Unravelling the effect of strand orientation on exciton migration in conjugated polymers (original) (raw)
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ACS applied materials & interfaces, 2015
Conjugated polymers are widely used materials in organic photovoltaic devices. Owing to their extended electronic wave functions, they often form semicrystalline thin films. In this work, we aim to understand whether distribution of crystallographic orientations affects exciton diffusion using a low-band-gap polymer backbone motif that is representative of the donor/acceptor copolymer class. Using the fact that the polymer side chain can tune the dominant crystallographic orientation in the thin film, we have measured the quenching of polymer photoluminescence, and thus the extent of exciton dissociation, as a function of crystal orientation with respect to a quenching substrate. We find that the crystallite orientation distribution has little effect on the average exciton diffusion length. We suggest several possibilities for the lack of correlation between crystallographic texture and exciton transport in semicrystalline conjugated polymer films.
Excitonic Energy Migration in Conjugated Polymers: The Critical Role of Interchain Morphology
Excitonic energy migration was studied using single molecule spectroscopy of individual conjugated polymer (CP) chains and aggregates. To probe the effect of interchain morphology on energy migration in CP, tailored interchain morphologies were achieved using solvent vapor annealing to construct polymer aggregates, which were then studied with single aggregate spectroscopy. We report that highly ordered interchain packing in regioregular poly(3-hexylthiophene) (rr-P3HT) enables long-range interchain energy migration, while disordered packing in regioran-dom poly(3-hexylthiophene) (rra-P3HT), even in aggregates of just a few chains, can dramatically impede the interchain mechanism. In contrast to rr-P3HT, interchain energy migration in poly(3-(2′-methoxy-5′-octylphenyl)thiophene) (POMeOPT), a polythiophene derivative with bulky side chains, can be completely inhibited. We use simulated structures to show that the reduction in interchain coupling is not due simply to increased packing distance between backbones of different chains, but reflects inhibition of stacking due to side-chain-induced twisting of the contours of individual chains. A competition from intrachain coupling has also been demonstrated by comparing POMeOPT aggregates with different polymer chain sizes.
Temperature Dependence of Exciton Diffusion in Conjugated Polymers
The Journal of Physical Chemistry B, 2008
The temperature dependence of the exciton dynamics in a conjugated polymer is studied using time-resolved spectroscopy. Photoluminescence decays were measured in heterostructured samples containing a sharp polymer-fullerene interface, which acts as an exciton quenching wall. Using a 1D diffusion model, the exciton diffusion length and diffusion coefficient were extracted in the temperature range of 4-293 K. The exciton dynamics reveal two temperature regimes: in the range of 4-150 K, the exciton diffusion length (coefficient) of ∼3 nm (∼1.5 × 10 -4 cm 2 /s) is nearly temperature independent. Increasing the temperature up to 293 K leads to a gradual growth up to 4.5 nm (∼3.2 × 10 -4 cm 2 /s). This demonstrates that exciton diffusion in conjugated polymers is governed by two processes: an initial downhill migration toward lower energy states in the inhomogenously broadened density of states, followed by temperature activated hopping. The latter process is switched off below 150 K.
Excitation energy migration assisted processes in conjugated polymers
Synthetic Metals, 2004
An important characteristic of thin films of state-of-the-art conjugated polymers and small molecule materials is energetic and morphological disorder. For the photophysics in these electroactive thin films, the disorder has an important consequence: a single and multi-step (migration) excitation energy transfer processes of the charge-neutral states, such as singlet and triplet excitons (TEs) towards lower energetic sites can be observed prior to radiative recombination. Yet as found migration assisted processes, which are rather pronounced in the solid state of organic molecules and conjugated polymers can be held responsible for a large number of nonradiative deactivation channels as they enable quenching at chemical defects or other excited states such as polarons. Furthermore, the ability to migrate enlarges the action radius of singlet excitons (SEs) up to 20 nm in the solid state which makes it necessary to include the migration process for describing the overall energy transfer behaviour of conjugated polymers.
Understanding excitons in optically active polymers
Polymer International, 2008
We review the solid-state physics approach to electronic and optical properties of conducting polymers, and bring together the languages of solid-state theory for polymers and the quantum chemistry of oligomers. We consider polymers as generic one-dimensional semiconductors with specific features of strongly correlated electronic systems. Our model combines the large distance electron-hole motion within an exciton, governed by long-range Coulomb attraction with strong intramonomer electronic correlations, which results in effective intramonomer electron-hole repulsion. We exploit the dielectric screening to go beyond the single chain picture and to compare excitons for polymers in solutions and in films. Our approach allows the connecting, explaining, exploiting and organizing of such different experimental and numerical findings as shallow singlet and deep triplet excitons in phenylenes, anomaly in singlet-triplet exciton formation ratio, A g-B u crossing in polyenes and common 1/N energy dependencies in oligomers.
