Light-induced Electron Spin Resonance Research Articles

Charge Separation and Triplet Exciton Formation Pathways in Small Molecule Solar Cells as Studied by Time-resolved EPR Spectroscopy.

Organic solar cells are a promising renewable energy technology, offering the advantages of mechanical flexibility and solution processability. An understanding of the electronic excited states and charge separation pathways in these systems is crucial if efficiencies are to be further improved. Here we use light induced electron paramagnetic resonance (LEPR) spectroscopy and density functional theory calculations (DFT) to study the electronic excited states, charge transfer (CT) dynamics and triplet exciton formation pathways in blends of the small molecule donors (DTS(FBTTh2)2, DTS(F2BTTh2)2, DTS(PTTh2)2, DTG(FBTTh2)2 and DTG(F2BTTh2)2) with the fullerene derivative PC61BM. Using high frequency EPR the g-tensor of the positive polaron on the donor molecules was determined. The experimental results are compared with DFT calculations which reveal that the spin density of the polaron is distributed over a dimer or trimer. Time-resolved EPR (TR-EPR) spectra attributed to singlet CT states were identified and the polarization patterns revealed similar charge separation dynamics in the four fluorobenzothiadiazole donors, while charge separation in the DTS(PTTh2)2 blend is slower. Using TR-EPR we also investigated the triplet exciton formation pathways in the blend. The polarization patterns reveal that the excitons originate from both intersystem crossing (ISC) and back electron transfer (BET) processes. The DTS(PTTh2)2 blend was found to contain substantially more triplet excitons formed by BET than the fluorobenzothiadiazole blends. The higher BET triplet exciton population in the DTS(PTTh2)2 blend is in accordance with the slower charge separation dynamics observed in this blend.

Effect of spin localization on charge transport in low-bandgap bilayered ordered nanocomposites

Direct Light-Induced Electron Paramagnetic Resonance was used for comparative study of magnetic, relaxation and dynamic parameters of spin charge carriers, polarons and methanofullerene anion radicals, photoinduced in bulk heterojunctions of composites formed by low-bandgap copolymers, poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFO-DBT) and poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) with methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). A fraction of polarons is captured by deep spin traps reversibly initiated in the copolymer backbone due to its disordering. The number and energy depth of the traps are governed by the structure of copolymer matrix and photon energy. Both composites exhibit photo-response within photon energy/wavelength 1.32–3.14eV/940–395nm regions which is wider than those of other polymer composites. The magnetic, relaxation and dynamics parameters of the spin charge carriers were shown to be governed by the exchange interaction of their spins and photon energy. The specific morphology of the composites provokes extreme selectivity of these parameters to photon energy. The anisotropy of polaron mobility through bulk heterojunctions was shown to reflect the system dimensionality and is dependent on the photon energy. Compared to PFO-DBT, the structure of the PCDTBT composite is more organised, resulting in a decrease in the number of spin traps and a change in the order of the charge recombination process. The decay of free charge carriers is interpreted in terms of the trapping-detrapping spin diffusion in bulk heterojunctions.

Electron self-trapped at molybdenum complex in lead molybdate: An EPR and TSL comparative study

Single crystals of PbMoO4 were studied by means of light-induced electron paramagnetic resonance (EPR) and wavelength resolved thermally stimulated luminescence (TSL). The work is focused on the electron self-trapped at the (MoO4)2- complex. Detailed analysis of the EPR spectra of this complex allowed to determine a set of spin-Hamiltonian parameters including g factor and the 95,97Mo, 207Pb hyperfine tensors further used in calculations performed in terms of crystal field and LCAO-MO theories. They have shown a significant overlap of the Mo 5d1 and 6s6p ligand lead orbitals. The contributions of oxygen and lead to the molecular orbitals with an electron trapped at the (MoO4)3- complex were about 14% and 38%, respectively. The decay of the molybdenum center EPR intensity with annealing temperature provided information about thermal stability of the center, which thermally destroys already at 40K. Trap depth and frequency factor, Et = 0.05eV, and f0 = 104s−1, have been determined. The TSL glow curve displays two overlapping peaks in the 40–50K temperature region, corresponding to the disappearance of the EPR signal from the (MoO4)3- center. By partial cleaning technique and initial rise method it is shown that the lower temperature peak is related to a trap with thermal parameters similar to those determined for the (MoO4)3- center. This allows to correlate it to the thermal destruction of the (MoO4)3- center. The obtained results are discussed in a broader frame of charge trapping mechanism in all the family of tungstate and molybdate-based single crystals.

