Temperature Activation of the Photoinduced Charge Carrier

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Article

Temperature Activation of the Charge Carrier Generation Efficiency in Quaterthiophene:C Mixed Films 60

Christian Koerner, Hannah Ziehlke, Roland Gresser, Roland Fitzner, Egon Reinold, Peter Bäuerle, Karl Leo, and Moritz Riede J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 Nov 2012 Downloaded from http://pubs.acs.org on November 7, 2012

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The Journal of Physical Chemistry

Temperature activation of the charge carrier generation efficiency in quaterthiophene:C60 mixed films Christian Koerner,∗,† Hannah Ziehlke,† Roland Gresser,† Roland Fitzner,‡ Egon Reinold,‡ Peter Bäuerle,‡ Karl Leo,† and Moritz Riede† Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Straße 1, 01062 Dresden, Germany, and Institute of Organic Chemistry II and Advanced Materials, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany E-mail: [email protected]

KEYWORDS: photoinduced absorption; oligothiophene; organic solar cells; temperature activation; triplet exciton; exciton diffusion; charge carrier generation

∗ To

whom correspondence should be addressed für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Straße 1, 01062 Dresden, Germany ‡ Institute of Organic Chemistry II and Advanced Materials, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany † Institut

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Abstract We measure photoinduced excitations in a dicyanovinyl end-capped methylated quaterthiophene derivative in blends with the electron acceptor C60 , as already employed in organic photovoltaics. By using DFT calculations and analyzing the recombination characteristics of the excited states revealed by photoinduced absorption (PIA) spectroscopy, the absorption peaks are assigned to triplet exciton, cation, and anion transitions. We determine the temperature dependent generation and recombination behavior of triplet excitons and cations in the mixed layer. At 10 K, we observe an enhanced triplet exciton generation rate compared to the pristine donor layer due to back recombination from a charge-transfer (CT) state at the donor-acceptor interface. With increasing temperature, the triplet generation rate first increases which is ascribed to an enhanced singlet exciton migration to this interface. Above 150 K, the triplet generation rate declines due to the beginning CT exciton separation, leading to the generation of free charge carriers. This temperature activated behavior is ascribed to a temperature activated increase of charge carrier mobility, facilitating CT exciton splitting.

Introduction In recent years, the power conversion efficiencies (PCE) of organic solar cells (OSC) have rapidly increased to remarkable and product relevant values 1,2 . Nevertheless, the PCEs still have to increase to broadly establish OSCs on the market. Therefore, much emphasis is put into research on basic processes between light absorption and charge carrier extraction. Those studies include e.g. the optimization of thin film morphology for charge transport 3 , the adjustment of energy levels towards higher open-circuit voltages 4,5 , and the insertion of interlayers to improve contacts 6 . Likewise, the parameters determining the efficiency of the dissociation step between neutral excitons and free charge carriers at the interface between electron donor (D) and acceptor (A) molecules are critical for the performance of OSCs. One of the parameters determining this efficiency is the free energy gain within the charge separation process, ∆GCS , which is usually expressed by the energy difference between the initial singlet exciton, ES , and the energy of the separated charges, defined 2 ACS Paragon Plus Environment

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D A 11 by the ionization potential of the donor (EIP ) and the electron affinity of the acceptor (EEA ) :

D A ∆GCS = ES − (EIP − EEA )

(1)

This energy difference is supposed to provide the excess energy to overcome the Coulomb binding of the subsequently formed CT exciton 7–13 . However, the role of this excess energy within the charge generation process in D/A systems has not been fully elucidated, yet 14–17 . Moreover, strong electric fields are also supposed to have an influence on the dissociation process. Such a mechanism was already proposed by Onsager for the dissociation of ion pairs 18 and later modified by Braun for the dissociation of coupled charge transfer states 19 . This model also includes a temperature activated term additional to other parameters like the initial charge pair distance or the binding energy, corresponding to a barrier for charge separation. However, the direct influence of temperature has not been investigated in much detail up to now. In this work, we investigate the influence of temperature on the charge generation process. Using PIA spectroscopy, we are able to study the recombination dynamics of long-lived excitations (triplet excitons and polarons, i.e. charged molecules such as cations or anions) with lifetimes between µs and ms. In contrast to ultra-fast transient techniques, quasi-steady-state PIA spectroscopy is not sensitive to fast processes like singlet or CT exciton dynamics, which occur on the ps or ns timescale. However, in some material systems, the recombination of CT states into the long-living donor or acceptor triplet state was found to be a major loss mechanism for charge carrier generation in OSC 20,21 . Thus, a detailed investigation of the triplet exciton and free charge carrier population and dynamics can indirectly provide access to the efficiency of charge transfer and charge carrier generation at the D/A interface. We use the class of dicyanovinyl-capped oligothiophenes as a model system to study structureproperty relationships by systematic chemical variations in the molecular structure. These materials have been investigated by our group over the last ten years and have led also to very high power conversion efficiencies approaching 7 % 4,21–26 .



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A quaterthiophene derivative similar to the one investigated here was previously investigated with PIA by Schueppel et al. 21 . The measurements were, however, only conducted at 10 K to retain measurable signals. The authors observed an increased triplet exciton generation in blends with C60 compared to a pristine donor layer of butyl substituted DCV4T (DCV4T-Bu). This finding was explained by a back recombination of CT states to the donor triplet state at the D/A interface. We investigate the chemically similar methylated quaterthiophene derivative DCV4T-Me (see Fig. 1) as the donor material, which was introduced by Fitzner et al. 26 . In blends with C60 , this material showed power conversion efficiencies of 3.8 % in a non-optimized bulk heterojunction (BHJ) structure. To investigate the influence of temperature on the charge carrier generation process, we perform temperature dependent PIA measurements between 10 and 290 K. These measurements give insight into the process cascade from singlet exciton formation in the donor to the generation of free charge carriers at the D/A interface. Due to its energetically favorable position, the DCV4TMe triplet state and its population mechanisms play a crucial role within this evaluation. We find that the generation of free charge carriers in these D/A blends is temperature activated. This behavior is assigned to a concurrent increase of the charge carrier mobility at higher temperatures which facilitates CT exciton dissociation.

