Long-lived Polarons Research Articles

Experimental evidence for ultrashort-lived spin polarons in EuSe

We investigated the time evolution of the density of giant spin polarons in a magnetic semiconductor. Spin polarons (SPs) were photoexcited and observed using time-resolved Faraday rotation. We find the existence of two types of SPs, a short-lived spin polaron with a lifetime of around τsl = 0.5 ns and a long-lived spin polaron with a lifetime of τll = 0.45 ± 0.03 µs, at T = 5 K. The stark difference of three order of magnitude between these lifetimes suggests that in the long lived SP the electron-hole pair is relaxed and its recombination is forbidden. The short-lived SP is probably associated with SP recombination before such relaxation has occured. An extraordinary finding is that the magnitude of τsl, as well as its decrease with increasing temperature, reproduces exactly the characteristic time for SP growth. This suggests that the thermal fluctuations, responsible for SP magnetic moment growth, are also responsible for increasing the recombination probability of SPs.

A novel concept and design for highly efficient photoelectrocatalytic materials with high performance, stability, and charge transport properties: development of an innovative next-generation green technology.

In semiconductors, generating charges via catalysis is a highly challenging task and characteristic of heterojunction photoanodes. A dithiophene-4,8-dione spin-coated film layer has a positive effect on the holes (positive charge carriers) for a long time in BHJ films in the solid state of materials. The photoexcited holes created in the BHJ film can persist for long periods of time, which is beneficial for catalytic reactions. In this study, a photoanode is electrically coupled to a hydrogen gas-evolving platinum cathode. When the photoanode is electrically coupled to a H2 gas evolving Pt cathode, curiously long-lived hole polaron states are observed on the timescale of seconds under operational conditions. These long-lived holes play a crucial role in enhancing the hydrogen peroxide oxidation performance of the film overlayer spin-coated onto the photoanode. The spin-coated film overlayer on the photoanode achieves the best oxidation performance for hydrogen peroxide of approximately 6.5 mA cm-2 at 1.23 VRHE without the need of a catalyst. This demonstrates the effectiveness of the overlayer in improving the catalytic performance of the photoanode with a better efficiency of 17.5% when using 851 nm excitation. This indicates that a relatively high percentage of incident photons at that specific wavelength is converted into photocurrent by the photoanode. This approach can lead to more efficient oxidation catalysis as demonstrated in the case of hydrogen peroxide oxidation.

Synergistic Contribution of Oligo(ethylene glycol) and Fluorine Substitution of Conjugated Polymer Photocatalysts toward Solar Driven Sacrificial Hydrogen Evolution.

To separately explore the importance of hydrophilicity and backbone planarity of polymer photocatalyst, a series of benzothiadiazole-based donor-acceptor alternating copolymers incorporating alkoxy, linear oligo(ethylene glycol) (OEG) side chain, and backbone fluorine substituents is presented. The OEG side chains in the polymer backbone increase the surface energy of the polymer nanoparticles, thereby improving the interaction with water and facilitating electron transfer to water. Moreover, the OEG-attached copolymers exhibit enhanced intermolecular packing compared to polymers with alkoxy side chains, which is possibly attributed to the self-assembly properties of the side chains. Fluorine substituents on the polymer backbone produce highly ordered lamellar stacks with distinct π-π stacking features; subsequently, the long-lived polarons toward hydrogen evolution are observed by transient absorption spectroscopy. In addition, a new nanoparticle synthesis strategy using a methanol/water mixed solvent is first adopted, thereby avoiding the screening effect of surfactants between the nanoparticles and water. Finally, hydrogen evolution rate of 26000 µmolg-1 h-1 is obtained for the copolymer incorporated with both OEG side chains and fluorine substituents under visible-light irradiation (λ> 420nm). This study demonstrates how the glycol side chain strategy can be further optimized for polymer photocatalysts by controlling the backbone planarity.

Quantum-Zeno Fermi polaron in the strong dissipation limit

The interplay between measurement and quantum correlations in many-body systems can lead to novel types of collective phenomena which are not accessible in isolated systems. In this work, we merge the Zeno-paradigm of quantum measurement theory with the concept of polarons in condensed-matter physics. The resulting quantum-Zeno Fermi-polaron is a quasi-particle which emerges for lossy impurities interacting with a quantum-degenerate bath of fermions. For loss rates of the order of the impurity-fermion binding energy the quasi-particle is short lived. However, we show that in the strongly dissipative regime of large loss rates a long-lived polaron branch re-emerges. This quantum-Zeno Fermi-polaron originates from the nontrivial interplay between the Fermi-surface and the surface of the momentum region forbidden by the quantum Zeno projection. The situation we consider here is realized naturally for polaritonic impurities in charge-tuneable semiconductors and can be also implemented using dressed atomic states in ultracold gases.

