magnitude or a smaller intraexcitonic interaction strength with temperature increasing from 80 to 340 K and (2) an order of magnitude smaller interexcitonic interaction strength compared to another prototypical 2D semiconductor named transition-metal dichalcogenides (TMDCs) with a comparable steady-state exciton binding energy. We further discuss the interplay between the long- and short-range exciton-phonon interaction and conclude that the exciton-phonon interaction strength is in an intermediate regime and the exciton polaron is momentarily trapped in 2D perovskites, that is, a dynamic exciton polaron.Finally, we highlight prospective opportunities with ligand and cation engineering to regulate the exciton-phonon interaction and exciton polaron properties in 2D perovskites, which have strong implications toward future rational design for 2D perovskite-based efficient photovoltaics or light-emitting devices with high color purity.

Spatiotemporally Coupled Electron-Hole Dynamics in Two Dimensional Heterostructures.

Coulomb interactions play a crucial role in low-dimensional semiconductor materials, e.g., 2D layered semiconductors, dictating their electronic and optical properties. However, fundamental questions remain as to whether and how Coulomb interactions affect the charge or energy flow in 2D heterostructures, which is essential for their light-electricity conversions. Herein, using ultrafast spectroscopy, we report real space coupled electron-hole dynamics in 2D heterostructures. We show in (WSe2/)WS2/MoTe2 with a controlled energy gradient for the hole and a near flat band for electron transfer, the fate of the electron is controlled by the hole in coupled dynamics. The interfacial electron transfer from WS2 to MoTe2 follows the hole closely and can be facilitated or suppressed by dynamic Coulomb interaction. In parallel to the band alignment, this study reveals the critical role of Coulomb interactions on the fate of photogenerated charges in 2D heterostructures, providing experimental evidence for coupled electron-hole dynamics and a new knob for steering nanoscale charge or energy transfer process.

Ultrafast Spontaneous Localization of a Jahn-Teller Exciton Polaron in Two-Dimensional Semiconducting CrI3 by Symmetry Breaking.

The excited state species and properties in low-dimensional semiconductors can be completely redefined by electron-lattice coupling or a polaronic effect. Here, by combining ultrafast broadband pump-probe spectroscopy and first-principles GW and Bethe-Salpeter equation calculations, we show semiconducting CrI3 as a prototypical 2D polaronic system with characteristic Jahn-Teller exciton polaron induced by symmetry breaking. A photogenerated electron and hole in CrI3 localize spontaneously in ∼0.9 ps and pair geminately to a Jahn-Teller exciton polaron with elongated Cr-I octahedra, large binding energy, and an unprecedentedly small exciton-exciton annihilation rate constant (∼10-20 cm3 s-1). Coherent phonon dynamics indicates the localization is mainly triggered by the coherent nuclear vibration of the I-Cr-I out-of-plane stretch mode at 128.5 ± 0.1 cm-1. The excited state Jahn-Teller exciton polaron in CrI3 broadens the realm of 2D polaron systems and reveals the decisive role of coupled electron-lattice motion on excited state properties and exciton physics in 2D semiconductors.

Vacuum-Vapor-Deposited 0D/3D All-Inorganic Perovskite Composite Films toward Low-Threshold Amplified Spontaneous Emission and Lasing.

Vacuum vapor deposition (VVD) is a promising way to advancing the commercialization of perovskite light sources owing to its convenience for wafer-scale mass production and compatibility with silicon photonics manufacturing infrastructure. However, the light emission performance of VVD-grown perovskites still lags far behind that of the conventional solution-processed counterparts due to their inferior luminescence properties. Here, a 0D/3D cesium-lead-bromide perovskite composite film is prepared on Si/SiO2 substrates through composition modulation with the VVD method, which exhibits an ultralow amplified spontaneous emission (ASE) threshold down to 14.3 µJ cm-2 in the optimal films, which is on par with that of the solution-processed counterparts. Meanwhile, they also display intriguing operational stability with negligible emission intensity decay under continuous excitation above ASE threshold for 4 h in the air. The outstanding ASE performance mainly originates from the reduced trap density and weakened electron-phonon coupling in the 3D CsPbBr3 phase enabled by the incorporation of the 0D Cs4 PbBr6 phase. Finally, by integrating the composite film with the distributed feedback (DFB) cavity, DFB lasing is achieved with a low threshold of 18.2 µJ cm-2 under nanosecond-pulsed laser pumping, which highlights the potential of VVD-processed perovskites for developing high-performance lasers.