Effect of spin traps on charge transport in low-bandgap copolymer:fullerene composites

Light-Induced EPR study of magnetic, relaxation and dynamic parameters of spin charge carriers background photoinduced in bulk heterojunctions of composites formed by poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFO-DBT) and poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) with methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester is described. A part of polarons is captured by deep spin traps whose number and energy depth are governed by the structure, morphology of a copolymer matrix and also by the photon energy. Both the composites exhibit photo-response within photon energy/wavelength 1.32–3.14 eV/940-395 nm region which is wider than that of other polymer composites. Magnetic, relaxation and dynamics parameters of spin charge carriers were shown to be governed by their exchange interaction and photon energy. Specific morphology of the composites causes selectivity of these parameters to the photon energy. It was shown that the anisotropy of spin mobility through bulk heterojunctions reflects the system dimensionality and is governed by the photon properties. The replacement of the PFO-DBT backbone by the PCDTBT matrix leads increases the ordering of a copolymer, decreases the number of spin traps and changes a mechanism of charge recombination. The decay of free charge carriers was interpreted in terms of the trapping-detrapping spin diffusion in bulk heterojunctions.

(Invited) Excited State of Y-Nitride Clusterfullerene: Luminescence and EPR Spectroscopy Study

Empty fullerenes are known to have very fast intersystem crossing with near 100% yield of the T1 state, which results in very low quantum yield of fluorescence and relatively short fluorescence lifetimes (less than 1 ns). In endohedral metallofullerenes, the intersystem crossing is even faster due to the internal heavy-atom effect, and hence fluorescence can hardly be expected. However, Y3N@C80 is known to exhibit reasonably high yield of fluorescence (1%) and unusually long lifetimes of 800 ns.1, 2 Such unusual photophysical parameters indicate that simple fluorescence mechanism cannot explain experimental facts. In this contribution we describe steady-state and lifetime luminescence as well as light-induced electron paramagnetic resonance studies of Y3N@C80 at different temperatures aimed at clarification of photophysical properties of Y3N@C80. 1. Bharadwaj, L.; Novotny, L., Plasmon-Enhanced Photoemission from a Single Y3N@C80 Fullerene. J. Phys. Chem. C 2010, 114, 7444-7447. 2. Toth, K.; Molloy, J. K.; Matta, M.; Heinrich, B.; Guillon, D.; Bergamini, G.; Zerbetto, F.; Donnio, B.; Ceroni, P.; Felder-Flesch, D., A Strongly Emitting Liquid-Crystalline Derivative of Y3N@C80: Bright and Long-Lived Near-IR Luminescence from a Charge Transfer State. Angew. Chem.-Int. Edit. Engl. 2013, 52, 12303-12307.

Novel Anthrathiophene-Based Small Molecules as Donor Material for Organic Photovoltaics: Synthesis and Light-Induced EPR Study

Abstract A novel anthrathiophene-based compound, 1,4-bis((5-(6,11-dioxoanthra[2,1-b]thiophene-2-yl)thien-2-yl)ethynyl)-2,5-bis(octyloxy)benzene, was synthesized and characterized. The optical absorption spectrum of the synthesized compound in film is strongly red-shifted as compared to the solution spectrum. The energies of frontier orbitals measured by cyclic voltammetry show that this compound can act as electron donor in a composite with the widely used fullerene derivative PCBM. This is confirmed by light-induced electron transfer from it to PCBM evidenced from light-induced EPR spectroscopy. The spectroscopic data suggest that anthrathiophene is a promising platform for synthesis of small-molecular electron donors for organic solar cells.

