Light-induced Electron Spin Resonance Research Articles

Light-Induced Triplet-Triplet Electron Resonance Spectroscopy.

We present a new technique, light-induced triplet–triplet electron resonance spectroscopy (LITTER), which measures the dipolar interaction between two photoexcited triplet states, enabling both the distance and angular distributions between the two triplet moieties to be determined on a nanometer scale. This is demonstrated for a model bis-porphyrin peptide that renders dipolar traces with strong orientation selection effects. Using simulations and density functional theory calculations, we extract distance distributions and relative orientations of the porphyrin moieties, allowing the dominant conformation of the peptide in a frozen solution to be identified. LITTER removes the requirement of current light-induced electron spin resonance pulse dipolar spectroscopy techniques to have a permanent paramagnetic moiety, becoming more suitable for in-cell applications and facilitating access to distance determination in unmodified macromolecular systems containing photoexcitable moieties. LITTER also has the potential to enable direct comparison with Förster resonance energy transfer and combination with microscopy inside cells.

Charge photogeneration and Recombination in Ternary Organic Photovoltaic Blend PCDTBT/PC60BM/ICBA Studied by EPR Spectroscopy

Using the stationary and pulsed EPR methods, the ternary composite PCDTBT and two fullerene acceptors PCDTBT/PC60BM/ICBA 1:1:1, as well as the corresponding binary composites PCDTBT/PC60BM 1:2 and PCDTBT/ICBA 1:2, were studied at a temperature of 80 K. Modeling these spectra allows us to estimate the contributions of PC60BM and ICBA to the light-induced EPR signal of the PCDTBT/PC60BM/ICBA ternary composite as 0.7:0.3. The absence of new lines in the EPR spectrum of the ternary composite, in comparison with the corresponding binary ones, means that the mechanism of the molecular alloy of PC60BM and ICBA, as previously assumed, is not operative in this system, and the most probable scenario is the existence of two parallel heterojunctions PCDTBT/PC60BM and PCDTBT/ICBA. This conclusion is confirmed by modeling the decay curves of the light-induced EPR upon turning off the light, as well as the out-of-phase electron spin echo from the charge transfer state (the main intermediate of the photoelectric conversion) in these composites. It is noteworthy that in the ternary composite with the same fullerene acceptors, but with a different polymer donor (P3HT), the molecular alloy mechanism of two acceptors is realized (Angmo et al. in J Mater Chem C 3: 5541–5548, 2015). It is likely that the polymer donor has a decisive influence on the morphology and electron-transport properties of such ternary composites. It should be noted that the methods of light-induced EPR and out-of-phase ESE were used for the first time to study ternary donor–acceptor composites.

Light-Induced EPR Study of the Effect of Coumarin Trace Additives on Spin Dynamics in the P3DDT/PC61BM Polymer Composite

The paper presents the results of an EPR study of the magnetic, relaxation, and dynamic parameters of spin charge carriers photogenerated by light in photovoltaic polymer composites, based on poly(3-dodecylthiophene) (P3DDT) and 6,6-phenyl-C61-butyric acid methyl ester (PCBM), in the presence of coumarin admixed to the system. It has been shown that the concentration of polarons and fullerene radical anions is characterized by a nonmonotonic dependence on the energy of incident photons with extrema at 1.9 and 2.7 eV. The largest increase in the concentration of mobile charge carriers was observed in composites with a coumarin content of 3 or 6 wt %. A significant slowdown of the carrier recombination process in the composites containing 3–6 wt % coumarin was observed after switching off the light. The temperature dependences of the concentrations of mobile and localized charge carriers for P3DDT/PCBM samples with a coumarin content of 3 wt % showed enhancement of the exchange interaction between charge carriers. The data obtained lead to the conclusion that coumarin affects spin interactions in the polymer composite.

Effects of small-molecule-doping on spin-assisted processes in P3DDT:PC61BM photovoltaics

