Hole Relaxation in Polymer:Fullerene Solar Cells Examined by the

Article pubs.acs.org/JPCC

Hole Relaxation in Polymer:Fullerene Solar Cells Examined by the Simultaneous Measurement of Time-of-Flight and Time-Resolved Microwave Conductivity Yoshiki Shimata† and Akinori Saeki*,†,‡ †

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

‡

S Supporting Information *

ABSTRACT: To comprehensively examine charge carrier transport and relaxation, we report a new technique combining time-of-flight (TOF) and time-resolved microwave conductivity (TRMC) using a harmonic cavity. The TOF analysis affords long-range hole mobilities of 10−4−10−2 cm2 V−1 s−1 for polymer:methanofullerene bulk heterojunction (BHJ) films (P3HT, PffBT4T, and PCPDTBT blended with PCBM), while TRMC transients are simultaneously recorded under an external electric field. The latter exhibits the acceleration of decay with increasing bias voltage. By analyzing the transient photocurrents and TRMC decays based on a diffusion theory with bulk charge recombination, hole relaxation is observed as a function of time (nanoseconds to microseconds) or distance (∼micrometers). Contrasting results are found among the BHJ films, which are consistent with the optimal thickness of the organic solar cells and provide the basis to interpret charge carrier dynamics from the spatiotemporal viewpoint.

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(EFISH).32,40,41 TRTS and time-resolved gigahertz spectroscopy constitute noncontact measurements, which can avoid contact issues; however, these methods are mostly incompatible with a device comprising metal or conducting electrodes. Exceptional examples of microwave spectroscopy under an external bias involve electron spin resonance (ESR),42 reflectance measurements,43 field-induced TRMC,44 and stripline resonators.45 However, the time resolution is of the order of microseconds or longer, which is not sufficient to probe the rapid dynamics of charge carriers. In this article, the hole relaxation in BHJ films was examined using a newly developed TOF−TRMC system. Transient photocurrent (TPC) is measured in a layered device under an external bias to evaluate conventional TOF mobilities. TRMC transients are simultaneously measured in an identical setup. To the best of our knowledge, this is the first study of a TOF− TRMC technique to measure the nanosecond−microsecond hole relaxation during the transport of the holes to the electrode.

INTRODUCTION Thus far, organic photovoltaics (OPV) are among the most interesting optoelectronic materials. Their power conversion efficiencies have been reported to increase continuously.1−5 Bulk heterojunctions (BHJs) comprise an electron donor and an electron acceptor, which simultaneously improve energy conversion via three steps: photoabsorption, charge separation and transport, and collection.6,7 In addition, owing to the facile processing and versatile tunability of their optical or electrochemical properties, diverse chemical structures are obtained, affording advantages such as low-cost manufacturing, flexibility, and lightweight devices.8,9 Although a large number of studies have reported the key roles of fullerene aggregates,10,11 delocalization,12−14 interface,15−18 coherence,19−22 and dimensionality,23,24 the charge separation and transport mechanisms, particularly relaxation, still need to be addressed. Charge carrier mobility is a crucial parameter for electronic devices, which determines current flow25,26 and a balance of positive and negative charges;27,28 it is also associated with the charge separation at the donor−acceptor interface.29−33 Typical techniques to evaluate nontemporal charge carrier mobility include space-charge-limited current (SCLC), field-effect transistor (FET), and time-of-flight (TOF) measurements, which are referred to as direct-current (dc) methods. In contrast, time-dependent mobilities (or conductivities) are evaluated by alternating-current (ac) methods, e.g., timeresolved terahertz spectroscopy (TRTS),34−36 time-resolved microwave conductivity (TRMC) using gigahertz microwaves,37−39 and electric field-induced second harmonic © XXXX American Chemical Society

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EXPERIMENTAL SECTION Regioregular poly(3-hexylthiophene) (P3HT; >98% head-totail regioregularity) and PCBM (PC61BM and PC71BM) (purity >99%) were purchased from Aldrich and Frontier Carbon Inc., respectively. PCPDTBT and PffBT4T were purchased from 1Received: May 30, 2017 Revised: August 7, 2017

