Photoinduced Dynamics of Charge Separation: From Photosynthesis

Article pubs.acs.org/JPCB

Photoinduced Dynamics of Charge Separation: From Photosynthesis to Polymer−Fullerene Bulk Heterojunctions Jens Niklas,† Serge Beaupré,‡ Mario Leclerc,‡ Tao Xu,§ Luping Yu,§ Andreas Sperlich,∥ Vladimir Dyakonov,∥ and Oleg G. Poluektov*,† †

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Department of Chemistry, Laval University, Quebec City, Quebec G1V 0A6, Canada § Department of Chemistry and James Franck Institute, University of Chicago, Chicago, Illinois 60637, United States ∥ University of Würzburg and Bavarian Centre for Applied Energy Research (ZAE Bayern), D-97074 Würzburg, Germany ‡

ABSTRACT: 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.

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the range of ∼30 Å. This stabilizes the radical pair (RP) on the time scale from milliseconds up to seconds, which is needed for the following slow process of chemical transformation. Even though a variety of different photosystems have been studied, these general principles have been found everywhere.1,3−6 Charge separation processes in OPVs are often described in terms of solid-state physics.8−11 This process starts by lightinduced (singlet) exciton generation. This strongly bound neutral exciton diffuses and upon arrival at an interface can break down into two spin-carrying charged counterparts. This state is referred to as the charge-transfer (CT) state. The dissociation of this Coulomb-bound CT state into two separated charges is prerequisite of high charge carrier photogeneration yield. The two separated charges and the polarization induced in their surrounding are often called positive polaron (or “hole”) and negative polaron. In contrast to the typically planar structure used in semiconductor photovoltaics, molecular BHJ are intermixed and hence rather disordered systems, where there is no clearly defined interface between different phases of materials. Another important

INTRODUCTION In natural photosynthesis, as found both in bacteria and plants, light-induced charge separation occurs with a quantum yield of essentially unity.1,2 This efficiency is so far unmatched by any artificial system where light-induced charge separation happens. In photosynthetic systems, this efficient separation of charges is achieved with the aid of cascaded energy levels and ion screening.1,3−6 Organic photovoltaic (OPV) systems, by contrast, seem to possess none of these elements, yet surprisingly, some OPV devices work very efficiently.7 Hence, several fundamental questions of how the organic bulk heterojunction (BHJ) cell enables efficient long-lived and long-range charge separation remain unanswered. In photosynthesis, the absorption of a photon creates an excited singlet state, which is highly reducing, and electron transfer (ET) to the next redox-active cofactor is thermodynamically favorable. The initial charge separation is followed by further transfer of the electron thermodynamically downhill via multiple redox-active cofactors where the free energy is reduced, until the recombination rate is decreased by orders of magnitude. The positive charge remains largely where the initial charge separation took place. Electron transfer occurs by hopping from one acceptor to the next one and stabilizing the charge-separated (CS) state. Typically, after three to four ET steps, the distance between positive and negative charge is in © 2015 American Chemical Society

Special Issue: John R. Miller and Marshall D. Newton Festschrift Received: November 3, 2014 Revised: January 16, 2015 Published: January 19, 2015 7407

DOI: 10.1021/jp511021v J. Phys. Chem. B 2015, 119, 7407−7416

Article

The Journal of Physical Chemistry B

Figure 1. Schematic representation of typical photoinduced energy- and charge-transfer processes in molecular donor−acceptor systems and photosystems. As discussed in the text, only the secondary radical pairs can be detected by EPR. The lifetime of primary radical pairs is too short and beyond the time resolution of the EPR technique. D, donor; A, acceptor; ISC, intersystem crossing; S−T0, singlet−triplet mixing mechanism.