Figure 1: Chemical structure of the methylated quaterthiophene derivative DCV4T-Me.

Experimental DCV4T-Me (2,2’-[3,4,3”’,4”’-tetramethyl-2,2’:5’,2”:5”,2”’-quaterthien-5,5”’-diylbis(methane-1-yl-1-ylidine)]dimalononitrile) was synthesized as described in Ref. 26 and purified once by thermal gradient sublimation. C60 was purchased from Bucky, USA, and purified twice by thermal gradient sublimation. Samples are prepared on precleaned glass substrates by thermal evaporation in 4 ACS Paragon Plus Environment

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ultra-high vacuum (UHV) with a base pressure of < 10−8 mbar. The blend layer is coevaporated at a ratio of 1:1 by volume with a total thickness of 60 nm. The densities used for controlling the layer thickness during evaporation are 1.37 g/cm3 for DCV4T-Me and 1.54 g/cm3 for C60 . The samples are encapsulated by glass-glass encapsulation under nitrogen atmosphere subsequent to the preparation step. Details concerning the technique of PIA spectroscopy and the setup details are published elsewhere 24 . Further details about the density-functional theory (DFT) calculations are given in Ref. 27.

Results For PIA investigations, the photoactive blend layer is excited by a modulated laser beam with a frequency ω . The resulting change of the transmission, ∆T , of a broad band probe light beam is detected using the lock-in technique. The spectra are divided by the total transmission, T , to compensate for the transfer function of the setup. We also investigate a pure DCV4T-Me layer as a reference to check the influence of the acceptor C60 in the blend films. Detailed information about the recombination process are obtained by measuring the modulation frequency dependence of the in-phase (IP) and out-of-phase (OP) component of the PIA signal. The simplest description of the recombination process is a monomolecular recombination model following the differential equation

dn/dt

= G(t) − n/τ for an excited state density, n,

and the excited state lifetime, τ . The time dependent generation rate, G(t), is expressed by G(t) = ηα IL (1 + cos ω t) = g0 (1 + cos ω t) with the absorption coefficient of the primary photoexcitation at the exciting laser wavelength, α , the laser intensity (as photon flux), IL , and the quantum efficiency, η , of the specific excited state. This model can be used to describe the recombination behavior of triplet excitons as long as second order processes are negligible. The



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frequency dependence of the IP- and OP-signal is then described by 28

= σ dg0

τ 1 + (ωτ )2

(2a)

= σ dg0

ωτ 2 , 1 + (ωτ )2

(2b)

∆T − T

IP

and

∆T − T

OP

where σ is the absorption cross section of the observed transition and d the layer thickness. In the quasi-steady-state limit (ωτ ≪ 1), the observed PIA IP-signal, − ∆T T IP , is written as: ∆T = σ dg0 τ = geff τ . − T IP

(3)

In this work, we summarize all of these prefactors in an effective generation rate, geff . In the case of charged excitations, this recombination model is not suitable to describe the bimolecular nature of charge carrier recombination. The literature provides several analytical solutions to the differential equations describing a bimolecular recombination process 28–31 . However, in some cases non-analytical models are used to account for the dispersive recombination of charge carriers in organic materials, a problem that cannot be solved by analytical means. We use the model proposed by Epshtein et al. 32 , where the frequency dependence of the complex amplitude, R, is given as

R=

R0 1 + (iωτ )δ

,

(4)

with the steady-state amplitude R0 and the dispersivity parameter δ (0 < δ ≤ 1). The IP (OP) component is determined by the real (imaginary) part of Eq. 4. As both models have to match for δ = 1 (non-dispersive), R0 can be expressed equivalent to Eq. 2 by geff τ . In the case of bimolecular recombination, the lifetime in Eq. 4 can be expressed by the intensity dependent bimolecular lifetime τB = √1

β g0

∝ I −1/2 with the bimolecular recombination coefficient, β 33 . The

type of recombination (e.g. monomolecular or bimolecular) can be determined from the intensity 6 ACS Paragon Plus Environment

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dependence of the PIA signal. For monomolecular recombination, both the IP and the OP signal γ show a linear increase with excitation intensity ( ∆T T IP/OP ∝ I with γ = 1). In the case of bimolecular recombination, two different regimes have to be considered: ωτB ≫ 1 (high frequency

or low intensity) and ωτB ≪ 1 (low frequency or high intensity). In the first case, the IP component increases with γ = 3/2, whereas the OP component increases with γ = 1. In the second case, the IP component increases with γ = 1/2, whereas the OP component saturates. In the case of dispersive bimolecular recombination (Eq. 4), the exponent depends on the dispersivity parameter

δ . For ωτB ≫ 1, the IP component increases with γ = 1/2(2 + δ ), whereas the OP component increases with γ = 1/2(1 + δ ) < 1. For ωτB ≪ 1, the IP component becomes independent of δ (γ = 1/2). The OP component does not saturate like in the non-dispersive case, but increases gently with γ = 1/2(1 − δ ). For δ = 1, the predictions of the dispersive model match the pure bimolecular model. This analysis can be performed in the case of non-overlapping transitions. However, as soon as more than one transition contributes to the PIA signal at one specific energy, deviations from the above analysis will occur when the recombination mechanisms of both transitions are different. To circumvent this uncertainty, we analyze the intensity dependence of the fitted lifetime and effective generation rate. For monomolecular recombination, the lifetime is independent on intensity, whereas it decays with I −1/2 in the case of bimolecular recombination. The effective generation rate, geff , increases directly proportional to the excitation intensity in both cases.

