Charge Separation in PCPDTBT:PCBM Blends from an EPR

Article pubs.acs.org/JPCC

Charge Separation in PCPDTBT:PCBM Blends from an EPR Perspective Felix Kraffert,† Robert Steyrleuthner,† Steve Albrecht,‡ Dieter Neher,‡ Markus C. Scharber,§ Robert Bittl,† and Jan Behrends*,† †

Berlin Joint EPR Lab, Fachbereich Physik, Freie Universität Berlin, D-14195 Berlin, Germany Institute of Physics and Astronomy, University of Potsdam, D-14476 Potsdam, Germany § Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University, A-4040 Linz, Austria ‡

ABSTRACT: Using time-resolved electron paramagnetic resonance (EPR) spectroscopy in conjunction with optical excitation we study charge separation in conjugated polymers blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). A direct comparison between samples comprising poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT) and their analogues containing poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)2,1,3-benzothiadiazole)-5,5′-diyl] (Si-PCPDTBT) reveals a remarkable influence of the bridging atom (carbon vs silicon) in the polymer on the EPR spectra. While the EPR signatures of photogenerated positive polarons in C- and Si-bridged PCPDTBT are virtually identical, significant differences are observed with respect to the spin-relaxation behavior. The spin− lattice relaxation time of positive polarons in C-PCPDTBT at low temperature (T = 80 K) is found to be more than two orders or magnitude longer than in the Si-bridged polymer derivative. This surprisingly slow relaxation can be rationalized by polarons trapped in defect states that seem to be absent (or are present in a substantially smaller concentration) in blends comprising SiPCPDTBT. Transient EPR signals attributed to charge transfer (CT) states and separated polarons are smaller in the blends with C-PCPDTBT as compared to those with the silicon-bridged polymer. We propose that triplet formation occurs via the CT state, thus diminishing the probability that the CT state forms free charge carriers in blends of C-PCPDTBT with PCBM. This hypothesis is confirmed by direct detection of triplet excitons in C-PCPDTBT:PCBM blends. The shape of the transient EPR spectra reveals that the triplet excitons are, in contrast to those formed in pristine polymer films, not generated by direct intersystem crossing but result from back electron transfer through CT state recombination. The strong triplet signal is not observed in blends containing the Si-bridged polymer, indicating efficient singlet exciton splitting and subsequent charge carrier separation at the Si-PCPDTBT/PCBM interface.

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slightly improved charge transport properties,3 which were attributed to an enhanced π−π stacking. Solar cells incorporating Si-PCPDTBT as absorber material were the first photovoltaic devices utilizing low-bandgap copolymers that achieved certified efficiencies beyond 5%.4,5 Meanwhile the material development has progressed further, and maximum efficiencies have doubled.6 Despite these rapid developments fundamental questions related to the conspicuously improved photovoltaic performance of Si-PCPDTBT as compared to its carbon-bridged analogue (C-PCPDTBT) remain open to date. When blended with the electron acceptor [6,6]-phenyl C71-butyric acid methyl ester ([70]PCBM), Si-PCPDTBT exhibits a higher crystallinity and improved transport properties, resulting in

INTRODUCTION Solar cells based on organic semiconductors are about to become a serious competitor on the route toward cost-effective large-scale solar energy conversion. The maximum power conversion efficiencies of conjugated polymer:fullerene solar cells have steadily increased in recent years and by now approach those obtainable with, e.g., thin-film silicon technology.1 This efficiency increase was induced by significant advances in material development. An important step was the introduction of low-bandgap copolymers that allow an efficient usage of the solar spectrum. An early representative of this material class is the polymer poly[2,6-(4,4-bis(2-ethylhexyl)4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT).2 Small modifications of this material, i.e. the replacement of a carbon atom in the conjugated system by a silicon atom, led to poly[(4,4′-bis(2ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl] (Si-PCPDTBT) with © XXXX American Chemical Society

Received: September 24, 2014 Revised: November 5, 2014

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dx.doi.org/10.1021/jp509650v | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. States possibly involved in charge separation along with their trEPR fingerprints as well as potential excitation transfer pathways. The schematic illustrates the correlation between the inter charge-carrier distance and width of the EPR spectrum for each detectable state. The question mark indicates that, to the best of our knowledge, no unambiguous assignment of an EPR signal from a polymer:fullerene blend to a triplet CT state has been reported yet.

spectroscopy can directly probe the presence of CT states without the necessity that they recombine to yield a detectable signal.10 EPR can distinguish between photogenerated positive polarons (P+) in the donor material and negative polarons (P−) in the acceptor material and is thus capable of monitoring successful free charge carrier formation in bulk heterojunctions.11 Following the transient EPR (trEPR) signal as a function of time after a laser flash, which initiates exciton generation, provides the possibility to study the conversion of photoinduced CT states into separated polarons.12,13 A distinction between signals from free (noninteracting) charge carriers (cf. Figure 1) and long-lived weakly coupled CT states after spin state thermalization is, however, often impossible on the basis of the EPR spectrum alone. Further, EPR and related techniques, most notably optically detected magnetic resonance spectroscopy, are suitable to study triplet excitons14 and to disentangle signals that overlap in optical spectra.15 Triplet excitons play a particularly important role in polymer:fullerene blends based on low-bandgap organic semiconductors.16,17 Here we employ low-temperature (T = 80 K) continuouswave EPR (cwEPR) and trEPR spectroscopy to study photoexcited species in PCPDTBT:PCBM blends with particular emphasis on the role of the bridging atom (Si vs C). Organic solar cells using [70]PCBM exhibit higher efficiencies than their counterparts based on [6,6]-phenyl C61-butyric acid methyl ester ([60]PCBM). The superior photovoltaic performance can largely be attributed to the fact that the optical absorption is stronger in the range λ < 650 nm (where the absorption of PCPDTBT is weak) for [70]PCBM than for [60]PCBM. This results in higher currents because photons absorbed by the acceptor material additionally

