Light-induced Electron Spin Resonance Signal Research Articles

Investigation of Charge Accumulation States in Polymer Solar Cells using Light-Induced Electron Spin Resonance Spectroscopy

Charge accumulation states in organic solar cells were studied in detail by a microscopic characterization with light-induced electron spin resonance (ESR) spectroscopy under device operation using a typical polymer solar cell of indium-tin oxide (ITO)/poly (3,4-ethylenedioxythiophene):poly (4-styrenesulfonate) (PEDOT:PSS)/regioregular poly(3-hexylthiophene) (P3HT):fullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM)/LiF/Al. Two light-induced ESR signals with a narrow and broad linewidth were observed where the broad component with a slow accumulation rate clearly correlated with the deterioration of the device performance. From the ESR analysis, the charge accumulation state causing the device deterioration is ascribed to holes at P3HT polymer-chain ends with residual bromines in amorphous regions in the active layer. Preventing the charge accumulation seems indispensable to develop highly durable polymer solar cells with high performance.

Direct observation of UV-induced accumulated charges in inverted-type polymer solar cells with TiOx layer by electron spin resonance

ABSTRACTWe present an electron spin resonance (ESR) study of inverted-type polymer solar cells with the structure of indium tin oxide (ITO)/amorphous titanium oxide (TiOx)/regioregular poly(3-hexylthiophene) (P3HT):phenyl-C61-butryic acid methyl ester (PCBM)/Au. Two light-induced ESR signals with narrow and broad linewidth were observed by light irradiation. We found a strong correlation between typical light-soaking effects and the ESR signal with the broad linewidth under UV light irradiation. From the fact that the light-soaking effect requires UV irradiation, the ESR signal with the broad linewidth is found to be related with the light-soaking effect.

Effects of Aging and Annealing on the Density of Trap States in Organic Photovoltaic Materials

The concentration of charge carriers in bulk heterojunction solar cells is limited by recombination processes whose rates may be affected by trapping in deep localized states within the band gap. Trapping reduces carriers mobility and causes dispersive charge transport with negative impact on device performance. Aging and thermal annealing may cause variations of the trap state distribution in the photoactive materials. Variable-temperature light-induced electron spin resonance (LESR) measurements are one method for revealing the distribution of deep trap states (DOS). The LESR signal intensity is in fact proportional to the concentration of trapped charges that survive recombination long enough for ESR detection at the chosen experimental temperature. We used LESR to study the effect of aging and thermal annealing on drop-casted blend films composed of regioregular poly(3-dodecylthiophene-2,5-diyl) (rrP3DDT) as charge donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor materials. The trapped carriers concentration reduced slightly after aging and strongly after annealing; lower recombination rates after aging tended to be restored after annealing. We ascribe the variations in charge concentration to two effects: (1) a decrease in charge generation efficiency due to blend demixing, causing less exciton splitting and charge separation; and (2) the presence of oxygen contaminant acting as additional trap states. The experimental DOS can be modeled by Gaussian curves whose parameters (average and standard deviation) are influenced by aging and annealing. Aging shifts the average binding energy to higher values and increases Gaussian width (i.e., energetic disorder). Annealing tends to restore the average energy to the original value, while curve width is invariant or increases. We conclude that the effects of aging and annealing on the DOS are due to the presence of reversible oxygen contamination and to the changes in the blend film microscopic morphology. Both are favorably modified by thermal annealing, although some (apparently irreversible) features of the energetic disorder deserve further investigation.

Time-Resolved Photoluminescence and Light-Induced Electron Spin Resonance Studies of Photo-Induced Charge Transfer in New Polyazomethines:Fullerene for Organic Solar Cells

