Excitonic Polaron Research Articles

Water tetrahedron intercalated with hydronium ion (H3O+) as molecular reactor producing deuterium under X-ray absorption

Features of X-ray absorption and emission spectra of water and D2O are interpreted in terms of polaronic exciton theory under interaction with the incident photon. The interaction generating electric field Eel=1.21678×1010V/m leads to energy levels splitting of hole and electronic polarons with the energy gap ΔEsp=2.764566eV that is constrained by water polarizability. Kinetic energy accumulation by the electron coupled with hole polaron occurs under Compton interaction with the incident photon. It is found that Compton wavelength (λC/2π) plays some important role in the processes of the energy accumulation, fifteen-step of which (15λC/2π) is required to obtain polaronic exciton radius providing kinetic energy 534.556eV. The latter admits to evaluate polaronic exciton levels taking into account inelastic effects for water tetrahedron intercalated with H3O+. The obtained energetic diagram provides the emission spectrum interpretation in terms of proton-proton fusion process with pep-neutrino and deuterium producing. The mass of pep-neutrino evaluated as a mass defect of the precessing polaronic exciton under Compton interaction is found to be 3.43026×10−37kg.

Polaron self-localization in white-light emitting hybrid perovskites

Ultra-broadband photoluminescence in two-dimensional (2D) perovskites (EDBE)PbX4 (where X = Cl, Br) is shown to originate from self-trapped polaronic excitons localised at specific inorganic lattice sites.

Polaronic exciton and its energy levels in water structure

Abstract Polaronic exciton behavior is considered in water structure under elastic and inelastic interaction of water with the incident light. The consideration allows deriving water polarizability from kinetic energy estimation using oscillators corresponding to structural molecular parameters and features of the interaction with photon. Kinetic energy estimations of polaronic exciton successfully explain few excitation energies of gaseous and liquid water including 7.4499 and 21.38 eV that are found in a good agreement with Eex = 7.447 eV (λ = 166.6 nm) and near 56 nm (22 eV), which was observed under inelastic X-ray scattering. The energy estimations imply the view of several energetic levels of polaronic exciton depending on the local interactions in water. Water polarizability (α) estimated as the difference in behavior under elastic and inelastic interaction with the incident photon, is found to be 1.4693 A3 that is slightly different from the adopted α = 1.470 A3.

The Root Causes of the Limited Stability of Solution‐Coated Small‐Molecule Organic Light‐Emitting Devices: Faster Host Aggregation by Exciton–Polaron Interactions

The degradation mechanism is compared in organic light‐emitting devices (OLEDs) fabricated by solution‐coating to that in vacuum‐deposited OLEDs. Devices comprising various host materials made by vacuum‐deposition or solution‐coating are investigated. Changes in devices electroluminescence (EL) spectra during prolonged electrical driving are compared and analyzed. Hole‐only devices are also utilized, and employed to study the effects of charges and excitons, separately and combined. The results reveal that the faster degradation of solution‐processed devices relative to their vacuum‐deposited counterparts under electrical stress is due to a faster aggregation of the host materials. Interactions between excitons and polarons in the emitting layers of the devices induce this aggregation phenomenon. Although this phenomenon affects both vacuum‐deposited and solution‐coated emitting layers, it is found to occur much faster in the later. The findings shed light on the root causes of the limited stability of solution‐processed OLEDs.

Direct Observation of Electron-Phonon Coupling and Slow Vibrational Relaxation in Organic-Inorganic Hybrid Perovskites.

Quantum and dielectric confinement effects in Ruddlesden-Popper 2D hybrid perovskites create excitons with a binding energy exceeding 150 meV. We exploit the large exciton binding energy to study exciton and carrier dynamics as well as electron-phonon coupling (EPC) in hybrid perovskites using absorption and photoluminescence (PL) spectroscopies. At temperatures <75 K, we resolve splitting of the excitonic absorption and PL into multiple regularly spaced resonances every 40-46 meV, consistent with EPC to phonons located on the organic cation. We also resolve resonances with a 14 meV spacing, in accord with coupling to phonons with mixed organic and inorganic character. These assignments are supported by density-functional theory calculations. Hot exciton PL and time-resolved PL measurements show that vibrational relaxation occurs on a picosecond time scale competitive with that for PL. At temperatures >75 K, excitonic absorption and PL exhibit homogeneous broadening. While absorption remains homogeneous, PL becomes inhomogeneous at temperatures <75K, which we speculate is caused by the formation and subsequent dynamics of a polaronic exciton.