Identification of optical transitions To identify the peaks obtained by PIA spectroscopy, we perform time-dependent density functional theory (TD-DFT) calculations for DCV4T-Me. Similar calculations were already published by Schueppel et al. for the quaterthiophene derivative without any side chains (DCV4T) 21 . In addition to the transition energies, E, and oscillator strengths, f , of singlet excitons, triplet excitons and radical cations, we futhermore calculate these parameters for the radical anions, as those are 7 ACS Paragon Plus Environment


also supposed to be generated in neat donor layers. For full consistency, we repeat the calculations for DCV4T as well and compare the results in Tab. 1. The results for DCV4T-Me do not signifi− + − Table 1: Calculated transition energies for Sn ←S0 , Tn ←T1 , D+ n ←D0 and An ←A0 optical transitions for DCV4T and DCV4T-Me. For each excitation species, the transition energy, E, and the corresponding oscillator strength, f , of the first few transitions are shown.

2.17 2.57 3.11 3.28 3.40 0.67 1.48 1.62 1.94 2.08

2.018 0.000 0.000 0.309 0.008 0.009 0.000 2.237 0.000 0.000

DCV4T-Me E (eV) f 2.24 2.59 3.13 3.30 3.41 0.64 1.47 1.61 1.91 2.08

n

1.806 0.005 0.038 0.349 0.005 0.013 0.000 2.260 0.000 0.000

+ D+ n ←D0

1 2 3 4 5 2 3 4 5 6

DCV4T E (eV) f

− A− n ←A0

Sn ←S0

n

Tn ←T1

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DCV4T E (eV) f

1 2 3 4 5 1 2 3 4 5

1.00 1.58 1.83 1.96 2.19 0.81 1.53 1.64 1.66 2.24

0.304 0.000 1.882 0.000 0.019 0.373 0.000 1.764 0.000 0.063

DCV4T-Me E (eV) f 1.00 1.59 1.81 1.91 1.93 0.79 1.53 1.63 1.64 2.25

0.314 0.000 1.819 0.000 0.003 0.384 0.000 1.650 0.000 0.078

cantly differ from the values derived for the unsubstituted DCV4T, indicating that the molecules’ intrinsic energetic properties are not significantly different. This finding is not unexpected, as the alkyl side chains are not included in the π -system of the molecular backbone.

The optical transitions of a pure DCV4T-Me film measured by PIA spectroscopy are shown in Fig. 2a. The spectra resemble the measurements with the butylated DCV4T by Schueppel et al. 21 . There, the most prominent transition at 1.39 eV was assigned to the T4 ←T1 transition, which is the most intense triplet-triplet transition (see Tab. 1). The value predicted by DFT calculations is slightly higher (1.6 eV). This mismatch in energy is attributed to a polarization of the environment of the molecules in a thin film, in contrast to the conditions of an isolated molecule assumed for DFT calculations. In a mixed layer with C60 , the authors observed a triplet absorption signal that was more than doubled compared to the pristine layer. This was explained with an additional indirect population of the oligothiophene triplet state from a CT state at the DCV4T-Bu:C60 interface. Moreover, an additional peak at 0.90 eV was attributed to the quaterthiophene cation. This assign8 ACS Paragon Plus Environment

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Figure 2: (color online) a) PIA spectra of a 30 nm DCV4T-Me neat layer (dashed line) and a 60 nm DCV4T-Me:C60 blend layer (1:1 vol. ratio) (solid line) measured at 10 K. The IR region is shown in the inset on larger scale. Additionally, the evolution of the normalized spectra upon increasing the temperature from 10 to 290 K is shown for the blend (b) and the neat layer (c), respectively. The main cation and triplet transitions in the blend layer are marked by gray dotted lines. All measurements were performed at an excitation intensity of approximately 130 mW/cm2 .



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ment was confirmed by cation spectroscopy in solution, revealing cationic transitions at 0.93 eV and 1.62 eV, respectively 21 . For DCV4T-Me, we observe that the main triplet transition is slightly shifted to 1.36 eV. The indirect population mechanism of the donor triplet state is also present for DCV4T-Me. This can be concluded from the strong enhancement of the triplet transition in the blend layer spectrum compared to that of the pristine donor layer. For this process to happen, the triplet state energy must be smaller than the CT state energy. Due to the absence of a distinctive CT emission feature, we have to estimate the CT state energy from the D and A energy levels. Usually, the upper limit of the CT state energy is estimated using the difference between the HOMO of the donor and the LUMO of the acceptor molecule. In thin films, these terms are replaced by the ionization potential and the electron affinity, respectively. For the ionization potential of DCV4T-Me, a value of 5.7 eV was measured by ultraviolet photoelectron spectroscopy. The electron affinity of 4.0 eV for C60 is taken from literature 34 . Therefore, we set the upper limit of the CT state energy to 1.7 eV. Assuming a binding energy for the CT state in the order of 0.3 eV 35 , we expect the CT state energy at approximately 1.4 eV. The triplet state energy is also difficult to access due to the absence of a distinct phosphorescence signal. Schueppel et al. calculated the energies of the T1 triplet levels for the series of non-alkylated DCVnTs with n=3. . . 6 and determined values between 1.29 and 1.26 eV for the T1 state energy. The energy of the triplet level does not change significantly along this series because the triplet exciton is strongly localized on the central one or two thiophene rings 36 . Thus, we do not expect any change of the triplet level on adding methyl side chains at the terminal thiophene units. Due to the polarization effects in thin film, the calculated energies are supposed to decrease by an amount of 0.1-0.2 eV (as observed for the triplet and cation transitions). Overall, we estimate the CT state to lie higher in energy than the DCV4T-Me triplet state by approximately 0.2-0.3 eV. Coming back to the PIA spectra in Fig. 2, we find two additional transitions at 0.93 and 1.55 eV. Together with the calculated values for cation transitions (1.00 and 1.81 eV) and the cation spectroscopy results by Schueppel et al., we identify these peaks with the oligothiophene low-energy