bulk heterojunction solar cells that are more efficient than those using C-PCPDTBT:[70]PCBM blends.3 This discrepancy has often been attributed to improved bulk properties such as the charge carrier mobility and more favorable phase separation. However, fundamental differences between both materials seem to exist regarding local properties with significant impact on charge transfer and charge separation close to the donor/ acceptor interface. In particular, photoluminescence quenching experiments revealed a long-lived emissive component in CPCPDTBT:PCBM (C-blend) that was not observed in SiPCPDTBT:PCBM (Si-blend). This signal was suggested to arise from the decay of a long-lived charge transfer (CT) state at the heterojunction interface.3 As the singlet exciton diffusion lengths in both polymers were found to be almost identical,7 an explanation for the difference in the microscopic charge separation behavior solely based on different bulk properties of both polymers seems unlikely. This emphasizes the need for an improved understanding of the microscopic processes occurring at the donor/acceptor interface. Spin-sensitive techniques can help to provide information on excitation transfer pathways because many states involved in charge separation are accessible to electron paramagnetic resonance (EPR) spectroscopy and exhibit clearly distinguishable signals. Figure 1 gives an overview of the photoinduced states possibly involved in charge separation along with their EPR signatures. The present knowledge of charge separation in polymer: fullerene blends is largely based on findings from optical spectroscopy. Photoluminescence and electroluminescence experiments turned out to be particularly useful to study the impact of CT states on the luminescence spectrum.8,9 However, these techniques require that the CT states recombine and are thus rather insensitive to long-lived CT states. In contrast, EPR B



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Figure 2. EPR signals from photogenerated polarons. (a) Comparison between light-induced X-band cwEPR spectra of the Si-blend (red) and the C-blend (blue). The dark signal (black) of the Si-blend is shown for reference. The dark signal of the C-blend is similar to that of the Si-blend. The inset shows a magnification of the polymer P+-signal of the C-blend. (b) Light-induced Q-band cwEPR spectra (dots) and simulations (solid lines) of the C- and Si-blend. The light off signal was subtracted from the light on spectrum. Simulation results are shown in green for the polymer signal and in blue for the PCBM signal. Illumination was provided by a laser diode emitting at λ = 638 nm (T = 80 K, field modulation frequency 100 and 10 kHz for X- and Q-band, respectively).

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EXPERIMENTAL DETAILS For the fabrication of the EPR samples the materials (pure SiPCPDTBT and pure C-PCPDTBT for the pristine film samples and a mixture (1:3) of the polymers with [60]PCBM for the blend samples) were dissolved in 1,2dichlorobenzene without processing additives. About 30 μL of the solution were loaded into EPR quartz tubes with an outer diameter of 3 mm, and the solvent was evaporated under vacuum, leaving a film on the inner sample tube wall. Subsequently, the tubes were filled with Helium up to a pressure of 500 mbar and sealed using a blowtorch. The second type of samples, referred to as thin film samples throughout this article, were spin-coated (1500 rpm for 40 s) from solution (weight ratio 1:3 for C-PCPDTBT:[60]PCBM and 1:1.5 for Si-PCPDTBT:[60]PCBM) onto quartz substrates. Chlorobenzene and 1,2-dichlorobenzene were used as processing solvents for C-PCPDTBT:PCBM and SiPCPDTBT:PCBM, respectively. The solvents as well as the weight ratios were chosen such that the properties of the resulting films resemble those of absorber layers in optimized solar cells (without processing additives).2,18,19 Finally, the thin films were dried in vacuum for 30 min, placed into EPR quartz tubes (outer diameter 5 mm) in helium atmosphere and sealed using a blowtorch. Low-temperature (T = 80 K) cwEPR spectra were recorded for different illumination conditions using a laboratory-built combined X-/Q-band EPR spectrometer. Magnetic-field modulation in combination with lock-in detection was employed, resulting in derivative spectra. Spectra measured after inserting the sample into the spectrometer without illumination are referred to as dark spectra. Measurements taken during illumination and after switching off the laser (638 nm laser diode from Mitsubishi Electronics ML520G54) and an additional waiting time of 3 min are referred to as light on and light off spectra, respectively. The magnetic field axis was calibrated using an NMR Gaussmeter (Bruker ER 035M), and the (constant) offset between the positions of the

