Charge Transfer Excitons Research Articles

Luminescent switching behavior achieved by adjusting cocrystal charge transfer excitons under acid stimulation

This study presents acid stimuli responsive cocrystal systems with dynamically controllable luminous characteristics, demonstrating highly sensitive fluorescence switching, offering a successful method for developing smart materials.

Empirical Model of the Charge Carriers’ Photogeneration in Organic Solar Cells

A model of the photocurrent generation of charge carriers in blends of donor (D) and acceptor (A)materials structured on the nanoscale is considered. The absorption of a quantum of light in one of thesematerials creates a molecular exciton, which can reach the interface between the D and A phases and form aninterfacial charge transfer (CT) exciton on this interface, which dissociates into an electron-hole pair. Theprobabilities of the dissociation of CT excitons into free current carriers are calculated as a function of theelectric field and the thermalization length of the electron-hole pair.

Time-domain observation of interlayer exciton formation and thermalization in a MoSe2/WSe2 heterostructure

Vertical heterostructures of transition metal dichalcogenides (TMDs) host interlayer excitons with electrons and holes residing in different layers. With respect to their intralayer counterparts, interlayer excitons feature longer lifetimes and diffusion lengths, paving the way for room temperature excitonic optoelectronic devices. The interlayer exciton formation process and its underlying physical mechanisms are largely unexplored. Here we use ultrafast transient absorption spectroscopy with a broadband white-light probe to simultaneously resolve interlayer charge transfer and interlayer exciton formation dynamics in a MoSe2/WSe2 heterostructure. We observe an interlayer exciton formation timescale nearly an order of magnitude (~1 ps) longer than the interlayer charge transfer time (~100 fs). Microscopic calculations attribute this relative delay to an interplay of a phonon-assisted interlayer exciton cascade and thermalization, and excitonic wave-function overlap. Our results may explain the efficient photocurrent generation observed in optoelectronic devices based on TMD heterostructures, as the interlayer excitons are able to dissociate during thermalization.

Observation of aligned dipoles and angular chromism of exciplexes in organic molecular heterostructures

The dipole characteristics of Frenkel excitons and charge-transfer excitons between donor and acceptor molecules in organic heterostructures such as exciplexes are important in organic photonics and optoelectronics. For the bilayer of the organic donor 4,4′,4′′-tris[(3-methylphenyl)phenylamino]triphenylamine and acceptor 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine molecules, the exciplexes form aligned dipoles perpendicular to the Frenkel excitons, as observed in back focal plane photoluminescence images. The angular chromism of exciplexes observed in the 100 meV range indicates possible delocalization and angle-sensing photonic applications. The blue shift of the peak position and increase in the linewidth of photoluminescene spectra with increasing excitation power are caused by the repulsive aligned exciplex dipole moments with a long lifetime (4.65 μs). Electroluminescence spectra of the exciplex from organic light-emitting diodes using the bilayer are blue-shifted with increasing bias, suggesting unidirectional alignment of the exciplex dipole moments. The observation of exciplex dipole moment alignments across molecular interfaces can facilitate the controlled coupling of exciton species and increase efficiency of organic light-emitting diodes.

Critical role of empty in-gap states in the photocatalytic water splitting on carbon nitride nanosheets

Introducing nitrogen vacancies and non-metal dopants is one popular strategy to improve the photocatalytic H2O splitting efficiency of carbon nitride nanosheets. Especially, Nat. Energy2021, 6, 388–397 reported a fascinating solar-to-hydrogen efficiency of 1.16% for the carbon nitride nanosheets containing nitrogen vacancies and boron dopants. However, effects of these defects on the reaction mechanism remain unknown. Through first-principles calculations we reveal that H2O splitting on perfect melon can only be realized in the excited state under the assistance of charge-transfer excitons, moreover this excited-state reaction is easily destroyed by aqueous solution; the key role of nitrogen vacancies and boron dopants should be that they introduce empty in-gap states which can either make the reaction realized in the ground state or avoid the negative effect of aqueous solution. However, the inter-triazine nitrogen vacancy can be poisoned and the reduction capability can be damaged by boron dopants. These two phenomena do not occur at two-coordinated nitrogen vacancies, but H2O splitting at this defect requires charge-transfer excitons. Above shortcomings may limit further improvement of melon on photocatalytic H2O splitting by doping nitrogen vacancies and boron. Introducing appropriate empty states might be the way to design good photocatalysts for splitting H2O.