Hot Hole Hopping in a Polyoxotitanate Cluster Terminated with Catechol Electron Donors

Fast hole hopping in Ti17cat4, a 1 nm diameter molecular polyoxotitanate cluster bearing four catechol ligands (Ti17(μ4-O)4(μ3-O)16(μ2-O)4(cat)4(OPri)16), was investigated by ultrafast spectroscopy and quantum dynamics simulations. The catechol moieties coupled to the TiO2 core of the cluster give rise to a charge-transfer band, the excitation of which promotes an electron from the highest occupied molecular orbital of the ligand to the inorganic core, resulting in the formation of {cat+•,Ti3+}, a vibrationally hot polaronic exciton. Dynamic depolarization measurements indicate that within less than 100 fs the Franck–Condon polaronic state formed at the interface evolves into a fully charge-separated state and the injected electron delocalizes over the quasi conduction band of the cluster. The positive charge (hole) resulting from the injection does not remain static either. The initial hole hopping between the catechol sites occurs with the rate of ∼5 × 1011 s–1 or more and competes with the intramolecul...

Polaronic exciton in self-organized assemblies of protonated meso-tetraphenylporphine dimers and water at room temperature

Assemblies consisting of protonated meso-tetraphenylporphine (TPP) dimers and water have been investigated by UV–vis and infrared (IR) spectroscopy and by atomic force microscopy (AFM) in thin layers. Features of electronic absorption spectra of the assemblies are interpreted in terms of hole polaron combined with exciton theory using quantum well with parameters obtained from the dimer structure. It appears to be hole polaron moving defines kinetic energy of polaronic exciton confined in a quantum well when the electron absorbs photon. Hole polaron characteristics such as polaron self-energy, energy of Frank–Condon transitions, and radius of hole polaron moving through water are found to be 1.38 eV, 0.2445 eV, and 0.246 Å, respectively. A doublet at 1944, 1960 cm−1 (0.2412, 0.2432 eV) observed in IR spectra matches the energy of Frank–Condon transitions. Excitation energies estimated using molecular parameters for polaronic excitons in pure water and in the TPP dimers are found in a good agreement with the experimental data.

Influence of the Guest on Aggregation of the Host by Exciton-Polaron Interactions and Its Effects on the Stability of Phosphorescent Organic Light-Emitting Devices.

The root causes of the differences in electroluminescence stability among phosphorescent organic light-emitting devices (PHOLEDs) utilizing different emitter guests are studied. The results show that the host material plays a more influential role in limiting device stability in comparison to the guest. During the operation of a PHOLED, the host undergoes aggregation as a result of interactions between the excitons and positive polarons. The rate of this aggregation is found to be the limiting factor for device lifetime and is influenced by the choice of the guest material and its concentration. Finally, it is shown that phase segregation between the host and the guest is an important aspect of the aggregation process. As a result of this segregation, energy transfer from the host to the guest becomes increasingly less efficient, resulting in the observed gradual loss in electroluminescence efficiency in the devices over time. The findings explain why PHOLEDs utilizing different guest materials but otherwise identical material systems can have significantly different lifetimes and provide an answer to a long-lasting question in the field.

Magneto-conductance characteristics of trapped triplet-polaron and triplet-trapped polaron interactions in anthracene-based organic light emitting diodes.

The effects of a magnetic field on the dissociation of triplet excitons by free charges (TCI) are well understood. However, the magneto-conductance (MC) characteristics of trapped triplet-polaron interactions (TtPI) and triplet-trapped polaron interactions (TPtI) within organic light emitting diodes (OLEDs) are not well understood. We have studied these interactions in an anthracene-based OLED. The electroluminescence spectra, current-voltage characteristics and magneto-electroluminescence indicated that the anthracene layer contained many defects that could trap either triplet excitons or polarons, which led to TPtI and TtPI. The MC curves at low temperature exhibited a complex line shape, which indicated that intersystem crossing, TPtI, TtPI, and TCI occurred simultaneously in the device. The individual MC characteristics of TPtI and TtPI were extracted from temperature dependant MC curves by fitting them to three empirical Lorentzian functions and one non-Lorentzian function. The MC of TPtI exhibited a negative sign, while that of TtPI exhibited a positive one, with characteristic magnetic fields (B0) of ∼10.5 and ∼15 mT, respectively. Both processes were prominent below 150 K and weakened with increasing temperature. TPtI was neglected above 200 K, while TtPI was observed even at ambient temperature. These results add significant insight into the magnetic field effects on triplet-polaron interactions.

Polaronic exciton binding energy in iodide and bromide organic-inorganic lead halide perovskites

The last 4 years have seen the rapid emergence of a new solar cell technology based on organic-inorganic lead halide perovskites, primarily CH3NH3PbI3 and related halides involving Cl and Br. Debate continues on the role of excitons and free carriers in these materials. Recent studies report values of exciton binding energy for the iodide ranging from 0.7 meV to 200 meV, with vastly different implications for device operation and design. In the present work, previously neglected polarons are shown likely to have a major impact in determining excitonic properties. Polaronic exciton binding energies calculated using effective longitudinal optical phonon energies, deduced from permittivity measurements, are shown consistent with experimental energies for good quality samples of CH3NH3PbI3 and CH3NH3PbBr3, as determined over a large temperature range from optical absorption data. Bandgaps determined simultaneously show a discontinuity at the orthorhombic to tetragonal phase transition for the iodide, but not for the bromide.