Exciton migration to chain aggregates in conjugated polymers: influence of side-chain substitution
Chemical Physics Letters, 2001
We have performed time-resolved photoluminescence (PL) measurements to study the transfer of excitons from noninteracting polymer chains to chain aggregates for thin ®lms of polyindeno¯uorene with dierent attached side chains. The transfer time is shown to be fast (%35 ps) and side-chain independent indicating that the substitution of dierent side chains does not aect the local concentration of aggregate sites within aggregate-containing domains, but rather the extent to which such domains are formed. We ®nd the aggregate emission eciency to be relatively high (J 20%) while still lower than the eciency of emission from non-interacting chains. Ó
Journal of Physical Chemistry B, 2014
Photoexcitation of conjugated poly-2,7-(9,9-dihexylfluorene) polyfluorenes with naphthylimide (NI) and anthraquinone (AQ) electron-acceptor end traps produces excitons that form charge transfer states at the end traps. Intramolecular singlet exciton transport to end traps was examined by steady state fluorescence for polyfluorenes of 17 to 127 repeat units in chloroform, dimethylformamide (DMF), tetrahydrofuran (THF), and p-xylene. End traps capture excitons and form charge transfer (CT) states at all polymer lengths and in all solvents. The CT nature of the end-trapped states is confirmed by their fluorescence spectra, solvent and trap group dependence and DFT descriptions. Quantum yields of CT fluorescence are as large as 46%. This strong CT emission is understood in terms of intensity borrowing. Energies of the CT states from onsets of the fluorescence spectra give the depths of the traps which vary with solvent polarity. For NI end traps the trap depths are 0.06 (p-xylene), 0.13 (THF) and 0.19 eV (CHCl 3). For AQ, CT fluorescence could be observed only in p-xylene where the trap depth is 0.27 eV. Quantum yields, emission energies, charge transfer energies, solvent reorganization and vibrational energies were calculated. Fluorescence measurements on chains >100 repeat units indicate that end traps capture ~50% of the excitons, and that the exciton diffusion length L D =34 nm, which is much larger than diffusion lengths reported in polymer films or than previously BNL-108317-2015-JA 2 known for diffusion along isolated chains. The efficiency of exciton capture depends on chain length, but not on trap depth, solvent polarity or which trap group is present.
Chain Conformation and Exciton Delocalization in a Push-Pull Conjugated Polymer
arXiv (Cornell University), 2023
Linear and nonlinear optical lineshapes reveal details of excitonic structure in semiconductor polymers. We implement absorption, photoluminescence, and transient absorption spectroscopies in DPP-DTT, an electron push-pull copolymer, to explore the relationship between their spectral lineshapes and chain conformation, deduced from resonance Raman spectroscopy and from ab initio calculations. The viscosity of precursor polymer solutions before film casting displays a transition that suggests gel formation above a critical concentration. Upon crossing this viscosity deflection concentration, the lineshape analysis of the absorption spectra within a photophysical aggregate model reveals a gradual increase in interchain excitonic coupling. We also observe a red-shifted and line-narrowed steady-state photoluminescence spectrum, along with increasing resonance Raman intensity in the stretching and torsional modes of the dithienothiphene unit, which suggests a longer exciton coherence length along the polymer-chain backbone. Furthermore, we observe a change of lineshape in the photoinduced absorption component of the transient absorption spectrum. The derivative-like lineshape may originate from two possibilities: a new excited-state absorption, or from Stark effect, both of which are consistent with the emergence of high-energy shoulder as seen in both photoluminescence and absorption spectra. Therefore, we conclude that the exciton is more dispersed along the polymer chain backbone with increasing concentrations, leading to the hypothesis that the polymer chain order is enhanced when the push-pull polymers are processed at higher concentrations. Thus, tuning the microscopic chain conformation by concentration would be another factor of interest when considering the polymer assembly pathways for pursuing large-area and high-performance organic optoelectronic devices.
Homocoupling Defects in a Conjugated Polymer Limit Exciton Diffusion
Advanced Functional Materials, 2019
Copolymers such as PCDTBT (poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole))) are commonly employed as donor material in bulk heterojunction solar cells. Recently, chemical defects such as homocouplings have been shown to form at the material synthesis stage, strongly reducing the short circuit current in organic photovoltaics. Here it is shown that both low molecular weight and homocoupling defects reduce the short circuit current of solar cells because of limited exciton diffusion. A model that unites and explains the influence of both chemical parameters with the distribution of conjugation lengths is proposed. The connection between limited exciton diffusion and short circuit current is revealed via kinetic Monte Carlo simulation of bulk heterojunctions. The findings are likely applicable for copolymers in general.