Charge Transfer Excitons Research Articles

Ultrafast Charge Transfer and Relaxation at a Donor-Acceptor Interface.

The efficiency of charge separation in organic photovoltaic materials is crucially determined by the underlying dynamics of the charge transfer (CT) excitons and their dissociation into free electrons and holes. To unravel the main principles of the underlying mechanism on a molecular level, we construct a toy model of electronically coupled donors interacting with a manifold of CT exciton states. In particular, we set up a ladder of CT site energies to model the exciton dissociation. To mimic the complexity of the exciton dynamics at the donor-acceptor interface, the electronic CT manifold is designed to include two vibrational modes that are vibronically coupled to the excitons. We examine the impact of the electronic and vibrational coherences and the structure of the vibronic manifold on the transfer efficiency and charge recombination. Optimal configurations of the vibronic CT manifold are revealed. In particular, the rate of charge recombination can be minimized when the transient dynamics are carefully explored. Such a toy model can be used as a guide for the design of organic materials for efficient photovoltaic devices.

Layer-resolved many-electron interactions in delafossite PdCoO2 from standing-wave photoemission spectroscopy

When a three-dimensional material is constructed by stacking different two-dimensional layers into an ordered structure, new and unique physical properties can emerge. An example is the delafossite PdCoO2, which consists of alternating layers of metallic Pd and Mott-insulating CoO2 sheets. To understand the nature of the electronic coupling between the layers that gives rise to the unique properties of PdCoO2, we revealed its layer-resolved electronic structure combining standing-wave X-ray photoemission spectroscopy and ab initio many-body calculations. Experimentally, we have decomposed the measured VB spectrum into contributions from Pd and CoO2 layers. Computationally, we find that many-body interactions in Pd and CoO2 layers are highly different. Holes in the CoO2 layer interact strongly with charge-transfer excitons in the same layer, whereas holes in the Pd layer couple to plasmons in the Pd layer. Interestingly, we find that holes in states hybridized across both layers couple to both types of excitations (charge-transfer excitons or plasmons), with the intensity of photoemission satellites being proportional to the projection of the state onto a given layer. This establishes satellites as a sensitive probe for inter-layer hybridization. These findings pave the way towards a better understanding of complex many-electron interactions in layered quantum materials.

Construction of binary donor–acceptor conjugated copolymer in g-C3N4 for enhanced visible light-induced hydrogen evolution

Grafting π-conjugated donor units into graphitic carbon nitride (g-C3N4) represents a promising alterative to improve its light absorption range and charge transfer kinetics. To this end, we here report an efficient intramolecular g-C3N4-based conjugated copolymers via the one-spot thermal copolymerization for not only reduces the bandgap to 2.56 eV and thus expand the visible-light harvesting capability, but also accelerates the separation ability of photogenerated excitons. Consequently, the copolymer of 3 wt% p-phenylenediamine with melamine exhibits extraordinary hydrogen (H2) evolution rate of 1040 μmol g−1 h−1 under visible light. Interestingly, it still shows hydrogen production (5.29 μmol g−1 h−1) at λ = 600 nm (orange light). Systematic characterizations have proved that the optimal sample has high photocatalytic activity, due to rapid charge transfer kinetics and exciton separation. Our work sheds light on a molecular design to regulate charge-transfer dynamics among of polymeric semiconductors towards efficient solar-to-H2 production.

Spin-orbital coupling and slow phonon effects enabled persistent photoluminescence in organic crystal under isomer doping