The different influence of singlet and triplet excitons in the degradation of phosphorescent organic light-emitting devices due to exciton–polaron-induced aggregation of host materials

The lifetime of a phosphorescent organic light-emitting device (PhOLED) may range over several orders of magnitude depending on the choice of the phosphorescent emitter guest material and its concentration. This behavior, we find, results primarily from different rates of exicton–polaron-induced aggregation (EPIA) of the host material due to exciton–polaron interactions. We investigate the influence of singlet versus triplet excitons in the EPIA of host materials and the degradation of PhOLEDs. The results show that there is a correlation between device electroluminescence degradation and the concentration of singlet excitons on the host during electrical driving. In contrast, we find no particular correlation between device degradation rate and the concentration of triplet excitons. The results therefore reveal that singlet excitons play the leading role causing the EPIA of host materials and device degradation by this mechanism.

Modeling of ultrafast time-resolved fluorescence applied to a weakly coupled chromophore pair.

We present theory for calculating the third-order non-linear response function of a molecular aggregate in the weak inter-chromophore coupling regime. This approach is based on the perturbative expansion of the system evolution with respect to the resonance coupling, while the system-bath interaction is treated non-perturbatively by means of cumulant expansion. An explicit expression for the time-resolved fluorescence signal is then obtained. This allows us to investigate the ultrafast time-dependent Stokes shift, signatures of coherent dynamics, and the excitonic polaron formation in the excited state of the aggregate. Numerical simulations of the time-resolved fluorescence spectra of a pair of coupled molecules demonstrate these effects.

The accumulation of diffusive charges in organic light-emitting diodes

We investigated the accumulation of diffusive charges within the emitting layer of organic light-emitting diodes (OLEDs). The presence of accumulated charges can be probed by both transient electroluminescence (transient EL) and impedance measurements (IM). As an example we studied OLED devices using a red phosphorescent emitter iridium(III)bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate) [Ir(MDQ)2(acac)] and green emitter Ir(ppy)3 co-doped into a mixed host matrix. The results deduced from both transient EL and IM measurements are in good consistency, enabling us to compare the accumulation of diffusive charges in different devices quantitatively. Experimental results indicate that the lifetime of OLED devices is reduced by the diffusive charges. We ascribe this to the exciton–polaron annihilation where the diffusive charges play an important role. The quantification of the accumulation of diffusive charges in the emitting layer may help to improve the lifetime of OLEDs.

Enhanced stability in inverted simplified phosphorescent organic light-emitting devices and its origins

We study the stability of simplified phosphorescent organic light-emitting devices in standard and inverted architectures. Results show that the inverted devices have higher electroluminescence stability, exhibiting almost three times longer lifetime relative to the widely used standard device architecture, while having the same current efficiency. Investigations reveal that inverted devices have a higher electron/hole ratio in the hole transport layer and a lower concentration of un-recombined positive charges in the emission layer. The results suggest that their higher stability is due to reduced degradation at the emission layer/electron transport layer interface as a result of reduced exciton–polaron interactions.

Excitonic polarons in low-dimensional transition metal dichalcogenides

We examine the excitonic polaron properties of common monolayer transition metal dichalcogenides (MoS2, MoSe2, WS2 and WSe2). The excitonic polaron is formed when excitons interact with acoustic or optical phonons via coupling to the deformation potentials associated with the conduction and valence bands. A unitary transformation which performs an approximate diagonalization of the exciton–phonon operator is used to evaluate the ground state energy of the excitonic polaron. We derive analytical expressions of the changes in the excitonic polaron energy and mass at small exciton wavevectors involving the deformation potential due to optical phonons. The polaronic effect of the monolayer transition metal dichalcogenides is examined by comparing changes in the energy gap shift and effective masses based on known deformation potential constants for carrier–phonon interactions. Our results indicate the occurrence of comparable energy shifts when the ground state exciton interacts with optical or acoustic phonons. We extend our calculations to explore the influence of exciton–lattice interactions on the binding energies and the self-trapping of excitons in two-dimensional layers of transition metal dichalcogenides.

Influence of interfacial area on exciton separation and polaron recombination in nanostructured bilayer all-polymer solar cells.