Scheme 1. Structures of the Monomeric Units of the Polymers P3HT, PCDTBT, and PTB7 and the Fullerene Derivative C60PCBM

tems.20−23 In semiconductor materials, Miller−Abrahams theory is commonly used to describe charge transport.24 Electron paramagnetic resonance (EPR) spectroscopy has been used extensively in the research on photosynthesis and now emerges also in OPV research as a very valuable method for the studies of charge and spin dynamics.25−34 It is ideally suited to investigate the electronic structure and dynamics of charged states in organic materials, since it exclusively monitors molecules with unpaired electrons, e.g., the radical cations and anions, RPs and triplet states, which are generated during lightinduced charge separation processes. That means that EPR is insensitive to the majority of molecules in the material, which do not take part in the charge separation processes. Timeresolved EPR (TR-EPR) has the additional advantage to monitor the charge dynamics on the nanoseconds to seconds time scale.26,27,35 The TR-EPR technique was crucial for understanding spin-dependent photoinduced energy and CT pathways in molecular donor−acceptor systems and photosynthesis.25,35,36 Typical pathways for the photoinduced charge process are schematically shown in Figure 1. After light-induced electron transition of the donor, D, to the excited singlet state, D*, the electron is transferred to the nearest acceptor, thus forming the primary singlet RP, S(D+···A−). Such RPs usually have a very short lifetime, i.e., from tens to hundreds of picoseconds, and are not directly observable by EPR. Owing to the short separation distance between D and A, the magnetic interaction within the primary RP is strong, and the singlet−

differencewith respect to inorganic semiconductor photovoltaicsis the significantly lower dielectric constant of the OPV materials which is similar to molecular donor−acceptor systems. This leads to less effective screening of the separated charges and thus requires larger distances to achieve “stable” charge separation. Finally, the charge transport mechanisms in both types of photovoltaic systems are quite different, being of hopping type in OPV materials and band-like in crystalline semiconductors. OPV materials are essentially molecular systems and the molecular/chemical terminology may be better suited for description of ET processes.9,12 In natural and artificial photosynthetic molecular systems the generation of the CT state is referred to as primary electron transfer. Only further ET steps, termed secondary electron transfer, lead to stable, longlived charge separation. These charges are termed radical cation and radical anion, respectively. Note that the latter terms are typical for the molecular/chemical viewpoint, while the polaron notation is widely used in the physics community. In the following, both terms will be used interchangeably. Electron-transfer processes in organic molecular systems are well understood within the Marcus theory of ET and its recent extensions.13−16 Predictions of Marcus theory were experimentally proven in the seminal works of John Miller and Gerhard Closs17−19 and later applied to study the dynamics of charge separation processes in proteins and model sys7408


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High-frequency EPR measurements were performed on a home-built D-band (130 GHz) spectrometer46,47 in the pulsed mode. EPR spectra were recorded by monitoring the electron spin−echo (ESE) intensity from a two microwave pulse sequence as a function of magnetic field. The duration of a π/2 microwave pulse was 40−60 ns, and typical separation times between microwave pulses were 150−300 ns. Light excitation was done directly in the cavity of the spectrometer with 532 nm laser light through an optical fiber (Nd:YAG Laser, INDI, Newport, operating at 20 Hz, and OPO, basiScan, GWU). Typical incident light intensities at the sample were 20 mW (1 mJ/pulse/cm2). Special precautions were taken to minimize sample degradation, which is facilitated by molecular oxygen in combination with light.48−51 In addition, both films and frozen solutions were studied at cryogenic temperatures. Data processing was done using Xepr (Bruker BioSpin, Rheinstetten) and Matlab 7.11.2 (MathWorks, Natick, MA) software. The magnetic parameters were obtained from theoretical simulation of the EPR spectra. These simulations were performed using the EasySpin software package (version 4.5.5).52

triplet (S−T0) conversion (mixing) is very efficient. If the lifetime of these RPs is short compared to the S−T0 conversion time, the pair can decay via two possible routes: to the singlet ground state or a further ET forming the secondary singlet RPs, S (D+···AAA−). If the RP lifetime is long and comparable to the S−T0 mixing rate, the singlet RP can be converted to a triplet RP, T(D+···A−). The latter has also two preferable escape routes: decay, preferably to the triplet state located on either the donor, TD, or the acceptor, TA, or forward ET to the secondary triplet RP, T(D+···AAA−). Here we report TR-EPR studies on CS states in BHJ systems containing the reference polymer, poly(3-hexylthiophene-2,5diyl) (P3HT),37,38 and two low-band-gap polymers, poly[[9-(1octylnonyl)-9H-carbazole-2,7-diyl]thiophene-2,5-diyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT)39,40 and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b] thiophenediyl]] (PTB7),41 in blends with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (C60-PCBM)42 (Scheme 1). The TR-EPR spectra exhibit strong polarization patterns for all polymer−fullerene blends, caused by non-Boltzmann population of the spin energy levels. The experimental data show close similarities of ET processes in OPV materials and photosynthetic systems. This allows us to analyze and describe ET processes based on the knowledge accumulated in research on photochemical CS in molecular systems. Analysis using the model of weakly coupled spincorrelated radical pairs (SCRPs)29,43−45 leads us to the conclusion that charge separation occurs as sequential CT between donor−acceptor molecules.

