Polaron Pairs Research Articles

Synthesis, Redox Properties and Structural Features of Expanded Isophlorins

The expanded derivatives of isophlorin provide sufficient examples for Huckel’s 4nπ anti-aromatic systems. They vary significantly in their electrochemical properties compared to the aromatic expanded porphyrins. In comparison to the small macrocycles, the large and planar expanded isophlorins exhibit reversible two-electron oxidation. The structure of these macrocycles also undergo reversible structural transformation between planar and figure of eight conformations accompanying the redox process. The oxidized product could be identified as aromatic dication or as a polaron pair depending on the size of the macrocycle. The number of non-pyrrolic heterocyclic units is crucial to stabilize aromatic, antiaromatic or neutral radical isophlorin. A stable 25π neutral radical identified in a one-pot synthesis, exhibits reversible one electron redox to form 24π anti-aromatic cation and a 26π aromatic anion. Synthesis, experimental details and structural features of expanded isophlorins will be highlighted in this presentation.

Electron Correlation Effects in Polaron-Pair Recombination in Conjugated Polymers**Supported by the National Natural Science Foundation of China under Grant No 11474218.

Within a Su–Schriffer–Heeger model modified to include electron-electron interaction and an external electric field, we investigate the dynamics of oppositely charged polarons in a polymer chain in the presence of both electron-phonon and electron-electron interactions under the influence of an external electric field. We adopt a multi-configurational time-dependent Hartree–Fock method for the time-dependent Schrödinger equation and the Newtonian equation of motion for a lattice. Our results show that the on-site Coulomb interaction is of fundamental importance and favors the recombination between the pairs of polarons, and the yield of excitons depends crucially on the strength of the on-site Coulomb interaction U. Furthermore, the influence of the nearest neighbor interaction V is also discussed.

Enhanced Photogeneration of Polaron Pairs in Neat Semicrystalline Donor-Acceptor Copolymer Films via Direct Excitation of Interchain Aggregates.

We investigate the photogeneration of polaron pairs (PPs) in neat films of the semicrystalline donor-acceptor semiconducting copolymer PCPDTBT. Carefully selecting the solution-processing procedures, we obtain films with different amounts of crystallinity and interchain aggregation. We compare the photogeneration of PPs between the films by monitoring their photoinduced absorption in ultrafast pump-probe experiments, selectively exciting nonaggregated or aggregated polymer chains. The direct photoexcitation of interchain π-aggregates results in prompt (<100 fs) charge generation. Compared to the case where nonaggregated chains are excited, we find an 8-fold increase in the prompt PP to singlet-exciton ratio. We also show that highly crystalline lamellar nanostructures not containing π-stacked or any light-absorbing aggregates do not improve the efficiency of PP photogeneration. Our results show that light absorption from interchain aggregates is highly beneficial for charge photogeneration in semiconducting polymers and should be taken into account when optimizing film morphologies for photovoltaic devices.

Polaron pair mediated triplet generation in polymer/fullerene blends.

Electron spin is a key consideration for the function of organic semiconductors in light-emitting diodes and solar cells, as well as spintronic applications relying on organic magnetoresistance. A mechanism for triplet excited state generation in such systems is by recombination of electron-hole pairs. However, the exact charge recombination mechanism, whether geminate or nongeminate and whether it involves spin-state mixing is not well understood. In this work, the dynamics of free charge separation competing with recombination to polymer triplet states is studied in two closely related polymer-fullerene blends with differing polymer fluorination and photovoltaic performance. Using time-resolved laser spectroscopic techniques and quantum chemical calculations, we show that lower charge separation in the fluorinated system is associated with the formation of bound electron-hole pairs, which undergo spin-state mixing on the nanosecond timescale and subsequent geminate recombination to triplet excitons. We find that these bound electron-hole pairs can be dissociated by electric fields.

An effect of molecular motion on carrier formation in a poly(3-hexylthiophene) film.

