Polaron Generation Research Articles

Exciton and Charge Generation in PC60BM Thin Films

Transient absorption spectroscopy is employed to contrast the photophysics of [6,6]-phenyl C61 butyric acid methyl ester (PC60BM) dispersed in a polystyrene matrix and as a neat film. For the dispersed PC60BM:polystyrene film, singlet excitons are observed that undergo intersystem crossing to triplet excitons. In contrast, in the neat PC60BM film, the transient absorption data indicate significant polaron generation, with photogenerated polarons exhibiting dispersive, bimolecular charge recombination on the nano- to microsecond time scales. These results are discussed in terms of their implications for charge generation from PC60BM light absorption in polymer/fullerene solar cells.

Application of Z-scan technique for the study of nonlinear absorption in chemically reduced LiNbO3 crystals

The nonlinear absorption (NLA) was studied by open-aperture Z-scan experiments in the chemically reduced nominally pure LiNbO3 crystals at cw-illumination with the red (644 nm) and green (514.5 nm) laser beams. The magnitude of the measured NLA is considerably different from the reported Z-scan results obtained in as-grown LiNbO3. The positive sign of NLA obtained with the red light has been related to the generation of the small bound polarons absorbing in red and near-IR ranges. Application of green light results in the light-induced transparency, i.e. the Z-scan traces show negative sign of NLA. Intensity dependence of Z-scan traces allows for conclusion that the photo-induced dissociation of small NbLi4+:NbNb4+ bipolarons and sequent generation of small polarons gives the dominating contribution to the nonlinear optical absorption in reduced crystals with a large bipolarons concentration.

In-Depth Investigation of the Optical Effects in Rationally Designed Phenoxazine-Based Polyazomethines with Activated Quenched Fluorescence

Novel polyazomethines containing a N-hexylphenoxazine electron donor (D) unit and various electron acceptor (A) moieties were developed. The D–A polymers were successfully synthesized by polycondensation of 2,5-bis(p-aminophenyl)-1,3,4-oxadiazole, 4,4′-(9-fluorenylidene)dianiline, or 2,7-diaminofluorene with N-hexylphenoxazine-3,7-dicarbaldehyde, under friendly conditions. The polymer processing conditions promoted different self-assemblies during evaporation that, in turn, definitely exerted a drastic influence on the photophysical behavior of polymers. Electronic absorption, fluorescence, and electrochemistry measurements revealed that the electron-withdrawing subunit directly connected to the phenoxazine core facilitates the charge transfer character and induces a fine-tuning of HOMO–LUMO energy levels. We brought evidence, for the first time, that polaron generation in pristine, undoped films of polyazomethines is achievable. Also, it was found that fluorescence quantum yields could be tailored with o...

Photoinduced giant magnetic polarons in EuTe

Photoinduced magnetic polarons in EuTe, with a magnetic moment of several hundred Bohr magnetons, were investigated as a function of pump intensity and temperature by pump-probe Faraday rotation. The quantum efficiency for optical generation of magnetic polarons is found to be 0.09. The pump-intensity dependence of the photoinduced Faraday rotation shows a sublinear increase, from which we deduce that the population of photoexcited polarons is limited by a maximum value of $4.5\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. This is four orders of magnitude less than the concentration of polarons that would completely fill the crystal, which suggests that the photoexcited polarons are anchored by defects. In addition to the generation of polarons, at high pump densities the modulated pump light also causes a small alternating heating of the illuminated region. The temperature dependence of the polaron magnetic moment is well described by the Curie--Weiss law. Above 100 K, polarons are thermally quenched with an activation energy of 11 meV.

Small-Size Effects on Electron Transfer in P3HT/InP Quantum Dots

The charge carrier photogeneration yield in hybrid polymer/nanocrystal solar cells strongly depends on the interplay between charge transfer across quantum dot (QD) organic capping layers and quantum confinement effects related to the QD size. Here we combine femtosecond transient spectroscopy and density functional theory (DFT) calculations to improve the understanding of charge transfer dynamics at P3HT/InP QD heterointerfaces as a function of core size (2.5 vs 4.5 nm) and length of the surface ligands (oleylamine vs pyridine). We find that, for large core QDs, the polaron generation yield in P3HT is enhanced by efficient exciton dissociation and charge transfer, and is limited by the length of the ligands. Conversely, for smaller size QDs, electron injection from P3HT to InP cores becomes inefficient due to the unfavorable interfacial energetics, even with short pyridine ligands. Thus, we suggest that both QD surface ligand functionalization and core size should be optimized simultaneously for the design of high-performance hybrid nanocrystal/polymer solar cells.