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RESULTS AND DISCUSSION Upon illumination, intense cw EPR signals around g ≈ ge were detected in the three different polymer−fullerene blends, as shown in Figure 2. We also note that only very weak EPR

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EXPERIMENTAL SECTION Sample Preparation. The organic polymers studied in this work were highly regioregular P3HT, PCDTBT, and PTB7 (Scheme 1). P3HT was obtained from Rieke Metals (Lincoln, NE); PCDTBT and PTB7 were synthesized as described previously.39−41 The soluble fullerene derivative C60-PCBM was obtained from Sigma-Aldrich (St. Louis, MO) and Solenne BV (Groningen, The Netherlands). Polymer−fullerene mixtures (1:2 or 1:4 weight ratios) were prepared under anaerobic conditions in a N2 drybox, using deoxygenated toluene or chlorobenzene as solvents (Sigma-Aldrich). The concentrations of the solutions were in 5−15 mg/mL range. The solutions were filled into EPR quartz tubes, sealed under N2-atmosphere, and then frozen quickly in liquid nitrogen. Films were prepared by slowly pumping the samples to remove the solvent gradually. Upon mixing of polymer and fullerene all further steps were performed under dimmed light. EPR Spectroscopy. Continuous-wave (cw) and pulsed Xband (9 GHz) EPR experiments were carried out with a Bruker ELEXSYS E580 EPR spectrometer (Bruker Biospin, Rheinstetten, Germany), equipped with a Flexline dielectric ring resonator (Bruker ER 4118X-MD5-W1) and a helium gasflow cryostat (CF935, Oxford Instruments, UK). The temperature controller was an ITC (Oxford Instruments, UK). Light excitation was done in situ with ns pulses of 532 nm laser light (Nd:YAG Laser with OPO, model Vibrant from Opotek, operating at 10 Hz), or a 300 W xenon lamp (LX 300F from Atlas Specialty Lighting with a PS300-13 300 W power supply from PerkinElmer). Typical incident light intensities at the sample were around 2 W for the lamp and 40 mW (2 mJ/ pulse/cm2) for the laser.

Figure 2. Light-induced X-band cw EPR of polymer−fullerene blends containing P3HT, PCDTBT, or PTB7 and C60-PCBM. (a) cw EPR spectra recorded as first derivative of the absorption signal. (b,c) Decay of the cw EPR signal after several laser flashes recorded for P3HT:C60PCBM blend at the maximum of the EPR signal (345.55 mT). Note 3 orders of magnitude difference of the time range between panels (b) and (c). Measurements were done at T = 50 K.

signals were observed prior to illumination, which we attribute to traces of paramagnetic impurities (not shown). Lightinduced EPR signals in similar polymer−fullerene blends were previously reported, line shape analyses were performed and magnetic resonance parameters were determined.32,34,53−55 The spectra (Figure 2a) consist of two signals, at low and high magnetic field, attributed to positive and negative light-induced polarons on the polymer chain and on the fullerene cage, respectively. The intensity of these signals decreases after 7409


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Figure 3. Time-resolved X-band cw EPR spectra of polymer−fullerene blends containing P3HT, PCDTBT, or PTB7 and C60-PCBM. Spectra were detected at 0.5 μs delay after laser flash. Black lines are experimental spectra in panels (a) and (b). Letters A and E indicate absorptive and emissive phases of the signals. In (b) the spectrum of P3HT:C60-PCBM blend is overlapped with TR-EPR spectra of photosystem I (P700+A1−) recorded under the similar conditions. No change in the polarization pattern or appearance of the broad signal, similar to that reported in ref 61, was observed at delay times of 0.5−3 μs. At delay times longer than 3 μs, thermalization processes considerably distort polarization pattern. Spectra were fit as the sum of the Gaussian lines with proper phase: emission for polymer and absorption for fullerene. A small admission, ca. 20% by amplitude, of derivative-type signal is needed to explain the line shape of PCDTBT:C60-PCBM spectra, while this contribution is almost negligible for PTB7:C60PCBM. In all cases the width of the fitted lines is 0.1−0.2 mT broader compared to the fitting of cw EPR spectra. Green lines show the simulation of the positive polaron on the polymer, blue lines show the simulation of the negative polaron on the fullerene, and red lines show the sum of the two simulations.