Construction of Z-scheme photocatalyst with p-n heterojunction of molecular semiconductors for enhanced photocatalytic water oxidation

Here, the donor-acceptor organic composites of n-type perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and p-type cobalt phthalocyanine (CoPc) semiconductors are firstly synthesized and optimized for photocatalytic oxygen evolution (POE) as catalyst. The POE rate of 12.30 mmol·g−1·h−1 with the apparent quantum yield of 4.00 % at 475 nm can be achieved under visible light (> 420 nm), as compared with 1.58 mmol.g−1.h−1 of PTCDA. Under the full UV-Vis light (> 320 nm), the rate is up to 17.65 mmol·g−1·h−1. The systematical experiments clearly reveal the generation of dual-channel energy transfer of fluorescence resonance from CoPc to PTCDA and electron transfer from oxygen groups in PTCDA to Co atom in CoPc under UV-Vis light irradiation. Consequently, the formation of Z-scheme p/n heterojunction in CoPc/PTCDA composite greatly enhances the POE performance due to the lifetime increase of metal-ligand charge transfer exciton in CoPc (44.83 vs. 2.95 ps) and perylene cationic exciton (20.13 vs. 2.13 ps).

Long-Range Charge Separation Enabled by Intramoiety Delocalized Excitations in Copolymer Donors in Organic Photovoltaic Blends.

For over two decades, most high-performance organic photovoltaics (OPVs) have been made with donor:acceptor bulk heterojunctions with domain sizes limited by exciton diffusion, where charge separation mostly takes place through the dissociation of the interfacial charge-transfer (xCT) excitons. Recently, nonfullerene acceptor (NFA)-based OPVs have shown excellent compatibility to device structures with large domains in active layers. However, it remains elusive how the excitations that are distant from the interfaces are converted into free charges. Here, we report the identification of a new charge separation channel in model copolymer/NFA blends mediated by intra-moiety delocalized excitations in both planar heterojunctions and donor-enriched bulk heterojunctions. The delocalized excitations induced by interchromophore electronic interactions in copolymer donors mediate the long-range charge separation and dissociate into free charges without forming the bound xCT states first, releasing the constraints associated with the short exciton diffusion length in organic materials. The long-range charge separation mechanism uncovered in this work, in cooperation with the short-range xCT-mediated pathway, holds the potential to further optimize OPVs with diverse device structures.

Strong Dipolar Repulsion of One-Dimensional Interfacial Excitons in Monolayer Lateral Heterojunctions.

Repulsive and long-range exciton-exciton interactions are crucial for the exploration of one-dimensional (1D) correlated quantum phases in the solid state. However, the experimental realization of nanoscale confinement of a 1D dipolar exciton has thus far been limited. Here, we demonstrate atomically precise lateral heterojunctions based at transitional-metal dichalcogenides (TMDCs) as a platform for 1D dipolar excitons. The dynamics and transport of the interfacial charge transfer excitons in a type II WSe2-WS1.16Se0.84 lateral heterostructure were spatially and temporally imaged using ultrafast transient reflection microscopy. The expansion of the exciton cloud driven by dipolar repulsion was found to be strongly density dependent and highly anisotropic. The interaction strength between the 1D excitons was determined to be ∼3.9 × 10-14 eV cm-2, corresponding to a dipolar length of 310 nm, which is a factor of 2-3 larger than the interlayer excitons at two-dimensional van der Waals vertical interfaces. These results suggest 1D dipolar excitons with large static in-plane dipole moments in lateral TMDC heterojunctions as an exciting system for investigating quantum many-body physics.