The influence of the 1,2-benzopyrone (BP) and 2,5-diphenyloxazole (PPO, DPO) additives on the formation, separation, motion and recombination of charge carriers in the poly(3-dodecylthiophene):[6,6]-phenyl-C61-butanoic acid methyl ester (P3DDT:PC61BM) organic photovoltaics was investigated by the light-induced electron paramagnetic resonance (LEPR) and NIR-Vis-UV spectroscopy within the wide temperature and photon energy range. These processes were interpreted in the framework of the exchange interaction of spin ensembles differently distributed in bulk heterojunctions of the P3DDT:PC61BM composite. The concentration, composition and dynamics of spin charge carriers were shown to be determined by the modification degree of the sample with small molecules, the energy of the light photons as well as the number, spatial distribution and energetic depth of spin traps formed in the disordered polymer matrix. Electronic functionality of the composite becomes better after its doping with BP and PPO additives up to optimal weight levels of 0.03 and 0.06, respectively. Such modification can improve the morphology/ordering of the composite that increases the number of highly mobile charge carriers due to the release of a part of carriers captured by energetically deep spin traps. This increases an exchange interaction between spin ensembles, reduces the number and depth of electron spin traps that, in turn, prevents the recombination of charge carriers and accelerates the power conversion.

Long‐Standing Optically Excited Magnetic States in ZnO Nanoparticles

The spectral dependence of the light‐induced magnetic states generated in zinc oxide nanoparticles by illumination during an electron paramagnetic resonance (EPR) experiment is studied for three incident light wavelengths (451, 405, and 370 nm). The minimal wavelength excitation is found to be in the visible violet region, very close to 405 nm for all of the light‐induced EPR lines. This points to a close relationship between their originating centers, despite the difference in their dynamics previously revealed by saturation recovery measurements. The study of the light‐induced signals decay with time (after illumination removal) and with temperature (under constant illumination) confirms the existence of two groups of lines characterized by different dynamics. The quite unusual persistence of these magnetic states after excitation removal opens promising possibilities for encoding information within semiconducting nanoparticles.

Organic Amines as Targeting Stabilizer at the Polymer/Fullerene Interface for Polymer:PC 61 BM Solar Cells

Herein, it is demonstrated that a small amount (0.05% in weight ratio) of polyethyleneimine (PEI) can effectively suppress the “burn‐in” degradation of both PTB7‐Th:PC61BM and P3HT:PC61BM cells, similar to the piperazine derivatives, suggesting that organic amines can serve as universal stabilizer in polymer:PC61BM solar cells. Light‐induced electron spin resonance (LESR) spectroscopy measurement shows a higher ESR signal intensity of PC61BM anions in 0.2% PEI‐doped film than in 1% piperazine‐doped film. Moreover, no piperazine is detected in a 10% (w/w) piperazine‐doped film by gas chromatography–mass spectrometry (GSMS). These results suggest that the residual piperazine in the blend film is very low, which can be understood by the high volatility of piperazine. Quantum calculations are performed on the intermolecular binding energy (EB) between polymer (using model repeating units), PC61BM, and piperazine molecules. Results reveal that piperazine prefers to localize at the polymer:fullerene interface by complexing with PC61BM (in P3HT:PC61BM) or PTB7‐Th (in PTB7‐Th:PC61BM system), which indicates that the photo dimerization of PC61BM which causes the “burn‐in” degradation of polymer:fullerene solar cells mainly happens at the donor/acceptor interface, and the organic amine serves as the targeting stabilizer at the interface.

Azetidinium as cation in lead mixed halide perovskite nanocrystals of optoelectronic quality

Previous theoretical calculations show that azetidinium has the right radial size to form a 3D perovskite with lead halides [G. Kieslich et al., Chem. Sci. 5, 4712 (2014)] and has been shown to impart, as the A-site cation of the ABX3 unit, beneficial properties to ferroelectric perovskites [B. Zhou et al., Angew. Chem., Int. Ed. 50, 11441 (2011)]. However, there has been very limited research into its use as the cation in lead halide perovskites to date. In this communication, we report the synthesis and characterization of azetidinium-based lead mixed halide perovskite colloidal nanocrystals. The mixed halide system is iodine and chlorine unlike other reported nanocrystals in the literature, where the halide systems are either iodine/bromine or bromine/chlorine. UV-visible absorbance data, complemented with photoluminescence spectroscopy, reveal an indirect-bandgap of about 2.018 eV for our nanocrystals. Structural characterization using transmission electron microscopy shows two distinct interatomic distances (2.98 Å ± 0.15 Å and 3.43 Å ± 0.16 Å) and non-orthogonal lattice angles (≈112°) intrinsic to the nanocrystals with a probable triclinic structure revealed by X-ray diffraction. The presence of chlorine and iodine within the nanocrystals is confirmed by energy dispersive X-ray spectroscopy. Finally, light-induced electron paramagnetic resonance spectroscopy with PCBM confirms the photoinduced charge transfer capabilities of the nanocrystals. The formation of such semiconducting lead mixed halide perovskites using azetidinium as the cation suggests a promising subclass of hybrid perovskites holding potential for optoelectronic applications such as in solar cells and photodetectors.

