Spin-dependent dynamics of polaron pairs in organic semiconductors (original) (raw)
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Polaron spin dynamics in high-mobility polymeric semiconductors
Nature Physics, 2019
Polymeric semiconductors exhibit exceptionally long spin lifetimes, and recently observed micrometre spin diffusion lengths in conjugated polymers demonstrate the potential for organic spintronics devices. Weak spin-orbit and hyperfine interactions lie at the origin of their long spin lifetimes, but the coupling mechanism of a spin to its environment remains elusive. Here, we present a systematic study of polaron spin lifetimes in field-effect transistors with high-mobility conjugated polymers as an active layer. We demonstrate how spin relaxation is governed by the charges' hopping motion at low temperatures, whereas an Elliott-Yafet-like relaxation due to a transient localization of the carrier wavefunctions is responsible for spin relaxation at high temperatures. In this regime, charge, spin and structural dynamics are intimately related and depend sensitively on the local conformation of polymer backbones and the crystalline packing of the polymer chains.
Tracking the coherent generation of polaron pairs in conjugated polymers
Nature communications, 2016
The optical excitation of organic semiconductors not only generates charge-neutral electron-hole pairs (excitons), but also charge-separated polaron pairs with high yield. The microscopic mechanisms underlying this charge separation have been debated for many years. Here we use ultrafast two-dimensional electronic spectroscopy to study the dynamics of polaron pair formation in a prototypical polymer thin film on a sub-20-fs time scale. We observe multi-period peak oscillations persisting for up to about 1 ps as distinct signatures of vibronic quantum coherence at room temperature. The measured two-dimensional spectra show pronounced peak splittings revealing that the elementary optical excitations of this polymer are hybridized exciton-polaron-pairs, strongly coupled to a dominant underdamped vibrational mode. Coherent vibronic coupling induces ultrafast polaron pair formation, accelerates the charge separation dynamics and makes it insensitive to disorder. These findings open up ne...
Dynamics of Photogenerated Polaron-Excitons in Organic Semiconductors
15th Brazilian Workshop on Semiconductor Physics, 2012
In this work we performed numerical simulations of one π-conjugated polymer chain subjected to photogeneration. Within the SSH model modified to include the Brazoviskii-Kirova symmetry breaking term, we investigate the dynamics of photoexcitations to address the generation mechanism of polaron-excitons using the unrestricted Hartree-Fock approximation. It was obtained that after the photoexcitation the system relaxes spontaneously into a polaron-exciton in a transient state in a range of 200 f s. Our results also show that charged polarons are generated directly after this transient state.
We present an extensive density functional theory (DFT) study on the neutral and charged electronic excitations in oligophenylene vinylenes including lowest singlet (S 1 ) and triplet (T 1 ) excitons and positive (P þ ) and negative (P -) polarons. We investigated the vibrational and electronic properties of molecules using five different DFT functionals from pure GGA to long-range-corrected hybrids and found an explicit correlation between the spatial extent of the state and the fraction of the orbital exchange. While solvent effects are found to be negligible for neutral (S 1 and T 1 ) excitons, they play an important role for charged (P þ and P -) species. S 1 states are observed to be spatially less localized compared to the polaronic wave functions (P þ and P -). This is in contrast to the T 1 states, which exhibit more spatial confinement in comparison to P þ and Pstates.
Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics
Polymeric semiconductors are materials where unique optical and electronic properties often originate from a tailored chemical structure. This allows for synthesizing conjugated macromolecules with ad hoc functionalities for organic electronics. In photovoltaics, donoracceptor co-polymers, with moieties of different electron affinity alternating on the chain, have attracted considerable interest. The low bandgap offers optimal light-harvesting characteristics and has inspired work towards record power conversion efficiencies. Here we show for the first time how the chemical structure of donor and acceptor moieties controls the photogeneration of polaron pairs. We show that co-polymers with strong acceptors show large yields of polaron pair formation up to 24% of the initial photoexcitations as compared with a homopolymer (η = 8%). π-conjugated spacers, separating the donor and acceptor centre of masses, have the beneficial role of increasing the recombination time. The results provide useful input into the understanding of polaron pair photogeneration in low-bandgap co-polymers for photovoltaics.