spins.25−27,36 This type of information cannot be obtained by ultrafast optical spectroscopy. The cw TR-EPR spectra obtained in the three polymer− fullerene blends at 0.5 μs delay after laser flash shown in Figure 3. All blends under study reveal electron spin-polarization effects. The spectrum of reference P3HT:C60‑PCBM blend (top) is in good agreement with previously reported spectra.62−65 Polarization manifests as an alternation of absorption (A) and emission (E) signals. Note that the ordinate (y-axis) in Figure 3 directly corresponds to the intensity of microwaves reflected from the sample inside the cavity resulting in respective minima and maxima. While the cw EPR spectra (Figure 2) of all blends look similar, their cw TREPR spectra (Figure 3) are strikingly different. In order to understand this difference we consider the origin of spinpolarization in TR-EPR. The observation of electron spin-polarization (due to nonBoltzmann distribution) is generally explained in the framework of the so-called chemically induced dynamic electron polarization (CIDEP) model and attributed to spin-dynamics within the primary RP, where strong magnetic interactions between positive and negative charges (radical ions) are present.66−72 Within this model, the electron spin-polarization builds up during the charge separation process, when positive and negative charges are still in close vicinity to each other. During the course of time the strong magnetic interactions within this RP mix the spin states of the radicals, thus forming a nonBoltzmann distribution, which is conserved upon further separation of the radicals. The conventional CIDEP effect, typically observed in liquids, is detected when radical ions are quickly separated to a distance at which the interaction between

illumination is ceased, while spectral line shapes are similar. The presence of the polaron signals after illumination reveals that electron and holes are not free but locally trapped. Figure 2b,c displays the temporal evolution of the cw EPR signal intensity for the P3HT:C60-PCBM blend after switching off the light. Note, EPR experiments are performed at “open circuit conditions” and charges cannot be extracted from the samples. The bimolecular recombination kinetics clearly demonstrates decay over a long time scale, which is due to a broad distribution of trapping energies for electrons and holes. This is a consequence of the heterogeneous character of BHJ. Long time scale (multiexponential) kinetics, also known as logarithmic or polychronic kinetics, are typically observed in heterogeneous systems with distributed kinetic parameters, such as activation/trapping energy and pre-exponential factors.56−60 This type of kinetics cannot be modeled by a single- or biexponential(s) at any given temperature. In order to study the evolution of the EPR spectra taking place at times shorter than milliseconds, we performed directdetection time-resolved experiments. TR-EPR spectroscopy has much lower time resolution in comparison to ultrafast optical spectroscopy and is around 500 ns for our TR-EPR spectrometer. However, despite its lower time resolution, the magnetic interactions in short-lived primary charge separated states (the precursor states) can influence the line shape of longer-lived CS states. Thus, the dynamics of the short-lived CS states (≪500 ns) may become indirectly accessible with TREPR. In addition, EPR spectroscopy can provide structural information about the longer-lived CS states through the measurements of the magnetic interactions between 7410