The organic solar cells (OSCs) are considered as promising, cost-effective way to harness solar energy. They are built of two organic materials, mostly: conjugated polymers and fullerene derivatives, e.g. [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), which are electron donor and electron acceptor, respectively. Upon illumination, electron-hole pairs called excitons are generated on a donor site. Through dissociation at the donor/acceptor interface, excitons separate to free charges: electrons pass into an acceptor site, holes stay on the donor site. This phenomenon is called a photo-induced charge transfer (CT) and is a crucial step in the OSCs operations.Techniques, which directly detect this phenomenon are: Time-Resolved Photoluminescence (TRPL) and Light-Induced Electron Spin Resonance (LESR) Spectroscopies. The idea of the TRPL experiment is to excite the sample by pulse laser and observe decay of its luminescence in time. In the case of effective photo-induced CT, the luminescence quenching is observed. The ESR technique is based on a magnetic resonance absorption, which only occurs for samples having unpaired electrons. As was mentioned above, the photo-induced CT generates free charges – holes and electrons, which appear in a LESR spectrum as two lines from positive polaron (P+) on a donor site and negative polaron (F-) on an acceptor site (1, 2).This article presents studies of the photo-induced CT in mixtures made of PCBM and two new polyazomethines: 25Th-cardo and 2252Th-DMB. Polymer 25Th-cardo was obtained from 2,5-thiophenedicarboxaldehyde and 4-[9-(4-aminophenyl)-9H-fluoren-9-yl]aniline, while 2252Th-DMB from 2,2':5',2''-terthiophene-5,5''-dicarboxaldehyde and 3,3′-dimethoxybenzidine (3). P3HT:PCBM mixture was used as reference.All measurements were done for pure components and polymer:PCBM mixtures. The TRPL was measured using a spectrometer (0.05 nm resolution) combined with a Hamamatsu Streak Camera (2.5 ps resolution) for samples excitated by titanium-sapphire laser pulses. The ESR was carried out using a Bruker ELEXSYS E580 X-band spectrometer (9.5 GHz/ 0.33T) equipped with a continuous-flow cryostat, allowing decrease of temperature down to 2.5 K. For all the samples dark, light on (LESR) and light off spectra were recorded. An illumination of the samples was performed directly in the microwave cavity of the ESR spectrometer by DH-2000 Deuterium-Tungsten Halogen Light Sources.For pure P3HT we observed a strong luminescence having maximum around 1.8 eV. This luminescence was quenched after adding a strong electron acceptor - PCBM. This behaviour is assigned to the effective photo-induced CT. Both polyazomethines has a luminescence with max. around 2 eV. For their mixtures with PCBM, the luminescence quenching is observed, but it is not so significant as in the case of P3HT:PCBM system. Moreover for 25Th-cardo:PCBM composite an additional luminescence appear in the range from 1.3 to 1.8 eV, which turned out being a PCBM luminescence.No dark ESR signal from all polymers:PCBM composite was detected. However, as the P3HT:PCBM and 2252Th-DMB:PCBM mixtures were illuminated, two overlaping lines appeared from P+ and F- polarons - confirming effective photo-induced CT. In the case of 25Th-cardo:PCBM mixture no LESR signal was recorded.We can conclude that photo-induced CT occurs in both polyazomethines:PCBM mixtures, however for 2252Th-DMB:PCBM it is much more effective than for 25Th-cardo:PCBM.This work shows that the TRPL and the LESR can be used as universal methods to detect the photo-induced CT in the donor:acceptor mixtures, thereby, can be helpfull tools for preliminary investigation of new materials for OSCs.This work was supported by the Foundation for Polish Science International PhD Projects Programme co-financed by the EU European Regional Development Fund and the NCBiR project PBS1/A5/27/2012.REFERENCES1. V. I. Krinichnyi, E. I. Yudanova and N. G. Spitsina., J. Phys. Chem. C, 144, 16756-16766 (2010).2. C. Carati, L. Bonoldi and R.Po, Phys. Rev. B, 84, 245205 (2011).3. A. Iwan, E. Schab-Balcerzak, K. P. Korona, S. Grankowska, M. Kaminska, Synthetic Met. (2013). http://dx.doi.org/10.1016/j.synthmet.2013.10.008

Charge Accumulation in Organic Solar Cells during Device Operation as Investigated by Electron Spin Resonance

An electron spin resonance (ESR) method was applied to bulk-heterojunction organic thin-film solar cells to investigate the accumulated charges in such devices under simulated solar irradiation. The device structure used was an indium–tin oxide/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)/regioregular poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM)/Pd/LiF/Al. We measured light-induced ESR (LESR) signals and device characteristics simultaneously using the same device. We observed two different LESR signals, namely, a narrow signal (g = 2.0023, ΔHpp ∼0.2 mT) and a broad signal (g = 2.0034, ΔHpp ∼1.5 mT). The origin of the narrow signal was confirmed to be the charge accumulation in P3HT. The origin of the broad signal was determined to be the charge accumulation in PEDOT and/or PSS by the spectral simulation of the LESR signal. Moreover, the charge accumulation in PEDOT and/or PSS was found to be related to voltage and current.