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(LE) and high-energy (HE) cation transition. Those features exhibit recombination and intensity dependencies that differ from those observed for the triplet transition at 1.36 eV (see Fig. 3). Moreover, Fig. 2b shows that these features exhibit a different temperature behavior than the triplet transition, also supporting the different origin. At temperatures above 150 K, the cation peaks observed in the blend layer also occur in the neat layer at 1.0 eV and around 1.6 eV (see Fig. 2c), showing a smaller amplitude and a small blue-shift as compared to the mixed layer. From this observation, we conclude that there are free charge carriers created by photoexcitation in the pristine donor layer as well. The exact process for this charge generation in the absence of the usual heterojunction interface as well as externally applied electric fields is under discussion. Nevertheless, this intrinsic charge carrier generation even in the absence of an electric field has already been reported for several material systems 37–40 . However, due to the absence of an electron acceptor in the neat layer, this observation demands a presence of absorption signatures of the DCV4T-Me radical anion. Its low energy transition is predicted by DFT calculations at 0.79 eV, roughly matching the small peak around 0.7 eV we observe with PIA. At higher temperatures, a high energy anion transition (predicted at 1.63 eV) may also contribute to the plateau between 1.45 and 1.75 eV. In contrast to our expectations, no signature of a C60 anion is observed in the mixed layer (C60 anion absorbs around 1.16 eV 12,41 ). However, the C60 anion absorption reported in literature is usually quite small, which may be due to a small absorption cross section of the C60 anion transition (cf. Eq. 2). Therefore, we expect that a potential absorption signature of the C60 anion in the DCV4T-Me:C60 blend layer is hidden behind the strong oligothiophene triplet absorption.

Generation and recombination dynamics of triplet excitons and cations The spectra presented only show the positions of the optical transitions in the DCV4T-Me neat layer and the mixed layer with C60 . With increasing measurement temperature, the PIA signal intensity decreases by two orders of magnitude from 10 to 290 K, mainly due to a decrease in excited state lifetime. For further investigation of changes in the recombination characteristics, the 11 ACS Paragon Plus Environment


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dependence of the absorption signal on the modulation frequency of the laser excitation has to be examined, to extract both, lifetime and generation rate. In this work, we use the monomolecular model (Eq. 2) for the triplet exciton and the dispersive model (Eq. 4) for charged excitations. Figures 3a-c show three examples of the IP and OP signals measured at the triplet transition (1.36 eV) and the two cation transitions (0.93 and 1.55 eV) at an excitation intensity of approximately 30 mW/cm2 . For each data set, the IP and OP signals are fitted simultaneously using Eq. 4. As apparent from Fig. 3a, the data are not modeled correctly by a single monomolecular decay, because the OP component exhibits a two-peak structure which is only reproduced assuming a second component with a different lifetime and generation rate. This recombination behavior has already previously been observed for polymers 42,43 , and also for terthiophene molecules 27 . The authors proposed that several spectrally overlapping excited species cause such a behavior. Therefore, we use two independent recombination species with different lifetimes and generation rates to fit the data. Whenever necessary, a third recombination component is used to accurately fit the data. A justification for this procedure is given below. The resulting fit parameters of the exemplary measurements at 10 K are given in Fig. 3a-c. The triplet transition exhibits one strong recombination component with a lifetime of 25 µs at 10 K. From Fig. 2, it is obvious that this transition is sufficiently intense and broad to contribute signal at both cation transitions, too. The flank of the central transition around 1.3 eV reaches out until 0.7 eV and slightly less at higher energies due to the ground state bleach region. Due to the small lifetime of the DCV4T-Me triplet state, the triplet contribution can therefore unambiguously be identified in both cation transitions. The lifetimes τ1 of the first recombination component determined from the fits in Fig. 3a-c are all on the order of 20 µs. It is self-evident that the generation rates geff,1 obtained from the fits at 0.93 and 1.55 eV are much smaller than those obtained at the triplet peak, because those measurements are taken at the side of the triplet transition. At this point, it is worthwile mentioning that the determined effective generation rates of triplet exciton and cation cannot be compared directly, because the respective absorption cross sections are not expected to be identical.


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The cation features exhibit a long lived component with a lifetime τ3 on the order of 1 ms, which is attributed to the cation of DCV4T-Me. The lifetimes of the cation component at the HE and LE transition are in good agreement, confirming the common origin. Moreover, all examples show an additional recombination component with lifetimes on the order of 100 µs. In the case of the triplet transition itself, the DFT calculations in Tab. 1 show that the DCV4T-Me triplet and anion main transitions exhibit similar transition energies which may cause a multiple recombination behavior as well. Triplet exciton and cation transitions can be distinguished by the intensity dependence of their lifetime, like discussed above. Such an analysis is exemplarily shown for 10 K in Figs. 3d-f. The triplet component shows a beginning transition from monomolecular to bimolecular recombination for intensities higher than 30 mW/cm2 , which is indicated by a slight decrease in triplet lifetime above this critical intensity (see Fig. 3d) 44 . This behavior is supposed to be due to second-order processes like e.g. triplet-triplet annihilation at high excitation intensity. Therefore, all recombination analysis herein is conducted at 30 mW/cm2 (marked as dashed black line in Fig. 3d-f), where the triplet recombination is still monomolecular. The DCV4T-Me cation exhibits a typical bimolecular recombination behavior with the predicted square-root decrease of the modeled lifetime. The intensity dependence of the HE and LE cation lifetimes at 10 K is also shown in Fig. 3e and f, respectively, with a gray line indicating the square-root dependence on intensity.