Influence of vibronic interaction of charge transfer excitons in PTB7/BTA-based nonfullerene organic solar cells.

We studied photoinduced charge transfer (CT) states and their dissociation processes at the donor/acceptor (D/A) interface of PTB7/BTAx (x = 1 and 3) nonfullerene organic thin-film solar cells using density functional theory (DFT) and time-dependent DFT calculations. We focused on the CT distances and electron coupling in the CT state generated by photoexcitation and the Huang-Rhys (HR) factors that describe the nonadiabatic processes associated with vibronic interactions. The PTB7/BTA3 system with a large short-circuit current density (JSC) exhibited a large charge CT distance and electronic coupling. Contrastingly, the PTB7/BTA1 system with a low JSC has a large HR factor because of the low-wavenumber vibrational modes in the CT state of the D/A complex and is prone to nonadiabatic relaxation to the ground state. Systematic theoretical analysis of the excitonic states in the D/A complex has provided insight into the control of CT exciton dynamics, namely JSC and electron-hole recombination.

Origins of the open-circuit voltage in ternary organic solar cells and design rules for minimized voltage losses

AbstractThe power conversion efficiency of ternary organic solar cells (TOSCs), consisting of one host binary blend and one guest component, remains limited by large voltage losses. The fundamental understanding of the open-circuit voltage (VOC) in TOSCs is controversial, limiting rational design of the guest component. In this study, we systematically investigate how the guest component affects the radiative and non-radiative related parts of VOC of a series of TOSCs using the detailed balanced principle. We highlight that the thermal population of charge-transfer and local exciton states provided by the guest binary blend (that is, the guest-component-based binary blend) has a significant influence on the non-radiative voltage losses. Ultimately, we provide two design rules for enhancing the VOC in TOSCs: high emission yield for the guest binary blend and similar charge-transfer-state energies for host/guest binary blends; high miscibility of the guest component with the low gap component in the host binary blend.

Manipulation of Charge Delocalization in a Bulk Heterojunction Material Using a Mid-Infrared Push Pulse.

In organic bulk heterojunction materials, charge delocalization has been proposed to play a vital role in the generation of free carriers by effectively reducing the Coulomb attraction via an interfacial charge transfer exciton (CTX). Pump-push-probe (PPP) experiments produced evidence that the excess energy given by a push pulse enhances delocalization, thereby increasing photocurrent. However, previous studies have employed near-infrared push pulses in the range ∼0.4-0.6 eV, which is larger than the binding energy of a typical CTX. This raises the doubt that the push pulse may directly promote dissociation without involving delocalized states. Here, we perform PPP experiments with mid-infrared push pulses at energies that are well below the binding energy of a CTX state (0.12-0.25 eV). We identify three types of CTXs: delocalized, localized, and trapped. The excitation resides over multiple polymer chains in delocalized CTXs, while it is restricted to a single chain (albeit maintaining a degree of intrachain delocalization) in localized CTXs. Trapped CTXs are instead completely localized. The pump pulse generates a "hot" delocalized CTX, which promptly relaxes to a localized CTX and eventually to trapped states. We find that photo-exciting localized CTXs with push pulses resonant to the mid-infrared charge transfer absorption can promote delocalization and, in turn, contribute to the formation of long-lived charge separated states. On the other hand, we found that trapped CTXs are non-responsive to the push pulses. We hypothesize that delocalized states identified in prior studies are only accessible in systems where there is significant interchain electronic coupling or regioregularity that supports either inter- or intrachain polaron delocalization. This, in turn, emphasizes the importance of engineering the micromorphology and energetics of the donor-acceptor interface to exploit the full potential of a material for photovoltaic applications.

The Impact of Spacer Size on Charge Transfer Excitons in Dion-Jacobson and Ruddlesden-Popper Layered Hybrid Perovskites.