Free carriers, polarons (P), in conjugated polymers play a key role in the performance of optoelectronic devices. Here, we present solid evidence that P can be predominantly generated from polaron pairs (PP) in a poly(3-hexylthiophene) (P3HT) film under zero electric field. P formation from PP strongly depends on temperature. The temperature dependence of P starts to change around 300 K. P3HT exhibits a thermal molecular motion named the α1 relaxation process, in which the twisting motion of thiophene rings is released, in this temperature region. Thus, it can be claimed that the twisting motion of P3HT thiophene rings is one of the determining factors of the photodynamics of P in P3HT films. This finding holds true for poly(thiophene)s with different alkyl lengths and should be considered in the design and construction of highly-functionalized organic devices based on poly(thiophene)s.

Time-Resolved Raman Spectroscopy of Polaron Pair Formation in Poly(3-hexylthiophene) Aggregates.

The ultrafast formation of bound charge pairs, or polaron pairs (PPs), in mixed-order aggregates of poly(3-hexylthiophene) was investigated using femtosecond stimulated Raman spectroscopy (FSRS). Spectral dynamics in the carbon-carbon stretching region reveal a significant photoinduced depletion in steady-state features associated with lamellar-stacked, ordered polymer regions upon 500 nm photoexcitation; this is followed by the appearance of red-shifted features attributable to PPs that is delayed by a few hundred femtoseconds. PP features decay with concomitant recovery of the steady-state Raman depletion over a few picoseconds. The vibrational spectrum of the PP obtained exhibits a modest red shift (<15 cm(-1)) and lower Raman activity relative to steady-state features in the C═C stretching region but similar features in other regions. In total, this work demonstrates the potential of time-resolved Raman as a morphologically selective and structurally sensitive probe for tracking ultrafast charge separation and recombination dynamics within polymer regions of conjugated materials.

Enhanced charge separation in ternary P3HT/PCBM/CuInS2 nanocrystals hybrid solar cells.

Geminate recombination of bound polaron pairs at the donor/acceptor interface is one of the major loss mechanisms in organic bulk heterojunction solar cells. One way to overcome Coulomb attraction between opposite charge carriers and to achieve their full dissociation is the introduction of high dielectric permittivity materials such as nanoparticles of narrow band gap semiconductors. We selected CuInS2 nanocrystals of 7.4 nm size, which present intermediate energy levels with respect to poly(3-hexylthiophene) (P3HT) and Phenyl-C61-butyric acid methyl ester (PCBM). Efficient charge transfer from P3HT to nanocrystals takes place as evidenced by light-induced electron spin resonance. Charge transfer between nanocrystals and PCBM only occurs after replacing bulky dodecanethiol (DDT) surface ligands with shorter 1,2-ethylhexanethiol (EHT) ligands. Solar cells containing in the active layer a ternary blend of P3HT:PCBM:CuInS2-EHT nanocrystals in 1:1:0.5 mass ratio show strongly improved short circuit current density and a higher fill factor with respect to the P3HT:PCBM reference device. Complementary measurements of the absorption properties, external quantum efficiency and charge carrier mobility indicate that enhanced charge separation in the ternary blend is at the origin of the observed behavior. The same trend is observed for blends using the glassy polymer poly(triarylamine) (PTAA).

Investigation of excitons fission and annihilation processes in Rubrene based devices by utilizing magneto-electroluminescence curves