contribute to the current. However, as shown by Morana et al., blends consisting of Si-PCPDTBT and either [60]PCBM or [70]PCBM reveal comparable morphologies.18 We thus use [60]PCBM in this comparative EPR study in order to benefit from the larger spectral separation between the P+- and P−signals, which makes it easier to distinguish between these signal contributions than in blends comprising [70]PCBM. The intensity of the light-induced cwEPR signals is in line with the previously observed superior photovoltaic performance of the Si-blend. An unexpectedly weak cwEPR signal of P+ in the C-blend is shown to result from long spin−lattice relaxation times (T1) of P+. Long T1-times are indicative of isolated states with localized wave functions such as polarons trapped in defects. Moreover, we find a surprisingly strong signal from triplet excitons located on the polymer in the Cblend. This signal is much more intense than the triplet signal observed in pristine C-PCPDTBT. The shape of the EPR spectrum directly shows that it originates from the frequently discussed back electron transfer from the CT state.16,17 Transient EPR measurements reveal clear CT signatures in the Si- and the C-blends. Since the ratio between the CT signal intensities and the signal intensity attributed to separated polarons is similar in both blends (i.e., a high concentration of CT states seems to lead to a high concentration of free photogenerated charges), we attribute the poorer solar-cell performance of the C-blend not to a long-lived CT state. Our results suggest that it is rather the high concentration of triplet excitons in the C-blend (but not in the Si-blend) that is responsible for the long-lived signal observed in previous photoluminescence quenching experiments.3 In accordance with previous results from optical measurements,17 we propose that triplet formation occurs via the CT state, thus diminishing the probability that the CT state forms free charge carriers in blends of C-PCPDTBT with PCBM. C



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remains whether the EPR signatures are identical or different for P+ in both polymer variants. We apply Q-band EPR to resolve the anisotropic g-matrices and get insight into the microscopic structure of P+ in both polymers. 2. Signal Intensity of P+ in Both Blends. Several parameters may influence the cwEPR signal intensity. Using pulse EPR following laser flash excitation we differentiate between spin relaxation and charge carrier lifetime effects in order to understand the different P+-signal intensities shown in Figure 2a. 3. Charge Separation Yield. The intensities of the P+- and − P -signals in the Si-blend are stronger than those in the Cblend. This is consistent with the fact that charge separation is more efficient in the Si-bridged material as evidenced by internal quantum efficiency measurements that consistently give higher values for Si-blends (75−80%, at room temperature)3 than for optimized C-blends (68−71%, at room temperature, blended with [70]PCBM and diiodooctane as processing agent).24 We will address the question why the charge separation works better in the Si-blend. Transient EPR allows us to particularly study CT and triplet states, which may play a key role in charge separation. Q-band EPR (34 GHz) provides higher resolution with respect to anisotropic g-matrices as compared to X-band EPR (9.6 GHz). The distinct peaks in the Q-band spectra shown in Figure 2b allow us to accurately determine the g-matrices for P+ by comparing the experimental spectra to Easyspin 25 simulations. The simulation results are shown in Table 1.

Gaussmeter and the sample was determined by a reference sample (nitrogen encapsulated in C60). A Bruker Elexsys E-580 pulse EPR (pEPR) spectrometer was used to determine spin-relaxation times of polarons in both PCPDTBT:PCBM blends. Optical excitation at λ = 650 nm (excitation of the donor material) was provided by a Nd:YAG laser (Spectra Physics LAB 150) equipped with a second and third harmonic generator in combination with an optical parametric oscillator (OPTA OPO BBO-355-vis/IR). The pulse length was 6 ns, and the fluence used in all pulsed experiments was approximately 1.0 mJ/cm2. Transient detection of EPR following laser excitation was performed using a laboratory-built spectrometer.20 Optical excitation at λ = 650 nm was provided by a Nd:YAG laser (Spectra Physics GCR-11) in combination with an optical parametric oscillator (same model as described above). Transient EPR signals were recorded after the laser flash by a digital oscilloscope (LeCroy WaveRunner 104MXi). Several EPR transients were accumulated for each field-position in the scan range of the static magnetic field. The time between subsequent laser pulses was 0.1 s. The nonresonant background signal induced by the laser flash was recorded far from the resonant signal and subtracted from each EPR transient. Cooling was provided by a laboratory-built helium-gas flow cryostat. All measurements were performed using a dielectric ring resonator (Bruker ER 4118X-MD5).

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RESULTS AND DISCUSSION Photoinduced Charge Carriers. Light-induced cwEPR was used to determine the EPR parameters of photogenerated long-lived polarons in C- and Si-bridged PCPDTBT:PCBM blends. The spectral signature of P− in PCBM is well-known from previous measurements on polymer:fullerene blends21,22 and substantially differs from the spectrum found for P+ in many OPV-relevant polymers.23 This allows us to reliably determine the spectrum of P+ in PCPDTBT using X-band (9.6 GHz) cwEPR. Figure 2a shows a light-induced cwEPR spectrum (light on−dark) of the C-blend (blue) in comparison to the Si-blend (red). EPR signals due to photogenerated charge carriers in the polymer phase as well as in the fullerene phase can clearly be resolved. The intensity of the signal attributed to P− on PCBM is stronger in the Si-blend than in the C-blend. Further, a comparison of the relative signal intensities of P+ and P− reveals a significant difference between both blends. The measurements demonstrate that it is possible to differentiate between the photogenerated polaron signals of P+ on PCPDTBT and P− on PCBM due to a difference in gmatrices. In X-band EPR the resonance lines of both polarons can approximately be described by isotropic g-factors. The gfactor of a polaron depends on its microscopic environment and can be considered a fingerprint of a paramagnetic center. Determining the g-factors of the P+- and P−-signal is an important prerequisite for the following investigations, i.e., by choosing the magnetic field appropriately we can selectively address either P+ or P− even in X-band EPR. The difference between the light-induced EPR signals of the C- and the Siblend poses several important questions related to charge separation in PCPDTBT:PCBM blends that we will address in the following sections: 1. EPR Signature of P+ in PCPDTBT. Despite the different signal intensities related to P+ in both blends the question