Quadrimolecular recombination kinetics of photogenerated charge carriers in the composites of regioregular polythiophene derivatives and soluble fullerene

Light-induced electron spin resonance (LESR) measurements have been performed on the composites of regioregular polythiophene derivatives and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in order to study the recombination kinetics of photogenerated charge carriers. We adopt two regioregular polymers with different side chains; head-to-tail poly(3-hexylthiophene) (RR-P3HT) and head-to-head poly(3-dodecynylthiophene-2,5-diyl) [HH-P3(C≡CDec)Th]. In both systems, two LESR signals due to positive polarons on the polymer (g∼2.002) and fullerene radical anions (g∼2.000) have been observed. Quadrimolecular recombination (QR) kinetics, previously reported for RR-P3HT/C60 composites, where two positive polarons and two radical anions recombine simultaneously, has been confirmed in both systems by the observation of Iex0.25 dependence of the LESR intensity on the excitation light intensity (Iex) and the decay curve of the LESR intensity. This process implies the formation of doubly-charged states such as bipolarons or polaron pairs on the polymer to attract two radical anions. Temperature dependence of the QR rate constant, γ, in both systems has exhibited a crossover of the transport mechanism from low temperature tunneling to high temperature hopping process, as in the case of RR-P3HT/C60 composites. In the RR-P3HT/PCBM composites, γ has exhibited marked dependencies on the PCBM concentration or annealing, which may be related to the change of the crystallinity of the phase-separated polymer and fullerene domains as well as their interface structures, affecting the carrier mobilities or the trap states at the interface. Associated change of the molecular orientation of RR-P3HT crystalline domains with the lamellar structure has been further confirmed from the anisotropic LESR signals of the cast films on the substrates, exhibiting a qualitative agreement with the reported x-ray or optical analyses. In the HH-P3(C≡CDec)Th/PCBM composite, γ has been smaller than those in the RR-P3HT/PCBM composites, reflecting the difference of local structures due to the different molecular structure. Furthermore, the hyperfine-determined LESR linewidth of the positive polaron has exhibited a smaller value than those in the RR-P3HT composite, implying the larger extension of the polaron wave function on the polymer chain, which is consistent with the highly coplanar structure of this polymer.

Electron spin resonance and electron nuclear double resonance of photogenerated polarons in polyfluorene and its fullerene composite

Electron spin resonance (ESR) and electron-nuclear double resonance (ENDOR) of photogenerated polarons in poly(9,9-dioctylfluorene) (PFO) and its composite with fullerene $({\text{C}}_{60})$ using variable photoexcitation energy up to 4.1 eV are reported. For PFO, a light-induced ESR (LESR) signal $(g=2.003)$ is observed below 60 K, and its transient response and excitation spectrum indicate that the observed spins are photogenerated polarons on PFO. For the ${\text{PFO-C}}_{60}$ composite, two LESR signals of photogenerated positive polarons on PFO $({g}_{1}=2.003)$ and radical anions on ${\text{C}}_{60}$ $({g}_{2}=1.999)$, respectively, are observed below 120 K, which are caused by photoinduced electron transfer from PFO to ${\text{C}}_{60}$. A remarkable enhancement of the LESR signals in the excitation spectrum at $\ensuremath{\sim}2.8\text{ }\text{eV}$ is observed compared with the case of pure PFO. The bimolecular-recombination kinetics of photogenerated charge carriers in the composite are confirmed by the dependence of the LESR on excitation-light intensity and by the decay dynamics. Light-induced ENDOR (LENDOR) signals are clearly observed for excitation around 2.8 eV owing to the highly efficient photoinduced electron transfer in the composite. Broad LENDOR shifts directly reflect the spin-density distribution of the polarons in PFO. We have determined its maximum shift using LENDOR-induced ESR, and have evaluated the maximum spin density on the carbon site coupled to the proton as 0.032. This value is consistent with the theoretical result obtained by Pariser-Parr-Pople (PPP) model, where the spatial extent of the polarons is calculated as $\ensuremath{\sim}3$ monomer units of PFO. The calculated LESR spectra of PFO based on the PPP model are consistent with the experimental spectra, which confirm the above spatial extension of the polaron in PFO.