Exciton dissociation and charge carrier recombination processes in organic semiconductors
The Journal of Chemical Physics, 2011
Exciton dissociation and charge carrier recombination processes in organic semiconductors JCP: BioChem. Phys. 5, 12B602 Note: Scale-free center-of-mass displacement correlations in polymer films without topological constraints and momentum conservation J. Chem. Phys. 135, 186101 (2011) Directivity and isotropic band-gap in 12-fold symmetry plasmonic quasi-crystals with small index contrast Appl. Phys. Lett. 99, 181104 (2011) Structure-dependent band dispersion in epitaxial anthracene films J. Chem. Phys. 135, 124709 Electronic structure of the poly(3-hexylthiophene):indene-C60 bisadduct bulk heterojunction Exciton dissociation and charge recombination processes in organic semiconductors, with thermal effects taken into account, are described in this paper. Here, we analyzed the mechanisms of polaronexcitons dissociation into free charge carriers and the consequent recombination of those carriers under thermal effects on two parallel π -conjugated polymers chains electronically coupled. Our results suggest that exciton dissociation in a single molecule give rise to localized, polaron-like charge carrier. Besides, we concluded that in the case of interchain processes, the bimolecular polaron recombination does not lead to an usual exciton state. Rather, this type of recombination leads to an oscillating dipole between the two chains. The recombination time obtained here for these processes are in agreement with the experimental results. Finally, our results show that temperature effects are essential to the relaxation process leading to polaron formation in a single chain, as in the absence of temperature, this process was not observed. In the case of two chains, we conclude that temperature effects also help the bimolecular recombination process, as observed experimentally.
Synthetic Metals, 2003
We report on an experimental study of spin-dependent exciton formation in films of -conjugated polymers. We employed a spectroscopic/magnetic resonance technique, namely the photoinduced absorption (PA)-detected magnetic resonance technique, and showed that non-geminate recombination pairs of polarons form singlet excitons more efficiently than triplets. The experimental proof for this is achieved by detection of a resonant decrease of triplet exciton absorption and resonantly enhanced singlet photoluminescence (PL). In addition, resonantly enhanced absorption is observed which we assign to a singlet exciton precursor. Our study has important implications regarding the efficiency that can be achieved in polymer light-emitting diodes (PLEDs).
Polaron-pair generation in poly(phenylene vinylenes)
Physical Review B, 1992
The effects of weak magnetic fields on the photoconductivity of poly(p-phenylene vinylene) (PPV) and two derivatives, poly(1, 4-phenylene-1, 2-dimethoxyphenyl vinylene) (DMOP-PPV) and poly(2-phenyl-1,4-phenylene vinylene) (PPPV), were observed within the temperature range 130-350 K. These effects are attributed to the formation of interchain pairs involving a negative polaron and a positive polaron. A polaron pair is formed as a result of interchain electron transfer from a molecular exciton. The lifetime of a pair is estimated to be within the range of 10 '-10 s. Thermal dissociation of a polaron pair produces free charge carriers, and recombination of the pair regenerates a singlet or triplet exciton on a single conjugated segment of a chain.
Dynamical evolution of polaron to bipolaron in conjugated polymers
Physical Review B, 2006
We investigate the insertion of holes in conjugated polymers bearing polarons. We use a Su-Schrieffer-Heeger model modified to include electron-electron interactions via an extended Hubbard Hamiltonian, a Brazovskii-Kirova symmetry breaking term, and an external electric field. We study the dynamics performing numerical calculations within the time-dependent unrestricted Hartree-Fock approximation. We find that adding a hole in a conducting polymer bearing a single positively charged polaron leads to the direct transition of polaron to bipolaron state. The transition produced is a single-polaron to biplaron transition whose excitation spectrum explains the experimental data. The competing mechanism of two polarons merging to form a bipolaron is not observed. We also find that depending on how fast the hole is inserted, a structure that contains a bipolaron coupled to a breather is created. The bipolaron-breather pair can be decoupled under the action of an external electric field. We determine the value of the critical electric field to untrap the bipolaron from the breather.
Spin transport and bipolaron density in organic polymers
Journal of Physics: Condensed Matter, 2009
We present a theory for spin-polarized transport through a generic organic polymer connected to ferromagnetic leads with arbitrary angle θ between their magnetization directions, taking into account the polaron and bipolaron states as effective charge and spin carriers. Within a diffusive description of polaron-bipolaron transport including polaron-bipolaron conversion, we find that the bipolaron density depends on the angle θ. This is remarkable, given the fact that bipolarons are spinless quasiparticles, and opens a new way to probe spin accumulation in organic polymers.