Synthesis of a Terpene-Based New Chiral Inducer and Preparation of an Asymmetric Polymer

A new chiral compound was synthesized based on L-borneol. A cholesteric liquid crystal (LC) electrolyte solution was prepared by adding it as a chiral inducer to a nematic LC. A chiral poly(3,4-ethylenedioxythiophene) (PEDOT*, * = asymmetry) film was prepared by electrochemical polymerization in the induced cholesteric LC. The PEDOT* film showed a maximum absorption band due to a π–π* transition in the UV-vis absorption and bisignate Cotton effect in the reduced state in circular dichroism (CD). The CD spectrum indicates that PEDOT* backbones form right-handed helical aggregation. However, the maximum optical absorption band due to π–π* transition of the PEDOT* decreases and a new absorption band appears at long wavelengths upon oxidation due to generation of polarons as charge carriers. Bisignate Cotton effect disappears and broadly negative CD signal appears at long wavelengths in the oxidized state.

A rhodanine flanked nonfullerene acceptor for solution-processed organic photovoltaics.

A novel small molecule, FBR, bearing 3-ethylrhodanine flanking groups was synthesized as a nonfullerene electron acceptor for solution-processed bulk heterojunction organic photovoltaics (OPV). A straightforward synthesis route was employed, offering the potential for large scale preparation of this material. Inverted OPV devices employing poly(3-hexylthiophene) (P3HT) as the donor polymer and FBR as the acceptor gave power conversion efficiencies (PCE) up to 4.1%. Transient and steady state optical spectroscopies indicated efficient, ultrafast charge generation and efficient photocurrent generation from both donor and acceptor. Ultrafast transient absorption spectroscopy was used to investigate polaron generation efficiency as well as recombination dynamics. It was determined that the P3HT:FBR blend is highly intermixed, leading to increased charge generation relative to comparative devices with P3HT:PC60BM, but also faster recombination due to a nonideal morphology in which, in contrast to P3HT:PC60BM devices, the acceptor does not aggregate enough to create appropriate percolation pathways that prevent fast nongeminate recombination. Despite this nonoptimal morphology the P3HT:FBR devices exhibit better performance than P3HT:PC60BM devices, used as control, demonstrating that this acceptor shows great promise for further optimization.

Formation, Optical Properties and Applications of Edge Gold-Coated Silver Nanoprisms

ABSTRACTA facile, high-yield synthesis of edge gold-coated silver nanoprisms (GSNPs) with a gold nanoframe as thin as 1.7 nm and their comprehensive characterizations by using various spectroscopic and microscopic techniques is introduced. The GSNPs exhibit remarkably higher stability than silver nanoprisms (SNPs) and are therefore explored as effective optical antennae for light-harvesting applications. When embedded into a bulk heterojunctions film of P3HT:PCBM, plasmonic GSNPs with a localized surface plasmon resonance (LSPR) around 500 nm can effectively act as optical antennae to enhance light harvesting in the active layer. As a result, we measured up to 7-fold enhancement in the polaron generation yield through photoinduced absorption spectroscopy. Owing to the high stability and strong field enhancement, the presented GSNPs feature great potential as plasmonic probes for photovoltaic applications and LSPR sensing.

Luminescence properties of magnetic polarons in EuTe: Theoretical description and experiments in magnetic fields up to 28 T

The recent discovery of a polaron-associated zero phonon line in the band-edge photoluminescence of high optical quality EuTe crystals opens up the prospect of answering long-standing questions about the polaron internal structure, thermal stability, and generation efficiency. Here, a Schr\"odinger equation for the polaron was formulated and resolved by using both variational and self-consistent methods. The theory is in good agreement with measurements of the zero phonon line as a function of magnetic field and temperature, and it could be applied to other polaronic systems. It is deduced that, in EuTe, at $0\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, a polaron carries a magnetic moment of $610{\ensuremath{\mu}}_{B}$, and its binding energy is 27 meV. However, this binding energy does not carry the usual meaning of thermal stability, because it decreases drastically when the sample is warmed up. For instance, at $T=100\phantom{\rule{4pt}{0ex}}\mathrm{K}$, the binding energy is already reduced to only 6 meV. The thermal destruction of a polaron is brought about by thermal fluctuations of the spin lattice that suppress the electron's self-energy. Photoluminescence excitation spectra of EuTe demonstrate that the photogeneration of polarons becomes increasingly inefficient when the energy of the pumping photon is increased above the band gap.