The macroscopic device performance of organic solar cells is governed by interface physics on a nanometer scale. A comb-like bilayer all-polymer morphology featuring a controlled enhancement in donor-acceptor interfacial area is employed as a model system to investigate the fundamental processes of exciton separation and polaron recombination in these devices. The different nanostructures are characterized locally by SEM/AFM, and the buried interdigitating interface of the final device architecture is statistically verified on a large area via advanced grazing incidence X-ray scattering techniques. The results show equally enhanced harvesting of photoexcitons in both donor and acceptor materials directly correlated to the total enhancement of interfacial area. Apart from this beneficial effect, the enhanced interface leads to significantly increased polaron recombination losses both around the open-circuit voltage and maximum power point, which is determined in complement with diode dark current characteristics, impedance spectroscopy, and transient photovoltage measurements. From these findings, it is inferred that a spatially optimized comb-like donor-acceptor nanonetwork alone is not the ideal morphology even though often postulated. Instead, the energetic landscape has to be considered. A perfect morphology for an excitonic solar cell must be spatially and energetically optimized with respect to the donor-acceptor interface.

Dynamics of exciton-polaron transition in molecular assemblies: the variational approach.

Dynamics of excitonic polaron formation in molecular systems coupled to an overdamped bath are investigated using the Dirac-Frenkel variational principle and Davydov D1 Ansatz. Using a two-site model system we show that a few qualitatively distinct relaxation regimes of an optically created exciton are possible, depending on the timescale of bath fluctuations. A slow bath always leads to adiabatic polaron formation. Non-adiabatic exciton self-trapping occurs when the system is strongly coupled to a fast bath. Weak coupling to such bath does not perturb the excitonic picture. The complex system-bath dynamics can then be mapped to an effective model where the resonant coupling between sites is quenched during relaxation. The timescale of the polaron formation can be defined by the timescale of resonant coupling quenching, and is found to directly correlate with the bath relaxation time.

Ultrafast Photoemission Spectroscopy of the Uranium DioxideUO2Mott Insulator: Evidence for a Robust Energy Gap Structure

Time-resolved photoemission spectroscopy utilizing a probe energy of 32.55 eV and a pump energy of 3.1 and 4.65 eV with 30 fs temporal resolution is used to study the carrier dynamics in the $5f$ Mott insulator uranium dioxide (${\mathrm{UO}}_{2}$). The Mott gap and on-site Coulomb interaction energies are measured directly as ${E}_{\text{gap}}=2.5\text{ }\text{ }\mathrm{eV}$ and ${U}_{C}=5\text{ }\text{ }\mathrm{eV}$, respectively, and the dynamics of the upper Hubbard band is mapped. The $f\text{\ensuremath{-}}f$Mott-Hubbard dynamics involves subpicosecond fluence-dependent relaxation, followed by decay via coupling to the lattice upon formation of excitonic polarons. Instead of an expected metallic transition, we observe a robust Mott gap structure, even at high pump fluences.

Simplified Organic Light-Emitting Devices Utilizing Ultrathin Electron Transport Layers and New Insights on Their Roles

The lifetime of organic light-emitting devices (OLEDs) can be limited by exciton-polaron interactions at the organic/organic interfaces. In this work, we show that simplified phosphorescent OLEDs (PHOLEDs) are subjected to this phenomenon. By reducing the exciton concentration at the emission layer (EML)/electron transport layer (ETL) interface by means of increasing the EML thickness, hence broadening the recombination zone, the device lifetime can indeed be improved. Moreover, we report a device that displays the same extended lifetime, but with only 1 nm thin ETL. Studying the roles of this ultrathin ETL in increasing device efficiency reveals that electron injection, hole blocking, and triplet exciton blocking are all important factors. Hole blocking of the ETL can be achieved by highest occupied molecular orbitals level mismatch, where a layer thickness as low as 1 nm is sufficient, or by low hole mobility of the ETL, where a much thicker layer is required (> 5 nm). This ultrathin ETL also enables devices with only 50 nm total organic stacks, which is more than 50% thinner than the typical. This structure opens up opportunities for much shorter processing time and lower fabrication costs in the OLED industry.

Degradation of Organic/Organic Interfaces in Organic Light-Emitting Devices due to Polaron–Exciton Interactions

We study the stability of common hole transport material/electron transport material (HTM/ETM) interfaces present in typical organic light-emitting devices (OLEDs) under various stress scenarios. We determined that these interfaces degrade rapidly, because of an interaction between HTM positive polarons and HTM singlet excitons. The phenomenon results in a deterioration in conduction across the interface, and contributes to the commonly observed increase in OLED driving voltage with electrical driving time. This interfacial degradation can be slowed if the exciton lifetime becomes shorter. The findings uncover a new degradation mechanism that is interfacial in nature, which affects organic/organic interfaces in OLEDs and contributes to their limited electroluminescence stability, and shed light on approaches for reducing it. Although this study has focused on OLEDs, we can expect the same degradation mechanism to affect organic/organic interfaces in other organic optoelectronic devices where both excitons and polarons are present in high concentrations, such as in organic solar cells or photodetectors.