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DOI: 10.1021/acs.jpcc.7b05212 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Material Inc.; solvents were purchased from Kishida Kagaku Corp. and used as received without further purification. An ∼30 nm thick ZnO layer was fabricated on a cleaned, patterned ITO layer (∼15 Ω/square, 150 nm thickness) with an Al guide electrode on a 1 mm thick quartz substrate by spin-coating using a ZnO precursor solution (0.1 g/mL zinc acetate dihydrate and 0.028 g/mL ethanolamine in 2-methoxyethanol). The substrate was annealed on a hot plate at 200 °C for 30 min. A BHJ layer were prepared in an N2-filled glovebox by dropcasting the solutions (0.2 wt % P3HT:PCBM = 1:1 in odichlorobenzene; 0.2 wt % PffBT4T:PCBM = 1:1.5 in odichlorobenzene; 0.1 wt % PCPDTBT:PC71BM = 1:3.5 in chlorobenzene with 3 vol % 1,8-diiodooctane). After the films were coarsely dried, they were subjected to vigorous drying overnight under vacuum. A gold electrode (100 nm) was deposited on the BHJ layer using a shadow mask. The BHJ layer thickness was measured using a surface profiler (Bruker model Dektak XT), affording 1.8 μm for P3HT:PCBM, 2.6 μm for PffBT4T:PCBM, and 2.3 μm for PCPDTBT:PC71BM. The decadic absorption coefficients were evaluated from the UV− vis absorption spectra of a thin film: 4.9 × 104 cm−1 for P3HT:PCBM, 2.4 × 104 cm−1 for PffBT4T:PCBM, and 2.4 × 104 cm−1 for PCPDTBT:PC71BM at 532 nm are observed. PC71BM instead of PC61BM was used for PCPDTBT to increase the absorption at 532 nm. A layered device was set in a harmonic resonant cavity and probed by continuous microwaves (∼9.1 GHz, ∼10 mW) generated using a signal generator (Rohde & Schwarz model SMB100A). Such a low microwave power affects neither diffusional motion of charge carriers nor TPC signal. The second-harmonic generation (SHG; 532 nm) of a Nd:YAG laser (Spectra-Physics Inc. GCR-100, 5−8 ns pulse duration, 10 Hz) was used as the excitation source (incident photon density, I0 = 4.3 × 1016 photons cm−2 pulse−1). TOF transients were measured using an oscilloscope (Tektronix model DPO4000, 50 Ω termination) under an external bias (ca. 0−20 V) applied using a source meter (Keithley model 2612A). TRMC transients through a cavity, an FET amplifier, and a detector were simultaneously recorded using the same oscilloscope. The Q-curve was measured using a calibrated microwave power meter (Rohde & Schwarz model NRP2) without an FET amplifier. The experiments were performed at 27 °C under air.

Figure 1. (a) Schematic of a system for the simultaneous measurements of TOF and TRMC using a harmonic cavity and a layered device. Blue ellipsoids represent the electric field generated by microwaves in the cavity. (b) Device structure for the TOF−TRMC measurement.

Analysis of TPC Transients. Figures 2a−c show the TPC profiles (ΔI) under external electric field (E) of 103−104 V cm−1. All curves exhibit typical dispersive decay with a kink point in the logarithmic plots, corresponding to the time-offlight τTOF (Figures S2−S4). The TRMC decays at E = 0 are mostly identical to those at the lowest E (Figure S5), and the τTOFs are independent of the incident photon density of excitation I0 (Figure S6). With increasing E, the intensity of ΔI increases, while τTOF decreases, both of which are directly related to the increase in the hole velocity at high voltage. In contrast, the intensity of the TRMC signal (ΔP/P: change of microwave power divided by its baseline) mostly remains constant, while decay is accelerated with increasing E (Figures 2d−f, the linear plots of TPC and TRMC are provided in Figure S7). The dependence on bias is more pronounced for crystalline polymers (i.e., P3HT and PffBT4T) compared to semicrystalline or rather amorphous polymer (PCPDTBT). The latter exhibits a prompt decay ( τTOF (posttransit).57,58 Initially, this power-law formula was examined using a constant α or time-dependent α(t) = (τ/t)β; however, a good fit was not obtained. Thus, the modified Scott model with a Gaussian distribution for the carrier packet is used,59 which has been applied to analyze the TPC of Fe-doped Bi12Ge(Si)O20.60 pv =