Polaron and Exciton Delocalization in Oligomers of High-Performance Polymer PTB7.

A key characteristic of organic photovoltaic cells is the efficient charge separation in the active layer. Sufficient delocalization of the positive polaron in organic photovoltaics is considered essential for the effective separation of the opposite charges and the suppression of recombination. We use light-induced EPR and ENDOR spectroscopy combined with DFT calculations to determine the electronic structure of the positive polaron in PTB7-type oligomers. Utilizing the superior spectral resolution of high-frequency (130 GHz) D-band EPR, the principal components of the g tensors were determined. Pulsed ENDOR spectroscopy at X-band allowed the measurement of 1H hyperfine coupling constants. A comparison of g tensors and 1H hyperfine coupling constants of the PTB7-type oligomers with the high-performance PTB7 polymer revealed a delocalization of the positive polaron in the polymer over about four monomeric units, corresponding to about 45 Å in length. Our current study thus not only determines the polaron delocalization length in PTB7 but also validates the approach combining EPR/ENDOR spectroscopy with DFT-calculated magnetic resonance parameters. This is of importance in those cases where oligomers of defined length are not easily obtained. In addition, the delocalization of the neutral triplet exciton was also determined in the oligomers and compared with polymer PTB7. The analysis revealed that the neutral triplet exciton is substantially more delocalized than the positive polaron, exceeding 10 monomeric units.

Molecular Oriented Charge Accumulation in High-Efficiency Polymer Solar Cells as Revealed by Operando Spin Analysis.

A low band-gap polymer, PTB7-Th, is one of the typical p-type semiconductors among the next-generation solar-cell materials that have achieved power conversion efficiencies of over 10%. However, the internal deterioration mechanism of high-efficiency polymer solar cells such as PTB7-Th-based cells is still an open issue and has been extensively studied. Here, we report a study with operando electron spin resonance (ESR) spectroscopy for PTB7-Th polymer solar cells with an n-type semiconductor PC71BM to clarify the internal deterioration mechanism at a molecular level. We have directly observed ambipolar charge accumulation with a face-on molecular orientation in the cells under simulated solar irradiation using an operando light-induced ESR technique. Moreover, we have found a clear correlation between the charge accumulation and performance deterioration of the cells. The charge accumulation sites have been clarified by the ESR analysis and density functional theory calculation. The prevention of such charge accumulation on the basis of the present finding would be important for the commercialization of high-efficiency polymer solar cells.

Synthesis of 2,2′-[2,2′-(arenediyl)bis(anthra[2,3-b]thiophene-5,10-diylidene)]tetrapropanedinitriles and their performance as non-fullerene acceptors in organic photovoltaics

Abstract Three novel molecules based on thiophene-fused 11,11,12,12-tetracyano-9,10-anthraquinodimethane (TCAQ) units linked through functionalized π-conjugated arene bridges are synthesized by three-step sequence of Sonagashira coupling, double thioannulation and Knoevenagel condensation. The energy of their frontier orbitals shows that these molecules are suitable for replacing fullerenes as electron acceptors in composites with conjugated donor polymers used in organic photovoltaics (OPV). The composite of polymer donor PCDTBT with fluorene-linked thiophene-fused TCAQs demonstrates good solubility in organic solvents and bulk heterojunction morphology with characteristic donor and acceptor domain size of several tens of nanometers, favourable for photoelectric conversion. This composite also exhibits prominent light-induced EPR signal indicating efficient photoinduced free charge generation. OPV devices based on this composite were fabricated by vacuum-free method. They show photoelectric conversion with maximum PCE of 0.71%.

Light-Induced Electron Paramagnetic Resonance Spectroscopy of Spin-Assisted Charge Transfer in Narrow-Bandgap Copolymer:Methanofullerene Composites

Magnetic resonance, relaxation, and dynamic parameters of polarons and methanofullerene radical anions photoinduced in photovoltaic composites formed by narrow-bandgap poly[(9,9-dioctylfluorenyl-2,...