Determination of the Average Native Background and the Light-Induced EPR Signals and their Variation in the Teeth Enamel Based on Large-Scale Survey of the Population

The aim of the study is to determine the average intensity and variation of the native background signal amplitude (NSA) and of the solar light-induced signal amplitude (LSA) in electron paramagnetic resonance (EPR) spectra of tooth enamel for different kinds of teeth and different groups of people. These values are necessary for determination of the intensity of the radiation-induced signal amplitude (RSA) by subtraction of the expected NSA and LSA from the total signal amplitude measured in L-band for in vivo EPR dosimetry. Variation of these signals should be taken into account when estimating the uncertainty of the estimated RSA. A new analysis of several hundred EPR spectra that were measured earlier at X-band in a large-scale examination of the population of the Central Russia was performed. Based on this analysis, the average values and the variation (standard deviation, SD) of the amplitude of the NSA for the teeth from different positions, as well as LSA in outer enamel of the front teeth for different population groups, were determined. To convert data acquired at X-band to values corresponding to the conditions of measurement at L-band, the experimental dependencies of the intensities of the RSA, LSA and NSA on the m.w. power, measured at both X and L-band, were analysed. For the two central upper incisors, which are mainly used in in vivo dosimetry, the mean LSA annual rate induced only in the outer side enamel and its variation were obtained as 10±2 (SD =8) mGy y-1, the same for X- and L-bands (results are presented as the mean±error of mean). Mean NSA in enamel and its variation for theupper incisors was calculated at 2.0±0.2 (SD =0.5) Gy, relative to the calibrated RSA dose-response to gamma radiation measured under non-power saturation conditions at X-band. Assuming the same value for L-band under non-power saturating conditions, then for in vivo measurements at L-band at 25 mW (power saturation conditions), a mean NSA and its variation correspond to 4.0±0.4 (SD =1.0) Gy.

Light-induced electron spin resonance as a probe for charge separation and recombination in hybrid donor:acceptor systems

Donor:acceptor systems are the core of many concepts for photovoltaic devices, because they provide a mean to separate photo-generated electron-hole pairs into extractable charge carriers. In organic photovoltaics, donor:acceptor systems consist of two different organic semiconductors. Alternatively, inorganic semiconductor nanocrystals are also suitable materials. Their use in combination with an organic material gives rise to hybrid systems. Charge separation being a key step in the energy conversion process, suitable methods are required to study this physical process. The present review article provides an overview on the use of light-induced electron spin resonance as a powerful method to investigate charge separation in donor:acceptor blends. Thereby, organic and hybrid material systems are compared. Furthermore, detailed investigations of recombination processes are considered as well.

Charge Trapping Defects in CdSe Nanocrystal Quantum Dots

Charge trapping due to defects in semiconductor quantum dots (QDs) is expected to challenge the applicability of QDs in future technologies. The efficient elimination of defects from QDs demands an understanding of their origin and of their impact on (photo)electronic properties. Here, we identify the presence of two charge states of a defect in CdSe QDs using electron paramagnetic resonance (EPR), combined with electronic tuning of QDs via chemically induced Ag doping. From light-induced EPR, we show that these defects have a central role in Fermi level pinning in ensembles of intrinsic QDs. By analyzing the dependence of the EPR signal of the defects on the concentration of Ag dopants, we further demonstrate that the defects act as effective electron traps in the QDs. Our study also provides support to the proposed behavior of Ag dopants in CdSe QDs, according to which Ag atoms are n-type dopants at concentrations below 2 Ag atoms per QD and become p-type dopants for higher Ag concentrations. From tempe...

Magnetoelectroluminescence in organic light-emitting diodes.

The magnetoelectroluminescence of conjugated organic polymer films is widely accepted to arise from a polaron pair mechanism, but their magnetoconductance is less well understood. Here we derive a new relationship between the experimentally measurable magnetoelectroluminescence and magnetoconductance and the theoretically calculable singlet yield of the polaron pair recombination reaction. This relationship is expected to be valid regardless of the mechanism of the magnetoconductance, provided the mobilities of the free polarons are independent of the applied magnetic field (i.e., provided one discounts the possibility of spin-dependent transport). We also discuss the semiclassical calculation of the singlet yield of the polaron pair recombination reaction for materials such as poly(2,5-dioctyloxy-paraphenylene vinylene) (DOO-PPV), the hyperfine fields in the polarons of which can be extracted from light-induced electron spin resonance measurements. The resulting theory is shown to give good agreement with experimental data for both normal (H-) and deuterated (D-) DOO-PPV over a wide range of magnetic field strengths once singlet-triplet dephasing is taken into account. Without this effect, which has not been included in any previous simulation of magnetoelectroluminescence, it is not possible to reproduce the experimental data for both isotopologues in a consistent fashion. Our results also indicate that the magnetoconductance of DOO-PPV cannot be solely due to the effect of the magnetic field on the dissociation of polaron pairs.