ESR study of the hydrogenated nanocrystalline silicon thin films

We studied the stability and light-induced paramagnetic centers in hydrogenated nanocrystalline silicon thin films (nc-Si:H) by electron-spin-resonance (ESR) and photothermal-deflection-spectroscopy (PDS). There is no measurable change in defect density upon illumination with white light with a light intensity of 300mWcm−2 for 300h. At low temperatures, upon illumination with sub-bandgap light, a light-induced ESR signal appears. This signal is similar to that in hydrogenated micro-crystalline silicon (μc-Si:H).

Charge transfer in fullerene – porphyrin-derived dyads studied with light-induced electron spin resonance

Dyads of fullerene-porphyrin type and adequate reference porphyrins were investigated with light-induced electron spin resonance (LESR) spectroscopy. Charge localization and photoinduced electron transfer in the dyads were studied in the dark and under light illumination. It was stated that the LESR signal increases stepwise with the incident light and decreases very slowly when the light is switched-off. Photogeneration of porphyrin cation-radicals was observed and analyzed. The principal parameters of the LESR lines of the ion-radicals are determined. It was stated that the main ESR parameters of the porphyrins are weakly dependent on number of involved porphyrin molecules; these parameters are not noticeably changed after the dyad formation.

Charge transfer in PDI-derived systems studied with light-induced electron spin resonance

Dissymmetrical perylene-3,4,9,10-tetracarboxydiimide building block (PDI) was used to obtain the dyads with fullerene (PDI-C 60) and tetrathiafulvalene (PDI-TTF). Photoinduced electron transfer in these dyads was investigated with light-induced electron spin resonance (LESR) spectroscopy. The LESR signals of the samples in CHCl 3 solutions were recorded in the dark and under light illumination, at room temperature and at about 80 K. It was stated that the LESR signal increases stepwise with the incident light and decreases slowly when the light is switched off. The photogeneration of PDI +, TTF − and C 60 − ion-radicals was observed and analyzed. The principal parameters of the LESR lines of the ion-radicals are determined.

Light-induced ESR in polycrystalline and thin films of [formula omitted

We studied on a chromium oxide Cr 2 O 3 to investigate near-infrared (NIR) light-induced magnetism function at room temperature. A X-band electron-spin resonance (ESR) spectrometer and a SQUID magnetometer were employed to clarify magnetic behavior in the Cr 2 O 3 under the illumination of NIR light ( λ = 1064 nm ). The light-induced ESR signal appears above 250 K and is remarkably enhanced around room temperature. Contrary, with visible green light ( λ = 532 nm ), the ESR profile does not show any change. The light-induced ESR is strongly dependent on the irradiation light wavelength. We conclude the light-induced ESR below T N may come from light-excited carriers associating with a reentrance from antiferromagnetic to paramagnetic spin order. Thin films were successfully prepared by a RF magnetron sputtering and a sol–gel method. The T N of the thin film is increased from 308 to about 400 K. We discuss intriguing properties on light-induced effect for both polycrystalline and thin films and state the chromate is a candidature to be applied to NIR detection industrial devices.

Long-Lived Photoinduced Charges in Donor−Acceptor Anthraquinone-Substituted Thiophene Copolymers

The photoinduced charge-transfer properties of a series of polyalkylthiophene copolymers, carrying anthraquinone substituents covalently linked to the conjugated backbone, have been studied in the solid state by photoinduced absorption (PA) and light-induced electron spin resonance (LESR) spectroscopy. The measurements indicate the formation of metastable charges arising from the photoinduced electron transfer from the polythiophene backbone to the anthraquinone moieties. At low temperatures (below 200 K), long-lived persistent charges are formed, exhibiting lifetimes that extend for several minutes; their recombination kinetics has been studied by following the formation and decay of the PA and LESR signals. The results are rationalized using a model originally proposed to describe the low-temperature recombination kinetics of long-lived photoexcited carriers in amorphous inorganic semiconductors. It is clearly evidenced that, in these polymers, the number of acceptor substituents in the chain, easily tuned by chemical tailoring, plays a key role in the photoexcitation scenario.