Table 1. Principal Values of the g-Matrices of Si- and CPCPDTBT Determined from the Simulated Q-Band Spectra g-value gx gy gz

Si-PCPDTBT

C-PCPDTBT

PCBM

PCBM, ref 21

2.0029(1) 2.0021(1) 2.0011(1)

2.0030(3) 2.0021(3) 2.0010(3)

2.0003(1) 2.0001(1) 1.9985(1)

2.0003(1) 2.0001(1) 1.9982(1)

Both polymers exhibit the same anisotropic g-matrix within the experimental error. The simulations are more accurate for Si-PCPDTBT due to a better signal-to-noise ratio of the measured spectrum. The ratio between the polymer and the PCBM signal intensity is again smaller for the C-blend in comparison to the Si-blend, as already seen in the X-band spectra (cf. Figure 2a). The anisotropic PCBM peak cannot be fully resolved due to substantial g-strain that causes a fielddependent line broadening. In accordance with previous reports the g-strain is most pronounced for the gz-component of P−.21 In the simulations g-strain was included by assuming a Gaussian g-distribution centered at the respective g-value. Our results demonstrate that the shape of the EPR spectra (but not the intensity) associated with P+ and P− are virtually identical in both materials. This indicates that the microscopic structure (e.g., localization) of photogenerated positive polarons is similar for both polymer derivatives. One could anticipate that the silicon atom in Si-PCPDTBT severely affects the gmatrix of the P+ due to its comparatively large atomic mass (and hence strong spin−orbit coupling). However, previous density functional theory calculations for the highest occupied molecular orbital have shown almost no charge density of the positive polaron on the Si-atom.19 This is consistent with the results of our cwEPR measurements. D



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Figure 3. Inversion recovery for P+ and P− in C-PCPDTBT:PCBM (left) and Si-PCPDTBT:PCBM (right) at 80 K (tdaf = 10 μs). A constant offset was added to the inversion recovery curves for clarity. (a) Result of an inversion recovery measurement on a millisecond time scale at the CPCPDTBT resonance position (see inset in the panel below). (b) Result of an inversion recovery experiment on a microsecond time scale. The blue graph shows the echo decay on the resonance of the PCBM signal (see inset), the red curve shows a measurement in the same time range at the CPCPDTBT resonance position. (c) Pulse sequence used for determining the spin−lattice relaxation times (T1) of light-induced polarons. The initial laser flash (λ = 650 nm) generates excitons in the polymer. The following microwave pulses correspond to an inversion recovery experiment. (d) The blue graph shows the echo decay on the resonance of the PCBM (P−) signal (see inset). The red curve is the corresponding result for the SiPCPDTBT (P+).

be 10(1) μs for the C-blend and 11(1) μs for the Si-blend. These values are in good agreement, indicating that T1 of P− in PCBM is independent of the choice of the polymer. In contrast, T1 for P+ in both polymers (red curves) differ significantly. Two different time rangesthe lower one for 0 to 60 μs (Figure 3b), the upper one (Figure 3a) for 0 to 60 ms between the π and π/2 pulseobtained at the C-PCPDTBT resonance position demonstrate this discrepancy. Only a small part of the spins relaxes in the time range of few μs. The major part relaxes with a time constant of 5.7(9) ms, which is almost a factor of 350 larger than the spin−lattice relaxation time of 17(1) μs found for the spins at the Si-PCPDTBT resonance position. Such a long T1-time results in saturation effects (bleaching) in cwEPR experiments even at moderate mw power levels typically used for the investigation of photogenerated charge carriers in polymer:fullerene blends. The pEPR results show that the low intensity of the P+-signal in the light-induced cwEPR spectra of the C-blend is not necessarily related to less efficient charge separation in the Cblend. They rather indicate that the surprisingly long spin− lattice relaxation time, which is more than 2 orders of magnitude longer than in the Si-polymer, affects the cwEPR signal of P+ measured in the C-blend. Possible candidates for paramagnetic species with spin− lattice relaxation times in the millisecond range are polarons trapped at defects in the C-polymer domains and at the donor/ acceptor interfaces. These deeply trapped P+ are relatively localized, which reduces the interaction with free polarons in more delocalized states and thus effectively decouples the trapped P+ from their environment. In consequence, the spinrelaxation times for the trapped P+ are longer than for the more mobile charge carriers and impedes their detection using cwEPR. The P+-signal observed in the cwEPR spectrum of the C-blend (cf. Figure 2) is attributed to a small fraction of mobile