Influence of Photogeneration Frequency on the Transport of Spin Charge Carriers in the Copolymer–Methanofullerene Composite: EPR Study

The photovoltaic composite formed by narrow-gap copolymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(bithiophene)] and methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) has been studied in a wide energy range of generating photons, 1.32–3.14 eV at T = 77 K, by the light-induced electron paramagnetic resonance. It has been shown that some polarons are captured by spin traps formed in the copolymer matrix, and the concentration and depth of such traps are determined by the photon energy. The recombination kinetics of polarons and fullerene radical anions after turning off the light can be described in the framework of a second-order bimolecular process. The formation of spin traps in the copolymer matrix and the exchange interaction between different spin packets cause the extreme sensitivity of magnetic resonance and electronic parameters of charge carriers to the number and energy of generating photons.

2,7-Disubstituted 1,3,6,8-tetraazabenzopyrenes: Synthesis, characterization, optical and electrochemical properties

The syntheses of 2,7-diamino-1,3,6,8-tetraazabenzopyrene framework was achieved through CuI-catalyzed double annulation of 1,4-diiodoanthraquinone with guanidine. In addition, the preparation of series of 2,7-disubstituted 1,3,6,8-tetraazabenzopyrenes functionalized by NR2, CO, Cl, alkoxy and phenoxy groups with good yields was achieved through the simple transformations such as diazotization, alkylation and nucleophilic aromatic substitution. The properties of 1,3,6,8-tetraazabenzopyrenes synthesized were studied by DFT-calculations, optical and photoluminescence spectroscopy, cyclic voltammetry and light-induced EPR spectroscopy. The luminescence of amino derivatives is quenched in acidic solutions depending on the degree of protonation.

Light-Induced Pulsed EPR Dipolar Spectroscopy on a Paradigmatic Hemeprotein.

Light‐induced pulsed EPR dipolar spectroscopic methods allow the determination of nanometer distances between paramagnetic sites. Here we employ orthogonal spin labels, a chromophore triplet state and a stable radical, to carry out distance measurements in singly nitroxide‐labeled human neuroglobin. We demonstrate that Zn‐substitution of neuroglobin, to populate the Zn(II) protoporphyrin IX triplet state, makes it possible to perform light‐induced pulsed dipolar experiments on hemeproteins, extending the use of light‐induced dipolar spectroscopy to this large class of metalloproteins. The versatility of the method is ensured by the employment of different techniques: relaxation‐induced dipolar modulation enhancement (RIDME) is applied for the first time to the photoexcited triplet state. In addition, an alternative pulse scheme for laser‐induced magnetic dipole (LaserIMD) spectroscopy, based on the refocused‐echo detection sequence, is proposed for accurate zero‐time determination and reliable distance analysis.

Thermally activated, light-induced electron-spin-resonance spin density reflected by photocurrents in a perovskite solar cell

Light-induced electron-spin-resonance (LESR) spin density in a perovskite solar cell (PSC) is ascribed to photo-induced charges accumulated at the interface of the perovskite and the hole transport layer (HTL). The LESR spin density was observed to be decreased as the temperature was lowered from room temperature to 170 K in a PSC; below ∼170 K, no LESR was found. Meanwhile, the photocurrent showed only an ∼25% decrease between room temperature and 170 K. The comparison of LESR in temperature-dependence with transient or steady photocurrents suggested that the decrease in photocurrent at the low temperatures stems mainly from trapping of delocalized holes at the interface of perovskite and HTL.

Light-induced EPR study of spin-assisted charge transport in PFOT:PC61BM composite

Magnetic resonance, relaxation and dynamic parameters of polaron spins and methanfullerene radical anions, initiated by the infrared-visible-ultraviolet photons in bulk heterojunctions of the composite formed by narrow-band poly[(9,9-dioctylfluorenyl-2.7-diyl)-co-(bithiophene)] (PFOT, F8T2) copolymer and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) globules were investigated by the direct Light-Induced Electron Paramagnetic Resonance (LEPR) spectroscopy. It was shown that some of the polarons are captured by spin traps formed in the copolymer matrix due to its disordering. The number, spatial distribution and energy depth of such traps depend on the structure and morphology of composite. It is shown that the main parameters of both charge carriers are determined by the exchange interaction of the spin ensembles, as well as the energy of the photons. The formation of spin traps in the copolymer matrix and the exchange interaction between different spin packets cause the extreme sensitivity of the composite's magnetic resonance and electronic parameters to the number and energy of the initiating photons. The predominant photoinitiation of localized polarons in the copolymer matrix was demonstrated. This process is substantially accelerated when the composite is illuminated by photons with the energy lying near 1.8 and 2.7 eV. The recombination of both charge carriers can be described in terms of a bimolecular process of the second order. It was found that the contributions of polarons and methanofullerene radical anions to the effective paramagnetic susceptibility increase substantially near the photon energy of 2.6 and 2.1 eV due to the exchange interaction of these charge carriers. It is shown that the mobility of polarons varies monotonically throughout all the photon energy range, whereas the librational spin dynamics of the methanofullerene globules is substantially accelerated near the photon energy of 2.0 and 2.7 eV.