Quadrimolecular recombination kinetics of photogenerated positive polarons and C 60− radical anions in regioregular poly(3-alkylthiophene)/C 60 composites measured by light-induced ESR

Light-induced electron spin resonance (LESR) measurements have been performed to study recombination kinetics of photogenerated charge carriers in the composites of regioregular poly(3-alkylthiophene) (RR-PAT) and C 60. Two transient LESR signals due to photogenerated positive RR-PAT polarons and C 60 − radical anions are remarkably enhanced for the excitation energy of ∼1.8 eV. We observe an ∼ I ex 0.25 power dependence of the prompt decay LESR component on excitation-light intensity I ex. This behavior is well explained by a model of quadrimolecular recombination (QR) of the photogenerated charge carriers. The QR is dominant at higher I ex while bimolecular recombination (BR) is relatively important at lower I ex. The time decay of the LESR intensity is also well explained by the QR model. The temperature dependence of the QR rate constant shows a crossover from tunneling to hopping behavior with increasing temperature; the activation energy for the hopping of 0.10 eV is obtained. In the regiorandom PAT/C 60 composites, only BR is observed.

Photoinduced Electron Transfer in Solid C60 Donor/Acceptor Complexes Studied by Light-Induced Electron-Spin Resonance

ABSTRACT Light-induced ESR (LESR) spectroscopy was used to study photoinduced electron transfer (PIET) in the crystals of the C60 donor/acceptor complexes. High-field LESR signals attributed to (g = 1.9984−1.9992 and ΔH = 3−6 G at 100 K) were observed in fullerene complexes with Leuco Crystal Violet (LCV·C60·C6H5Cl, 1), N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPDA·C60, 2), and bianthrone ((BA)2C60, 3), whereas a low-field LESR signal in 2 was attributed to TMPDA•+. Microwave power studies reveal two independent spins in 2 and show high relaxation rates for photoinduced spins on . The LESR signals consist of a major long-lived “persistent” component and a minor “prompt” one, which disappears immediately after light is off. The maximal intensity of the LESR signals was observed for the excitation of 1–3 below 2 eV. This range of photon energy corresponds to direct intermolecular charge transfer from donor to C60 molecules. PIET is temperature activated with Ea = 23, 43 and 49 meV for 1, 2, and 3, respectively. Photoinduced spin susceptibility attains the maximum at 30 K in 1 and decreases with temperature down to 4 K indicating a possible antiferromagnetic interaction of spins, while in 2 it increases with the temperature decrease down to 4 K.

Photoinduced charge transfer in composites of conjugated polymers and semiconductornanocrystals

Photoinduced electron transfer in composites of CdSe and InP nanocrystals (NC) with aconjugated polymer [2-methoxy-5-(3-,7-dimethyl-octyloxy)-1, 4-phenylene vinylene](OC1C10-PPV) is studied by means of light-induced electron spin resonance (LESR), photoluminescence(PL) as well as quasi steady-state photoinduced absorption (PIA) spectroscopy. Thequenching of PL, the occurrence of two new optical absorption bands, and the formation oflight-induced paramagnetic species on the polymer chain are altogether interpreted as anelectron transfer between a donor polymer and acceptor NC in the excited state. In particularthe LESR provides evidence of an electron transfer rather than energy transfer due to anoverlap of absorption and emission bands of NC and the conjugated polymer. Theg-factor of the signal is consistent with the cationic origin ofOC1C10-PPV radicals. Additionally, the LESR signal is sensitive to NC surface coating being smaller in blends with thetri-n-octylphosphineoxide–tri-n-octylphosphine (TOPO–TOP) coated CdSe surface and completely absent in blendswith TOPO–TOP–InP. LESR probes predominantly positive polarons on theconjugated polymer chain generated at room temperature in the course of anelectron transfer to NC. This is very different from the situation in blends ofOC1C10-PPV with fullerenes, another promising photovoltaic acceptor, where the photogenerated cationand anion were both observed in the LESR. In PIA we found contributions of both polymerpolarons and electrons on the NC. The recombination kinetics shows a broad distributionof lifetimes, which is characteristic for dispersive (diffusive) recombination processes withfractional power frequency dependence.

Optical and ESR studies on poly(3-alkylthiophene)/fullerene composites for solar cells

Optical measurements (absorption coefficients, photoluminescence) and light-induced electron spin resonance ( LESR ) technique were used to study photoinduced charge generation and charge transfer in conjugated polymer / fullerene composites. The quenching of fluorescence by C 60 gives a first indication of an effective charge transfer from P3DDT to C 60 . At 77 K two kinds of LESR signals were identified, one of polaron (P +. ) on the polymer chain and one of radical anion (C 60 -.) of fullerene. The relaxation rates of generated paramagnetic centres were estimated by microwave power saturation studies. Moreover for the first centre (P +. ) two contributions to the LESR signal were observed: a permanent one and a photoinduced one. The light intensity dependence of the photoinduced P +. and C 60 -. ESR signals is of bimolecular type (I 0.5 ). Solar cells based on P3DDT/ C 60 and P3DDT/ PCBM are prepared and characterised.