Discussion This recombination analysis, yielding the lifetimes and generation rates, is performed for a set of temperatures from 10 to 290 K (see Fig. 4a). Due to the small PIA signal at higher temperatures, the cation transitions can only be evaluated until 200 K. The lifetimes of both the triplet exciton and the cation decrease with increasing temperature by more than one order of magnitude. Surprisingly, the triplet generation rate first increases by a factor of 2 from 10 to 150 K. For higher temperatures, it decreases again to approximately its low temperature value. The generation rate of the DCV4T-Me cation increases exponentially in the 13 ACS Paragon Plus Environment


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Figure 3: (color online) a-c) Excitation frequency dependent measurements at the spectral position of the triplet exciton and the LE/HE cation transitions in the DCV4T-Me:C60 blend layer. IP and OP component are plotted as filled squares and open circles, respectively. The multicomponent fits are represented by red lines. For comparison, a single component fit using the dispersive recombination model (Eq. 4) is shown as blue dotted line for the triplet transition in a). All lifetimes τi and effective generation rates geff,i are given in the respective figures. The measurements were performed at 10 K with an excitation intensity of approximately 30 mW/cm2 (marked by the dashed black line in d-f). d-f) Intensity dependence of the determined triplet (τ1 in part a) and cation lifetimes (τ3 in parts b/c). In e) and f), the gray lines with a slope of -1/2 in the log-log-plot indicate the square-root decrease of lifetime with increasing intensity predicted assuming bimolecular recombination (vide supra).


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blend layer, starting above 150 K, where the triplet generation rate starts to decrease. Before continuing the discussion, we have to exclude a direct temperature activation of the ISC process in DCV4T-Me. The ISC efficiency depends on the energy difference between the contributing singlet and triplet level. Therefore, a temperature activated generation mechanism of triplet excitons is feasible if the triplet level is slightly higher in energy than the singlet exciton. Such processes have been reported for example for terthiophenes, showing a thermally activated decrease of the fluorescence quantum yield 45 . However, Beljonne et al. also reported that especially in acceptor substituted and longer oligothiphenes, like those used herein, the triplet levels are energetically well separated from the first excited singlet state and a thermal activation of the ISC process is therefore not expected 36 . Using the numbers from Tab. 1 for DCV4T-Me, we derive a large energetic distance of E(T4 ) − E(S1 ) ≈ 0.65 eV between the first excited singlet and the nearest triplet state using E(T1 ) ≈ 1.3 eV 21 . Thus, in accordance with the results of Beljonne et al. we do not expect a thermal activation of the ISC rate. Experimental evidence for this statement is drawn from the evaluation of the tripet generation rates in the pristine DCV4T-Me film, which gives constant values over the whole evaluable temperature range (until 200 K, see SI). The temperature dependent generation rates of DCV4T-Me triplet excitons and cations can be understood if we consider the pathways for triplet exciton generation and the morphology of the blend layer, as illustrated in Fig. 4b and c. For the following interpretation, we only consider excitations in DCV4T-Me, since its absorption is much higher than that of C60 at the exciting laser wavelength (532 nm). In a pristine donor layer, triplet excitons form via intermolecular ISC. The rate constant of this process is expected not to be temperature dependent for this material. As long as the phase sizes of donor and acceptor in the mixed layer are large enough that not every exciton reaches the D/A interface, this process also occurs in the mixed layer (process 1 in Fig. 4). However, triplet exciton generation via ISC will be suppressed if the singlet exciton reaches the D/A interface. There, it undergoes charge transfer by transferring its electron to the acceptor C60 . Thus, we observe efficient photoluminescence quenching in the mixed layer (see SI). However, the singlet exciton



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diffusion length LD (T ) is temperature dependent. This behavior was investigated by Mikhnenko et al. for the polymer MDMO-PPV 46 . The authors showed an increase in LD from 3 nm at 4 K to 4.5 nm at 293 K. They explained their results by thermally activated hopping of singlet excitons in the Gaussian density of states of a disordered organic material. Following their explanation, LD may become very short at low temperature, allowing only excitons in the direct vicinity of the D/A interface to undergo charge transfer (shaded blue area in Fig. 4b). Exciton diffusion lengths in oligothiophene derivatives such as the one presented here, have not been studied in great detail, yet. However, there are some estimations for LD at room temperature ranging between 5 and 10 nm 47,48 . Subsequent to the CT process, the CT exciton is supposed to be dissociated to free polarons. However, this process is not efficient at low temperatures and the CT excitons have to recombine. In addition to direct recombination to the ground state, they can also form an energetically favorable triplet exciton on the donor. This indirect triplet state population pathway leads to the observed increase of the triplet exciton generation rate in the mixed layer compared to the neat layer, already at 10 K. Upon increasing the temperature, more singlet excitons reach the D/A interface due to activated migration. Despite this enhanced formation of CT states, their dissociation remains inefficient, so that this increase in CT exciton generation leads to an increase in DCV4T-Me triplet generation by back transfer from the CT state (process 2). For temperatures above 150 K, the generation rate of cations increases exponentially, showing that the probability for CT exciton dissociation increases significantly (process 3). Concurrently, the generation rate of triplet excitons from CT excitons decreases as these are competing processes. Moreover, the direct generation of triplet excitons from intermolecular ISC decreases as well, because the probability for a singlet exciton to reach the D/A interface strongly increases. This statement is confirmed by a decrease of the residual photoluminescence signal for increasing temperature (see SI). From our measurements, we conclude that the dissociation of the CT state into free charge carriers is a temperature activated process, which is also in accordance with Onsager-Braun theory. Using an exponential function with an activation energy Ea , we model the cation generation rate,