That the energy of triplet exciton in Rubrene is about half of its singlet leads to energy resonance. This resonance not only allows two triplets to annihilate into a singlet, but also makes a singlet probably fission into two triplets in different molecules. On the other hand, the π-π conjugation of two Rubrene molecules could be formed during molecules stacking, and this spatial relationship will affect the charge transport property enormously. In this article, we use organic magnetic-field effect as a convenient approach to explore the influence of the energy resonant excited states in the Rubrene molecules and the π-π conjugation between the different molecules on the luminescence property of Rubrene. Firstly, we fabricate organic light emitting diodes based on pure Rubrene and modulate the thickness of Rubrene. Experimental measurements of these devices at room temperature exhibit that the thickness can affect the devices' magneto-electroluminescence (MEL) curves substantially. Values of high-field MEL increase with the thickness of Rubrene and gradually saturate after reaching 30 nm. This can be attributed to the fact that the ratio of π-π conjugation in Rubrene molecules to the stacking will grow with increasing thickness, and then saturate at a proper thickness. Subsequently, we modulate the concentration of Rubrene by doping Buthocuproine (BCP) in the active layer. Experimental results at room temperature show that the values of high-field MEL decrease as the concentration of Rubrene decreases. These results verify that the influence of π-π conjugation is not only on the MEL curves, but also on the singlet fission. Furthermore, all the MEL curves exhibit a high-field decay at low temperatures since the endothermic fission process in the Rubrene molecules becomes weaker as the temperature decreases, and the longer triplet lifetime at lower temperatures also enhances the process of triplet annihilation. Besides, the extensively existent intersystem crossing between singlet and triplet polaron pairs may affect these devices as well. Finally, the MEL curves of 20% Rubrene device at room temperature changing with various currents are successfully fitted through the combination of two exponential functions and a Lorentzian function. By means of the fitting, we confirm that the singlet exciton fission, the triplet-triplet exciton annihilation, and the intersystem crossing between singlet and triplet polarons coexist in the devices. Therefore, the varieties of these MEL curves can be attributed to the competition of these processes. The fittings reveal that the triplet-triplet exciton annihilation rate increases more obviously than the singlet exciton fission rate with increasing current. Compared with the rates of the two bimolecular interactions given before, the change of the intersystem crossing rate could be neglected because of its small magnitude. This work is helpful to expand the understanding of the internal mechanism of organic optoelectronic devices.

The role of triplet bipolarons in organic magnetoconductance

By constructing dynamic equations including polarons, singlet bipolarons and triplet bipolarons, we calculate triplet bipolarons dependent magnetoconductance (MC). It is indicated that MC can be changed by tuning the density of triplet bipolaron. And a reversal sign of MC at ultrasmall (1–2mT) is obtained due to interplay of hyperfine and exchange interactions in polaron pairs. Moreover, the theoretical calculations are well consistent with the experimental results.

Polarization and dissociation of a high energy photon-excited state in conjugated polymers

Abstract We use the tight-binding Su–Schrieffer–Heeger model for the one-dimensional conjugated polymers to explore the static polarization behavior of a high energy photon-excited state under the electric field. An obvious reverse polarization is obtained although the electric field is weak. With the increase of field strength, the degree of polarization increases first and then decreases. When the electric field is strong enough, the excited state is dissociated into the polaron pair. In addition, the effects of electron–electron interaction and interchain coupling are also discussed. The results indicate that the electron–electron interaction could weaken the reverse polarization of the high energy photon-excited state, which tends to be dissociated into a high-energy exciton and a pair of solitons with the including of interchain coupling.

Effect of axial halogen substitution on the performance of subphthalocyanine based organic photovoltaic cells

To probe the influence of molecular dipole on the open circuit voltage (VOC) of molecular heterojunction organic solar cells, we study axially fluorinated boron subphthalocyanine/fullerene (SubPc-F/C60) junctions. These exhibit an open-circuit voltage VOC=1.00V, a value closer to the HOMO–LUMO offset at the donor–acceptor interface=1.69eV than the VOC=1.06V measured for junctions between the archetypal chlorinated SubPc and C60, with corresponding HOMO–LUMO offset=1.84eV. Aside from the axial halogen substitution, the two compounds exhibit similar molecular structure and optical absorption. The energy levels and structure of the heteromolecular polaron pair are calculated, and the ideal organic diode model for SubPc-Cl is modified accordingly, successfully reproducing the experimental SubPc-F device characteristics. The reproducible difference in VOC is attributed to the different electric dipole strength between SubPc-F and SubPc-Cl and its influence on polaron pair dynamics at the heterojunction.

Ultrafast Terahertz Photoconductivity of Photovoltaic Polymer-Fullerene Blends: A Comparative Study Correlated with Photovoltaic Device Performance.