Toward Better Efficiency of Air-Stable Polyazomethine-Based Organic Solar Cells Using Time-Resolved Photoluminescence and Light-Induced Electron Spin Resonance as Verification Methods

Photoinduced charge transfer is a crucial process in the operation of organic solar cells. In this work we used two spectroscopic techniques, namely, time-resolved photoluminescence and light-induced electron spin resonance, which directly detect this phenomenon in blends composed of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and two air-stable polyazomethines with different chemical structures, one with thiophene ring and cardo moieties (25Th-cardo) and one with three thiophene rings (2252Th-DMB). For both polymers we observed a photoluminescence with a maximum around 2 eV. In a mixture with PCBM, the photoluminescence of both polymers was quenched. At the same time, the photoluminescence of PCBM was enhanced in a mixture with 25Th-cardo, whereas it was similar to the photoluminescence of pure PCBM in the mixture with 2252Th-DMB. Moreover, for 2252Th-DMB:PCBM, two overlapping lines of electron spin resonance were detected under illumination, one originating from the positive polaron and another fr...

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.

Microstructure and charge trapping assessment in highly reactive mixed phase TiO2 photocatalysts

The structural-microstructural characteristics and interfacial charge transfer are key issues to the development of efficient mixed phase TiO2 photocatalysts. In this work, the interplay of lattice deformation and microstrains as well as the identification of charge trapping sites and electron transfer were investigated for a series of nanostructured titania photocatalysts by X-ray powder diffraction analysis, Raman and electron paramagnetic resonance (EPR) spectroscopy. These mixed phase nanomaterials were selected as model sol-gel TiO2 systems based on their exceptional photocatalytic performance over a wide range of hazardous water pollutants (including degradation/mineralization of phenol, 2,4-dichlorophenoxyacetic acid and imazalil) under UV light. Lattice contraction with respect to the bulk anatase together with anisotropic microstrains was identified for the smallest (11nm) anatase nanoparticles. Both effects gradually relaxed with the increase of calcination temperature and the concomitant particle growth, with microstrains scaling linearly with the relative change of the c-axis lattice constant and the broadening of the main anatase Raman mode. The growth of anatase nanoparticles at 1023K with minimal lattice deformation and microstrains resulted in the optimal photocatalytic efficiency, outperforming the benchmark Aeroxide® P25 catalyst. This mixed phase catalyst comprised also larger, though more strained, rutile nanocrystals than P25, and presented an additional deeper electron trapping lattice site according to light-induced EPR measurements. More importantly, electron transfer from rutile to anatase lattice traps was identified by EPR under visible light in the mixed phase photocatalyst. The improved crystal quality of the anatase nanocrystals combined with the enhanced charge separation in anatase/rutile interfaces is concluded crucial to the design of competent solar photocatalytic nanomaterials.

Light-Induced EPR Spectroscopy of Charge Transfer in Low-Band-Gap PCDTBT:PC71BM Bulk Heterojunctions

The light-induced electron paramagnetic resonance (LEPR) study of spin charge carrier steady-state initiated by IR–Vis photons in an organic composite of a low-band-gap poly(N-9’-heptadecanyl-2, 7-carbazole-alt-5, 5-(4’, 7’-di-2-thienyl-2’, 1’, 3′-benzothiadiazole)) with a [6, 6]-phenyl-C 71-butyric acid methyl ester is reported. LEPR spectra of the composite were deconvoluted, and the main resonance parameters of polarons and anion radicals of methanofullerene were determined. The reversible formation in polymer backbone of spin traps, whose number, distribution, and depth is governed by the photon energy, has been shown. Part of the photoinduced charge carriers is pinned by such traps, resulting in the formation in the composite of polymer and fullerene domains with different band gaps and photon sensitivity. Relaxation and dynamics parameters of spin charge carriers were shown to depend on the energy of exciting photons. The mechanisms of polaron diffusion along and between polymer chains, as well as librations of methanofullerene cages in a polymer matrix, were proposed. Our results suggest that the use of C70-counterions as an electron accepter in the composite provokes its layered morphology. This hinders the formation of spin traps, increases the path length for charge carriers, and, therefore, accelerates their spin-assisted relaxation and dynamics through bulk heterojunctions within the visible and infrared regions of the sun spectrum with comparable effectiveness.

