Electroabsorption Signal Research Articles

Probing Polaron Environment in a Doped Polymer via the Photoinduced Stark Effect

Conjugated polymers (CPs) constitute a unique class of materials with applications ranging from energy storage to bioelectronics, thanks in part to their ability to be easily doped to produce mobile charge carriers. While methods for doping CPs are well-developed, a deeper understanding of the interplay between the local environment and polaronic properties is currently lacking. Using transient absorption (TA) spectroscopy, we demonstrate that photoexciting polarons in a lightly doped 3,4-propylenedioxythiophene-co-3,4-ethylenedioxythiophene (ProDOT-co-EDOT) polymer film produces a Stark effect that is sensitive to their local environments. The resulting electroabsorption (EA) signal, which originates from a shift in the band gap of nearby polymer chains due to the photoinduced electric field, indicates that polaron excitation spatially separates the hole from its counterion as the hole thermalizes within 200 fs. Changes in the spectral position of the EA signal when tuning the excitation wavelength across the P1 and P2 polaron bands, combined with a dynamic blue shift, suggest a variation in field strength that senses different extents of the local environment. We propose the photoinduced Stark effect in TA experiments as a sensitive probe of the local environment of polarons in doped CPs that can also be used to refine electronic structure models describing their polaronic states and transitions.

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.

Dynamics of Internal Electric Field Screening in Hybrid Perovskite Solar Cells Probed Using Electroabsorption

Electric fields arising from the distribution of charge in metal-halide perovskite solar cells are critical for understanding the many weird and wonderful optoelectronic properties displayed by these devices. Mobile ionic defects are thought to accumulate at interfaces to screen electric fields within the bulk of the perovskite semiconductor on application of external bias, but tools are needed to directly probe the dynamics of this process. Here, we show that electroabsorption measurements allow the electric field within the active layer to be tracked as a function of frequency or time. The magnitude of the electroabsorption signal, corresponding to the strength of the electric field in the perovskite layer, falls off for externally applied low-frequency voltages or at long times following voltage steps. Our observations are consistent with drift-diffusion simulations, impedance spectroscopy, and transient photocurrent measurements. They indicate charge redistribution on timescales ranging from 10 ms to 100 s, depending on the device interlayer material, perovskite composition, dominant charged defect, and illumination conditions. The method can be used on typical solar-cell structures and has the potential to become a routine characterization tool for optimizing hybrid perovskite devices.

Investigation of internal fields in organic semiconductors in the presence of traps

In an organic semiconductor optoelectronic device, the built-in field within the active layer is typically determined by the difference in contact potentials of the device. However, the presence of space charges and trap states contribute to the electric field within the thin film. Depending on the maximum applied forward voltage, the trap states can be charged, inducing hysteresis in the optoelectronic response of the system. In this work, we investigate the electric fields inside organic photovoltaic device structures, in the presence of traps, using electroabsorption (EA) spectroscopy. Comparing simulations with our experimental results, we explained the origin of hysteresis in the electroabsorption signal as a function of applied DC bias. We solved Poisson’s equation to estimate the densities of trapped carriers in the active layers. The filled trap densities in poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)] (PPDT2FBT) were found to be ∼1×1017 and ∼6×1016 cm−3, respectively. From the transient EA measurements, the estimated values of energies of the trap states with respect to the HOMO level were 0.82 and 0.76 eV in P3HT and 0.70 and 0.64 eV in PPDT2FBT, which indicated the presence of midgap traps in these organic semiconductor thin films. Such trap induced changes in the internal fields within the active layers, affect the mobility and carrier transport in the organic optoelectronic devices. The midgap traps lead to exciton quenching and also act as non-radiative recombination centers, resulting in reduction in luminescence efficiency of the active layers.