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Figure 4: (color online) a) Temperature dependence of lifetime τ (squares) and effective generation rate geff (circles) of the DCV4T-Me triplet exciton transition at 1.36 eV (top) and the LE cation transition at 0.93 eV (bottom) measured at an excitation intensity of approximately 30 mW/cm2 . The dashed lines are a guide to eye. b) Schematic sketch of the DCV4T-Me (blue):C60 (yellow) mixed layer and the processes 1-3 governing the three temperature regimes marked in a) and described in the text. The blue-shaded area marks the effective D/A interface region, from which the singlet excitons ( ) X can reach the real D/A interface, where the excitons can be split to form a CT exciton (represented by a red bound around electron and hole). Triplet excitons are marked by

. T With increasing temperature, this region expands further into the pristine DCV4T-Me phase due to the increased exciton diffusion length. c) Energy level diagrams for visualization of the dominant (red) and secondary (dotted black) steps following singlet excitation in the donor for the three temperature regimes discussed in the text.



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Figure 5: (color online) Exponential fit of the temperature dependent cation generation rate from Fig. 4 with an activation energy Ea according to the formula given in the figure. The determined fit parameters are also stated. following the formula given in Fig. 5. This results in an activation energy of 39 meV, which is higher than the thermal energy kB T at room temperature (25 meV). Thus, it remains questionable if thermal energy alone is responsible for the dissociation of the CT states at the D/A interface. Several factors are reported in literature to influence charge carrier generation efficiency at the D/A interface, for example the free-energy difference driving charge separation 9,11–13 , macroscopic electric fields 49 , the dielectric constant of the blend 50 , the strength of electronic interactions at the donor acceptor interface 51 , the film nanomorphology/domain size 9,50,53–56 , and inter/intramolecular charge/wavefunction delocalization 9,53,56–58 . Among those, only the charge carrier mobility is directly temperature dependent. We therefore conclude that the observed increase in DCV4T-Me cation generation with increasing temperature is not only a direct result of additional thermal energy, but most probably due to an increase in charge carrier mobility driving charge separation. It is intuitive to state that the dissociation of CT states depends on the ability of charge carriers to move away from the interface 15,59 . Charge carrier motion in rather amorphous or nanocrystalline organic materials is governed by hopping transport. The hopping process from 18 ACS Paragon Plus Environment

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one site to another is determined by the site distance and the energy difference between two sites. This energy difference is macroscopically expressed in terms of an energetic disorder, which is in the range of 50 to 150 meV for organic materials 60–62 . Thus, the activation energy of 39 meV, which is determined herein, is a reasonable value for the temperature activation of charge carrier motion in a disordered energetic landscape. Simply spoken, an increase in temperature facilitates motion of charge carriers away from the interface, which might be expressed macroscopically by an increase in charge carrier mobility which in turn promotes CT state dissociation. In this sense, many of the impact factors mentioned above refer to the same principle mechanism in those blend layers: nanomorphology, domain size, charge carrier delocalization, and charge carrier mobility may all refer to the same fundamental issue, namely the ability of the charge carrier to move (away from the interface).

Conclusion We show photoinduced absorption measurements on DCV4T-Me in pristine and mixed layers with fullerene C60 . The spectra are assigned to specific triplet exciton, cation, and anion transitions using DFT calculations. The analysis of the generation and recombination behavior via excitation modulation frequency dependent measurements reveals an increased triplet exciton generation rate in the blend compared to the pristine donor layer. We assign this finding to an additional population path via recombination from a CT state at the D/A interface. This population mechanism is most efficient at low temperatures (T < 150 K) due to a suppressed CT state dissociation. At higher temperatures, several competing processes are populating the oligothiophene triplet state (intramolecular intersystem crossing, CT state recombination) or hindering triplet generation by dissociating CT states into free charge carriers. At temperatures above 150 K, an exponential increase in charge carrier generation rate is observed and assigned to an increase in charge carrier mobilities with higher temperatures. We determine an activation energy of 39 meV, which is com-



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parable to the energetic disorder in organic materials. Therefore, our analysis provides insight not only into the population and recombination behavior of directly accessible triplet excitons and charge carriers, but also indirectly to the dissociation efficiency of the CT state.

Acknowledgement We thank Max Tietze for UPS measurements. Funding was granted from the Deutsche Forschungsgemeinschaft (DFG) within the Priority Program SPP1355, which we gratefully acknowledge.

References (1) Dou, L.; You, J.; Yang, J.; Chen, C.-C.; He, Y.; Murase, S.; Moriarty, T.; Emery, K.; Li, G.; Yang, Y. Nature Photonics 2012, 6, 180–185. (2) heliatek GmbH, press release 2012, (3) Wynands, D.; Levichkova, M.; Riede, M.; Pfeiffer, M.; Baeuerle, P.; Rentenberger, R.; Denner, P.; Leo, K. Journal of Applied Physics 2010, 107, 014517. (4) Riede, M.; Uhrich, C.; Widmer, J.; Timmreck, R.; Wynands, D.; Schwartz, G.; Gnehr, W.-M.; Hildebrandt, D.; Weiss, A.; Hwang, J.; Sundarraj, S.; Erk, P.; Pfeiffer, M.; Leo, K. Advanced Functional Materials 2011, 21, 3019. (5) Yamamoto, S.; Orimo, A.; Ohkita, H.; Benten, H.; Ito, S. Advanced Energy Materials 2012, 2, 229–237. (6) Ratcliff, E. L.; Zacher, B.; Armstrong, N. R. The Journal of Physical Chemistry Letters 2011, 2, 1337–1350. (7) Drori, T.; Sheng, C.-X.; Ndobe, A.; Singh, S.; Holt, J.; Vardeny, Z. V. Physical Review Letters 2008, 101, 37401. (8) Drori, T.; Holt, J.; Vardeny, Z. V. Physical Review B 2010, 82, 75207. 20 ACS Paragon Plus Environment