When periodically packing the intramolecular donor-acceptor structures to form ferroelectric-like lattice identified by second harmonic generation, our CD49 molecular crystal shows long-wavelength persistent photoluminescence peaked at 542 nm with the lifetime of 0.43 s, in addition to the short-wavelength prompt photoluminescence peaked at 363 nm with the lifetime of 0.45 ns. Interestingly, the long-wavelength persistent photoluminescence demonstrates magnetic field effects, showing as crystalline intermolecular charge-transfer excitons with singlet spin characteristics formed within ferroelectric-like lattice based on internal minority/majority carrier-balancing mechanism activated by isomer doping effects towards increasing electron-hole pairing probability. Our photoinduced Raman spectroscopy reveals the unusual slow relaxation of photoexcited lattice vibrations, indicating slow phonon effects occurring in ferroelectric-like lattice. Here, we show that crystalline intermolecular charge-transfer excitons are interacted with ferroelectric-like lattice, leading to exciton-lattice coupling within periodically packed intramolecular donor-acceptor structures to evolve ultralong-lived crystalline light-emitting states through slow phonon effects in ferroelectric light-emitting organic crystal.

Strong Photophysical Diversity and the Role of Charge Transfer Excitons in Transition Metal Phthalocyanine β-Phases

The photophysics of the three transition metal phthalocyanines NiPc, CuPc and ZnPc in their β-phase has been studied using complementary experimental techniques: photoluminescence, optical absorption and electron-energy-loss spectroscopy. We demonstrate that the strongly different photoluminescence signal can be understood based on the different exciton dispersions, and that tiny differences in the structure and thus the intermolecular orbital overlap is able to model these differences. Our results clearly indicate a significant admixture of charge-transfer excitations to the lowest exciton state. They additionally demonstrate the need of a detailed microscopic understanding of organic semiconductors for their application potential.

Factors That Prevent Spin-Triplet Recombination in Non-fullerene Organic Photovoltaics

Managing the dynamics of spin-triplet electronic states is crucial for achieving high-performance organic photovoltaics. Here we show that the replacement of fullerene with non-fullerene acceptor (NFA) molecules leads to suppression of triplet recombination and thus more efficient charge generation. This indicates that the relaxation of charges to the local triplet exciton state, although energetically allowed, is outcompeted by the thermally activated separation of interfacial charge-transfer excitons (CTEs) in the NFA-based system. By rationalizing our results with Marcus theory, we propose that triplet recombination in the fullerene system is driven by the small energy difference and strong electronic couplings between the CTE state and the lowest-lying triplet exciton state (T1) of fullerene acceptor molecules. In contrast, the large energy difference and small electronic couplings between these states in the NFA-based blends lead to sufficiently slow triplet relaxation rate compared to the charge separation rate (≪1010 s-1), thus preventing triplet recombination.

New [2,2]Fluorenophanes Give Insights into Asymmetric Charge Transfer-Mediated Exciton Delocalization along the π-π Packing Direction.

A series of new [2,2]fluorenophanes has been synthesized and characterized; among them, molecules of crystallographically asymmetric anti-[2.2](1,4)(4,1)fluorenophane (K2C-2) aggregate to form one-dimensional supramolecular chain structures through effective intermolecular π-π overlapping. This, in combination with the synergistic intramolecular π-π interaction, leads to prominent dual emission mediated by charge transfer (CT) exciton delocalization. Support of this new insight is given by mapping the transition density along the π-π packing direction where the intramolecular excitation and intermolecular CT coexist in K2C-2.

Exciton Heating Versus Cooling: Charge Separation Driven by Entropy and Delocalization at Interfaces with Planar Molecules