Long-range transport and ultrafast interfacial charge transfer in perovskite/monolayer semiconductor heterostructure for enhanced light absorption and photocarrier lifetime.

Atomically thin two-dimensional transition metal dichalcogenides (TMDs) have shown great potential for optoelectronic applications, including photodetectors, phototransistors, and spintronic devices. However, the applications of TMD-based optoelectronic devices are severely restricted by their weak light absorption and short exciton lifetime due to their atomically thin nature and strong excitonic effect. To simultaneously enhance the light absorption and photocarrier lifetime of monolayer semiconductors, here, we report 3D/2D perovskite/TMD type II heterostructures by coupling solution processed highly smooth and ligand free CsPbBr3 film with MoS2 and WS2 monolayers. By time-resolved spectroscopy, we show interfacial hole transfer from MoS2 (WS2) to the perovskite layer occurs in an ultrafast time scale (100 and 350 fs) and interfacial electron transfer from ultrathin CsPbBr3 to MoS2 (WS2) in ∼3 (9) ps, forming a long-lived charge separation with a lifetime of >20 ns. With increasing CsPbBr3 thickness, the electron transfer rate from CsPbBr3 to TMD is slower, but the efficiency remains to be near-unity due to coupled long-range diffusion and ultrafast interfacial electron transfer. This study indicates that coupling solution processed lead halide perovskites with strong light absorption and long carrier diffusion length to monolayer semiconductors to form a type II heterostructure is a promising strategy to simultaneously enhance the light harvesting capability and photocarrier lifetime of monolayer semiconductors.

Photoexcitation-controlled self-recoverable molecular aggregation for flicker phosphorescence.

Chemical systems with external control capability and self-recoverability are promising since they can avoid additional chemical or energy imposition during the working process. However, it remains challenging to employ such a nonequilibrium method for the engineering of optoelectronic function and for visualization. Here, we report a functional molecule that can undergo intense conformational regulation upon photoexcitation. It enables a dynamical change in hydrophobicity and a follow-up molecular aggregation in aqueous media, accordingly leading to an aggregation-induced phosphorescence (AIP) behavior. This successive performance is self-recoverable, allowing a rapid (second-scale cycle) and long-standing (>103 cycles) flicker ability under rhythmical control of the AIP. Compared with traditional bidirectional manipulations, such monodirectional photocontrol with spontaneous reset profoundly enhances the operability while mostly avoiding possible side reactions and fatigue accumulation. Furthermore, this material can serve as a type of luminescent probe for dynamically strengthening visualization in bioimaging.

One-Dimensional Superlattice Heterostructure Library.

Axially, epitaxially organizing nano-objects of distinct compositions and structures into superlattice nanowires enables full utilization of sunlight, readily engineered band structures, and tunable geometric parameters to fit carrier transport, thus holding great promise for optoelectronics and solar-to-fuel conversion. To maximize their efficiency, the general and high-precision synthesis of colloidal axial superlattice nanowires (ASLNWs) with programmable compositions and structures is the prerequisite; however, it remains challenging. Here, we report an axial encoding methodology toward the ASLNW library with precise control over their compositions, dimensions, crystal phases, interfaces, and periodicity. Using a predesigned, editable nanoparticle framework that offers the synthetic selectivity, we are able to chemically decouple adjacent sub-objects in ASLNWs and thus craft them in a controlled approach, yielding a library of distinct ASLNWs. We integrate therein plasmonic, metallic, or near-infrared-active chalcogenides, which hold great potential in solar energy conversion. Such synthetic capability enables a performance boost in target applications, as we report order-of-magnitude enhanced photocatalytic hydrogen production rates using optimized ASLNWs compared to corresponding solo objects. Furthermore, it is expected that such unique superlattice nanowires could bring out new phenomena.