Reduced Recombination and Capacitor-like Charge Buildup in an Organic Heterojunction

Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination-poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated. Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient Stark effect, caused by nanoscale electric fields of ∼487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ∼2000 times less than predicted by Langevin theory. This in turn leads to the buildup of electric charge in donor and acceptor domains-away from the interface-resistant to bimolecular recombination. Interestingly, this signal is only experimentally obvious in thick films due to the different scaling of electroabsorption and photoinduced absorption signals in transient absorption spectroscopy. Rather than inhibiting device performance, we show that devices up to 600 nm thick maintain efficiencies of >8% because domains can afford much higher carrier densities. These observations suggest that with particular nanoscale morphologies the bulk heterojunction can go beyond its established role in charge photogeneration and can act as a capacitor, where adjacent free charges are held away from the interface and can be protected from bimolecular recombination.

Distinct Effects of External Electric Field on Interfacial and Bulk-like Water Confined in Reverse Micelles

Nanoscopically confined water under an electric field is widely found in various fields of chemistry and biology. Despite their importance, it remains to be clarified how such water molecules respond to an electric field. In this study, we use reverse micelles as a model system for water confinement and measure the electric field effects on water (D2O) molecules inside reverse micelles with infrared (IR) electroabsorption (EA) spectroscopy. Conventional IR and IR EA spectra of water in anionic and nonionic reverse micelles are interpreted in terms of interfacial and bulk-like water molecules in the reverse micelles. The IR EA signals of these water species that are attributable to field-induced reorientation show opposite signs, indicating that they have distinctly different vibrational properties mainly because of the confined environment. Our results demonstrate that IR EA spectroscopy holds promise for gaining novel insights into the behaviors of confined water such as hydration water and water in poro...

Dipole Moment and Polarizability of Tunable Intramolecular Charge Transfer States in Heterocyclic π-Conjugated Molecular Dyads Determined by Computational and Stark Spectroscopic Study.

The annulation of two redox-active molecules into a compact and planar structure paves the way toward a new class of electronically versatile materials whose physical properties can be tuned via a substitution of one of the constituting moieties. Specifically, we present tetrathiafulvalene-benzothiadiazole donor-acceptor molecules. The critical role played by the dielectric properties of these molecules is evident by the large spectral shifts of the ground-state absorption spectra in a range of solvents. Stark spectroscopy is performed to determine experimentally dipole and polarizability change over transitions in the visible range with particular attention to the transition from the highest-occupied molecular orbital (HOMO) to the lowest-unoccupied molecular orbital (LUMO). The experimental results are compared to the results of time-dependent density functional theory calculations, and we reciprocally validate results from calculation and experiment. This allows us to filter out effective models and reveal important insights. The calculations are initially performed in the gas phase and subsequently a polarizable continuum model is adopted to probe the influence of the solvent on the molecular dielectric properties. The results show a large charge displacement from the HOMO to the LUMO and confirm the intramolecular charge transfer nature of the lowest-energy transition. Substitution of the acceptor moiety with electron-withdrawing groups results in changes to the experimentally determined molecular properties consistent with the effects predicted by computational results. The dominant contribution to the electroabsorption signal is due to the change in dipole moment, which is measured to be roughly 20 D for all samples and forms a small angle with the transition dipole moment in a toluene solvent environment.

Femtosecond Dynamics of Photoexcited C60 Films.

The well known organic semiconductor C60 is attracting renewed attention due to its centimeter-long electron diffusion length and high performance of solar cells containing 95% fullerene, yet its photophysical properties remain poorly understood. We elucidate the dynamics of Frenkel and intermolecular (inter-C60) charge-transfer (CT) excitons in neat and diluted C60 films from high-quality femtosecond transient absorption (TA) measurements performed at low fluences and free from oxygen or pump-induced photodimerization. We find from preferential excitation of either species that the CT excitons give rise to a strong electro-absorption (EA) signal but are extremely short-lived. The Frenkel exciton relaxation and triplet yield strongly depend on the C60 aggregation. Finally, TA measurements on full devices with applied electric field allow us to optically monitor the dissociation of CT excitons into free charges for the first time and to demonstrate the influence of cluster size on the spectral signature of the C60 anion.

Unveiling the Nature of Charge Carrier Interactions by Electroabsorption Spectroscopy: An Illustration with Lead-Halide Perovskites.