Spin Relaxation Behavior. Signal intensities in cwEPR can be severely affected by spin-relaxation times. In particular, long T1-times can lead to saturation effects with respect to the particular transition and reduce the signal intensity measured in a cwEPR experiment.26 To identify the reason for the very low P+-signal in the C-blend, pEPR measurements are helpful because of their capability of directly determining spinrelaxation times. Correlating the initial laser excitation with a microwave pulse sequence enables the investigation of polarons at a chosen time after exciton generation. A similar strategy was recently applied to study the relaxation behavior of polarons in polymer:fullerene blends comprising polythiophene and PCBM.27 Figure 3c illustrates the light induced inversion recovery experiment used to determine the T1-times shown in Figure 3. The sequence starts with exciton generation by a laser flash followed by a first π microwave pulse after a constant delay tdaf (delay after flash). This microwave pulse inverts the polarization with respect to the static magnetic field. During the waiting time τ the spins relax back to their ground state, leading to a decay of the polarization. The following π/2−π sequence (Hahn echo sequence) is used to detect the remaining polarization after tdaf. The readout is realized by the integration over the echo signal marked in red. The blue curves in Figure 3 show the inversion recovery signal along with monoexponential fits recorded at the resonance position of P− (in PCBM) for both blends. There are some minor deviations between the experimental data and the fits, indicating that the true relaxation behavior cannot accurately be reproduced using monoexponential fits. However, since we are only interested in a rough estimate of the relaxation times and not in a detailed analysis of the underlying relaxation mechanism, monoexponential fits were chosen. The magnetic field resonance positions of the measurements are indicated in the inset. The PCBM T1-time was determined to E



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Figure 4. Transient EPR measurement on (a) C-PCPDTBT:PCBM and (b) Si-PCPDTBT:PCBM. The plots show slices along the magnetic field axis at fixed times after the laser flash (E, emissive = negative signal; A, absorptive = positive signal). The upper plot shows the complete 2D (field vs time) trEPR spectrum to illustrate the dynamics of the charge separation process. The red dashed line visualizes the noninteracting polaron spectrum determined by light-induced cwEPR (integrated spectrum).

Note that trEPR is generally insensitive to separated (and noninteracting) polarons for short delays after the flash. This is due to the fact that the EPR signal of separated polarons results from an imbalance of the spin-up and spin-down levels for each individual polaron. In thermal equilibrium this imbalance is given by the Boltzmann distribution, leading to a larger population of the spin-down level. However, immediately after singlet exciton dissociation both energy levels for each polaron are populated equally, and thermal equilibrium is only gradually restored by spin relaxation on a time scale given by T1. This fact readily explains why for the C-blend the P+-signal at long delays after the flash (12 μs) is very weak in comparison to the P−signal. This observation is in agreement with the cw and pulse EPR measurements (cf. Figure 2 and Figure 3) and can be explained by the long spin-relaxation time of P+ in PCPDTBT. In the trEPR spectra (Figure 4) signals attributed to CT states and separated polarons can be observed for both blends. In case of the Si-blend the transition from the CT state to the P+ and the P− occurs on the same time scale as in the C-blend. The broader structure covering the range from 341 to 345 mT, which superimposes the CT state and polaron signals, is possibly related to triplet excitons in large PCBM domains. Because of the limited field range we observe only the central features of the (broader) PCBM triplet exciton signal. The fact that the signal intensities originating from P+ and P− at late times are larger in the Si-blend as compared to the C-blend is consistent with the observation that charge separation is more efficient in the Si-blend as compared to the C-blend. Note that the differences between the trEPR signal intensities of the Cand the Si-blends are much larger than those observed in cwEPR (Figure 2a) and pEPR measurements (Figure 3). The same behavior is found for thin-film samples. The reason for this discrepancy is not clear yet, but it could be related to the fact that it is generally complicated to compare absolute signal intensities measured by trEPR and cwEPR spectroscopy.30 Further, signals from triplet CT states can possibly affect the intensities measured in the trEPR spectra, particularly in the case of the C-blend. A comparison of the C- and Si-blends reveals that the ratio between the signal intensities related to the initial CT state and the non- or weakly interacting polarons is similar in both blends. This supports the interpretation that the CT state is an intermediate state on the way toward separated charge carriers; i.e., a high concentration of CT states seen in trEPR leads to a

charge carriers with spin-relaxation times in the microsecond regime (see Figure 3b). Trap states were shown to be present in PCPDTBT:PCBM blends and influence the charge transport properties in a negative way.28 Filling these traps by molecular doping can significantly improve the charge carrier mobility.29 Charge Transfer States. In contrast to cwEPR and pEPR measurements, where stationary paramagnetic species may give rise to background signals, transient EPR is sensitive to lightinduced signals only. The advantage of recording a whole transient conveniently allows the observation of dynamical processes like charge transfer and charge separation. The benefit of trEPR in comparison to other transient optical measurements is related to its capability of reliably distinguishing between different EPR-active species such as strongly coupled triplet excitons, weakly bound CT states and separated polarons. The dynamics and conversion mechanisms can be studied and the contributing signals distributed over the g-axis can be assigned to their molecular environment. The transition from CT states into centers with purely absorptive EPR signals is measured in a small field range (ca. 6 mT) around the resonance position of the blends. Both blends show these transitions as demonstrated in Figure 4. Initially, the trEPR spectrum exhibits emissive and absorptive components indicative for spin-correlated polaron pairs. This signal can be attributed to CT states based on the following arguments: As we observe a non-Boltzmann polarization pattern, which results from the fact that the precursor state is a singlet exciton, we can assign the signal to geminate polaron pairs originating from the same exciton. The presence of spin− spin coupling demonstrates that both charge carriers reside in close proximity. In consequence, the polaron pairs experience Coulomb attraction. Further, the g-values indicate that the signal arises from pairs located at the donor/acceptor interface (P+ in PCPDTBT and P− in PCBM). These observations suggest that the spin-correlated polaron-pair signal observed for short delays after the flash essentially originates from CT states. For longer delays after the laser flash, the trEPR spectrum gradually changes into a purely absorptive spectrum (expected for separated polarons as well as long-lived weakly coupled CT states with thermalized spin states). The absorptive spectrum for long delays consists of two components, one related to P+ in PCPDTBT and the other attributed to P− in PCBM. Both components are centered at different resonance positions due to g-value differences. F