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dipole interaction, this value allows estimating the distance between radicals in the observable RP state within the point dipole approximation as ca. 25−30 Å. According to ref 78, at such distances the exchange coupling in non-covalently bound molecular systems is several orders of magnitude smaller than the dipolar interaction and hence does not need to be considered. Thus, the CIDEP model is not applicable to explain EA polarization in PCDTBT:C60-PCBM and PTB7:C60-PCBM spectra. On the other hand, transformation of the polarization pattern from EAEA to EA is known for molecular donor−acceptor systems and can be rationalized within the theory of sequential ET, which is a combination of conventional CIDEP and SCRP theories and assumes the presence of an intermediate RP with lifetime long enough to allow for singlet−triplet mixing.79−83 During the well-known sequential ET process in natural photosynthetic proteins, the electron first hops to the adjacent acceptor, forming the primary RP, then to the intermediate acceptor, forming a sequential RP, and after that to secondary RP. The lifetime of the primary RP is too short (less than 10 ps) to contribute to the polarization pattern.1,4,5 On the other hand, the lifetime of the intermediate RP is typically hundreds of picoseconds,1,4,5 and, being comparable to the S−T0 mixing time, can contribute to the distortion of the observable SCRP polarization pattern (Figure 1). The longer the lifetime of the sequential RP, the stronger is the transformation of the EAEA to EA pattern.79−85 It is also interesting to compare the SCRP EPR spectrum of photosystem I (PS I) and P3HT:C60-PCBM film (see Figure 3b, top). The distance between donor and acceptor radicals in the secondary, observable RP of PS I is ∼25 Å,86−88 i.e., similar to our estimation for the BHJ material. Not only the width of the TR-EPR spectra, but also the polarization pattern EAEA, are the same for these two systems. It was shown that in natural and artificial photosynthetic assemblies this spin-polarization pattern means that the SCRP is in a singlet state; i.e., the RP was created from a singlet excited state.44,45,73−77 Based on this comparison, one can conclude that in the case of P3HT:C60PCBM films, the RPs, S(D+···A−), are created from the excited singlet state (singlet exciton) as well, while the triplet exciton does not contribute to the charge separation. This is again similar to molecular systems where primary charge separation outcompetes the intersystem crossing (ISC) to the triplet state (Figure 1). Further evidence for a strong analogy between spindependent ET dynamics in photosynthetic systems and OPV materials comes from the observation of triplet excitons generated by S−T0 mixing.89−91 The pathway for the formation of this type of triplet state is schematically shown in Figure 1. Triplet excitons generated by this mechanism exhibit a different polarization pattern than triplet excitons generated by the ISC mechanism (Figure 4). The S−T0-type triplet was initially observed in modified natural photosynthetic centers and later also found in a number of artificial donor−acceptor systems.23,89,90 Figure 4a shows the high-frequency TR-EPR spectra recorded in the pulsed mode of the chlorophyll triplet state, formed by ISC and S−T0 mixing. There is an apparent difference between the EEEAAA (ISC) and AEEAAE (S−T0 or RP) polarization pattern of the triplet spectra. The S−T0 triplet polarization pattern is considered to be a fingerprint of longlived intermediate RP states and, as a consequence, a measure of efficiency of charge separation.

them became negligible. A similar effect can be observed in solid materials, like OPVs, upon fast separation of spin carrying charges. It was established that two mechanisms are responsible for a spin-polarization pattern, namely, triplet mechanism and radical pair mechanism.69−72 In the triplet mechanism the polarization is transferred from the precursor triplet state and reveals itself in radical signals of the same polarization, either AA or EE (two absorptive or two emissive signals, respectively). In the radical pair mechanism the precursor state is singlet and population is built due to the resonance frequency offset between the two radicals. This leads to the formation of the socalled antiphase doublet spectra with polarization AE or EA (absorption-emission or emission-absorption, respectively) upon “complete” charge separation. Note that primary RPs (precursor) are not directly observable by EPR due to their short lifetime (≤ns), and only secondary, well separated and longer-lived ion pairs can be observed by EPR (see above). The TR-EPR P3HT:C60-PCBM spectrum (Figure 3) reveals an EAEA polarization pattern, and thus cannot be explained within the above-mentioned conventional CIDEP model. These types of spectra are commonly detected in molecular donor−acceptor assemblies and in photosynthetic systems, like photosystems of plants and bacteria.28,29,31,36,73,74 To explain such spin-polarization patterns the spin-correlated radical pair (SCRP) model was developed.44,45,73−77 The SCRP model is valid under the assumption that charge separation is very fast compared to spin-mixing within the primary/intermediate RP and magnetic interactions between spins in the secondary (observable) RPs are totally responsible for the polarization pattern of the spectra. This pattern depends on the spin state of the precursor that can be in singlet or triplet state. The nonBoltzmann spin polarization sustains in the TR-EPR spectra for the shorter of the RP lifetime or the electronic spin−lattice relaxation time, T1e. To summarize, the significant difference between the two polarization mechanisms is the following: the standard CIDEP mechanism is applicable for those radicals, which are well separated (typically over 30 Å) and magnetic interactions between them are negligible. In the SCRP model, the radicals are separated by a shorter distances, less than ca. 30 Å, and the magnetic interaction between them must be taken into account. Thus, the EAEA spin-polarization pattern of P3HT:C60PCBM spectra can be explained only within the SCRP model, assuming appreciable magnetic interactions between spins in the charge separated state (secondary RPs). On the other hand, the polarization pattern of the TR-EPR spectra of PCDTBT: C60-PCBM and PTB7:C60-PCBM, shown in Figure 3a, middle and lower traces, respectively, is not EAEA, but is closer to EA. Figure 3b shows how the TR-EPR line shape can be fitted as a sum of an emission line of the low-field donor signal and an absorption line of the high-field acceptor signal. A small admission, ca. 20% by amplitude, of derivative-type signal is needed to explain the line shape of PCDTBT:C60-PCBM spectra, while this contribution is almost negligible for PTB7: C60-PCBM. While EA-type polarization patterns could qualitatively be explained within the conventional CIDEP model, the validity of the assumption is related to the strength of magnetic interaction between spins within the RP. This interaction can be evaluated from the broadening of the individual components of the SCRP spectra (Figure 3) compared to the cw EPR spectra (Figure 2), which is in the range of 0.1−0.2 mT for all three OPV blends. Assuming only electron−electron dipole− 7411