Unravelling the nature of the interactions between photogenerated charge carriers in solar energy conversion devices is key to enhance performance. In this perspective, we discuss electroabsorption spectroscopy (EAS), as the spectral bandshape of the electroabsorption (EA) signal directly depends on the strength of the charge carrier interactions. For instance, the electroabsorption response in molecular or confined excitonic systems can be modelled perturbatively yielding the Stark effect. In contrast, most solids exhibit weaker interactions, and a perturbative approach cannot be taken in general. For solids with negligible charge carrier interactions, one resorts to the Franz-Keldysh theory of a continuum in a field, that, in the low-field limit, simplifies to the low-field FKA effect. Alternatively, when the continuum approximation breaks down, the problem of a Wannier exciton in a field has to be solved, and numerical methods emerged as the best solution. We illustrate our discussion with two examples involving lead-halide perovskites, a new, high-stake solar cell material. In the first example, we discuss the lineshape of the electroabsorption response for thin-films of lead-iodide perovskite, that sustains the photogeneration of free carriers. In the second example, we address a confined excitonic case with lead-bromide perovskite nanoparticles, and demonstrate the presence of so-called charge-transfer excitons.

Unconventional electroabsorption in monolayer MoS2

Signal modulation in optoelectronics is obtained by modulation of either the refractive index or the absorbance by an electric field. However, electromodulators have not kept up with the miniaturization of other electronic and optical components. Here we show a strong transverse electroabsorption signal in a monolayer of the 2D semiconductor MoS2. The electroabsorption spectrum is dominated by an apparent linewidth broadening of around 15% at a modulated voltage of only Vpp = 0.5 V. Contrary to known variants of the Stark effect, the broadening increases linearly with the applied field strength and arises from a linear variation of the distance between the strongly overlapping exciton and trion resonances. The achievable modulation depths exceeding 0.1 dB nm−1 bear the scope for extremely compact, ultrafast, energy-efficient electroabsorption modulators for integrated photonics, including on-chip optical communication.

Theory of Stark spectroscopy transients from thin film organic semiconducting devices

Herein, we propose a model to describe picosecond-nanosecond charge separation and nongeminate recombination in organic semiconductors. The model is used to explain time-resolved electroabsorption (EA) measurements performed on diodes made from phenyl-C61-butyric acid methyl ester. We find that the measured shape of the EA transient is due to a combination of microscopic carrier dynamic effects such as carrier trapping, as well as macroscopic effects such as band bending caused by the nonuniform poloron generation profile across the device. We demonstrate that the initial fast phase of the EA transient is due to hot free carriers being able to move freely within the device; over time these hot free carriers cool and become trapped giving rise to the second slower phase of the transient. We further show that the commonly observed dependence of the EA signal on probe wavelength can be explained in terms of the spatial overlap of electrostatic potential within the device and the optical mode of the probe light. Finally, we discuss the implications of these results for pump-probe experiments on thin organic films.

Room temperature terahertz electro-optic modulation by excitons in carbon nanotubes

We have investigated the electroabsorption of aligned micelle-wrapped single-walled carbon nanotubes induced by intense terahertz (THz) pulses by using THz-pump and optical-probe spectroscopy at room temperature. The electroabsorption signal is enhanced at exciton resonances near the optical communication wavelength. The signal showed a quadratic dependence on the THz electric field and instantaneously followed the THz oscillation (∼1 ps). The observed ultrafast electro-optic modulation with a THz bandwidth suggests the potential functionality of carbon nanotubes in high-speed electro-optic devices.

Electroabsorption study of ordered polyfluorene thin films: Origin of oscillatory structure near the bottom of the continuum state

The excited-state structure of ordered ($\ensuremath{\beta}$-phase) poly(9,9-dioctylfluorene) (F8) thin films has been investigated by electroabsorption (EA) spectroscopy. The EA spectra of a number of $\ensuremath{\pi}$ conjugated polymers, including disordered (glassy) F8, are well explained by a theoretical essential-state model, which consists of three characteristic excited states and the ground state [Y. Kawabe et al., Phys. Rev. B 44, 6530 (1991)]. However, it is found that the essential-state model fails to explain the EA spectrum of $\ensuremath{\beta}$-phase F8 because of the appearance of an oscillatory EA signal near the bottom of the continuum state. From the applied electric-field dependence and the directional dependence of the field and polarization of the light, it has been revealed that the oscillation results from several intrachain excited states, some of which are not observed in glassy F8 because of considerable inhomogeneous broadening. To reproduce the EA spectrum of $\ensuremath{\beta}$-phase F8 by a sum-over-states calculation, at least seven excited states, in addition to the ground state, have to be taken into account.