Edge-Gold-Coated Silver Nanoprisms: Enhanced Stability and Applications in Organic Photovoltaics and Chemical Sensing

We report a facile synthetic route for edge-gold-coated silver nanoprisms (GSNPs) and their comprehensive optical and structural characterization. The GSNPs exhibit remarkably high stability toward chemical etching and excellent performance as both optical antennae for light-harvesting applications and refractive index sensors. We show that when embedded into a photovoltaic bulk heterojunction film of poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM), plasmonic GSNPs act as optical antennae to substantially enhance light absorption in the active organic solar cell layer. We measure a ≈7-fold enhancement in the polaron generation yield through photoinduced absorption spectroscopy. Owing to the high stability, large sensitivity factors, and strong field enhancement effect, these GSNPs exhibit great potential as optical probes for sensing and photovoltaic applications. We also show that the refractive index sensing figure of merit (FoM) of GSNPs can reach 4.05 RIU–1 and suggest based on...

Ion Transport and Switching Speed in Redox-Gated 3-Terminal Organic Memory Devices

The dynamics of redox gated organic memory devices based on dynamic doping of polythiophene were examined in detail in order to improve “write” and “erase” speed and determine ultimate performance. A 3-terminal geometry similar to a field effect transistor provided a source/gate circuit which reversibly oxidized a polythiophene polymer to cause a large increase in conductance between the source and drain electrodes. The devices were cycled for >1000 complete R/W/R/E cycles, and operated at relatively low voltage compared to commercial “flash” memory. The “write” and “erase” speeds were improved by a factor of >100 by using a spin-coated electrolyte layer and by small increases in device temperature. The influence of charging current and polaron propagation on response time were determined to be minor, with the rate limiting process being identified as the rate of conducting polaron generation. The main factor determining the W/E time was the mobility of ions in the polyethylene oxide electrolyte layer, which resulted in resistance losses during the application of the S-G “write” pulse. Response time was strongly dependent on the atmosphere, with water or acetonitrile vapor significantly increasing the rate of polaron generation. The results are important for design of molecular memory devices based on dynamic doping, and indicate likely avenues for further performance improvements.

Towards optimisation of photocurrent from fullerene excitons in organic solar cells

Photocurrent from fullerene excitons in polymer–fullerene solar cells is optimised by employing a series of low bandgap diketopyrrolopyrrole-containing polymers with differing molecular weights. The low LUMO level of this donor polymer prevents efficient charge generation from polymer excitons, allowing us to focus on charge and photocurrent generation specifically from fullerene excitons. We employ femtosecond transient absorption spectroscopy and transmission electron microscopy to show a correlation between fullerene domain size and the kinetics of polaron generation from fullerene excitons, and relate these observations to device efficiency. Charge generation from fullerene excitons is shown to occur on the 0.5 ns timescale, limited by the kinetics of diffusion of fullerene excitons to domain interfaces with donor polymers. For devices employing the highest molecular weight donor polymer, ∼7 mA cm−2 of photocurrent is observed from fullerene excitons, leading to an overall device efficiency of 5.2%.

Exciton and Hole-Transfer Dynamics in Polymer: Fullerene Blends

Ultrafast hole transfer dynamics from fullerene derivative to polymer in bulk heterojunction blends are studied with visible-pump - IR-probe spectroscopy. The hole transfer process is found to occur in 50/300 fs next to the interface, while a longer 15-ps time is attributed to exciton diffusion towards interface in PC71 BM domains. High polaron generation efficiency in P3HT blends indicates excellent intercalation between the polymer and the fullerene even at highest PC71 BM concentration thereby yielding a valuable information on the blend morphology.

Z-scan study of nonlinear absorption in reduced LiNbO3 crystals

The nonlinear absorption (NLA) was studied by open-aperture Z-scan experiments in the chemically reduced nominally pure LiNbO3 crystals at cw-illumination with the red (644 nm) and green (514.5 nm) laser beams. The magnitude of the measured NLA is considerably different from the reported Z-scan results obtained in as-grown LiNbO3. The positive sign of NLA obtained with the red light has been related to the generation of the small bound polarons absorbing in red and near-IR ranges. Application of green light results in the light-induced transparency, i.e., the Z-scan traces show negative sign of NLA. Intensity dependence of Z-scan traces allows for conclusion that the photo-induced dissociation of small NbLi4+:NbNb4+ bipolarons and sequent generation of small polarons gives the dominating contribution to the nonlinear optical absorption in reduced crystals with a large bipolarons concentration.