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material is a sequential process. Moreover, from the fact that a RP-triplet was recorded only in the PTB7:C60-PCBM blend, but not in blends containing P3HT or PCDTBT, we can conclude that the lifetime of the early (not observable) RPs in the PTB7 blend is longer than in the P3HT or PCDTBT blends. Recently the observation of S−T0 triplets in BHJ and its negative effect on the efficiency of solar cells was discussed.93,94 While, in general, the formation of the S−T0-polarized triplet states indicates low charge separation efficiency, we detected this triplet only in the PTB7:C60-PCBM blends, which demonstrates the best device efficiency of all three systems studied here. However, this observation should be interpreted with care. The experiment shown in Figure 4 was performed at high magnetic field (4.6 T) of the D-band EPR spectrometer. At 14 times lower field of 0.33 T using a conventional X-band EPR spectrometer, the triplet signal is very weak and shows spin-polarization mainly due to the ISC triplet formation mechanism (not shown). This can be explained by an increased rate of singlet−triplet mixing at 4.6 T (Figure 1), as this rate is proportional to the magnetic field.43,80,82 Thus, the RP-triplet state is formed efficiently at high magnetic field but may not be present at the normal functioning conditions of solar cells devices. The observation of spin-polarization phenomena in OPV blends and close similarities with the same phenomena in photosynthetic systems, allows us to make a number of important conclusions, which are based on the accumulated knowledge in theoretical and experimental studies spindependent ET in molecular donor−acceptor systems.29,43−45,79−83 First, the analysis of polarization patterns points to the sequential, stepwise charge separation processes. Only the secondary charge-separated (RP, CT) state is observable by EPR. At least one CS state must be a precursor of the observable RP. In natural and artificial photosynthetic systems there are at least two RPs, called primary and intermediate, which are not directly observable by EPR due to their short lifetime. Schematically, these steps of CS process in OPV are shown in Figure 5. While in the natural photosynthetic systems the ET occurs due to electron hopping/tunneling, we do not have definite evidence that in BHJ the electron counterpart is solely responsible for charge separation on the time scale of nanoseconds. It might be that after the initial charge separation, the next step is due to the hopping/tunneling of hole or both, electrons and hole. For simplicity and in analogy with photosynthesis we depicted in Figure 5 only the case when ET is due to the electron hopping. Second, while in principle the lifetime of the intermediate (not observable) RPs can be extracted from the modeling of the polarization pattern of the observable SCRP,80−85 this is not feasible for OPV materials due to the presence of structural and energetic heterogeneity as well as a large number of unknown magnetic parameters, which are required for theoretical treatment. However, because the mechanism of the light-induced ET in most molecular systems is largely the same, and similar TR-EPR signatures of charge separated states in OPV materials and natural photosynthetic assemblies (hence similar magnetic interactions within the RP) are detected (Figures 3 and 4), we make the following estimations of intermediate RP lifetimes. Based on the simulation reported in80−85 we can conclude that the lifetime of intermediate CS state in the case of P3HT is within hundreds of picoseconds, as