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Figure 5. Transient EPR spectra of (a) pristine C-PCPDTBT and (b) pristine Si-PCPDTBT (T = 80 K, λ = 650 nm). Both spectra reveal characteristic triplet signals with a shape that changes with time. The dashed lines represent simulated spectra for triplet excitons generated by intersystem crossing from the singlet excited state (simulation parameters for C-PCPDTBT: D = −950 MHz, E = 130 MHz, px = 0.33, py = 0.33, pz = 0.67, dx = 0.3 × 106 s−1, dy = 0.16 × 106 s−1, dz = 0.73 × 106 s−1. Si-PCPDTBT: D = −1050 MHz, E = 130 MHz, px = 0.07, py = 0.27, pz = 0.67, dx = 0.7 × 106 s−1, dy = 0.3 × 106 s−1, dz = 0.7 × 106 s−1). Details on the simulation parameters can be found in the text.

can partly be attributed to a difference in temperature (80 K vs room temperature). In fact, temperature-dependent trEPR measurements show that the CT state dissociation becomes faster upon increasing the temperature. However, other factors can influence the experimentally determined CT state lifetimes as well. We note that luminescence measurements require CT states to recombine in order yield a detectable signal. As a result, techniques based on optical emission are rather insensitive to long-lived CT states (with low recombination probability). In contrast, trEPR directly detects CT states without the necessity that they annihilate and thus tends to be more sensitive to long-lived CT states. In consequence, provided that there is a broad distribution of CT state lifetimes, it is conceivable that optical emission measurements preferentially see the short-lived CT states, whereas trEPR spectroscopy selectively detects the long-lived subspecies. The fact that we observed different intensities of the EPR signals from CT states and separated polarons for both blends raises the question on the reason for this difference. If we assume that all photons are absorbed in both blend films and lead to the formation of singlet excitons, there must be loss mechanism providing an alternative excitation transfer pathway that does not eventually lead to free charge carriers. This mechanism seems to be particularly relevant for charge separation in the C-PCPDTBT:PCBM blend. There are a number of possible loss mechanisms that can affect the charge carrier generation yield, among them (nongeminate) recombination of separated P+ and P−, (geminate) CT state recombination, fast singlet exciton recombination and excitation transfer pathways that involve the formation of triplet excitons either in the polymer or the PCBM phase (cf. Figure 1). In the following we will utilize the characteristic EPR signatures of triplet excitons in order to elucidate their role in loss processes in PCPDTBT:PCBM blends. Triplet Excitons in Pristine PCPDTBT Films. The EPR signatures of weakly bound or uncoupled species like CT states and separated polarons in PCPDTBT:PCBM blends cover a magnetic field range of about 1.5 mT (see Figure 4). As illustrated in Figure 1, the width of the EPR signal increases with the coupling strength between both constituents of a charge carrier pair and approaches 70 mT for triplet excitons in

high concentration of carriers that can be collected in a solar cell.31−33 Thus, our results suggest that the CT state seen in the trEPR spectrum does not result from long-lived charge carrier pairs that cannot be separated and are lost for solar cell operation. The purely absorptive (i.e., positive) spectrum observed for long delays after the laser flash is consistent with the spectrum expected for separated polarons in thermal equilibrium. The red dashed lines in Figure 4 demonstrate the similarities between the late trEPR signal and the cwEPR spectrum attributed to the noninteracting polaron signal. It was recently suggested that a net emissive (i.e., negative) signal measured 0.5 μs after light excitation results from separated charges.34 As it is evident from Figure 4a, we also detect a net emissive signal for short delays (see spectra for t = 0.3, 0.5, and 1.4 μs). However, taking into account the full temporal variation of the trEPR spectra until 12 μs after the laser flash, we think that this signal does not to originate from separated polarons. Our data suggest that it is rather due to CT states at the donor/acceptor interface, whereas a purely absorptive signal attributed to separated polarons is detected for delays beyond 10 μs. Figure 4a shows again the P+-signal, which is strongly suppressed because of the extremely long T1-realxation time. Figure 4b visualizes that in case of the Si-blend both polarons P+ and P− can be identified in the spectrum. However, the line shape and spectral position of the P+-signal in the Si-blend differs slightly between the trEPR and cwEPR. This can possibly be attributed to the fact that at the chosen experimental conditions (T = 80 K, essentially “open circuit conditions”) not all polarons are fully separated at 12 μs. This interpretation is consistent with what has been observed in other materials. In particular, a good agreement between the trEPR spectrum at 30 μs measured at T = 100 K and the corresponding integrated cwEPR spectrum was observed for blends comprising poly(3-hexylthiophene) and PCBM.12 Figure 4 shows that at T = 80 K characteristic CT signatures can be detected even for times larger than 1 μs. This result seems contradictory to the fact that time-resolved luminescence measurements of the CT state emission yield lifetimes of a few nanoseconds for several polymer:fullerene blends at room temperature.35,36 The discrepancy between the CT state lifetimes as determined by trEPR and optical spectroscopy G