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(9) Shoaee, S.; An, Z.; Zhang, X.; Barlow, S.; Marder, S. R.; Duffy, W.; Heeney, M.; McCulloch, I.; Durrant, J. R. Chemical Communications 2009, 1, 5445. (10) Shoaee, S.; Clarke, T. M.; Huang, C.; Barlow, S.; Marder, S. R.; Heeney, M.; McCulloch, I.; Durrant, J. R. Journal of the American Chemical Society 2010, 132, 12919. (11) Ohkita, H.; Cook, S.; Astuti, Y.; Duffy, W.; Tierney, S.; Zhang, W.; Heeney, M.; McCulloch, I.; Nelson, J.; Bradley, D. D. C.; Durrant, J. R. Journal of the American Chemical Society 2008, 130, 3030. (12) Clarke, T. M.; Ballantyne, A. M.; Nelson, J.; Bradley, D. D. C.; Durrant, J. R. Advanced Functional Materials 2008, 18, 4029. (13) Veldman, D.; Ìpek, O.; 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. Journal of the American Chemical Society 2008, 130, 7721. (14) Lee, J.; Vandewal, K.; Yost, S. R.; Bahlke, M. E.; Goris, L.; Baldo, M. A.; Manca, J. V.; Voorhis, T. V. Journal of the American Chemical Society 2010, 132, 11878. (15) Pensack, R. D.; Asbury, J. B. The Journal of Physical Chemistry Letters 2010, 1, 2255. (16) Marsh, R. A.; Hodgkiss, J. M.; Friend, R. H. Advanced materials 2010, 22, 3672. (17) Bakulin, A. A.; Rao, A.; Pavelyev, V. G.; van Loosdrecht, P. H. M.; Pshenichnikov, M. S.; Niedzialek, D.; Cornil, J.; Beljonne, D.; Friend, R. H. Science 335, 1340. (18) Onsager, L. Physical Review 1938, 54, 554. (19) Braun, C. L. The Journal of Chemical Physics 1984, 80, 4157. (20) Veldman, D.; Meskers, S. C. J.; Janssen, R. A. J. Advanced Functional Materials 2009, 19, 1939.



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(21) Schueppel, R.; Schmidt, K.; Uhrich, C.; Schulze, K.; Wynands, D.; Brédas, J.; Brier, E.; Reinold, E.; Bu, H.-B.; Baeuerle, P.; Maennig, B.; Pfeiffer, M.; Leo, K. Physical Review B 2008, 77, 1. (22) Schulze, K.; Riede, M.; Brier, E.; Reinold, E.; Bäuerle, P.; Leo, K. Journal of Applied Physics 2008, 104, 074511. (23) Wynands, D.; Levichkova, M.; Leo, K.; Uhrich, C.; Schwartz, G.; Hildebrandt, D.; Pfeiffer, M.; Riede, M. Applied Physics Letters 2010, 97, 073503. (24) Ziehlke, H.; Burtone, L.; Koerner, C.; Fitzner, R.; Reinold, E.; Bäuerle, P.; Leo, K.; Riede, M. Organic Electronics 2011, 12, 2258. (25) Fitzner, R.; Reinold, E.; Mishra, A.; Mena-Osteritz, E.; Ziehlke, H.; Körner, C.; Leo, K.; Riede, M.; Weil, M.; Tsaryova, O.; Weiß, A.; Uhrich, C.; Pfeiffer, M.; Bäuerle, P. Advanced Functional Materials 2011, 21, 897. (26) Fitzner, R.; Elschner, C.; Weil, M.; Uhrich, C.; Körner, C.; Riede, M.; Leo, K.; Pfeiffer, M.; Reinold, E.; Mena-Osteritz, E.; Bäuerle, P. Advanced Materials 2012, 24, 675. (27) Ziehlke, H.; Fitzner, R.; Koerner, C.; Gresser, R.; Reinold, E.; Bäuerle, P.; Leo, K.; Riede, M. K. Journal of Physical Chemistry A 2011, 115, 8437. (28) Dellepiane, G.; Cuniberti, C.; Comoretto, D.; Musso, G. F.; Figari, G.; Piaggi, A.; Borghesi, A. Physical Review B 1993, 48, 7850. (29) Botta, C.; Luzzati, S.; Tubino, R.; Bradley, D. D. C.; Friend, R. H. Physical Review B 1993, 48, 14809. (30) Westerling, M.; Vijila, C.; Österbacka, R.; Stubb, H. Chemical Physics 2003, 286, 315. (31) Westerling, M.; Vijila, C.; Österbacka, R.; Stubb, H. Physical Review B 2004, 69, 245201.