Figure 4. Time-resolved electron spin−echo detected D-band spectra recorded after laser flash using a two-pulse microwave sequence. Letters A and E indicate absorptive and emissive phases of the signals. T = 50 K. (a) Photosynthetic reaction center (RC) protein from purple bacterium Rhodobacter sphaeroides and pigments isolated from it. For the isolated donor pigment of the RC (bacteriochlorophyll a), the triplet state is formed by the intersystem crossing mechanism (black, upper spectrum), giving rise to an EEEAAA polarization pattern. If the lifetime of the primary/sequential RP in photosynthetic protein is prolonged by prereduction of the secondary acceptor (a quinone), the triplet state is formed by the S−T0 mechanism, which is characterized by a AEEAAE pattern (red, bottom). Note that this AEEAAE pattern is unique; it cannot be formed by any type of intersystem crossing. Delay after laser flash, 1 μs. (b) Light minus dark electron spin−echo detected D-band difference spectra of PTB7:C60PCBM blend. Light spectrum was recorded at 1 μs delay after laser flash, dark spectrum 100 μs. T = 50 K. The AEEAAE polarization pattern, identical to that of the photosynthetic protein, confirms the radical pair (S−T0) mechanism of triplet formation discussed in panel (a). Two peaks in the shaded region of the spectrum are due to the C60-PCBM triplet state and, probably, small contribution of the SCRP signal. Green line shows a simulation of the PTB7 triplet spectrum formed by the S−T0 mechanism. Simulation parameters are |D| = 1260 MHz; |E|= 165 MHz; g1 = 2.0050; g2 = 2.0045; g3 = 2.0025. Note that the scales of the (a) and (b) graphs are different for better comparison.

The polarized triplet state detected in PTB7:C60-PCBM blend (Figure 4b) is strikingly similar to that detected in quinone prereduced photosynthetic reaction center (RC) protein from purple bacterium Rhodobacter sphaeroides92 (Figure 4a). This provides further evidence that ET in OPV 7412


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orientation is a general feature and also present in PCDTBT: C60-PCBM and PTB7:C60-PCBM blends. Fourth, the lower limit of the lifetime of RP stabilized at distances of 25−30 Å can be estimated from the decay of the correlated RP state, which is measured by cw TR-EPR. For all our systems this lifetime is at least 3 μs at 50 K. While the typical stabilization time for molecular systems is much longer than the decay of spin polarization, we cannot measure this time directly due to the strong background signal from longlived polaron species and broad distribution of the recombination rates in BHJ materials (Figure 2b, c). It is interesting to note that the lifetime of the intermediate RP in the studied materials correlates with the delocalization length of the hole (positive polaron) along the polymer chain. As it was shown34 the delocalization length decreases in the series P3HT, PCDTBT, PTB7 from 60 Å to approximately 40 Å, while according to our analysis, the lifetime is increasing in the same sequence. A plausible explanation of this dependence might be that smaller delocalization length leads to stronger Coulombic interaction between separated charges and thus longer stabilization of the intermediate RP. Charge delocalization in a CT-state has been proposed as an efficient mechanism enabling charges to overcome the Coulomb attraction and thus to achieve long-range charge separation in OPV materials.96,97 Interestingly, the delocalization of the positive charge over two (bacterio)chlorophyll molecules is well known in natural photosynthetic systems as a mechanism to prevent charge recombination at short CS distances of the primary and intermediate RPs, as well as to adjust the redox potential of the donor site.1,4,5 This is just a part of the driving force behind ET in photosynthesis. In photosystem I (like in OPVs), the primary and intermediate acceptors are all the same type of molecules: chlorophylls in PS I and fullerenes in OPV. The driving force of the sequential ET in PS I is the difference of redox potentials among the chlorophyll molecules, which is created by specific interactions with the protein surrounding. We suggest that a similar mechanism is also at work in OPV materials. Here, the intermediate acceptor is also a fullerene molecule, which is supposed to have the same redox potential. However, the electron affinity of the fullerene molecule at the polymer− fullerene interface in BHJ may be lower than that of fullerenes further in the bulk. Another part of the driving force for the sequential ET may come from entropic contributions. These contributions do not need to be taken into account in the case of the quasi one-dimensional ET in natural photosynthetic systems. However, they could be substantial in BHJ with its higher-dimensional ET.10,98