Ultrafast photoinduced carrier dynamics in prototypical low band gap polymer:fullerene photovoltaic blend films PTB7:PC70BM and P3HT:PC70BM is investigated using ultrafast terahertz (THz) spectroscopy. The subpicosecond and few-picosecond decays of THz-probed photoconductivities for both compounds are observed, attributed to the rapid formation of polaron pairs by exciton-exciton annihilation and subsequent polaron pair annihilation, respectively. The transient THz photoconductivity spectra of PTB7:PC70BM are well described by the Drude-Smith (DS) model, directly yielding the important charge transport parameters such as charge carrier density, momentum scattering time, and effective localization. By comparison with P3HT:PC70BM, we find that in PTB7:PC70BM the mobile charge carrier photoconductivity is significantly enhanced by a factor of 1.8 and prevails for longer times after charge formation, due to both improved mobile charge carrier yield and lower charge localization. In PTB7:PC70BM, a strong dependency of electron momentum scattering time on electron density was found, well parametrized by the empirical Caughey-Thomas model. The difference in ultrafast photoconductivities of both P3HT:PC70BM and PTB7:PC70BM is found to correlate very well with the performance of photovoltaic devices based on those materials.

Dynamics of charge separation at an organic donor-acceptor interface

The key process in organic photovoltaic cells is the charge separation at organic donor-acceptor interfaces. However, exactly how the charges separate into free charge carriers still remains a puzzle. We present here simulations of the electron dynamics of this process using a nonadiabatic Ehrenfest method. From these simulations, we give a direct illustration of the charge separation process. The results show that the delocalization of the electronic states involved plays a critical role in order to overcome the Coulomb attraction of the charge transfer (CT) exciton. Charge separation only occurs for sufficiently strong intermolecular interactions. Alternatively, the CT exciton relaxes into a bound polaron pair. The results also show that the ``excess energy'' of the hot CT exciton facilitates the charge separation process to a certain degree.

Properties of oligothiophene polycations

The different electronic states of trications and tetracations have been studied for oligomers made of n 3,4-ethylenedioxythiophene units, where n ranges from 2 to 30. Results have been compared with those obtained for neutral, monodications and dications of the same n-oligomers. For the longer oligomer (n ≥ 24) trications, the doublet and quartet states are isoenergetic, while the triplet and quintet states are isoenergetic for tetracations of the same length. Analysis of the geometry reveals three separated polarons for the doublet and quartet states of tricationic n-oligomers, even though that of the former state has been attributed to a heavy spin contamination. In contrast, the structure of tetracations shows double bipolaron character for the singlet state and two polaron pairs for both the triplet and quintet states. Analyses of both charge and spin density distributions support the polaronic or bipolaronic behavior predicted by molecular geometry for the different electronic states. Results have been also used to evaluate the ionization potentials and the π-π* lowest transition energy of infinite polymer chains. Copyright © 2014 John Wiley & Sons, Ltd.

DISCERNMENT OF POSSIBLE ORGANIC MAGNETIC FIELD EFFECT MECHANISMS USING POLYMER LIGHT-EMITTING ELECTROCHEMICAL CELLS

We report studies of magnetic field effect (MFE) in polymer light-emitting electrochemical cells (PLEC) using the "super-yellow" poly-(phenylene vynilene) (SY-PPV) polymer in vertical and planar device configurations. The purpose is to discern the existing MFE mechanisms in organic light emitting diodes (OLEDs) where the current and electroluminescence are strongly modulated by a small applied magnetic field. In particular, we investigate the mutual relationship between magneto-conductance (MC) and magneto-electroluminescence (MEL) by studying the role of polaron density dissociated from polaron pairs (PP) on these magnetic responses. In general, the dissociated polaron density is determined by the PP dissociation rate and the PP density. For the planar PLEC, which possesses a small dissociation rate, we observe small and negative MC at all applied voltages regardless of the emission intensity, while MEL becomes positive when electroluminescence quantum efficiency increases. The MC has a much narrower width than the MEL, indicating that the MC and MEL do not share a common origin. However, MC reverses and has the same width as MEL when the device is exposed to a threshold laser power. For the vertical PLEC, characterized by a large dissociation rate, MC and MEL are positive and have the same width. We discuss the results using the existing MFE mechanism in OLEDs. We show that the PP model can explain the positive MEL and MC, while the negative MC can be explained by the bipolaron model. Finally, we present a possibility to complete an all-organic PLEC magnetic sensor by using an inkjet printer.