Revealing ultrafast relaxation dynamics in six-thiophene thin film and single crystal

The excited state dynamics of six-thiophene (6T) thin film and single crystal was studied by time-resolved fluorescence and femtosecond transient absorption techniques under different excitation conditions. The dominant process in 6T system is the generation of ion pairs, dissociated to polarons staying for up to 130−3700 ns in film and 1 ms in crystal. Singlet fission (SF) was directly observed from the upper vibrational levels of the first excited singlet state S1 in 6T thin film/single crystal within 30 fs, which competes with intramolecular vibrational relaxation. Triplet state lifetime is dramatically shortened from 6T single crystal (1 ms) to thin film (4 ns) due to triplet-triplet annihilation, which is influenced by structural defects in amorphous regions of 6T film. Compared with 6T crystal, the partially ordered semicrystalline morphology of 6T film suggests the impact of well-structured crystallinity and molecular packing on the photocarriers transport dynamics and lifetime of triplet state. Moreover, excitation to upper excited singlet state (4.96 eV) leads to a higher yield of polarons (factor of 4), while SF in film/crystal is fully suppressed. Existence of long-lived photogenerated polarons may count π-conjugated oligomers as promising materials for developing organic-molecule-based optoelectroinc devices.

Analyzing a Bose polaron across resonant interactions

Recently, two independent experiments reported the observation of long-lived polarons in a Bose-Einstein condensate, providing an excellent setting to study the generic scenario of a mobile impurity interacting with a quantum reservoir. Here, we expand the experimental analysis by disentangling the effects of trap inhomogeneities and the many-body continuum in one of these experiments. This makes it possible to extract the energy of the polaron at a well-defined density as a function of the interaction strength. Comparisons with quantum Monte-Carlo as well as diagrammatic calculations show good agreement, and provide a more detailed picture of the polaron properties at stronger interactions than previously possible. Moreover, we develop a semi-classical theory for the motional dynamics and three-body loss of the polarons, which partly explains a previously unresolved discrepancy between theory and experimental observations for repulsive interactions. Finally, we utilize quantum Monte-Carlo calculations to demonstrate that the findings reported in the two experiments are consistent with each other.

Type-II Dirac semimetal stabilized by electron-phonon coupling

There is major interest, in condensed matter physics, in understanding the role of topology: remarkable progress has been made in classifying topological properties of non-interacting electrons, and on understanding the interplay between topology and electron–electron interactions. We extend such studies to interactions with the lattice, and predict non-trivial topological effects in infinitely long-lived polaron bands. Specifically, for a two-dimensional many-band model with realistic electron–phonon coupling, we verify that sharp level crossings are possible for polaron eigenstates, and prove that they are responsible for a novel type of sharp transition in the ground state of the polaron that can occur at a fixed momentum. Furthermore, they result in the appearance of Dirac cones stabilized by electron–phonon coupling. Thus, electron–phonon coupling opens an avenue to create and control Dirac and Weyl semimetals.

Long Lived Photoexcitation Dynamics in π-Conjugated Polymer/PbS Quantum Dot Blended Films for Photovoltaic Application.

We used continuous wave photoinduced absorption (PIA) spectroscopy to investigate long-lived polarons in a blend of PbS quantum dot and regio-regular poly (3-hexylthiophene) (RR-P3HT). The charge transfer from RR-P3HT to PbS as well as from PbS to RR-P3HT were observed after changing the capping ligand of PbS from a long chain molecular to a short one. Therefore, PbS could be used to extend the working spectral range in hybrid solar cells with a proper capping ligand. However, we found that the recombination mechanism in the millisecond time region is dominated by the trap/defects in blended films, while it improves to a bimolecular recombination partially after ligand exchange. Our results suggest that passivating traps of nanocrystals by improving surface ligands will be crucial for relevant solar cell applications.

Long-Lived Photoinduced Polarons in Organohalide Perovskites.

The long diffusion length of charge carriers in the CH3NH3PbI3 perovskite is one of the most relevant properties for explaining the high photovoltaic efficiency of perovskite solar cells. As a possible mechanism for the large diffusion length of electrons and holes, several authors suggested a reduced coulomb attraction of the carriers due to the formation of polarons. Here we performed continuous wave far-infrared photoinduced absorption (PIA) experiments on CH3NH3PbI3; spectral changes are associated with local deformation of the lattice around the photogenerated long-lived charges, a typical signature of photoinduced polarons. Ab initio calculations show confinement of charge carriers at the interface between structural domains characterized by a different tilting of the PbI6 octahedra. The differential IR spectrum between unperturbed and perturbed simulated structures shows a close pattern to the experimental PIA. Positive and negative charges are confined in different varieties of the perovskites coherent with the low recombination and long diffusion length of photogenerated carriers.