Article

Figure 6. Results of transient EPR measurements on thin films of (a) C-PCPDTBT:PCBM and (b) Si-PCPDTBT:PCBM (T = 80 K, λ = 650 nm). The red dashed lines show simulated spectra of triplet excitons (see text for details). While the spectrum of the C-blend exhibits a strong triplet signal similar to the one found in the pristine polymer for long delays after the laser flash, the spectrum of the Si-blend is dominated by CT state and free polaron signal contributions in the central part of the spectrum. The triplet signature is much weaker than in the C-PCPDTBT blend.

A close inspection of the spectra reveals that the zero-fieldsplitting parameters D and E are not identical for the C- and the Si-bridged material, implying slightly different triplet localizations in both polymer derivatives. Note that the initial increase of the triplet signal is affected by the insufficient time resolution of our setup, which complicates the comparison of the signal intensities of the triplet signals for 0.7 and 4 μs. Apart from both polymers also pristine PCBM exhibits a clear triplet signature, but with a substantially smaller dipolar coupling constant. This difference in D, which results from the fact that the triplet state on PCBM is more delocalized, directly translates into a difference in the width of the spectra, so that both triplets can easily be distinguished. The coupling parameters for triplet excitons in PCBM are similar to those found for triplets in pure C60 films.39,40 We note that the D values observed here for both PCPDTBT derivatives are significantly smaller than the D value found for poly(3hexylthiophene), indicating a more delocalized triplet exciton on PCPDTBT.15 Triplet Excitons in PCPDTBT:PCBM Blends. When Cand Si-PCPDTBT are blended with PCBM, charge transfer at the donor/acceptor interface occurs, finally leading to free polarons that can be collected at the electrodes of a solar cell. The exciton dissociation typically happens on ultrafast time scales ( 4 μs) suggests that the decay mechanisms are identical in both cases. Having carefully considered the various triplet formation processes, we are left to conclude that the most probable process leading to the strong triplet signal observed in the Cblend (Figure 6a) is the BET mechanism. This is in agreement with previous findings based results from time-resolved optical spectroscopy.17 The shape of the trEPR spectra for short delays confirms that the triplets are formed by BET via recombination of CT states. A quantification of the losses due to BET is complicated to infer from the trEPR data because a number of rate coefficients and relaxation processes can affect the temporal behavior of the trEPR signals. However, a comparison between the signal intensities and the signal-to-noise ratio of the spectra shown in Figure 6 suggests that the BET rates in the C- and Si-blends differ by at least a factor of 10. While BET provides a credible explanation for the triplet spectrum of the C-blend for short delays, it does not readily account for the temporal change of the shape of the line shape, finally leading to a triplet signal that resembles the signal found in the pristine polymer film. In order to identify the reason for the dynamical behavior of the spectrum, we performed simulations with the aim to reproduce the spectra shown in Figure 6a for all delays with one set of parameters. In our model we include some of the pathways illustrated in Figure 1 and formulate a corresponding set of coupled rate equations for the populations of the triplet sublevels as well as the CT state sublevels. We assume that the singlet excitons generated in the C-PCPDTBT phase efficiently dissociate at the donor/acceptor interface and are thus transformed into CT states. Because of conservation of angular momentum only the CT state sublevels with singlet character are populated, whereas the CT state sublevels with pure triplet character (|CT+⟩ = |↑↑⟩ and |CT−⟩ = |↓↓⟩) remain empty. In the CT state singlet/triplet

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CONCLUSIONS Combining the information obtained from continuous wave, pulse and transient EPR spectroscopy in conjunction with optical excitation we have studied charge separation in PCPDTBT:PCBM blends with particular emphasis on the role of the bridging atom (C or Si). The EPR signatures of photogenerated positive polarons in C- and Si-bridged PCPDTBT are virtually identical. This is evidenced by Qband EPR measurements revealing similar principal values of the g-matrices for both materials. Yet, we find significant differences with respect to the spin-relaxation behavior. The spin−lattice relaxation time of P+ in C-PCPDTBT is found to J



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be several milliseconds and thus more than 2 orders of magnitude longer than in the Si-bridged polymer variant. This surprisingly slow relaxation can be rationalized by polarons trapped in defect states that seem to be absent (or are present in a substantially smaller concentration) in blends comprising Si-PCPDTBT and PCBM. Transient EPR signals from CT states as well as signals attributed to separated polarons could be detected in both blends. The ratio between the CT signal intensity and the signal intensity assigned to separated polarons is similar, indicating that irrespective of the polymer used in the blend a high concentration of CT states seems to lead to a high concentration of free photogenerated charges. In consequence, we believe that the less efficient charge carrier generation in the C-blend is not related to a long-lived CT state. Further, our results support the interpretation that the CT state is an intermediate state on the way toward separated charge carriers. The smaller signal intensities related to CT states and separated polarons in the C-blend can be explained by a loss mechanism involving the generation of triplet excitons via BET (see Figure 7) in accordance with previous findings from time-resolved

Freie Universität Berlin acknowledges financial support from the DFG (SPP 1601) and the Helmholtz Association (EnergieAllianz Hybrid-Photovoltaik). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Henryk Kalbe (Freie Universität Berlin) for his help in the early stage of the experiments.