The recent development of nonfullerene acceptors (NFAs) has resulted in an abrupt increase in the organic photovoltaic (OPV) efficiency. It remains unclear why charge separation (CS) can occur at a high yield in NFA OPVs despite the small energy-level offset at the donor-acceptor (D-A) interface. $\mathrm{In}$ order to resolve this issue, zinc phtholacyanine (ZnPc) and its fluorinated derivative (${\mathrm{F}}_{8}\mathrm{Zn}\mathrm{Pc}$) are used to build a model D-A interface for understanding the mechanism underlying the efficient CS in NFA OPVs. At this interface, we find that bound charge-transfer (CT) excitons can undergo a heating process in which the enthalpy increases at a surprisingly fast rate. Specifically, the CT exciton gains 0.3 eV from the environment in merely 10 ps. This fast exciton heating, which eventually leads to CS, resembles the CS behavior observed in NFA bulk heterojunctions (BHJs). Interestingly, when the pair of D-A molecules switches from a face-on to an edge-on orientation, the CT-exciton heating process is reversed to the typical hot CT-exciton cooling process in which the enthalpy decreases with time. By using sub-band-gap excitation, we further find that the exciton heating can occur directly from cold CT excitons without the need of excess energy provided by the energy-level offset. The competition between the exciton heating and cooling is explained by a model that incorporates the effect of entropy, electron delocalization, and structural anisotropy. It is found that for the face-on orientation, the small contact area between the delocalized electron and hole within the CT exciton significantly reduces the number of cold CT states, which increases the entropic driving force and drives the reaction into the heating direction. This finding provides insight on why the entropy-and-delocalization-driven CS would preferentially occur in NFA BHJs. The relative orientation of the NFA molecular stack and the polymer chain, together with bulky side groups, make the delocalized electron and hole in contact with each other via isolated pointlike junctions, which is the key for activating the exciton heating and subsequent CS.

All-optical control of charge transfer and interlayer excitons in transition metal dichalcogenide heterostructures

An all-optical approach to control interlayer charge transfer and interlayer exciton dynamics is demonstrated using transition metal dichalcogenide heterostructures of ${\mathrm{MoSe}}_{2}/{\mathrm{MoS}}_{2}$ and ${\mathrm{MoSe}}_{2}/{\mathrm{WS}}_{2}$ as examples. In the three-pulse pump-probe experiments, a control pulse injects photocarriers and produces an electric field due to the interlayer charge separation. A pump pulse excites new carriers and their dynamics under the influence of the control-injected carriers and field is time-resolved by measuring differential reflectance of a third probe pulse. In ${\mathrm{MoSe}}_{2}/{\mathrm{MoS}}_{2}$, the interlayer electron transfer time from ${\mathrm{MoSe}}_{2}$ to ${\mathrm{MoS}}_{2}$ decreases with increasing the control-injected carrier density and electric field. The dipole moment of the interlayer excitons is reduced by the control pulse, which can be utilized for ultrafast and all-optical control of interlayer excitons. The recombination of the interlayer excitons is enhanced by the control pulse, which increases the spatial overlap of the electron and hole wave functions. The effect of the control pulse on the interlayer excitons is confirmed in the ${\mathrm{MoSe}}_{2}/{\mathrm{WS}}_{2}$ heterostructure, although its effect on the electron transfer time is not resolvable. These results reveal useful information to understand and control interlayer charge transfer dynamics and provide tools for ultrafast manipulation of electrons in van der Waals heterostructures.

Pressure-dependent interfacial charge transfer excitons in WSe2-MoSe2 heterostructures in near infrared region

In this letter, we report our theoretical study on pressure-dependent interfacial charge transfer excitons in WSe2-MoSe2 van der Waals heterostructures in near infrared region (NIR). It is found that pressure can control not only the degree of interfacial charge transfer, but also the orientation of interfacial charge transfer in 2D heterostructures. Pressure can efficiently promote the separation of interfacial charge transfer between two layers of the heterostructures. The variation of pressure results in the changing of band gap, the effective mass, as well as the intrinsic carrier concentration in WSe2-MoSe2 van der Waals heterostructures. The pressure-induced red shifted for interfacial charge transfer excitons is also obtained. Furthermore, the reason why pressure results in fluorescence quenching is strong van der Waals interactions. This discovery will provide a new method for reversible non-destructive control of the electrical properties of two-dimensional semiconductors.

Using Si/MoS2 Core-Shell Nanopillar Arrays Enhances SERS Signal.