In Situ Synthesis and Characterization of Poly(aryleneethynylene)-Grafted Reduced Graphene Oxide.

Using highly soluble bromo-functionalized reduced graphene oxide (RGBr) as a key graphene template for surface-directing Sonogashira-Hagihara polymerization, a novel soluble poly(arylene-ethynylene)-grafted reduced graphene oxide, hereafter abbreviated as PAE-g-RGO, was prepared in situ. The entirely different electron distribution of LUMO and HOMO of PAE-g-RGO suggested the existence of a charge-transfer (CT) state (PAE(.-) -RGO(.+) ). The negative ΔGCS value (-2.57 eV) indicates that the occurrence of the charge separation via (1) RGO* in o-DCB is exothermic and favorable. Upon irradiation with 365 nm light, the light-induced electron paramagnetic resonance (LEPR) spectrum of PAE-g-RGO showed a decrease in the spin-state density owing to photoinduced intramolecular electron transfer events in this system. A sandwich-type Al/PAE-g-RGO/ITO device showed representative bistable electrical switching behavior. The nonvolatile memory performance was attributed to the CT-induced conductance changes, which was supported by molecular computation results and conductive atomic force microscopy (C-AFM) images.

Influence of morphology of low-band-gap PCDTBT:PC71BM composite on photoinduced charge transfer: LEPR spectroscopy study

Light-induced electron paramagnetic resonance (LEPR) study the steady-state of spin charge carriers initiated by Vis-NIR irradiation with the energy (wavelength) of 1.32–2.73eV (940–455nm) in organic composite of a low-band-gap poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (PCDTBT) with a [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) within a wide temperature range is reported. LEPR spectra of the PCDTBT:PC71BM composite were deconvoluted and the main resonance parameters of polarons and anion radicals of methanofullerene were determined. The reversible formation of spin traps in polymer backbone, whose number, distribution and depth governed by the photon energy, has being shown. A part of photoinduced charge carriers is pinned by such traps resulting in formation of domains with different band gaps and photon sensitivity. Relaxation and dynamics parameters of all the charge carriers determined separately by the steady-state saturation method were shown to depend extremely on the energy of exciting photons. Polaron diffusion along polymer chains was analyzed in terms of spin interaction with the lattice phonons of crystalline domains embedded into amorphous polymer matrix. Activation of polaron hopping over the energetic barrier is characteristic for the charge transfer between polymer chains. Small-angle librations of methanofullerene cages in the polymer matrix were shown to follow the Markus model. Our results suggest that C70-counter-ions embedded into the PCDTBT backbone strengthen an overlapping of molecular orbitals and provoke a layered morphology of appropriate composite. This regularizes the polymer matrix, hinders the formation of spin traps and accelerates spin dynamics that in turn facilitates charge transport through bulk heterojunctions. It was shown that the spin-assisted photon-electron conversion is realized in the composite within the visible and near-infrared range of the sun spectrum with comparable efficiency.

Light-induced electron spin resonance study of galvinoxyl-doped P3HT/PCBM bulk heterojunctions