Unusual features of the quaterthiophene electro‐absorption spectrum

AbstractA previously used model describing the coupling between Frenkel and charge‐transfer (CT) excited states in a crystal with molecules arranged in close‐packed planes of herringbone pattern, is generalized to include some CT states engaging the molecules from the different planes. The corresponding Hamiltonian is parametrized for quaterthiophene and numerically diagonalized, to yield ultimately the electro‐absorption (EA) spectrum. Contrary to our recent expectations, the contribution from the newly introduced CT states is only minor. It turns out that the high‐energy EA signal of quaterthiophene is dominated by the contributions from the CT states with the electron transferred along the longer lattice period in the close‐packed plane, in contrast to the more familiar case of sexithiophene, where these states barely contribute. The reasons of this difference are pinpointed and discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electroabsorption Spectra of Quadrupolar and Octupolar Dyes in Solution: Beyond the Liptay Formulation

We present theoretical models for electroabsorption (EA) spectra of polar and multipolar (quadrupolar and octupolar) organic chromophores in solutions. Based on essential-state models for the electronic structure, we obtain closed expressions for the EA spectra of these dyes. For polar dyes, we regain the well-known Liptay result, which relates the EA spectrum to the linear absorption spectrum and its first and second derivatives. For multipolar dyes, an additional contribution is found, which is due to the field-induced absorption toward dark states: when the dark states are close in energy to allowed states, this term leads to spurious second-derivative-like structures in the EA spectrum. This is particularly well-apparent for quadrupolar dyes where second-derivative contributions to the EA signal are symmetry-forbidden. The extension of essential-state models to account for slow degrees of freedom, including molecular vibrations and polar solvation, leads to a more-realistic description of optical spectra but hinders the analytical treatment of EA. However, numerically exact EA spectra can be obtained along the same lines that have been recently developed for linear and two-photon absorption spectra of (multi)polar organic chromophores in solution. Essential-state models offer the possibility for a joint analysis of linear and nonlinear spectra (including EA) of (multi)polar dyes: reliable information can then be extracted also on molecular properties for systems where overlapping signals from allowed and dark states would hinder the standard analysis of EA spectra.

The influence of subgap features in the electromodulation and built-in voltage measurements of polyfluorene blue light-emitting diodes with anodic charge injection layers

We report on electroabsorption (EA) and built-in voltage (VBI) measurements of polymer light-emitting diodes with the general structure ITO/PEDOT:PSS/emitting polymer/LiF∕Ca∕Al where ITO is indium tin oxide, PEDOT:PSS is poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate), and the emitting polymer is either poly(9,9-dioctylfluorene), poly(9, 9-dioctyl-fluorene-alt-bis-N,N′-(4-butyl-phenyl)-bis-N,N′-phenyl-1, 4-phenylenediamine), or poly(9,9-dioctylfluorene-alt-N-(4-butylphenyl)-diphenylamine). We find that the EA nulling voltage, i.e., the dc bias at which the EA signal vanishes, depends on the frequency of the ac voltage and on the incident photon wavelength. Such dependence poses a problem for accurate measurement of the built-in voltage (VBI), which is the voltage generated between the electrodes upon equilibration of the Fermi levels through the heterostructure. We find that the EA signal is mixed with a smaller intensity signal which can be ascribed to excited state absorption (ESA). We propose a method for separating the excited state absorption signal and producing accurate VBI measurements. We also demonstrate that in our devices the ESA contribution to the electromodulated (ΔT∕T) signal is negligible with respect to the accuracy with which VBI can be determined from the nulling voltage of ΔT∕T and can thus be safely ignored.