⎡ (v − v )2 ⎤ 1 0 ⎥ exp⎢ − 2 σv 2π 2σv ⎣ ⎦

⎡ ⎛ t ⎞ β ⎤⎧ j(t ) = A exp⎢ −⎜ ⎟ ⎥⎨1 − ⎣ ⎝ τ ⎠ ⎦⎩ ⎪

⎪

∫0

(1) t

⎡ (1/t ′ − 1/t )2 ⎤ ⎫ 0 ⎥ dt ′⎬ × exp⎢ − 2σI 2 ⎦ ⎭ ⎣

1 σIt ′2 2π

⎪

⎪

(2)

In eq 1, σv and v0 represent the standard deviation and mean carrier velocity, respectively. A, τ, and β (0 ≤ β ≤ 1) represent the scaling factor, decay lifetime, and power factor for a stretched exponential function, respectively. The first term in C


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The Journal of Physical Chemistry C relatively flat at shorter than τTOF, which progressively decrease beyond this time. In contrast, the TRMC curves converge to the baseline at around τTOF. This convergence is readily attributed to the collection of holes at the Au electrode. TRMC decays are always more rapid than TPC decays at less than τTOF, suggesting that the local hole mobilities probed by gigahertz electromagnetic waves undergo relaxation. Alternatively, nongeminate, bulk charge recombination can also lead to the decay of the TRMC transient but does not contribute to the TPC. It has been previously reported that the TPC profile of a P3HT:PCBM thin film (∼170 nm) is mostly identical to that of transient absorption spectroscopy (TAS) within ∼40 μs at the short-circuit condition, where the recombination loss is negligible.61 On the other hand, a large difference between the slow-decaying TPC and fast-decaying TAS has been found in a dye-sensitized solar cell (DSSC), in which the authors claimed that this is due to the charges that will not be collected, but instead lost to recombination.62 In our TOF−TRMC experiments, a high excitation intensity and thick film lead to a large recombination loss, which needs to be taken into consideration. To consider bulk recombination, two differential equations based on one-dimensional diffusion under an external electric field (E > 0) and the second-order recombination of holes and electrons, respectively, are formulated. ∂np

= Dp

∂ 2np 2

− γnpnn − Eμp

∂np ∂x

(3)

∂nn ∂n ∂n = Dn 2n − γnpnn + Eμn n ∂t ∂x ∂x

(4)

∂t

∂x

is a sole fitting parameter that is assumed to be constant regardless of the external bias. The effect of external bias on the generation yield of the initial charge pair is negligible in efficient BHJ systems.18,67 Geminate recombination typically occurs within a nanosecond,63 which is considerably shorter than the time resolution of TOF−TRMC measurements (typically 40 ns). Within this time resolution, the deformation of the initial exponential distribution of holes and electrons is also negligible because the diffusion length of the charge carriers defined by the square root of DΔt is as small as ∼10 nm for μ = 1.0 × 10−3 cm2 V−1 s−1 (D = 2.6 × 10−5 cm2 s−1) and Δt = 40 ns. Equations 3 and 4 were numerically solved by a finite difference method, and φEOP was screened to minimize the residual sum of squares for the TRMC decay at the lowest E. Figure 4a shows the fit of the hole decay in P3HT at E = 5.6 ×

Figure 4. (a) Normalized probability of holes in P3HT:PCBM at E = 5.6 × 103 V cm−1 (blue) to E = 5.6 × 104 V cm−1 (orange). Colored lines are obtained by solving the diffusion rate equations (eqs 3 and 4). Black line represents the normalized TRMC decay under E = 5.6 × 103 V cm−1. (b) The reconstructed spatiotemporal profiles of hole and electron (inset) densities in P3HT:PCBM under E = 5.6 × 104 V cm−1. The color change with an arrow represents the elapsed time (blue: 0 s; red: 2.95 μs ≈ 1/3τTOF).

2

Here, np (nn) is the density of holes (electrons), Dp (Dn) is the diffusion constant of holes (electrons), which is related to its mobility via Einstein’s relation (μp = eDp/kBT, where e is the unit charge of a single electron, kB the Boltzmann constant, and T the absolute temperature, 300 K), and γ is the bimolecular (second-order) rate constant. The zero-field hole mobility obtained by TOF experiments was used to calculate Dp. In the same manner, μp was varied according to the E-dependent TOF mobilities. The Dn and μn of electrons in the polymer:PCBM blends are assumed to be constant; their values have been taken from previously reported studies: μn = 1 × 10−3 cm2 V−1 s−1 for P3HT:PCBM, 2 7 , 5 0 8.1 × 10 − 4 cm 2 V − 1 s − 1 for PffBT4T:PCBM, 53 and 4 × 10 −4 cm 2 V −1 s −1 for PCPDTBT:PC71BM.54 Notably, electron mobility is not a decisive parameter because electrons are mostly stationary at the surface and are immediately collected by the ZnO/ITO electrode after pulse exposure. The initial condition regarding the hole and electron distributions is expressed by the Beer− Lambert law (decadic absorption coefficients ε are appended in the Experimental Section) as follows: np(x , t = 0) = nn(x , t = 0) = ϕEOPεI010−εx log 10

