Light-induced Electron Spin Resonance Research Articles

Electron Paramagnetic Resonance Tracing of Electronic Transfers in Push–Pull Copolymers/PCBM or Nanocrystal Composites

Transfers of photoexcited electrons between poly(thieno[3,4-c]pyrrole-4,6-dione)-based copolymers (push–pull type) and fullerene or nanocrystals were studied by light-induced electron paramagnetic resonance (EPR). EPR tracing methodology has been used: EPR signatures of the various species taking part in the electron-transfer processes were determined and used to monitor potential charge transfers in the studied composites. The EPR tracing method combined with DFT calculations revealed that excited electrons are not only promoted from the HOMO of the polymer but also from its HOMO – 1 and even HOMO – 2, which are either exclusively centered on the push moiety or delocalized over both the push and pull units.

Charge Recombination in P3HT/PC70BM Composite Studied by Light-Induced EPR

A composite of conductive polythiophene P3HT and soluble fullerene derivative PC70BM (the material widely used as the active layer in organic photovoltaics devices) was studied by light-induced EPR (LEPR). In contrast to P3HT/PC60BM composite, LEPR signal in P3HT/PC70BM can be detected in a wide temperature range up to room temperature. This signal was attributed to charge carriers P3HT+ and PC70BM–. The dependence of the intensity of LEPR signal on light intensity and the decay of LEPR signal upon switching light off are interpreted in frame of trap-limited bimolecular recombination model with finite rate of back electron transfer in [P3HT+ PC70BM–] encounter complex at polymer–fullerene interface. The apparent recombination order was found to be close to p = 3.5 for the temperature range from 100 K to room temperature. For temperatures above 150 K Arrhenius behavior of effective recombination rate constant was obtained with the activation energy Ea = 0.16 ± 0.01 eV, which is larger than analogous values for P3HT/PC60BM reported previously. This difference is attributed to the change of polymer/fullerene interface induced by the replacement of PC60BM by PC70BM.

The role of spin exchange in charge transfer in low-bandgap polymer: Fullerene bulk heterojunctions.

Formation, relaxation and dynamics of polarons and methanofullerene anion radicals photoinitiated in poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]:-[6,6]-phenyl-C61-butyric acid methyl ester (PCDTBT:PC61BM) bulk heterojunctions were studied mainly by light-induced EPR (LEPR) spectroscopy in wide photon energy and temperature ranges. Some polarons are pinned by spin traps whose number and depth are governed by the composite morphology and photon energy. The proximity of the photon energy and the polymer bandgap reduces the number of such traps, inhibits recombination of mobile charge carriers, and facilitates their mobility in polymer network. Spin relaxation and charge carrier dynamics were studied by the steady-state saturation method at wide range of temperature and photon energy. These processes were shown to be governed by spin exchange as well as by the photon energy. Charge transfer in the composite is governed by the polaron scattering on the lattice phonons of crystalline domains embedded into amorphous polymer matrix and its activation hopping between polymer layers. The energy barrier required for polaron interchain hopping exceeds that of its intrachain diffusion. Anisotropy of polaron dynamics in the PCDTBT:PC61BM composite is less than that of poly(3-alkylthiophenes)-based systems that evidences for better ordering of the former. Lorentzian shape of LEPR lines of both charge carriers, lower concentration of spin traps as well as behaviours of the main magnetic resonance parameters were explained by layer ordered morphology of polymer matrix.

Correlation between Hole Accumulation and Deterioration of Device Performance in Polymer Solar Cells as Investigated by Light-Induced Electron Spin Resonance

Light-induced electron spin resonance (LESR) study of polymer solar cells has been performed to investigate accumulated hole carriers in these devices under device operation. We analyzed clear correlation between the number of accumulated holes in regioregular poly(3-hexylthiophene) (P3HT) evaluated by LESR and the deterioration of device performance (Voc, Jsc) observed using the same device under simulated solar irradiation. The effects of hole accumulation with deep trapping levels formed in P3HT at the organic interfaces on the performance are examined by considering interfacial electric dipole layers and charge-carrier scattering by accumulated holes.