Figure 5. Highly simplified scheme of the charge separation steps near the polymer/fullerene interface. For simplicity and in analogy with photosynthesis we depicted only the case when ET is due to the electron hopping. Arrows indicate initial electron-transfer steps. Charge separation starts with ET to the nearest fullerene molecule and delocalization of the positive charge along the polymer chain. This state has a lifetime of several picoseconds. After subsequent ET steps within several nanoseconds, the electron and hole are stabilized at distances of ca. 25−30 Å.

spin-polarization is purely of SCRP-type and is not influenced by S−T0 mixing. For PCDTBT it is in the subnanosecond− nanoseconds range and for PTB7, where maximum distortion of the polarization due to S−T0 mixing was observed, it is about 10 ns. This estimation correlates with the observation of the S− T0 triplet exclusively in PTB7:C60-PCBM blends, which has the EPR signature of a long-lived intermediate RP-state as precursor for the triplet state. The longer lifetime may be rationalized by previously reported95 specific interactions of C60-PCBM molecules with the PTB7 polymer chain (side chains) at the polymer/fullerene interface. We speculate that this interaction modifies the redox potential of the fullerene molecules close to the interface, thus modulating driving force for secondary ET to the neighboring fullerene molecules. Third, the fact that SCRP is detected in the P3HT:C60PCBM blends leads to an important conclusion regarding molecular order in these heterogeneous BHJ systems. In the case of completely disordered systems (random distribution of the interspin vectors in the RP, which is expected for heterogeneous systems) and weakly coupled RP (magnetic interactions less than the line width, as observed here) the polarized SCRP spectrum should vanish due to averaging over all orientations.77 The observation of strong spin polarization confirms partial orientation order of the donor and acceptor molecules in P3HT:C60-PCBM blends at least near the polymer/fullerene interface within 30 Å. The case of PCDTBT and PTB7 is more difficult, as polarization is already built up in the precursor RP. This non-Boltzmann polarization is transferred to the observable, secondary RP. As discussed above, spins within observable RP are weakly coupled by dipolar interactions and to acquire a final polarization the averaging over possible orientation is needed. Whether this will lead also to the complete disappearance of the SCRP spectra has not been strictly shown before. However, this will certainly diminish the intensity of the SCRP spectra. As the observable intensity in all three cases was roughly the same, compare to the stationary spectra in Figure 2a, we believe that partial

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CONCLUSION The detection of strong electron spin polarization in the timeresolved EPR spectra of organic photovoltaic materials points to the significance of electron spin dynamics for the efficient functioning of organic solar cells.99 The physics of spin polarization in natural photosynthesis and molecular donor− acceptor systems is well understood and described within the models of spin-correlated radical pairs and sequential electron transfer. This allows us to make a comparison with charge separation processes in OPVs and molecular donor−acceptor systems. The polarization pattern of the SCRPs in polymer− fullerene blends reveals that the early charge separation process is a sequential stepwise charge transfer. The first step is the exciton dissociation and ET to the fullerene molecule next to 7413


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the polymer, leading to the formation of the primary radical pair. In order to outcompete the recombination processes, this state cannot live longer than several picoseconds. Importantly, efficient delocalization of the positive polaron on the polymer donor, which has been reported96 to happen on the same time scale, is a major mechanism to overcome the Coulomb attraction and thus recombination, and facilitate forward electron/hole transfer. This forward ET generates an intermediate radical pair, with the lifetime which depends upon the system and increases in the series P3HT, PCDTBT, PTB7 from several hundred picoseconds to several nanoseconds. The third step is charge transfer to the secondary radical pair with a separation of ca. 25−30 Å and lifetimes of at least 3 μs. Only the secondary radical pairs are observable by EPR. This work demonstrates that advanced time-resolved EPR spectroscopy, in combination with well-developed models of the ET process in molecular systems, can be a powerful approach for understanding the light-induced charge generation, separation, and recombination in OPV materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under contract no. DE-AC02-06CH11357 at Argonne National Laboratory (J.N. and O.G.P.). A.S. and V.D. were supported by the DFG SPP “New frontiers in sensitivity for EPR spectroscopy”, under contract DY18/11. This material is based upon work supported as part of the Argonne-Northwestern Solar Energy Research (ANSER) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award no. DE-SC0001059 (L.Y.). The synthesis of PCDTBT (M.L. and S.B.) was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada.

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Photoinduced Dynamics of Charge Separation: From Photosynthesis

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