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(1) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (Version 42). Prog. Photovoltaics 2013, 21, 827−837. (2) Mühlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z. G.; Waller, D.; Gaudiana, R.; Brabec, C. High Photovoltaic Performance of a LowBandgap Polymer. Adv. Mater. 2006, 18, 2884−2889. (3) Scharber, M. C.; et al. Influence of the Bridging Atom on the Performance of a Low-Bandgap Bulk Heterojunction Solar Cell. Adv. Mater. 2010, 22, 367−370. (4) Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Li, G.; Yang, Y. Synthesis, Characterization, and Photovoltaic Properties of a Low Band Gap Polymer Based on Silole-Containing Polythiophenes and 2,1,3Benzothiadiazole. J. Am. Chem. Soc. 2008, 130, 16144−16145. (5) Dennler, G.; Scharber, M. C.; Brabec, C. J. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 2009, 21, 1323−1338. (6) Li, G.; Zhu, R.; Yang, Y. Polymer Solar Cells. Nat. Photonics 2012, 6, 153−161. (7) Mikhnenko, O. V.; Azimi, H.; Scharber, M.; Morana, M.; Blom, P. W. M.; Loi, M. A. Exciton Diffusion Length in Narrow Bandgap Polymers. Energy Environ. Sci. 2012, 5, 6960−6965. (8) Hallermann, M.; Haneder, S.; Da Como, E. Charge-Transfer States in Conjugated Polymer/Fullerene Blends: Below-Gap Weakly Bound Excitons for Polymer Photovoltaics. Appl. Phys. Lett. 2008, 93, 053307. (9) Tvingstedt, K.; Vandewal, K.; Gadisa, A.; Zhang, F. L.; Manca, J.; Inganas, O. Electroluminescence from Charge Transfer States in Polymer Solar Cells. J. Am. Chem. Soc. 2009, 131, 11819−11824. (10) Pasimeni, L.; Maniero, A. L.; Ruzzi, M.; Prato, M.; Da Ros, T.; Barbarella, G.; Zambianchi, H. Photoinduced Electron Transfer in Sexithiophene/Fullerene Derivative Blends: Evidence of Long-Lived Spin Correlated Radical Pairs. Chem. Commun. 1999, 429−430. (11) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Photoinduced Electron-Transfer from a Conducting Polymer to Buckminsterfullerene. Science 1992, 258, 1474−1476. (12) Behrends, J.; Sperlich, A.; Schnegg, A.; Biskup, T.; Teutloff, C.; Lips, K.; Dyakonov, V.; Bittl, R. Direct Detection of Photoinduced Charge Transfer Complexes in Polymer Fullerene Blends. Phys. Rev. B 2012, 85, 125206. (13) Kobori, Y.; Noji, R.; Tsuganezawa, S. Initial Molecular Photocurrent: Nanostructure and Motion of Weakly Bound ChargeSeparated State in Organic Photovoltaic Interface. J. Phys. Chem. C 2013, 117, 1589−1599. (14) Liedtke, M.; Sperlich, A.; Kraus, H.; Baumann, A.; Deibel, C.; Wirix, M. J. M.; Loos, J.; Cardona, C. M.; Dyakonov, V. Triplet Exciton Generation in Bulk-Heterojunction Solar Cells Based on Endohedral Fullerenes. J. Am. Chem. Soc. 2011, 133, 9088−9094. (15) Swanson, L. S.; Shinar, J.; Yoshino, K. Optically Detected Magnetic-Resonance Study of Polaron and Triplet-Exciton Dynamics in Poly(3-Hexylthiophene) and Poly(3-Dodecylthiophene) Films and Solutions. Phys. Rev. Lett. 1990, 65, 1140−1143. (16) Distler, A.; Kutka, P.; Sauermann, T.; Egelhaaf, H. J.; Guldi, D. M.; Di Nuzzo, D.; Meskers, S. C. J.; Janssen, R. A. J. Effect of PCBM on the Photodegradation Kinetics of Polymers for Organic Photovoltaics. Chem. Mater. 2012, 24, 4397−4405.

Figure 7. Concluding scheme: excitation transfer pathways in SiPCPDTBT:PCBM and C-PCPDTBT:PCBM. Thick solid arrows indicate the processes discussed here. The relative comparison of the back electron transfer rates is an estimate based on the signal intensities and the signal-to-noise ratio of trEPR spectra from thin-film samples.

optical spectroscopy. This hypothesis is confirmed by direct detection of triplet excitons in C-PCPDTBT:PCBM blends. The shape of the trEPR spectra reveals that the triplet excitons are, in contrast to those formed in pristine polymer films, not generated by direct ISC, but result from BET through CT state recombination. The strong triplet signal is not observed in blends containing the Si-bridged polymer, indicating an efficient singlet exciton splitting and subsequent charge carrier separation at the Si-PCPDTBT/PCBM interface. This interpretation is in accordance with the previously observed superior performance of solar cells based on Si-bridged PCPDTBT in comparison to their counterparts using the Cbridged variant.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(J.B.) E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. K



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Charge Separation in PCPDTBT:PCBM Blends from an EPR

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