Light-induced ESR studies of polarons in regioregular poly(3-alkylthiophene)-fullerene composites

Light-induced electron spin resonance (LESR) has been employed to study photo-generated positive and negative polarons on poly(3-alkylthiophene) (PAT) and fullerene (C 60 ) in regioregular PAT-C 60 composites, respectively, using variable photo-excitation energy up to 4.1 eV. A remarkable enhancement of prompt LESR signal is found in excitation spectrum at around 1.8 eV, which is consistent with the enhancement of the photoconductivity, while persistent LESR signal is weakly dependent on photo-excitation energy due to small numbers of deep trap sites. In addition, light-induced electron-nuclear double resonance (LENDOR) measurements have been successfully performed on the composites. Two different LENDOR signals with broad and narrow linewidth are observed due to PAT and C 60 polarons, respectively.

Molecular Complexes of Fullerenes C 60 and C 70 with Saturated Amines

New molecular complexes of fullerenes C 60 and C 70 with leuco crystal violet (LCV, 1–3); leucomalachite green (LMG, 4–6); crystal violet lactone (CVL, 7); N, N, N′, N′-tetrabenzyl- p-phenylenediamine (TBPDA, 8, and 9); N, N, N′, N′-tetramethyl- p-phenylenediamine (TMPDA, 10, and 11); triphenylamine (TPA, 12, and 13); and substituted phenotellurazines (EPTA and TMPTA, 14, and 15) have been synthesized. Crystal structures have been solved for C 60 complexes with LMG (5, 6) TBPDA (8), TMPDA (10), and TPA (12). The C 60 molecules form closely packed double layers in 5 and 6, hexagonal layers in 10 and quasi-three-dimensional layers in 8 and 12. The substitution of disordered solvent molecules in the complexes with LMG (4, 5) by naphthalene ones results in the ordering of the C 60 molecules. According to IR-, UV–visible–NIR and ESR-spectroscopy the complexes have a neutral ground state. The spectra of 1–8, and 10 show intense charge transfer bands in the visible and NIR-range. On photoexcitation by white light (light-induced ESR (LESR) spectroscopy), 1 and 10 were shown to have an excited ionic state. The LESR signals were generated at light energies <2.25 eV indicating that the excited states in the complexes are realized mainly by direct charge transfer from donor to the C 60 molecule.

ESR studies of photogenerated polarons in regioregular poly(3-alkylthiophene)–fullerene composite

Electron spin resonance (ESR) studies of photogenerated polarons in composite of regioregular poly(3-octylthiophene) (PAT8) and fullerene (C 60) using variable photoexcitation energy up to 4.1 eV are reported. Two transient light-induced ESR (LESR) signals of photogenerated positive and negative polarons on PAT8 ( g 1=2.002) and C 60 ( g 2=1.999), respectively, are observed below 200 K, which are caused by photoinduced electron transfer between PAT8 and C 60. A remarkable enhancement of the LESR signals in excitation spectrum at around 1.8 eV is observed compared with the case of pure regioregular PAT8, which is consistent with the enhancement of the photoconductivity at around 1.8 eV where optically forbidden transition of C 60 occurs. Spin–lattice relaxation time of the polaron spins of PAT8 is obtained considering inhomogeneous broadening of the ESR linewidth due to the hyperfine interactions, which is at least approximately 18 times longer than that of C 60. Prompt and persistent LESR signal components are observed by transient response of the LESR signal. The prompt contribution shows a monotonic increase with increasing excitation-light intensity, while the persistent contribution is found to be excitation intensity independent due to deep traps of the photogenerated polarons.

Light-induced ESR study of quinquethiophene (5T)

Light-induced ESR (LESR) has been measured on quinquethiophene (5T) using variable excitation energy. The obtained g-value (2.003) of the dark ESR and LESR signals is close to that of π-electron radical cation (positive polaron) in oligothiophenes. The action spectra of the LESR signal show a monotonic increase above 3.5 eV up to 4.1 eV, despite of the lower optical density reported. These behaviors together with the transient response of the LESR signals suggest that the observed spins are optically generated polarons. Temperature dependence of the LESR signal indicates a motional narrowing behavior of the light-induced spins at higher temperatures.