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

How Photoinduced Crosslinking Under Operating Conditions Can Reduce PCDTBT‐Based Solar Cell Efficiency and then Stabilize It

Bulk heterojunction (BHJ) photovoltaic devices made of PCDTBT (poly[N‐9′‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]) and PC70BM ([6,6]‐phenyl‐C70‐butyric acid methyl ester) are among the most efficient and stable devices studied so far. However, during a short regime called “burn‐in”, a significant decrease of power conversion efficiency was observed. A study of the photochemical mechanisms involved in the PCDTBT:PCBM active layer exposed to light in encapsulated systems is presented. It is found that the photochemical reactions resulting from the absorption of light by PCDTBT involve crosslinking between the 2,7 carbazole unit of PCDTBT and the fullerene unit of PCBM. Those reactions stabilize the BHJ by avoiding the formation of microsized PCBM crystals known to cause failure of BHJ solar cells. Using classical electron paramagnetic resonance spectroscopy (EPR) (without illumination), paramagnetic defects along the polymer chains have been detected. The kinetics of defects intensity show a burn‐in trend. The evolution of their relaxation times upon aging is in good agreement with a structural change (crosslinking) of the BHJ observed from the nanomechanical properties. Finally, light‐induced electron paramagnetic resonance (LEPR) measurements performed on aged samples revealed that electron transfer is not significantly affected upon aging, confirming thus the stabilization of the BHJ in solar cell operating conditions.

Charge transfer and recombination in organic/inorganic hybrid composites with CuInS2 nanocrystals studied by light-induced electron spin resonance

Charge separation is an important elementary step in solar cells with donor/acceptor heterojunctions. In this work, light-induced electron spin resonance (LESR) is used to study the charge transfer and recombination of photo-generated charge carriers in inorganic/organic hybrid composites of CuInS2 nanoparticles with the conjugated polymer poly(3-hexylthiophene) (P3HT) or the electron acceptor phenyl-C61-butyric acid methyl ester (PCBM), respectively. After synthesis, the CuInS2 nanoparticles are surrounded by a ligand shell composed of relatively long-chained organic molecules. A ligand shell modification with hexanethiol was applied to the CuInS2 nanoparticles to study the influence of the ligand shell on the charge separation process. The effect of the hexanethiol treatment on the initial ligand shell present after synthesis was analyzed by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). We demonstrate that charge transfer in both the CuInS2/P3HT and CuInS2/PCBM composites is possible and in case of P3HT strongly influenced by the ligand shell. The recombination kinetics of photo-generated charge carriers reveals that trapping is especially governed by the organic semiconductor. Laboratory solar cells with CuInS2/P3HT or CuInS2/PCBM as the photoactive layer are presented as well.

A solution-processable polymer-grafted graphene oxide derivative for nonvolatile rewritable memory

A new conjugated polymer, poly{[9,9-di(triphenylamine) fluorene]-[9,9-dihexylfluorene]-[4,4′-(9H-fluorene-9,9-diyl) dibenzenamine]}(PTHF), was synthesized by the Suzuki coupling reaction and used to react with the graphene oxide (GO) containing surface-bonded acyl chloride moieties to give a solution-processable GO–PTHF. Covalent attachment of PTHF onto GO resulted in the partial fluorescence quenching of the parent polymer, suggesting energy and/or electron transfer between the excited singlet states of the PTHF moiety and the GO moiety. The absolute photoluminescence (PL) quantum yields (ΦPL) of the samples were changed from 0.59 for PTHF to 0.31 for GO–PTHF. A light-induced electron paramagnetic resonance (LEPR) technique was employed to study the photoinduced spins in the conjugated GO–PTHF. Upon irradiation with a 432 nm laser, both the polaron signals of the polymer chain at g = 2.1132, 2.0665 and 2.0197, and the signals of radical anions on GO at g// = 1.9726 and g⊥ = 1.9747 were kept unchanged, while the signal at g = 2.0002 almost completely disappeared due to a typical photoinduced selective quenching of the triplet state or a photoinduced spin polarization transfer process acting with a radical–triplet pair mechanism. A memory device based on GO–PTHF, in which the GO–PTHF film was sandwiched between the indium–tin oxide and Al electrodes, can be switched to the ON state under a negative electrical sweep, and can also be reset to the initial OFF state by a reverse (positive) electrical sweep.

Structure–property relationship of anilino-squaraines in organic solar cells

Soluble molecular semiconductors are a promising alternative to semiconducting polymers in the field of organic photovoltaics. Here, three custom-made symmetric 1,3-bis(N,N-alkylated-2,6-dihydroxy-anilino)squaraines containing systematic variations in their molecular structures are compared regarding their applicability as donor materials in bulk-heterojunction solar cells. The terminal substitution pattern of the squaraines is varied from cyclic over linear to branched including a stereogenic center. Single crystal structures are determined, and, in the case of chiral squaraine, unusual formation of stereoisomer co-crystals is revealed. The thin film absorbance spectra show characteristic signatures of H- and J-bands or hint at the formation of tautomers. The general feasibility of these model compounds for photovoltaic applications is studied by light-induced electron spin resonance spectroscopy. The impact of the different molecular substitution patterns on aggregation behavior and, consequently, their optoelectronic solid state properties including charge carrier mobility and finally the solar cell performance are investigated.