Excited-states spectroscopies and its magnetic field effect of π-conjugated polymer-fullerene blends with below-gap excitation

We used continuous wave photoinduced absorption (PIA) spectroscopy to investigate long-lived polarons in blend of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly (3-hexylthiophene) (RR-P3HT) with below-gap excitation. Compared to above-gap excitation, the polarons exhibited thermally activated and multi-trap limited bimolecular recombination too, but with narrower distribution profile in polaron states in millisecond time regime. However, magnetophoto-induced absorption (MPA) of polarons presented much weaker magnetic field effect in below-gap excitation (∼0.5mT) than in above-gap excitation (order of ∼10mT), although the underline mechanism is same as spin mixing of polaron pairs by the hyperfine interaction at small field region.

POLARON DYNAMICS. Long-lived photoinduced polaron formation in conjugated polyelectrolyte-fullerene assemblies.

The efficiency of biological photosynthesis results from the exquisite organization of photoactive elements that promote rapid movement of charge carriers out of a critical recombination range. If synthetic organic photovoltaic materials could mimic this assembly, charge separation and collection could be markedly enhanced. We show that micelle-forming cationic semiconducting polymers can coassemble in water with cationic fullerene derivatives to create photoinduced electron-transfer cascades that lead to exceptionally long-lived polarons. The stability of the polarons depends on the organization of the polymer-fullerene assembly. Properly designed assemblies can produce separated polaronic charges that are stable for days or weeks in aqueous solution.

Photoinduction of long-lived polarons

Polaron Dynamics Photosynthetic complexes and organic photovoltaics can rapidly create separated charges upon photoexcitation. However, unproductive charge recombination often occurs in the human-made system. This is in part because the charge acceptor and donor structures are much larger. Huber et al. created aqueous micelles that pair conjugated polyelectrolyte charge donors with fullerene acceptors at a much smaller interface. They observed the photoinduced formation of polarons—stable pairs of separated charges—with lifetimes of several days. Science , this issue p. [1340][1] [1]: /lookup/doi/10.1126/science.aaa6850

Plasmonic Ag@oxide nanoprisms for enhanced performance of organic solar cells.

Localized surface plasmon resonance (LSPR), light scattering, and lowering the series resistance of noble metal nanoparticles (NPs) provide positive effect on the performance of photovoltaic device. However, the exciton recombination on the noble metal NPs accompanying above influences will deteriorate the performance of device. In this report, surface-modified Ag@oxide (TiO2 or SiO2 ) nanoprisms with 1-2 nm shell thickness are developed. The thin film composed of P3HT/Ag@oxides and P3HT:PCBM/Ag@oxides is investigated by absorption, photoluminescence (PL), and transient absorption spectroscopy. The results show a significant absorption, PL enhancement, and long-lived photogenerated polaron in the P3HT/Ag@TiO2 film, indicating the increase of photogenerated exciton population by LSPR of Ag nanoprisms. In the case of P3HT/Ag nanoprisms, partial PL quench and relatively short-lived photogenerated polaron are observed. That indicates that the oxides layer can effectively avoid the exciton recombination. When the Ag@oxide nanoprisms are introduced into the active layer of P3HT:PCBM photovoltaic devices, about 31% of power conversion efficiency enhancement is obtained relative to the reference cell. All these results indicate that Ag@oxides can enhance the performance of the cell, at the same time the ultrathin oxide shell prevents from the exciton recombination.

Time resolved excitation dynamics in emeraldine base

Using femtosecond pump–probe transient absorption spectroscopy (wavelengths 330–800nm), we observed and explained excited state, charge transfer and polaron state dynamics in emeraldine base form of polyaniline in dimethyl sulfoxide solution. The excited state created by a pump pulse (700nm) in the quinoid absorption Q-band loses its initial symmetry by subsequent energy transition to a charge transfer state within 50fs. The hot charge transfer state either recombines non-radiatively into the ground state with decay time constant 0.55ps or transfers into a relaxed state corresponding to the relaxed phenyl geometry during ca 2.1ps. This relaxed state shows a prolonged lifetime of about 6.5ps before its recombination to the ground state. However, a small amount of long-lived polarons with lifetime of about 2ns in air and with lifetime longer than 6ns in the solution bubbled with dry N2 was detected.