The Effect of Ageing on Exciton Dynamics, Charge Separation, and Recombination in P3HT/PCBM Photovoltaic Blends

AbstractA study of how light‐induced degradation influences the fundamental photophysical processes in the active layer of poly(3‐hexylthiophene)/[6,6]‐phenyl C61‐butyric acid methyl ester (P3HT/PCBM) solar cells is presented. Non‐encapsulated samples are systematically aged by exposure to AM 1.5 illumination in the presence of dry air for different periods of time. The extent of degradation is quantified by the relative loss in the absorption maximum of the P3HT, which is varied in the range 0% to 20%. For degraded samples an increasing loss in the number of excitons within the P3HT domains is observed with longer ageing periods. This loss occurs rapidly, within the first 15 ps after photoexcitation. A more pronounced decrease in the population of polarons than excitons is observed, which also occurs on a timescale of a few picoseconds. These observations, complemented by a quantitative analysis of the polaron and exciton population dynamics, unravel two primary loss mechanisms for the performances of aged P3HT/PCBM solar cells. One is an initial ultrafast decrease in the polaron generation, apparently not related to the exciton diffusion to the polymer/fullerene interface; the second, less significant, is a loss in the exciton population within the photoexcited P3HT domains. The steady‐state photoinduced absorption spectra of degraded samples exhibits the appearance of a signal ascribed to triplet excitons, which is absent for non‐degraded samples. This latter observation is interpreted considering the formation of degraded sites where intersystem crossing and triplet exciton formation is more effective. The photovoltaic characteristics of same blends are also studied and discussed by comparing the decrease in the overall power conversion efficiency of solar cells.

Light-Harvesting Mechanism in Polymer/Fullerene/Dye Ternary Blends Studied by Transient Absorption Spectroscopy

The light-harvesting mechanism in polymer:fullerene:dye ternary blends was comprehensively studied by transient absorption spectroscopy. The ternary blend films consist of poly(3-hexylthiophene) (P3HT), a fullerene derivative (PCBM), and a silicon phthalocyanine derivative (SiPc) as a light-harvesting dye. Upon dye excitation of P3HT:PCBM:SiPc blends, P3HT polarons and SiPc anions were generated from SiPc excitons, and then electrons were shifted from SiPc to PCBM, suggesting that SiPc molecules are selectively located in a disordered P3HT matrix mixed with PCBM molecules at the P3HT/PCBM interface. Upon polymer excitation, P3HT excitons decayed more rapidly in ternary blends than in binary blends with an identically rapid formation of SiPc photobleaching, suggesting efficient energy transfer from P3HT to SiPc because of the large spectral overlap between P3HT emission and SiPc absorption, followed by slightly delayed generation of P3HT polarons and SiPc anions from SiPc excitons. Subsequently, the photobleaching disappeared while the P3HT polaron band remains unchanged, suggesting the charge shift from SiPc anion to PCBM. All these photovoltaic conversion processes are much more rapid than backward reactions, and therefore highly efficient light harvesting is achieved in ternary blend solar cells. This is because the SiPc molecules are selectively localized at the P3HT/PCBM interface with an interfacial coverage of 40%. Remarkably, the SiPc molecules are closely contact to both P3HT and PCBM at the interface in solvent-annealed ternary blends, which is an ideal interfacial structure for the efficient photovoltaic conversion.

Influence of Phase Segregation on Recombination Dynamics in Organic Bulk‐Heterojunction Solar Cells

AbstractThe recombination dynamics of charge carriers in organic bulk‐heterojunction (BHJ) solar cells made of the blend system poly(2,5‐bis(3‐dodecylthiophen‐2‐yl)thieno[2,3‐b]thiophene) (pBTCT‐C12):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM) with a donor–acceptor ratio of 1:1 and 1:4 are studied here. The techniques of charge‐carrier extraction by linearly increasing voltage (photo‐CELIV) and, as local probe, time‐resolved microwave conductivity are used. A difference of one order of magnitude is observed between the two blends in the initially extracted charge‐carrier concentration in the photo‐CELIV experiment, which can be assigned to an enhanced geminate recombination that arises through a fine interpenetrating network with isolated phase regions in the 1:1 pBTCT‐C12:PC61BM BHJ solar cells. In contrast, extensive phase segregation in 1:4 blend devices leads to an efficient polaron generation that results in an increased short‐circuit current density of the solar cells. For both studied ratios a bimolecular recombination of polarons is found using the complementary experiments. The charge‐carrier decay order of above two for temperatures below 300 K can be explained on the basis of a release of trapped charges. This mechanism leads to delayed bimolecular recombination processes. The experimental findings can be generalized to all polymer:fullerene blend systems allowing for phase segregation.