We report the effects of doping of P3HT/PCBM layers with spin 1/2 radicals of galvinoxyl (Gx) based on light-induced electron spin resonance (LESR), photoluminescence-detected magnetic resonance (PLDMR), and post-annealing experiments.LESR showed both a P3HT+ and PCBM− signal for undoped P3HT/PCBM; however, as Gx doping increased (above ∼1 wt%), only the P3HT+ signal was evident in the LESR spectra, with no PCBM− signal.The PLDMR exhibited a strong narrow signal at g = 2.002 that originates from nongeminate polaron pairs; no triplet PLDMR signal has been observed throughout the whole range of Gx concentrations (x = 0, 0.1, 1, 2, 4, 12 wt%). Adding Gx to ∼3 wt% led to a decrease of the PL-enhancement.There was big difference between the slow-dried P3HT/PCBM samples and the post-annealed samples. For the slow-dried samples, efficiency monotonously decreased with Gx additives. When post-annealed, however, an enhancement in η was observed at ∼2 wt% for P3HT/PCBM(1:2) samples.The LESR spectra for post-annealed samples revealed disappearance of Gx spin signals, and thus no spin interactions with PCBM− spins. It is unlikely that the increase of efficiency after Gx doping of P3HT/PCBM solar cell is due to an increase of triplet states.

Photoinduced Dynamics of Charge Separation: From Photosynthesis to Polymer-Fullerene Bulk Heterojunctions.

Understanding charge separation and charge transport is crucial for improving the efficiency of organic solar cells. Their active media are based on organic molecules and polymers, serving as both light-absorbing and transport layers. The charge-transfer (CT) states play an important role, being intermediate for free carrier generation and charge recombination. Here, we use light-induced electron paramagnetic resonance spectroscopy to study the CT dynamics in blends of the polymers P3HT, PCDTBT, and PTB7 with the fullerene derivative C60-PCBM. Time-resolved EPR measurements show strong spin-polarization patterns for all polymer-fullerene blends, confirming predominant generation of singlet CT states and partial orientation ordering near the donor-acceptor interface. These observations allow a comparison with charge separation processes in molecular donor-acceptor systems and in natural and artificial photosynthetic assemblies, and thus the elucidation of the initial steps of sequential CT in organic photovoltaic materials.

Enhanced charge separation in ternary P3HT/PCBM/CuInS2 nanocrystals hybrid solar cells.

Geminate recombination of bound polaron pairs at the donor/acceptor interface is one of the major loss mechanisms in organic bulk heterojunction solar cells. One way to overcome Coulomb attraction between opposite charge carriers and to achieve their full dissociation is the introduction of high dielectric permittivity materials such as nanoparticles of narrow band gap semiconductors. We selected CuInS2 nanocrystals of 7.4 nm size, which present intermediate energy levels with respect to poly(3-hexylthiophene) (P3HT) and Phenyl-C61-butyric acid methyl ester (PCBM). Efficient charge transfer from P3HT to nanocrystals takes place as evidenced by light-induced electron spin resonance. Charge transfer between nanocrystals and PCBM only occurs after replacing bulky dodecanethiol (DDT) surface ligands with shorter 1,2-ethylhexanethiol (EHT) ligands. Solar cells containing in the active layer a ternary blend of P3HT:PCBM:CuInS2-EHT nanocrystals in 1:1:0.5 mass ratio show strongly improved short circuit current density and a higher fill factor with respect to the P3HT:PCBM reference device. Complementary measurements of the absorption properties, external quantum efficiency and charge carrier mobility indicate that enhanced charge separation in the ternary blend is at the origin of the observed behavior. The same trend is observed for blends using the glassy polymer poly(triarylamine) (PTAA).

Improvement of P3HT–ICBA solar cell photovoltaic characteristics due to the incorporation of the maleic anhydride additive: P3HT morphology study of P3HT–ICBA and P3HT–ICBA–MA films by means of X-band LESR

A light-induced electron spin resonance (LESR) X-band study of poly(3-hexylthiophene)/indene-C60 bisadduct P3HT/ICBA flexible solid films containing the additive maleic anhydride (MA) is presented. An increase of P3HT crystalline domain orientation in P3HT:ICBA:MA in comparison with P3HT:ICBA films was confirmed by the angular LESR spectra dependence of P3HT positive polarons. It was assumed that the average increase of power conversion efficiency in P3HT:ICBA:MA solar cells films, relative to P3HT:ICBA, is connected with the more effective crystalline P3HT phase orientation due to the MA sublimation from the composites blends during annealing. The relative average increase of power conversion efficiency of SC films containing MA in comparison to pure P3HT:ICBA blends is estimated to be a factor of (1.15) higher, while the concentrations of functional composites (polymer/fullerene) in blends made with MA decrease by up to 25–30%.