Organic materials can tune the optical properties in layered (2D) hybrid perovskites, although their impact on photophysics is often overlooked. Here, we use transient absorption spectroscopy to probe the Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) 2D perovskite phases. We show the formation of charge transfer excitons in DJ phases, resulting in a photoinduced Stark effect which is shown to be dependent on the spacer size. By using electroabsorption spectroscopy, we quantify the strength of the photoinduced electric field, while temperature-dependent measurements demonstrate new features in the transient spectra of RP phases at low temperatures resulting from the quantum-confined Stark effect. This study reveals the impact of spacer size and perovskite phase configuration on charge transfer excitons in 2D perovskites of interest to their advanced material design.

Engineering push–pull structural versatility in highly functional carbazole-based hole transporting materials design for efficient perovskites solar devices

This study proposes a push–pull molecular engineering strategy to design highly functional hole transport materials (HTMs) for perovskite solar cells (PSCs). The strategy involves introducing acceptor-anchor groups via benzene spacer to the versatile carbazole core containing dimethoxytriphenylamine side groups, resulting in a series of nine newly designed HTM moieties (BEN-A1 to BEN-A9). Quantum simulation protocols using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods were employed to investigate the designed HTMs. Our results demonstrate that the designed HTMs exhibit promising charge separation and transport throughout the molecule with lower spatial hole-electron overlap at the carbazole core, resulting in 98% intrinsic charge transfer and small exciton binding energy (0.003–0.108 eV). The HTMs also exhibit stabilized HOMO energy levels (by up to 0.13 eV) approaching the threshold (−5.38 eV) with appropriate HTM/Perovskite energy levels alignment, suggesting excellent charge extraction and higher VOC. Optical analysis shows that our proposed HTMs exhibit large stokes shift values (82–108 nm) and transparent absorption in broader visible region, enabling full utilization of light for perovskite layer for photocurrent generation, efficient energy conversion, reduced thermalization losses, and improved spectral selectivity. The HTMs exhibit smaller hole reorganization energy (0.111–0.137 eV) and higher transfer integral, indicating robust hole mobility owing to enhanced carbazole core functionality. Furthermore, higher negative solvation-free energy values (−16.19 to −20.89 kcal/mol) and elevated dipole moments imply better solubility and surface-wetting properties. Overall, this study broadens our understanding of push–pull molecular engineering for versatile carbazole-based HTMs, which have immense prospects for efficient and functional application in PSCs.

Momentum dependence of the spectral weight in late transition metal phthalocyanine β-phases

The exciton dispersion in metal-free-phthalocyanine and cobalt-phthalocyanine as well as the momentum dependence of the spectral weight of the Q-band excitation in zinc-, copper-, nickel-, cobalt-, and metal-free-phthalocyanine β-phase thin films were evaluated using electron energy loss spectroscopy. We demonstrate that the lowest exciton in β-H2Pc and β-CoPc shows a negative dispersion which can be modeled taking into account charge transfer exciton coupling. The analysis of the momentum dependent spectral weight indicates substantial quadrupolar character of the Q-band excitations, which also points towards the admixture of charge transfer excitons.

Synchronous Light Harvesting and Energy Storing Organic Cathode Material 1,4-Dihydroxyanthraquinone for Lithium-Ion Batteries

The electrode materials for lithium-ion batteries which offer integrated conversion of light and storage of electricity are auspicious for compact and stand-alone energy systems. Herein, coin-cell lithium batteries based on organic compound 1,4-dihydroxyanthraquinone coupling porous carbon derived from MOFs (1,4DHAQ&ZIF-8C) are demonstrated. The proposed material simultaneously acts as lithium storing and photoactive electrode. Along with the electrochemical lithium-ion reactivity, 1,4DHAQ&ZIF-8C offers notable photoelectric properties, its specific capacity increases to 285mAh g-1 (1.58 times of the specific capacity without light) when illuminated, and the lithium-ion mobility improves to 0.313cm2 s-1 (up by 55%), meanwhile its charge transfer resistance decreases to 95.6Ω (down to 39%). Moreover, theoretical excited state analysis and microsecond measurements reveal the existence states of photogenerated carriers, demonstrating obvious intramolecular charge transfer and nanosecond-scale exciton lifetime (∼5ns). Our findings suggest valuable directions to advance the current strategies for the assembly of efficient photo-rechargeable lithium-ion batteries.