Two-dimensional layered material Molybdenum disulfide (MoS2) exhibits a flat surface without dangling bonds and is expected to be a suitable surface-enhanced Raman scattering (SERS) substrate for the detection of organic molecules. However, further fabrication of nanostructures for enhancement of SERS is necessary because of the low detection efficiency of MoS2. In this paper, period-distribution Si/MoS2 core/shell nanopillar (NP) arrays were fabricated for SERS. The MoS2 thin films were formed on the surface of Si NPs by sulfurizing the MoO3 thin films coated on the Si NP arrays. Scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were performed to characterize Si/MoS2 core-shell nanostructure. In comparison with a bare Si substrate and MoS2 thin film, the use of Si/MoS2 core-shell NP arrays as SERS substrates enhances the intensity of each SERS signal peak for Rhodamine 6G (R6G) molecules, and especially exhibits about 75-fold and 7-fold enhancements in the 1361 cm−1 peak signal, respectively. We suggest that the Si/MoS2 core-shell NP arrays with larger area could absorb more R6G molecules and provide larger interfaces between MoS2 and R6G molecules, leading to higher opportunity of charge transfer process and exciton transitions. Therefore, the Si/MoS2 core/shell NP arrays could effectively enhance SERS signal and serve as excellent SERS substrates in biomedical detection.

How the Size and Density of Charge-Transfer Excitons Depend on Heterojunction’s Architecture

We have characterized the size, intensity, density, and distribution of charge-transfer (CT) excitons as a function of the acceptor–donor architecture of prototypical organic interfaces. This characterization was done by computational analysis of 17 models of varying numbers, positions, and orientations of the donor and acceptor molecules. The models’ building blocks were phenyl-C61-butyric acid methyl ester (PCBM) fullerene acceptors and dual-band donor polymers composed of thiophene, benzothiadiazole, and benzotriazole subunits. The electronic structure of the donor–acceptor complexes was computed with the time-dependent long-range-corrected density-functional tight-binding method and analyzed with the fragment-based one-electron transition density matrix. In all models, the complexes with edge-on orientation have denser spectra of low-energy CT states lying below the absorption bands compared to the complexes with face-on orientation. This CT-state distribution in edge-on complexes provides a gate to efficiently populate cold CT excitons. Moreover, the cold CT excitons have a higher degree of charge separation in the edge-on than in the face-on complexes. The CT amount and the CT exciton size generally increase with the energy of the CT states, although the electron remains localized on a single molecule in cold CT states. Delocalization over two PCBM molecules was observed for high-energy CT states. The exciton size also depends on the orientation. Larger excitons are produced by the delocalization of the electrons perpendicularly to the interface. When the delocalization is parallel, the smaller electron–hole distances yield moderately sized CT excitons.

Quantum Dynamics of Exciton Transport and Dissociation in Multichromophoric Systems.

Due to the subtle interplay of site-to-site electronic couplings, exciton delocalization, nonadiabatic effects, and vibronic couplings, quantum dynamical studies are needed to elucidate the details of ultrafast photoinduced energy and charge transfer events in organic multichromophoric systems. In this vein, we review an approach that combines first-principles parameterized lattice Hamiltonians with accurate quantum dynamical simulations using advanced multiconfigurational methods. Focusing on the elementary transfer steps in organic functional materials, we address coherent exciton migration and creation of charge transfer excitons in homopolymers, notably representative of the poly(3-hexylthiophene) material, as well as exciton dissociation at polymer:fullerene heterojunctions. We emphasize the role of coherent transfer, trapping effects due to high-frequency phonon modes, and thermal activation due to low-frequency soft modes that drive a diffusive dynamics.

Chemical Bonding as a New Avenue for Controlling Excited-State Properties and Excitation Energy-Transfer Processes in Zinc Phthalocyanine-Fullerene Dyads.