Sub-3 nm Aluminum Nanocrystals Exhibiting Cluster-Like Optical Properties.

Metal nanoclusters with distinct photophysical and photochemical properties have drawn intense research interests for their applications in optoelectronics, catalysis, and biomedicine. Herein, strong evidence is provided that light metal is capable of generating comparable optical responses of noble metal nanoclusters, but at much shorter wavelength. Air-stable, size-uniform, sub-3 nm aluminum nanocrystals are prepared with simple solution based synthetic procedures, with photoluminescence located in the ultraviolet range and short exciton lifetime. Partial modulation of the photoluminescence is achieved, indicating the key role of surface oxides. This work is envisioned to inspire new frontiers of nanocluster research with light metals.

Highly Efficient Multiple Exciton Generation and Harvesting in Few-Layer Black Phosphorus and Heterostructure.

Multiple exciton generation (MEG) in semiconductors that yields two or more excitons by absorbing one high-energy photon has been proposed to break the Shockley-Queisser limit and boost photon-to-electron conversion efficiency. However, MEG performance in conventional bulk semiconductors or later colloidal nanocrystals is far from satisfactory. Here, we report efficient MEG in few-layer black phosphorus (BP), a direct narrow bandgap two-dimensional (2D) semiconductor with layer-tunable properties. MEG performance improves with decreasing layer number and reaches 2.09Eg threshold and 93% efficiency for two-layer BP, approaching energy conservation limit. The enhanced MEG can be attributed to strong Coulomb interaction and high density of states in 2D materials. Furthermore, MEG of BP shows negligible degradation in vertical heterostructure and multielectron can be extracted by interfacial transfer with near unity yield. These results suggest 2D semiconductors as an ideal system for next generation highly efficient light emission and charge transfer devices.

Simple Non-Fused Electron Acceptors Leading to Efficient Organic Photovoltaics.

Despite the remarkable progress achieved in recent years, organic photovoltaics (OPVs) still need work to approach the delicate balance between efficiency, stability, and cost. Herein, two fully non-fused electron acceptors, PTB4F and PTB4Cl, are developed via a two-step synthesis from single aromatic units. The introduction of a two-dimensional chain and halogenated terminals for these non-fused acceptors plays a synergistic role in optimizing their solid stacking and orientation, thus promoting an elongated exciton lifetime and fast charge-transfer rate in bulk heterojunction blends. As a result, PTB4Cl, upon blending with PBDB-TF polymer, has enabled single-junction OPVs with power conversion efficiencies of 12.76 %, representing the highest values among the reported fully unfused electron acceptors so far.

Pillar[5]arene-Based Solid-State Supramolecular Polymers with Suppressed Aggregation-Caused Quenching Effects and Two-Photon Excited Emission.

Herein we develop a new pillar[5]arene-mediated supramolecular polymerization strategy to control the assembly of dyes 4,7-di-2-thienyl-2,1,3-benzothiadiazole and 4,7-di-2-thienyl-2,1,3-benzoselenadiazole in the solid state. The resulting supramolecular polymeric structures drive the dye aggregates in the solid state from parallel alignments to highly ordered "head-to-tail" structures, which effectively suppresses excimer formation and thus aggregation-caused quenching effects and enhances the solid-state emission efficiency by almost 1 order of magnitude. This, together with two-photon excited emission, endows these solid-state polymers with exciting potential in optoelectronic, photonic, and bioimaging applications.