103 V cm−1 as well as the curves calculated using the same φEOP (1 × 10−3) with increasing E. Figure S12 shows the calculated decays of PffBT4T (φEOP = 1 × 10−3) and PCPDTBT (φEOP = 3 × 10−4). These small φEOPs at high I0 are consistent with the previous reports (φEOP < 10−2),30,38,68,69 since the transient maxima of TRMC divided by I0: (ΔP/P/I0)max is progressively decreased with increasing I0 (Figure S13). The ratio of (ΔP/P/ I0)max at I0 = 1016 cm−2 pulse−1 (the intensity of the present experiments) and 1011 cm−2 pulse−1 (nongeminate recombination within the time resolution is assumed negligible) is calculated to be ∼10−3 by extrapolating the observed dependence. In addition, the yields of collected charge carrier (φTPC) obtained by integrating TPC profiles (ΔI) are found to be 2.4 × 10−4−5.9 × 10−4 at the lowest E (Figure S14). These values are close to the obtained φEOPs, indicating the consistency of our experiments and analysis. The normalized probability in Figure 4a is expressed by Np(t)/Np(0), where Np(t) is the density of hole (cm−2) integrated along the depth (x), which is calculated as follows:

(5)

where φEOP is the charge generation yield at the end of pulse (time resolution of the present TRMC). The boundary conditions constitute the extraction of electrons and the reflection of holes at x = 0 and np (nn) = 0 at x = ∞ (the calculations were stopped before the holes reach the Au electrode). The γs are taken from the literatures that evaluate the identical polymer:PC(71)BM blend films at the low excitation by using transient absorption spectroscopy (γ = 1.5 × 10−12 cm3 s−1 for P3HT,63 1 × 10−11 cm3 s−1 for PffBT4T,64 and 1 × 10−10 cm3 s−1 for PCPDTBT).65,66 Accordingly, φEOP

Np(t ) =

∫0

∞

np(x , t ) dx

(6)

Nn(t) is also defined in the same manner (Np(0) = Nn(0)). The initial decay speeds at less than ∼100 ns are similar, while the time to reach saturation is gradually shortened with increasing E. Figure 4b shows the spatiotemporal evolution of the hole and electron densities in P3HT at high E. The electrons are promptly quenched via bulk recombination and collection at D


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The Journal of Physical Chemistry C