Internal electric fields in vacuum-evaporated organic films as studied by electroabsorption spectroscopy

Electroabsorption (EA) spectroscopy (Stark effect spectroscopy) has been applied to provide information on the internal electric fields occurring in thin films of organic compounds sandwiched between two semitransparent aluminium electrodes (Al/organic film/Al/glass substrate) under ambient conditions. A wide range of molecular materials, commonly used in organic light-emitting diodes, has been investigated. The analysis of the (1 ω)EA signals measured on the first harmonic of the applied sinusoidal electric field frequency ( ω) indicates a sample asymmetry and the presence of internal electric fields ( F i) in organic films with estimated values of F i between 4 × 10 4 and 3 × 10 5 V/cm. The possible origin of internal electric fields is discussed. The observed features of F i can be consistently explained by the asymmetrical distribution of space charge in the organic film associated with different defect (of structural and/or impurity origin) characteristics on the substrate and non-substrate side of the sample.

Microscopic calculation of polariton effects in molecular crystals – application to sexithiophene electroabsorption spectrum

AbstractAn explicit formula expressing the refractive index in terms of exciton energies and oscillator strengths is used to calculate the absorption spectrum of the crystal at zero and non‐zero electric field; the electroabsorption signal is calculated as their difference, without any a priori assumptions regarding the shape of the individual EA fingerprints. The approach is applied for the sexithiophene crystal. The results show that an isolated Frenkel exciton always yields a first‐derivative EA signal, regardless of its intensity, and that this conclusion is not affected by polaritonic effects. For the specific case of sexithiophene, a satisfactory theoretical reproduction of the EA spectrum is possible only when the low‐energy charge‐transfer states are included. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polariton effects in electroabsorption of molecular crystals with several molecules in the unit cell – Sexithiophene

Polariton effects in a model molecular crystal with several molecules in the unit cell are described within a semiclassical formalism taking complete account of the strong electrodynamic coupling between excitons and the photon field. Explicit formulas are derived for the specific cases of two and four molecules in the unit cell, and applied to calculate numerically the absorption spectrum of the sexithiophene crystal. The input data are almost entirely non-empirical, based on the results of previous calculations. The electroabsorption (EA) signal of the crystal is calculated as the difference between the absorption spectra at non-zero and at zero electric field, without any a priori assumptions regarding the shape of the individual EA fingerprints. The results confirm the previous conclusion that an isolated Frenkel exciton always yields a first-derivative EA signal, regardless of its intensity, and that this conclusion is not affected by polaritonic effects. For the specific case of sexithiophene, the experimental absorption spectrum is well reproduced, but the EA signal of the Frenkel states, in spite of full inclusion of polariton couplings, is underestimated by orders of magnitude. The results demonstrate that a satisfactory theoretical reproduction of the sexithiohene EA spectrum is possible only when low-energy charge-transfer states are included, as was done in a recent paper.

Theoretical interpretation of electro-absorption spectra for intense optical transitions

The interaction of a light wave with a molecular crystal subjected at the same time to the influence of static electric field is analyzed. The coupling of the crystal to the radiation field is described in terms of classical electrodynamics, the molecular transition moments being represented by oscillating dipoles. The molecular parameters that enter the classical equations of motion (transition energy and oscillator strength), modified by the static electric field, are derived from the corresponding zero-field values using quantum-mechanical perturbation theory. Subsequently, the field-induced change of the absorption spectrum [electro-absorption (EA) signal] is calculated as the difference between the absorption spectra at nonzero and at zero modulating field. The approach is valid for any allowed transition, irrespective of its intensity. The results demonstrate that, while the absorption spectra of very intense transitions exhibit substantial peculiarities (such as orientational dispersion and polariton effects), the relationship between the absorption and electro-absorption spectra is always the same, regardless of the oscillator strength; specifically, the EA signal of a nondegenerate Frenkel exciton follows the first derivative of the corresponding absorption band. These conclusions are discussed in the context of recent literature on this subject.