Structural Origin of the Midgap Electronic States and the Urbach Tail in Pnictogen-Chalcogenide Glasses.

We determine the electronic density of states for computationally generated bulk samples of amorphous chalcogenide alloys As xSe100- x. The samples were generated using a structure-building algorithm reported recently by us. Several key features of the calculated density of states are in good agreement with experiment: The trend of the mobility gap with arsenic content is reproduced. The sample-to-sample variation in the energies of states near the mobility gap is quantitatively consistent with the width of the Urbach tail in the optical edge observed in experiment. Most importantly, our samples consistently exhibit very deep-lying midgap electronic states that are delocalized significantly more than what would be expected for a deep impurity or defect state; the delocalization is highly anisotropic. These properties are consistent with those of the topological midgap electronic states that have been proposed by Zhugayevych and Lubchenko as an explanation for several puzzling optoelectronic anomalies observed in the chalcogenides, including light-induced midgap absorption and electron spin resonance signal, and anomalous photoluminescence. In a complement to the traditional view of the Urbach states as a generic consequence of disorder in atomic positions, the present results suggest these states can be also thought of as intimate pairs of topological midgap states that cannot recombine because of disorder. Finally, samples with an odd number of electrons exhibit neutral, spin 1/2 midgap states as well as polaron-like configurations that consist of a charge carrier bound to an intimate pair of midgap states; the polaron's identity, electron or hole, depends on the preparation protocol of the sample.

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.

Disentangling overlapping high-field EPR spectra of organic radicals: Identification of light-induced polarons in the record fullerene-free solar cell blend PBDB-T:ITIC

We present a combined high-field EPR and DFT study of light-induced radicals in the bulk heterojunction blend of PBDB-T:ITIC, currently one of the highest efficiency non-fullerene donor:acceptor combinations in organic photovoltaics. We demonstrate two different approaches for disentangling the strongly overlapping high-field EPR spectra of the positive and negative polarons after charge separation: (1) relaxation-filtered field-swept EPR based on the difference in T1 spin-relaxation times and (2) field-swept EDNMR-induced EPR by exploiting the presence of 14N hyperfine couplings in only one of the radical species, the small molecule acceptor radical. The approach is validated by light-induced EPR spectra on related blends and the spectral assignment is underpinned by DFT computations. The broader applicability of the spectral disentangling methods is discussed.

Mechanistic Insight into the Photocatalytic Working of Fluorinated Anatase {001} Nanosheets

Anatase nanosheets with exposed {001} facets have gained increasing interest for photocatalytic applications. To fully understand the structure-to-activity relation, combined experimental and computational methods have been exploited. Anatase nanosheets were prepared under hydrothermal conditions in the presence of fluorine ions. High resolution scanning transmission electron microscopy was used to fully characterize the synthesized material, confirming the TiO2 nanosheet morphology. Moreover, the surface structure and composition of a single nanosheet could be determined by annular bright-field scanning transmission electron microscopy (ABF-STEM) and STEM electron energy loss spectroscopy (STEM-EELS). The photocatalytic activity was tested for the decomposition of organic dyes rhodamine 6G and methyl orange and compared to a reference TiO2 anatase sample. The anatase nanosheets with exposed {001} facets revealed a significantly lower photocatalytic activity compared to the reference. In order to understand the mechanism for the catalytic performance, and to investigate the role of the presence of F–, light-induced electron paramagnetic resonance (EPR) experiments were performed. The EPR results are in agreement with TEM, proving the presence of Ti3+ species close to the surface of the sample and allowing the analysis of the photoinduced formation of paramagnetic species. Further, ab initio calculations of the anisotropic effective mass of electrons and electron holes in anatase show a very high effective mass of electrons in the [001] direction, having a negative impact on the mobility of electrons toward the {001} surface and thus the photocatalysis. Finally, motivated by the experimental results that indicate the presence of fluorine atoms at the surface, we performed ab initio calculations to determine the position of the band edges in anatase slabs with different terminations of the {001} surface. The presence of fluorine atoms near the surface is shown to strongly shift down the band edges, which indicates another reason why it can be expected that the prepared samples with a large amount of {001} surface, but with fluorine atoms near the surface, show only a low photocatalytic activity.