Continuous-wave photoinduced absorption studies in polythiophene and fullerene blended thin films

The generation and recombination dynamics of photocarriers (polarons) in poly(3-hexylthiophene) and $[6,6]$-phenyl-${\mathrm{C}}_{61}$-butyric acid methyl ester blended thin films have been investigated by means of continuous-wave photoinduced absorption spectroscopy. Continuous-wave photoinduced absorption spectroscopy has the advantage of being used to investigate long-lived excited species with lifetimes that range from microseconds to milliseconds. In this time range, three kinds of excited species are recognized in the blends: trapped, mobile, and delocalized polarons. From the modulation-frequency dependence, the mean lifetimes of the three types of polarons at low temperature were estimated. It is known that polarons are not generated directly by the pump beam but rather through Coulombically bound radical pairs. We have found that this two-step model for polaron generation can be recognized on a microsecond time scale by continuous-wave photoinduced absorption measurements.

Underneath the fascinations of carbon nanotubes and graphene nanoribbons

As a new class of materials, carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) have been continuing fascinating the community with properties that can be seen from neither bulk graphite nor diamond. Although the physics and chemistry of these carbon allotropes have been intensively investigated from various perspectives, the laws governing the fascinations and their interdependence remain as yet undetermined. From the perspectives of bond and nonbond formation, dissociation, relaxation, vibration, and the associated energetics and dynamics of charge repopulation, polarization, densification, and localization, this article aims to show that the broken-bond-induced local bond contraction and bond strength gain, quantum entrapment and densification of charge and energy, polarization of the unpaired edge dangling σ-bond electrons, and the formation of the pseudo-π-bond between the dangling σ-bond electrons along the edges are responsible for the anomalies. Theoretical reproduction of the experimentally observed elastic modulus enhancement, melting point depression, layer-number, strain, pressure and temperature induced Raman shift, C 1s core-level positive shift, work function reduction, band gap expansion, edge and defect selective generation of the Dirac-Fermi polarons and the associated magnetism consistently confirmed that the shorter and stronger bonds between undercoordinated carbon atoms modulate locally the atomic cohesive energy, the Hamiltonian, and hence the detectable bulk properties. The polarization of the unpaired dangling σ-bond electrons by the densely, deeply, and locally entrapped core and bonding electrons generates the massless, magnetic and mobile Dirac-Fermi polarons at sites surrounding vacancies and zigzag-GNR edges. The pseudo-π-bond formation between the nearest dangling σ-bond electrons along the armchair-GNR and the reconstructed zigzag-GNR edges discriminates them from the zigzag-GNR edges or vacancies in the electronic and magnetic properties. Consistency between predictions and observations confirmed that the C–C bond contracts by up to 30% with a 152% bond strength gain at the edges with respect to those in the bulk diamond.

Observation of bi-polarons in blends of conjugated copolymers and fullerene derivatives

From a fundamental and application point of view it is of importance to understand how charge carrier generation and transport in a conjugated polymer (CP):fullerene blend are affected by the blend morphology. In this work light-induced electron spin resonance (LESR) spectra and transient ESR response signals are recorded on non-annealed and annealed blend layers consisting of alkyl substituted thieno[3,2-b]thiophene copolymers (pATBT) and the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) at temperatures ranging from 10 to 180 K. Annealing of the blend sample leads to a reduction of the steady state concentration of light-induced PCBM anions within the blend at low temperatures (T = 10 K) and continuous illumination. This is explained on the basis of the reducing interfacial area of the blend composite on annealing, and the high activation energy for electron diffusion in PCBM blends leading to trapped electrons near the interface with the CP. As a consequence, these trapped electrons block consecutive electron transfer from an exciton on a CP to the PCBM domain, resulting in a relatively low concentration charge carriers in the annealed blend. Analysis of the transient ESR data allows us to conclude that in annealed samples diamagnetic bi-polaronic states on the CPs are generated at low temperature. The formation of these states is related to the generation and interaction of multiple positive polarons in the large crystalline polymer domains present in the annealed sample.