addition, PffBT4T monotonically decreases with the normalized time (Figure 5b), but high values are retained at 0.9−0.6 even at time/τTOF = 1 (1−7 μs in real-time units). As the bulk charge recombination and E-dependent macroscopic TOF mobilities are compensated, the vertical axes in Figures 5a−c can be correlated to the relaxation of TRMC hole mobility. The mostly constant, unity value found for PffBT4T at the lowest E means no hole relaxation. Interestingly, PCPDTBT exhibits distinct, rapid decays, which decrease to the baseline at the start (time/τTOF < 10−1), indicative of considerable relaxation in this semicrystalline polymer (Figure 5c). The relaxation of the charge carriers has been investigated by the photo-CELIV technique on the 1−100 μs scale. For instance, the hole mobility in regiorandom P3HT decreases by following t−0.42,71 and the electron mobility in ambipolar PF8TBTT obeys a similar power law of time via trapping.72 Notably, these mobilities are macroscopic values observed by the linear increase of voltage with a delay time. More importantly, ultrafast time-resolved studies, which utilize short light pulses of THz34,73 or EFISH,32,40 reveal the relaxation of mobilities in polymer:PCBM blends. The relaxation of charge carriers into the density of states (DOS) or trapping has been reported to be responsible for the picosecond decay.40 In addition, this relaxation is concomitant with the charge delocalization, which facilitates the separation of excitons into free charges.74 Although the equilibrium in DOS is mostly completed at an early time scale,75 a slow relaxation is still observed at greater than 1 ns.32,40 Analogous to these electromagnetic wave spectroscopy studies similar to TRMC, the hole relaxation observed in the TOF−TRMC measurement is possibly related to the trapping and resultant lower shift of the mean mobility. Such a slow, but more pronounced, relaxation has been reportedly observed for PTB7:PC71BM by integral-mode photocurrent measurements using a power law of relaxation (μ = μ0t−α) corresponding to trapping.76 On one hand, the mean mobility obtained from resistance-dependent photovoltage (RPV) measurements of a carbazole polymer (PCDTBT:PC71BM) exhibits no dependence on the electric field and the film thickness.77 The authors claimed that the loss of carrier density to the trap states, rather than hot-carrier thermalization with DOS, occurs at a delayed time scale.77 These studies are in agreement with the reasoning provided for the observed hole relaxation. The E dependence of (ΔP/P)/ΔIana/Np is in qualitative agreement with the negative dependence of TOF mobility, while the degree of the former is less significant than the latter. Notably, the TOF mobility is a mean value averaged over the micrometer-scale transport corresponding to the film thickness (temporal profile is not accessible), and it is sensitive to the mobile carriers that reach the back electrode first. Meanwhile, the TRMC mobility, which is reflective of microscopic oscillatory motions of charges, corresponds to a value averaged over the distribution of mobilities at each delay time. Therefore, the higher E dependence of TOF compared to TRMC is likely related to the fact that mobile holes are more susceptible to trapping than the less mobile ones. Notably, TRMC divided by TAS without external E was reported to be constant for P3HT:PCBM up to 10 μs, indicative of no relaxation of hole mobility.78 This is contradictory to the results obtained for the same blend film; however, the presence of an external bias and/ or the extraction of electrons in TOF−TRMC can account for the difference. This explanation is supported by the two facts: (1) (ΔP/P)/ΔIana/Np changes to a flat profile (no relaxation)

the electrode without any significant change from the initial exponential function. In sharp contrast, holes are moved to a longer distance under the external bias, and their distribution is transformed from the initial exponential shape to a Gaussian one. This result is in good agreement with the assumption made in the TPC analysis (Gaussian distribution for the carrier packet, eq 1). Accordingly, the saturation of Np(t)/Np(0) at a delayed time is rationalized by the spatial separation of the holes and electrons under E, which suppresses charge recombination. Hole Relaxation Deduced from TOF−TRMC. The TRMC transients (ΔP/P) were divided by the analyzed curve of TPC (ΔIana, Figure 3b) and Np = Np(t)/Np(0) (Figure 4a) for compensating the decrease of holes as a result of the bulk recombination and trapping detected by TPC.70 The time was further normalized by τTOF to cancel the variations of film thickness, translational TOF mobility, and its E dependence. Figures 5a−c show the resultant (ΔP/P)/ΔIana/Np, which is

Figure 5. Normalized ΔP/P (TRMC) divided by ΔIana (fitted curve of TPC with eq 2) and Np of (a) P3HT:PCBM, (b) PffBT4T:PCBM, and (c) PCPDTBT:PC71BM as a function of the normalized time (time/τTOF). The profiles of PCPDTBT:PC71BM at time/τTOF > 0.02−0.1 are not shown because of the large noise at the baseline level. (d) The spatiotemporal profiles of normalized (ΔP/P)/ΔIana/Np under the highest E (red: 5.6 × 104 V cm−1; blue: 3.9 × 104 V cm−1; green: 8.7 × 104 V cm−1). The distance from an anode (ZnO/ ITO) is converted from the normalized time by multiplying the film thickness. The profile of PCPDTBT:PC71BM at a distance >0.1 μm is cut for the same reason.

proportional to the TRMC mobility, as a function of time/τTOF. The ΔP/P/ΔIana without the inverse of Np shows a very rapid decrease even at E = 0 and an increase of ΔP/P/ΔIana (= activation of hole mobility) with increasing E, which is unable to be reasonably explained by considering the previous TRMC results without electrodes, negative bias dependence of TOF mobility (μh), and the optimal thickness of solar cells (vide inf ra). A decay due to bulk recombination is apparently involved in ΔP/P at the high excitation intensity and thick film, which needs to be compensated. The (ΔP/P)/ΔIana/Np of P3HT decreases to ∼0.2 at time/τTOF = 1 (9−26 μs in realtime units) with no significant dependence on E (Figure 5a). In E


