Formation Of Charge-transfer Excitons Research Articles

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.

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.

Crucial Role of Defect States in the Ultralong Phosphorescence of Organic Molecular Crystals

Ultralong organic phosphorescence (UOP) in pure organic molecular crystals has attracted a lot of interest recently. There is much debate on the emission mechanism of this UOP. Two recent experimental works published in Nat. Photonics 2019, 13, 406-411 and Nat. Mater. 2021, 20, 175-180 attribute UOP in the 2,4,6-trimethoxy-1,3,5-triazine (TMOT) crystals and the carbazole crystals to H-aggregation of the TMOT molecules or the formation of charge-transfer excitons between the carbazole and impurity molecules. Our first-principles many-body Green's function theory calculations show that the lowest triplet states of these two crystals are in fact the localized defect states originating from the twisted TMOT molecules and the impurities, respectively. Energies of the H-aggregation-induced exciton and the charge-transfer exciton are too high to account for UOP. UOP should be mainly due to the little orbital overlap between the localized defect state and the delocalized band edges of the crystal. Strong intermolecular interactions suppress nonradiative decay of the triplet exciton localized on the defect.

Charge-transfer biexciton annihilation in a donor-acceptor co-crystal yields high-energy long-lived charge carriers.

Organic donor–acceptor (D–A) co-crystals have attracted much interest due to their important optical and electronic properties. Co-crystals having ⋯DADA⋯ π-stacked morphologies are especially interesting because photoexcitation produces a charge-transfer (CT) exciton, D˙+–A˙−, between adjacent D–A molecules. Although several studies have reported on the steady-state optical properties of this type of CT exciton, very few have measured the dynamics of its formation and decay in a single D–A co-crystal. We have co-crystallized a peri-xanthenoxanthene (PXX) donor with a N,N-bis(3-pentyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis(dicarboximide) (Ph4PDI) acceptor to give an orthorhombic PXX–Ph4PDI ⋯DADA⋯ π-stacked co-crystal with a CT transition dipole moment that is perpendicular to the transition moments for Sn ← S0 excitation of PXX and Ph4PDI. Using polarized, broadband, femtosecond pump–probe microscopy, we have determined that selective photoexcitation of Ph4PDI in the single co-crystal results in CT exciton formation within the 300 fs instrument response time. At early times (0.3 ≤ t ≤ 500 ps), the CT excitons decay with a t−1/2 dependence, which is attributed to CT biexciton annihilation within the one-dimensional ⋯DADA⋯ π-stacks producing high-energy, long-lived (>8 ns) electron–hole pairs in the crystal. These energetic charge carriers may prove useful in applications ranging from photovoltaics and opto-electronics to photocatalysis.

Coherent Charge Transfer Exciton Formation in Regioregular P3HT: A Quantum Dynamical Study.

The ultrafast formation of charge transfer excitons (CTXs) in regioregular poly(3-hexyl thiophene) (rrP3HT) domains is elucidated by electronic structure and quantum dynamical studies of an aggregate model system comprising five stacked quaterthiophene units. Using a multistate vibronic coupling Hamiltonian parametrized by TDDFT calculations for 13 electronic states of Frenkel and CTX type, along with 78 vibrational modes, quantum dynamical simulations are carried out using the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method. In line with time-resolved spectroscopic results [ De Sio , A. ; et al. Nat. Commun. 2016 , 7 , 13742 ], it is found that CTX formation occurs immediately upon photoexcitation, accompanied by sustained regular oscillations with a ∼22 fs periodicity. These coherent features, whose presence may seem surprising in a high-dimensional aggregate or thin film material, can be traced back to a dominant vibronic signature of CC stretch-type high-frequency modes. These vibrational signatures are found to be enhanced due to a collective vibronic response that is prompted by the initial generation of a delocalized bright exciton and its subsequent relaxation, by internal conversion, to a polaronic local exciton ground state.

Particle plasmon‐induced charge trapping at heterointerfaces in PCDTBT:PC70BM blends

ABSTRACTWe investigate the influence of particle plasmons on exciton and charge generation and recombination processes in the blend of poly (9‐(1‐octylnonyl)‐9H‐carbazole‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl) (PCDTBT) and [6,6]‐phenyl‐C70butyric acid methyl ester (PC70BM). The particle plasmons are generated from gold nanoparticles, which are embedded into PCDTBT:PC70BM blend. For the blend with gold nanoparticles, we observe enhance light harvesting. Despite the enhanced light collection, we find that the quasi‐steady‐state charge generation has not been influenced by the particle plasmons. However, the generation and recombination of long‐lived (sub‐millisecond) polaron paris have been significantly enhanced: from untrapped state in the pristine blend to the trapped state in the gold nanoparticle‐embedded blend. This result implies that the plasmon‐influenced polarons are trapped at the broadband geminate polaron pair (GPP) state. This state acts as an intermediate state, which either leads to the formation of charge transfer excitons (CTXs) or free charge carriers. In our case, the particle plasmon‐influenced polarons are trapped in the GPP state, which leads to the formation of CTXs. For this reason, we do not observe the enhanced charge generation in PCDTBT:PC70BM blend with particle plasmon resonance. Finally, we revealed that the long‐lived polarons mainly resulted from the localization by particle plasmons. The macroscopic modification in the blend film made negligible contributions to this influence. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 940–947