Whether chemical bonding can regulate the excited-state and optoelectronic properties of donor-acceptor dyads has been largely elusive. In this work, we used electronic structure and nonadiabatic dynamics methods to explore the excited-state properties of covalently bonded zinc phthalocyanine (ZnPc)-fullerene (C60 ) dyads with a 6-6 (or 5-6) bonding configuration in which ZnPc is bonded to two carbon atoms shared by the two hexagonal rings (or a pentagonal and a hexagonal ring) in C60 . In both cases, the locally excited (LE) states on ZnPc are spectroscopically bright. However, their different chemical bonding differentiates the electronic interactions between ZnPc and C60 . In the 5-6 bonding configuration, the LE states on ZnPc are much higher in energy than the LE states on C60 . Thus, the excitation energy transfer from ZnPc to C60 is thermodynamically favorable. On the other hand, in the 6-6 bonding configuration, such a process is inhibited because the LE states on ZnPc are the lowest ones. More detailed mechanisms are elucidated from nonadiabatic dynamics simulations. In the 6-6 bonding configuration, no excitation energy transfer was observed. In contrast, in the 5-6 bonding configuration, several LE and charge-transfer (CT) excitons were shown to participate in the energy-transfer process. Further analysis reveals that the photoinduced energy transfer is mediated by a CT exciton, such that electron- and hole-transfer processes take place in a concerted but asynchronous manner in the excitation energy transfer. It is also found that high-level electronic structure methods including exciton effects are indispensable to accurately describe photoinduced energy- and electron-transfer processes. Furthermore, this work opens up new avenues for regulating the excited-state properties of molecular donor-acceptor dyads by means of chemical bonding.

Excitation Wavelength‐Dependent Charge Generation Dynamics in a Nonfullerene Organic Solar Cell Interface

Unraveling the charge generation dynamics at the donor–acceptor (D−A) interfaces is crucial for improving the photovoltaic performances of nonfullerene acceptor‐based organic solar cells (OSCs). Herein, time‐dependent density functional theory (TDDFT) based nonadiabatic dynamics simulations are used to explore the ultrafast photoinduced dynamics at a nonfullerene D–A PTB7@PDI interface. Based on the results, it is found that such an interface exhibits distinct charge generation processes upon excitation with different wavelengths. The excitation at ≈591 nm mainly results in the local exciton |PTB7∗>, whereas the charge transfer exciton |PTB7+PDI−> also has minor contribution. Later on, the electron transfer from PTB7 to PDI, i.e., channel I charge generation process, occurs in 1 ps. The situations are much more complex when the excitation is conducted using ≈487 nm light. The initial populated excitons include local excitons |PDI∗>, |PTB7∗>, and charge transfer exciton |PTB7+PDI−>, after which both channel I and channel II charge generation take place ultrafast. However, in both situations, the charge generation processes occur within a few picoseconds, which is consistent with previous experimental work. Such ultrafast charge generation processes in a wide range of solar spectra are one of the reasons responsible for the excellent photovoltaic properties of such OSCs.

Delocalized exciton formation in C60 linear molecular aggregates.

Organic semiconducting materials containing C60 molecules are efficient acceptors for planar perovskite solar cells. In this work, we theoretically investigate the optical and excitonic properties of C60 linear molecular aggregates (composed of 1 to 7 C60 molecules) via the real-time-propagation rt-TDDFT technique. In the case of a single C60 molecule, the photoabsorption peaks are dominated by localized molecular excitons. We furthermore demonstrate that, in the case of linear molecular aggregates, the photoabsorption peaks are contributed by localized molecular excitons, charge transfer excitons, and Wannier-like delocalized excitons. This result is different to the accepted theory that only localized molecular excitons or charge transfer excitons can be produced in organic semiconducting materials. This work provides additional insights into the exciton formation in C60 molecular aggregates and may help in the rational design of efficient solar cells.

Ultrafast photo-driven charge transfer exciton dynamics in mixed-stack pyrene-perylenediimide single co-crystals

Femtosecond transient absorption microscopy of organic donor–acceptor single co-crystals shows that photo-initiated charge transfer exciton diffusion as well as charge recombination rates depend critically on crystal morphology.

Optoelectronic properties of one-dimensional molecular chains simulated by a tight-binding model

Studying optical properties of organic materials is important due to the rapid development of organic light-emitting diodes, solar cells, and photon detectors. Here, for the first time, we have performed tight-binding calculations for singlet excitons, in combination with first-principles calculations of the excited states in molecular dimers, to describe the optical properties of a zinc-phthalocyanine one-dimensional molecular chain. We have included the intra-molecule and charge-transfer excitations and the coupling between them. Our calculations have successfully interpreted a body of experimental UV–visible optical spectra of transition-metal phthalocyanines. Compared with the previous ab initio calculations for a molecular dimer, the optical absorptions at the split peaks of the Q-bands can be comparable, which indicates the importance of the coupling between the intra-molecular and charge-transfer excitons.