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The Journal of Physical Chemistry C at low E (the blue line in Figures 5a and 5b) and (2) the TRMC transient without electrodes (blend films on a quartz, implying no electron extraction) exhibits a slower decay than that under low E. The electrons generated near the surface are collected by the ZnO/ITO electrode, leading to the faster TRMC decay in the TOF-TRMC device even at E = 0 than that of electrode-less film (Figure S15). The difference in the decays of a device and nondevice is more pronounced in PCPDTBT that most probably involves a large contribution of electrons to the TRMC signal (vide inf ra). Meanwhile, TQ1:PC71BM exhibits relaxation from picosecond (TRTS) to nanosecond−microsecond (TRMC/TAS), where the relaxation of holes is more significant than that of electrons.79 Notably, TOF−TRMC represents the simultaneous, independent evaluation of the translational and oscillational mobilities of charge carriers. Accordingly, (ΔP/P)/ΔIana/Np can be replotted against the travel distance by multiplying the time/τTOF and film thickness. Low-crystallinity PCPDTBT exhibits a prompt decay down to the baseline at ∼0.1 μm, while PffBT4T and P3HT exhibit moderate decay rates (Figure 5d). Face-on-rich PffBT4T exhibits slower relaxation compared to edge-on P3HT. The film thickness is one of the most essential parameters for solar cell optimization. A thick film is preferred from the viewpoint of light absorption for increasing short circuit current density (Jsc); however, fill factor (FF) and Jsc decrease for an extremely thick film because of the increased loss via charge trap and subsequent recombination. Interestingly, the degrees of relaxation observed in TOF−TRMC studies are in agreement with the optimal thickness of the solar cells: ∼100, ∼200, and ∼300 nm for PCPDTBT,54 P3HT,27 and PffBT4T,52, respectively. Namely, the slow relaxation of holes permits maximization of optimal thickness. PffBT4T is a highly crystalline, face-on-rich polymer with a high hole mobility.52 Indeed, it exhibits the highest TOF mobility (1.4 × 10−2 cm2 V−1 s−1), followed by P3HT and PCPDTBT. The (ΔP/P)/ΔIana/Np of PffBT4T for a thickness of 300 nm is ∼0.7, which is further recovered at low E. P3HT exhibits mostly comparable relaxation (∼0.7) at 200 nm, but it continues to decrease with the thickness. The exceptionally rapid decay observed for PCPDTBT (∼0 at 100 nm) is not solely explained by hole relaxation. The contribution of electron mobility to the TRMC signal is speculated to be responsible for the rapid decay of PCPDTBT. According to our calculation (Figure 4b and Figure S12), a majority of the electrons (95−97%) are rapidly swept out by the electrode at less than 1 μs. An even more rapid extraction time of 100 ps has been reported for a PCBM-rich BHJ film (APFO3:PCBM = 1:4) at high E.40 Indeed, a spike signal ( 90% Quantum Efficiency. Adv. Mater. 2014, 26, 1923−1928. (32) Vithanage, D. A.; Devižis, A.; Abramavičius, V.; Infahsaeng, Y.; Abramavičius, D.; MacKenzie, R. C. I.; Keivanidis, P. E.; Yartsev, A.; Hertel, D.; Nelson, J.; Sundström, V.; Gulbinas, V. Visualizing Charge Separation in Bulk Heterojunction Organic Solar Cells. Nat. Commun. 2013, 4, 2334. (33) Veldman, D.; Iṕ ek, Ö .; Meskers, S. C. J.; Sweelssen, J.; Koetse, M. M.; Veenstra, S. C.; Kroon, J. M.; van Bavel, S. S.; Loos, J.; Janssen, R. A. J. Compositional and Electric Field Dependence of the Dissociation of Charge Transfer Excitons in Alternating Polyfluorene Copolymer/Fullerene Blends. J. Am. Chem. Soc. 2008, 130, 7721− 7735. (34) Jin, Z.; Gehrig, D.; Dyer-Smith, C.; Heilweil, E. J.; Laquai, F.; Bonn, M.; Turchinovich, D. Ultrafast Terahertz Photoconductivity of Photovoltaic Polymer−Fullerene Blends: A Comparative Study Correlated with Photovoltaic Device Performance. J. Phys. Chem. Lett. 2014, 5, 3662−3668. (35) Ponseca, C. S.; Yartsev, A.; Wang, E.; Andersson, M. 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