Charge Transfer Excitons at van der Waals Interfaces

The van der Waals interfaces of molecular donor/acceptor or graphene-like two-dimensional (2D) semiconductors are central to concepts and emerging technologies of light-electricity interconversion. Examples include, among others, solar cells, photodetectors, and light emitting diodes. A salient feature in both types of van der Waals interfaces is the poorly screened Coulomb potential that can give rise to bound electron-hole pairs across the interface, i.e., charge transfer (CT) or interlayer excitons. Here we address common features of CT excitons at both types of interfaces. We emphasize the competition between localization and delocalization in ensuring efficient charge separation. At the molecular donor/acceptor interface, electronic delocalization in real space can dictate charge carrier separation. In contrast, at the 2D semiconductor heterojunction, delocalization in momentum space due to strong exciton binding may assist in parallel momentum conservation in CT exciton formation.

A simple organic diode structure with strong rectifying characteristics

A simple organic diode structure has been made based on N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB)/Fullerene (C60) heterojunction. Ultraviolet photoelectron spectroscopy measurements show an energy level alignment at the heterojunction as such that no potential barrier near this heterojunction to hinder the formation of charge transfer excitons, which recombines at a very fast rate. This fast exciton recombination at this heterojunction makes NPB/C60 behave like an ideal Ohmic contact and thus leads to an extremely high 7.8×104 rectifying ratio from the electrode/organic contacts.

Stable Bending Performance of Flexible Organic Light-Emitting Diodes Using IZO Anodes

We report luminescent characteristics and mechanical stability of a flexible organic light-emitting diode (FOLED) using an amorphous ZnO-doped In2O3 (a-IZO) anode with a low sheet resistance of and a high optical transparency of 86%. The FOLED consisting of a-IZO/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4, 8-diyl)] (F8BT)/LiF/Al exhibits the efficient luminescent characteristics, which are nearly identical with the photoluminescence spectrum of the organic emitting material in our FOLED. This observation clearly indicates that the luminescent characteristics of the FOLED are solely ascribed to molecular exciton formation within the F8BT layer, since exciplex and charge transfer exciton formation are strictly suppressed by both tunneling/thermionic injection of holes at the PEDOT:PSS/F8BT interface and enhanced hole transport. Furthermore, the use of the considerably flexible a-IZO anode and PEDOT:PSS acting as a strain-relief buffering material enables good retention of the efficient luminescent characteristics of the FOLED even after continuous bending of up to 1000 times.

Formation of the Donor–Acceptor Charge-Transfer Exciton and Its Contribution to Charge Photogeneration and Recombination in Small-Molecule Bulk Heterojunctions

We report on the formation of charge-transfer (CT) excitons and their effect on photocurrent dynamics in composites with a fluorinated anthradithiophene (ADT-TES-F) donor (D) and acceptors (A) with (i) various LUMO energy offsets with respect to that of the donor (ΔLUMO) and (ii) several different side groups that modify spatial D/A separation at the D/A interface. Exciplexes and nonemissive CT excitons were formed in composites with ΔLUMO <0.6 and >0.6 eV, respectively. A competition between fast charge carrier photogeneration and CT exciton formation was observed, with outcomes depending on the applied electric field (E) and on the D/A separation. At low E fields, CT formation was dominant, and up to a factor of 2 increase in charge photogeneration efficiency due to CT exciton dissociation was observed in composites with large spatial D/A separation compared with that in pristine D films. At high E fields, fast charge carrier photogeneration was dominant in all composites, and no improvement in charge photogeneration efficiency with respect to that in pristine D films was observed. Dramatic changes in charge recombination dynamics were observed depending on the spatial D/A separation. These contributed to a factor of 5–10 improvement in continuous-wave photocurrents in composites with large spatial D/A separation as compared with those in pristine D films.

The Relationship between the Electric Field-Induced Dissociation of Charge Transfer Excitons and the Photocurrent in Small Molecular/Polymeric Solar Cells

A bulk heterojunction system consisting of a soluble poly(p-phenylenevinylene) donor and a small molecular electron acceptor based on Vinazene (2-vinyl-4,5-dicyanoimidazole) has been investigated with respect to the correlation between field-induced emission quenching and free charge carrier formation. Transient and steady-state emission properties measured at different voltages indicate that emissive charge transfer (CT) excitons formed at the heterojunction are prone to direct field-induced dissociation. However, the split-up of these CT excitons cannot fully account for the photocurrents observed on the very same devices. It is proposed that the formation of free carriers involves a nonemissive precursor and that the recombination of this precursor does not proceed via the formation of CT excitons.

Soliton and Exciton Effects in Femtosecond Pump Probe Spectroscopy of Conjugated Polymers

Abstract Using a density matrix approach, the nonlinear optical response of conjugated polymers is examined taking into account many-body effects of the excitation process. The formation of charge transfer excitons and the creation of lattice distortions are studied in short polymer chains by solving the self consistent equations of motion for the reduced single electron density matrix and the lattice displacement. The coupling of electronic motions to lattice dynamics gives rise to the formation of excitonic polarons. which can be identified in the femtosecond pump-probe signal.