Charge transfer excitations, pair density waves, and superconductivity in moiré materials

Transition metal dichalcogenide (TMD) bilayers are a new class of tunable moir\'e systems attracting interest as quantum simulators of strongly-interacting electrons in two dimensions. In particular, recent theory predicts that the correlated insulator observed in WSe$_2$/WS$_2$ at half filling is a charge-transfer insulator similar to cuprates and, upon further hole doping, exhibits a transfer of charge from anion-like to cation-like orbitals at different locations in the moir\'e unit cell. In this work, we demonstrate that in this doped charge-transfer insulator, tightly-bound charge-2e excitations can form to lower the total electrostatic repulsion. This composite excitation, which we dub a trimer, consists of a pair of holes bound to a charge-transfer exciton. When the bandwidth of doped holes is small, trimers crystallize into insulating pair density waves at a sequence of commensurate doping levels. When the bandwidth becomes comparable to the pair binding energy, itinerant holes and charge-2e trimers interact resonantly, leading to unconventional superconductivity similar to superfluidity in an ultracold Fermi gas near Feshbach resonance. Our theory is broadly applicable to strongly-interacting charge-transfer insulators, such as WSe$_2$/WS$_2$ or TMD homobilayers under an applied electric field.

Excited-State Symmetry-Breaking Charge Separation Dynamics in Multibranched Perylene Diimide Molecules.

As one of the most promising nonfullerene acceptors for organic photovoltaics, perylene diimide (PDI)-based multibranched molecules with twisted or three-dimensional (3D) geometric structures have been developed, which effectively increase the power conversion efficiency (PCE) of organic solar cells. Understanding the structure-property relationships in multichromophoric molecular architectures at molecular and ultrafast time levels is a crucial step in establishing new design principles in organic electronic materials. For this, photodriven excited-state symmetry-breaking charge separation (SB-CS) of PDI-based multichromophoric acceptors has been proposed to improve the PCE by reducing the self-aggregation of the planar PDI monomer. Herein, we investigated the intramolecular excited-state SB-CS and charge recombination (CR) dynamics of two symmetric phenyl-methane-based PDI derivatives, a twist dimer PM-PDI2 (phenyl-methane-based PDI dimer) and a 3D configuration tetramer PM-PDI4 (phenyl-methane-based PDI tetramer), in different solvents using ultrafast femtosecond transient absorption (fs-TA) spectroscopy and quantum chemical calculations. The quantum chemical calculations and steady-state spectra show that the two PDI derivatives undergo conformational changes upon excitation, leading to their emission states that have the characteristics of partial charge-transfer (CT) exciton in all solvents. Based on the evolution of the fs-TA data, it is observed that the evolution from the CT state to SB-CS state is disfavored in a weak polar solvent, whereas clear SB-CS spectroscopic signatures of cationic and anionic PDI are observed in polar solvents. Faster CS and slower CR processes of PM-PDI4 are observed in comparison to those of PM-PDI2. The crowded space in the 3D structure shortens the distance between the branches, leading to a stronger electronic coupling at the lowest excited state and a larger negative Gibbs free energy change of PM-PDI4 relative to that of PM-PDI2, which benefits the charge separation among PDI units in PM-PDI4. Besides, the 3D structure of PM-PDI4 also restricts rotation to a surface crossing region between the excited state and ground state, thus inhibiting nonradiative CR process and increasing the CS state lifetime. Our results suggest that the kinetics of CS and CR processes are strongly related to the molecular geometric structure, and the excited-state symmetry breaking in the 3D structure acceptor has superior photogenerated charge and photovoltaic properties from the perspective of ultrafast dynamics.