High Intrachain Order Promotes Triplet Formation from Recombination of Long-Lived Polarons in Poly(3-hexylthiophene) J-Aggregate Nanofibers

Photoluminescence (PL) of single poly(3-hexylthiophene) (P3HT) J-aggregate nanofibers (NFs) exhibits strong quenching under intensity-modulated pulsed excitation. Initial PL intensities (I(0)) decay to steady-state levels (ISS) typically within ∼ 1-10 μs, and large quenching depths (I(0)/I(SS) >2) are observed for ∼ 70% of these NFs. Similar studies of polymorphic, H-aggregate type P3HT NFs show much smaller PL quenching depths (I(0)/I(SS) ≤ 1.2). P3HT chains in J-type NF π-stacks possess high intrachain order, which has been shown previously to promote the formation of long-lived, delocalized polarons. We propose that these species recombine nongeminately to triplets on time scales of >1 ns. The identity of triplets as the dominant PL quenchers was confirmed by subjecting NFs to oxygen, resulting in an instantaneous loss of triplet PL quenching (I(0)/I(SS) ∼ 1). The lower intrachain order in H-type NFs, similar to P3HT thin-film aggregates, localizes excitons and polarons, leading to efficient geminate recombination that suppresses triplet formation at longer time scales. Our results demonstrate the promise of self-assembly strategies to control intrachain ordering within multichromophoric polymeric aggregate assemblies to tune exciton coupling and interconversion processes between different spin states.

The influence of molecular interface modification on the charge dynamics of polymeric semiconductor:ZnO heterostructure

We demonstrate an enhancement of photocurrent in hybrid photovoltaic cells based on nanoparticles of zinc oxide (ZnO) and poly(3-hexylthiophene) (P3HT), through molecular interface modification with and without cis-bis(4,4′-dicarboxy-2,2′bipyridine) ruthenium (II) (N3-dye) and α-Sexithiophen-2-yl-phosphonic Acid (6TP) as interface modifiers. We identify the formation of long-lived polarons at P3HT:ZnO interface by means of quasi-steady-state photoinduced absorption (PIA) spectroscopy. Furthermore, by probing the pump-modulation-frequency-dependent PIA signal, we find that P3HT:ZnO-N3 and P3HT:ZnO-6TP exhibit more sharply decaying density with increasing modulation frequency, which is indicative of a longer average lifetime, approximating 1 ms as opposed to ∼0.2 ms without ZnO surface modification. This highlights the importance of the molecular interface modification in the steady-state polaron dynamics in hybrid solar cells.

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

AbstractReview: 176 refs.

Materials Design Considerations for Charge Generation in Organic Solar Cells

This article reviews some of our recent progress on materials design guidelines for photoinduced charge generation in bulk-heterojunction organic solar cells. Over the last 7 years, our group has employed transient absorption measurement to determine the relative quantum yields of long-lived polaron pairs for over 300 different organic Donor/Acceptor blend films. We have shown that this optical assay of charge separation can be a strong indicator of photocurrent generation efficiency in complete devices. In this review, we consider the lessons that can be drawn from these studies concerning the parameters that determine efficiency of this photoinduced charge separation in such solar cells. We consistently find, from studies of several materials series, that the energy offset driving charge separation is a key determinant of the efficiency of this charge generation, and thereby photocurrent generation. Moreover, we find that the magnitude of the energy offset required to drive charge separation, and the st...

Hole Transfer from Low Band Gap Quantum Dots to Conjugated Polymers in Organic/Inorganic Hybrid Photovoltaics.

We use photoinduced absorption (PIA) spectroscopy to investigate pathways for photocurrent generation in hybrid organic/inorganic quantum dot bulk heterojunction solar cells. We study blends of the conjugated polymer poly(2,3-bis(2-(hexyldecyl)quinoxaline-5,8-diyl-alt-N-(2-hexyldecyl)dithieno[3,2-b:2',3'-d]pyrrole) (PDTPQx-HD) with PbS quantum dots and find that positively charged polarons are formed on the conjugated polymer following selective photoexcitation of the PbS quantum dots. This result provides a direct spectroscopic fingerprint demonstrating that photoinduced hole transfer occurs from the photoexcited quantum dots to the host polymer. We compute the relative yields of long-lived holes following photoexcitation of both the polymer and quantum dot phases and estimate that more long-lived polarons are produced per photon absorbed by the polymer phase than by the quantum dot phase.

Photogeneration and Recombination in P3HT/PCBM Solar Cells Probed by Time-Delayed Collection Field Experiments

Time-delayed collection field (TDCF) experiments are performed on bulk heterojunctionsolarcellscomprisedofablendofpoly(3-hexylthiophene)and(6,6)-phenylC71 butyric acid methyl ester. TDCF is analogous to a pump-probe experiment using optical excitation and an electrical probe with a resolution of <100 ns. The number of free charge carriers extracted after a short delay is found to be independent of the electric field during illumination. Also, experiments performed with a variable delay between the optical excitation andtheelectricalprobedonotrevealanyevidenceforthegenerationofchargevia field-assisted dissociation of bound long-lived polaron pairs. Photocurrent transients are well fitted by computational drift diffusion simulations with only direct generation of free charge carriers. Withincreasingdelaytimesbetweenpumpandprobe,twolossmechanismsareidentified; first, charge-carriers areswept outofthe deviceby theinternalelectric field,and second,bimolecular recombination of the remaining carriers takes place with a reduced recombination coefficient.