Influence of spin–spin exchange on charge transfer in PANI-ES/P3DDT/PCBM composite

The first results of the light-induced EPR study of magnetic, relaxation and dynamic parameters of charge carriers stabilized in emeraldine salt form of polyaniline (PANI-ES) highly doped with sulfuric (SA) and para-toluenesulfonic (pTSA) acids, spin pairs background photoinitiated in bulk heterojunctions formed by poly(3-dodecylthiophene) with [6,6]-phenyl-C61-butyric acid methyl ester (P3DDT/PCBM) as well as charge carriers in appropriate PANI-ES/P3DDT/PCBM composites are described. Main magnetic resonance parameters for positively charged polarons in both polymer backbones and negatively charged methanofullerene ion-radical in the P3DDT/PCBM bulk heterojunctions were determined separately from deconvoluted LEPR spectra. These parameters were shown to be governed by the conformation of the PANI-ES chains which determines insulating and conducting forms of the PANI-SA and PANI-pTSA samples, respectively. The effective absorption peak-to-peak line width was described in terms of the exchange interaction of the polarons hopping along the neighboring solitary PANI and P3DDT nanoparticles. Paramagnetic susceptibility of such charge carriers was interpreted in the framework of the model of exchange coupled spin pairs differently distributed in parental polymer matrices. Exchange interaction increases dramatically as the PANI-SA network is replaced by the PANI-pTSA one in the triple composite. This deepens overlapping of wave functions of polarons in both polymer matrices and leads to the increase in the energy barrier which they overcome crossing a bulk heterojunctions and acceleration of spin–lattice relaxation in PANI-pTSA. Polaron diffusion in P3DDT was shown to govern spin susceptibility of methanofullerene anion radicals rotating between its chains.

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.

Time-Resolved EPR of Photoinduced Excited States in a Semiconducting Polymer/PCBM Blend

We studied by means of electron spin resonance (EPR) techniques the photoinduced processes in a poly[(9,9-bis(octyl)fluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl) (F8BT) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 (PCBM) blend. Using light-induced EPR (LEPR), we observed the generation of charge-separated states in the blend upon visible light excitation, and we detected the formation of the PCBM excited triplet state by using time-resolved EPR (TREPR) with microsecond time resolution. From spectral simulation of the TREPR spectrum we were able to identify a double pathway for the PCBM triplet generation: the normal intersystem crossing from PCBM excited singlet state (either directly produced by light absorption or populated by singlet–singlet energy transfer from the polymer) and a generation pathway via polaron pairs recombination. The experimental evidence of this process allows to assign the relative order of the energies of the polaron pairs and the triplet states and provides a mor...

Evaluation of Microscopic Properties of Organic Solar Cells by Light-Induced Electron Spin Resonance

We have investigated organic thin-film solar cells from the microscopic viewpoint with light-induced electron spin resonance (ESR). The utilized cell structures are indium–tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methylester (PCBM)/Pd/LiF/Al. We performed simultaneous measurements of light-induced ESR and device characteristics using the same device. Under simulated solar irradiation, the light-induced ESR intensity of a solar cell increases, while the short-circuit current and open-circuit voltage of the cell concomitantly decrease with increasing duration of irradiation. We have successfully observed a clear correlation between the light-induced ESR intensity and the device characteristics. Trapping sites of photogenerated hole carriers (positive polarons) are ascribed to P3HT by the analysis of the ESR signals.

Evaluation of Microscopic Properties of Organic Solar Cells by Light-Induced Electron Spin Resonance

Evaluation of Microscopic Properties of Organic Solar Cells by Light-Induced Electron Spin Resonance

Single-walled carbon nanotube/(Pb, Zn)-phosphate glass heterostructure: an optical sensor and efficient photocurrent converter

A light-sensitive heterostructure composite of single-walled carbon nanotubes (SWCNTs) and (Pb, Zn)-phosphate glass is synthesized and its spectroscopic and optoelectronic properties are studied. Absorption spectrum of SWCNTs in the composite is quite different from that of its film, which is consistent with our earlier observations that SWCNTs in such a composite suffer stress-induced conformational deformations and band structure modulation. UV/Vis light-induced electron spin resonance (LIESR) spectrum of the composite shows large enhancement of the signal due to the conduction electrons of the SWCNTs and concomitant appearance of a new signal in the lower field region with g = 2.45 and peak-to-peak half width ΔHpp = 9.0 mT, which is attributable to the lead hole (Pb3+). A phenomenon of creation of charge separation in the system due to the light-induced Pb3+ holes formation by releasing electrons from the interstitial Pb2+-sites and capture of those photo-generated electrons by the SWCNTs in the system, is thus observed. Similar LISER experiments using separately only UV (230–400 nm) and only Vis-light (>400 nm) demonstrate that the composite has good response to the UV and near visible light. Photoconduction studies, on the other hand, indicate that the SWCNTs, which exist in the system with a quasimetallic band structure, act as an efficient medium of transport for these charge carriers. The material thus shows the prospect of being used as a solar photocurrent converter.