Structure and Disorder in Squaraine–C60 Organic Solar Cells: A Theoretical Description of Molecular Packing and Electronic Coupling at the Donor–Acceptor Interface

Organic solar cells based on the combination of squaraine dyes (as electron donors) and fullerenes (as electron acceptors) have recently garnered much attention. Here, molecular dynamics simulations are carried out to investigate the evolution of a squaraine–C60 bilayer interface as a function of the orientation and order of the underlying squaraine layer. Electronic couplings between the main electronic states involved in exciton dissociation and charge (polaron pair) recombination are derived for donor–acceptor complexes extracted from the simulations. The results of the combined molecular‐dynamics−quantum‐mechanics approach provide insight into how the degree of molecular order and the dynamics at the interface impact the key processes involved in the photovoltaic effect.

ChemInform Abstract: Materials Design Considerations for Charge Generation in Organic Solar Cells

AbstractReview: 176 refs.

Temperature Effects on the Dynamics of Photoexcitations in Conjugated Polymers

Within a tight-binding electron–phonon interaction model modified to include temperature effects, the dynamics of photoexcitations in conjugated polymers are investigated using a nonadiabatic evolution method. For the photoexcited state from the highest occupied energy level (HOMO) to the lowest unoccupied energy level (LUMO), it is found that with increasing temperature, the stability of the polaron-excitons becomes weak. Furthermore, by showing time evolution of the localized energy levels at different temperatures, a blue-shift in the photoluminescence spectrum is observed. Yields of the polaron-excitons are calculated over the temperature range 5–300 K. It is found that the yield of polaron-excitons decreases with increasing temperature, and involves two different physical processes at lower and higher temperatures. For the photoexcited state from HOMO–1 to LUMO+1, we focus on the charged polaron pair lifetimes and the yield of polaron-excitons. The results show that the temperature effects shorten the lifetime of the charged polaron pair and cause the charged polaron pairs to easily merge into polaron-excitons, while the stability of the polaron-excitons formed from merging of the charge polaron pairs is influenced by temperature. As a result, the yield of the polaron-excitons first increases and then decreases in the processes.

Semi-Empirical Limiting Efficiency of Singlet-Fission-Capable Polyacene/Inorganic Hybrid Solar Cells

A detailed balance limiting efficiency calculation of a singlet fission capable organic/inorganic hybrid solar cell is presented. Incorporating experimental values for pertinent rates in the organic layer we find that the limiting efficiency of a tetracene/ideal inorganic semiconductor cell is 35.8%, exceeding the Shockley–Queisser limit. We further propose an “equivalent circuit” model of excitonic current, with three “cells” corresponding to the organic, inorganic, and interface species. By considering their relative contributions to the current density of the device, we discuss the major loss mechanisms and identify that the triplet lifetime and nonradiative decay of bound polaron pairs are major limitations to the development of a high-efficiency polyacene/inorganic solar cell. The results suggest that the efficiency of a tetracene/silicon hybrid solar cell could exceed the Shockley–Queisser limit.

Magnetic field effects on photoelectrochemical reactions of electrodes modified with thin films consisting of conductive polymers

The magnetic field effects (MFEs) on the photoelectrochemical reactions of electrodes modified with nanowires consisting of regioregular poly(3-hexylthiophene) (P3HT), disordered P3HT films without and with annealing treatment, and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) films were examined. In P3HT, the photocurrent increased in the presence of a magnetic field. The magnitude of the MFEs increased in the following order of modified electrodes: P3HT nanowires, P3HT films with annealing treatment, P3HT films without annealing treatment. The difference in the MFEs is most likely to be attributable to the capability of hole hopping between adjacent thiophenes. On the other hand, in MEH-PPV, the photocurrent decreased in the presence of a magnetic field. The MFEs on the photoelectrochemical reactions of electrodes modified with P3HT and MEH-PPV are explained by the polaron pair mechanism. The difference in the MFEs between P3HT and MEH-PPV is ascribed to the spin multiplicity of the polaron pairs.