In-plane and out-of-plane excitonic coupling in 2D molecular crystals

Understanding the nature of molecular excitons in low-dimensional molecular solids is of paramount importance in fundamental photophysics and various applications such as energy harvesting, switching electronics and display devices. Despite this, the spatial evolution of molecular excitons and their transition dipoles have not been captured in the precision of molecular length scales. Here we show in-plane and out-of-plane excitonic evolution in quasilayered two-dimensional (2D) perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) crystals assembly-grown on hexagonal boron nitride (hBN) crystals. Complete lattice constants with orientations of two herringbone-configured basis molecules are determined with polarization-resolved spectroscopy and electron diffraction methods. In the truly 2D limit of single layers, two Frenkel emissions Davydov-split by Kasha-type intralayer coupling exhibit energy inversion with decreasing temperature, which enhances excitonic coherence. As the thickness increases, the transition dipole moments of newly emerging charge transfer excitons are reoriented because of mixing with the Frenkel states. The current spatial anatomy of 2D molecular excitons will inspire a deeper understanding and groundbreaking applications of low-dimensional molecular systems.

Excitonic spectra of monoclinic anthracene-like 2D model

The paper contains symmetry analysis and classification of the excitons (Frenkel excitons - FEs - and Charge Transfer Excitons - CTEs) in anthracene-like two-dimensional model taking into account the symmetry of the monoclinic axis, the inversion center and the glide plane. The symmetry permitted combinations of CTEs and their coupling with dipole active FEs are studied. These circumstances govern the excitonic spectra and their manifestation in linear absorption. Green functions method is applied to find linear optical susceptibilities and excitonic spectra with polarization along the monoclinic axis or perpendicular to it.

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.

Interface engineering of charge-transfer excitons in 2D lateral heterostructures

The existence of bound charge transfer (CT) excitons at the interface of monolayer lateral heterojunctions has been debated in literature, but contrary to the case of interlayer excitons in vertical heterostructure their observation still has to be confirmed. Here, we present a microscopic study investigating signatures of bound CT excitons in photoluminescence spectra at the interface of hBN-encapsulated lateral MoSe2-WSe2 heterostructures. Based on a fully microscopic and material-specific theory, we reveal the many-particle processes behind the formation of CT excitons and how they can be tuned via interface- and dielectric engineering. For junction widths smaller than the Coulomb-induced Bohr radius we predict the appearance of a low-energy CT exciton. The theoretical prediction is compared with experimental low-temperature photoluminescence measurements showing emission in the bound CT excitons energy range. We show that for hBN-encapsulated heterostructures, CT excitons exhibit small binding energies of just a few tens meV and at the same time large dipole moments, making them promising materials for optoelectronic applications (benefiting from an efficient exciton dissociation and fast dipole-driven exciton propagation). Our joint theory-experiment study presents a significant step towards a microscopic understanding of optical properties of technologically promising 2D lateral heterostructures.

The Dynamics of Delocalized Excitations in Organic Solar Cells with Nonfullerene Acceptors

Recently, the performance of organic solar cells has been markedly improved benefiting from the development of nonfullerene acceptors (NFAs) with acceptor-donor-acceptor structures. Arising from the intermolecular electronic interactions between the electron donating and accepting units, intramoiety and interfacial delocalized excitations make a substantial contribution to the photocurrent generation. In this Perspective, we discuss recent studies on the excited-state dynamics responsible for the working mechanism in NFA-based organic solar cells and emphasize the dynamics of delocalized excitations in charge generation and recombination processes. The intramoiety delocalized excitations in NFAs enable charge separation without forming interfacial charge-transfer excitons first, allowing efficient photocharge generation in planar heterojunctions with reduced interfacial energy loss. We suggest a few research directions in elucidating the performance-limited processes toward the further optimization of NFA-based devices.