Light-Induced EPR Study of Charge Transfer in P3HT/PC71BM Bulk Heterojunctions

Radical pairs, polarons, and fullerene anion radicals photoinduced by photons with energy of 1.98–2.73 eV in bulk heterojunctions formed by regioregular poly(3-hexylthiophene) (P3HT) with (6,6)-phenyl-C71-butyric acid ester (PC71BM) methanofullerene have been studied by the direct light-induced EPR (LEPR) method at a wide temperature region. LEPR spectra of the P3HT/PC71BM composite were deconvoluted, and the main magnetic resonance parameters of these charge carriers have been determined. A part of photoinduced polarons is pinned by large-depth traps in the polymer matrix. It was shown that magnetic resonance, relaxation, and dynamics parameters of photoinduced charge carriers depend extremely on the energy of initiated photons. Relaxation and dynamics parameters of both the charge carriers were determined separately by the steady-state saturation method. Longitudinal diffusion of polarons was analyzed in terms of spin interaction with the lattice phonons of crystalline domains embedded into an amorphous polymer matrix. The interchain spin hopping is determined by the number and depth of the traps photoinitiated in the polymer matrix. Pseudorotation of methanofullerene molecules in a polymer matrix was shown to follow the activation Pike model. The replacement in the composite of PC61BM acceptors by PC71BM ones accelerates electron relaxation, hinders the formation of spin traps, and favors more ordered (crystalline) structure of bulk heterojunction that facilitates charge transfer in the P3HT/PC71BM composite.

Light-induced electron paramagnetic resonance evidence of charge transfer in electrospun fibers containing conjugated polymer/fullerene and conjugated polymer/fullerene/carbon nanotube blends

Electrospun sub-micron fibers containing conjugated polymer (poly(3-hexylthiophene), P3HT) with a fullerene derivative, phenyl-C61-butyric acid methylester (PCBM) or a mixture of PCBM and single-walled carbon nanotubes (SWCNTs) were studied by light-induced electron paramagnetic resonance spectroscopy. The results provide experimental evidence of electron transfer between PCBM and P3HT components in both fiber systems and suggest that the presence of a dispersing block-copolymer, which acts via physical adsorption onto the PCBM and SWCNT moieties, does not prevent electron transfer at the P3HT-PCBM interface. These findings suggest a research perspective towards utilization of fibers of functional nanocomposites in fiber-based organic optoelectronic and photovoltaic devices. The latter can be developed in the textile-type large area photovoltaics or individual fiber-based solar cells that will broaden energy applications from macro-power tools to micro-nanoscale power conversion devices and smart textiles.

Density of trap states in organic photovoltaic materials from LESR studies of carrier recombination kinetics

The density of trap states in donor/acceptor blends of organic photovoltaic materials is obtained from photogenerated charge-carrier recombination kinetics observed by light-induced electron spin resonance (LESR) at different temperatures. The method, based on an inversion calculation, is applied to experimental LESR data from two P3AT-PCBM blend films with different kinetic behavior, deriving density-of-states profiles and values of the trap depths in agreement with previous literature results obtained using other techniques.

Quadrimolecular Recombination of Persistent Photocarriers in Surface-Type Photocells of Regioregular Poly(3-hexylthiophene)/Methanofullerene Composites

Photocarrier recombination kinetics was studied in regioregular poly(3-hexylthiophene)/methanofullerene composites by measuring the photocurrent of surface-type photocells. The photocurrent exhibited fast and persistent components. The light-intensity dependence of the photocurrent and the analysis of decay curves of the persistent component indicated the dominance of quadrimolecular recombination (QR) between two positive carriers on regioregular polymers and two negative carriers on methanofullerenes in the time range of 0.1 to 100 s. No QR but bimolecular recombination was observed in cells containing regiorandom polymers, consistent with light-induced electron spin resonance studies. These results differ markedly from those of sandwich-type cells in which photocarriers flow out of the cell prior to the recombination, demonstrating an advantage of the surface-type photocell with larger electrode spacing in studying the recombination of long-lived photocarriers. At high methanofullerene concentrations above 50 wt %, the fast component predominates over the persistent one by annealing owing to percolation of the pathway for negative carriers.