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(32) Epshtein, O.; Nakhmanovich, G.; Eichen, Y.; Ehrenfreund, E. Physical Review B 2001, 63, 125206. (33) Epshtein, O.; Eichen, Y.; Ehrenfreund, E.; Wohlgenannt, M.; Vardeny, Z. Physical Review Letters 2003, 90, 1. (34) Zhao, W.; Kahn, A. Journal of Applied Physics 2009, 105, 123711. (35) Gélinas, S.; Paré-Labrosse, O.; Brosseau, C.-N.; Albert-Seifried, S.; McNeill, C. R.; Kirov, K. R.; Howard, I. A.; Leonelli, R.; Friend, R. H.; Silva, C. Journal of Physical Chemistry C 2011, 115, 7114. (36) Beljonne, D.; Cornil, J.; Friend, R. H.; Janssen, R. A. J.; Brédas, J. L. Journal of the American Chemical Society 1996, 118, 6453. (37) Dippel, O.; Brandl, V.; Bässler, H.; Danieli, R.; Zamboni, R.; Taliani, C. Chemical Physics Letters 1993, 216, 418. (38) Lanzani, G.; Rossi, L.; Piaggi, A.; Pal, A.; Taliani, C. Chemical Physics Letters 1994, 226, 547. (39) Cunningham, P. D.; Hayden, L. M. Journal of Physical Chemistry C 2008, 112, 7928. (40) Paquin, F.; Latini, G.; Sakowicz, M.; Karsenti, P.-L.; Wang, L.; Beljonne, D.; Stingelin, N.; Silva, C. Physical Review Letters 2011, 106, 1. (41) Janssen, R. A. J.; Hummelen, J. C.; Lee, K.; Pakbaz, K.; Sariciftci, N. S.; Heeger, A. J.; Wudl, F. The Journal of Chemical Physics 1995, 103, 788. (42) Aarnio, H.; Westerling, M.; Österbacka, R.; Svensson, M.; Andersson, M. R.; Pascher, T.; Pan, J.; Sundström, V.; Stubb, H. Synthetic Metals 2005, 155, 299. (43) Aarnio, H.; Westerling, M.; Österbacka, R.; Svensson, M.; Andersson, M. R.; Stubb, H. Chemical Physics 2006, 321, 127. 23 ACS Paragon Plus Environment


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(44) The decrease of the triplet lifetime with intensity, indicating a second order process, is more clearly visible at higher temperatures. However, for the sake of clarity, we omitted the respective figure in the main body of the paper and shifted it to the Supporting Information. (45) Rossi, R.; Ciofalo, M.; Carpita, A.; Ponterini, G. Journal of Photochemistry and Photobiology A: Chemistry 1993, 70, 59. (46) Mikhnenko, O. V.; Cordella, F.; Sieval, A. B.; Hummelen, J. C.; Blom, P. W. M.; Loi, M. A. Journal of Physical Chemistry B 2008, 112, 11601. (47) Uhrich, C. Strategien zur Optimierung organischer Solarzellen - Dotierte Transportschichten und neuartige Oligothiophene mit reduzierter Bandlücke. Ph.D. thesis, Technische Universität Dresden, 2008. (48) Wynands, D.; Männig, B.; Riede, M.; Leo, K.; Brier, E.; Reinold, E.; Bäuerle, P. Journal of Applied Physics 2009, 106, 054509. (49) Offermans, T.; van Hal, P.; Meskers, S.; Koetse, M.; Janssen, R. Physical Review B 2005, 72, 45213. (50) Mihailetchi, V. D.; Koster, L. J. A.; Blom, P. W. M.; Melzer, C.; de Boer, B.; van Duren, J. K. J.; Janssen, R. A. J. Advanced Functional Materials 2005, 15, 795. (51) Nelson, J.; Kirkpatrick, J.; Ravirajan, P. Physical Review B 2004, 69, 1. (52) Zhang, F.; Jespersen, K. G.; Björström, C.; Svensson, M.; Andersson, M. R.; Sundström, V.; Magnusson, K.; Moons, E.; Yartsev, A.; Inganäs, O. Advanced Functional Materials 2006, 16, 667. (53) Bartelt, A. F.; Strothkämper, C.; Schindler, W.; Fostiropoulos, K.; Eichberger, R. Applied Physics Letters 2011, 99, 143304. (54) McNeill, C. R.; Halls, J. J. M.; Wilson, R.; Whiting, G. L.; Berkebile, S.; Ramsey, M. G.; Friend, R. H.; Greenham, N. C. Advanced Functional Materials 2008, 18, 2309. 24 ACS Paragon Plus Environment

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(55) Quist, P. A. C.; Savenije, T. J.; Koetse, M. M.; Veenstra, S. C.; Kroon, J. M.; Siebbeles, L. D. A. Advanced Functional Materials 2005, 15, 469. (56) Mandoc, M. M.; Veurman, W.; Sweelssen, J.; Koetse, M. M.; Blom, P. W. M. Applied Physics Letters 2007, 91, 073518. (57) Deibel, C.; Strobel, T.; Dyakonov, V. Physical Review Letters 2009, 103, 1. (58) Schwarz, C.; Bässler, H.; Bauer, I.; Koenen, J.-M.; Preis, E.; Scherf, U.; Köhler, A. Advanced materials 2012, 24, 922. (59) Peumans, P.; Forrest, S. R. Chemical Physics Letters 2004, 398, 27. (60) Hertel, D.; Bässler, H. Chemphyschem 2008, 9, 666. (61) Martens, H.; Blom, P.; Schoo, H. Physical Review B 2000, 61, 7489. (62) Bässler, H. physica status solidi (b) 1993, 175, 15.



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Temperature activation of the charge carrier generation efficiency in oligothiophene:C60 mixed layers

Supporting Information

DCV4T-Me triplet exciton: second-order processes at high intensities The triplet exciton usually exhibits a monomolecular recombination behavior at low intensities, indicated by an intensity independent lifetime. At higher intensities, second-order processes like triplet-triplet annihilation shift the recombination to a characteristic bimolecular behavior with the lifetime being dependent on the excitation density and, hence, the laser intensity. Figure 3d in the main paper only shows the data at 10K, where the deviation from the monomolecular regime is not yet significant. However, as we compare the fit results to high temperature measurements, it is important that second-order processes can be neglected in any case. Figure S1 shows the intensity dependent fit results for the triplet exciton lifetime at temperatures between 10 and 290 K. It is clearly visible, that there is a transition from the monomolecular to the bimolecular regime above an intensity of about 30 mW/cm2 . This critical value is therefore used for all evaluations in the paper.


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Figure S1: Intensity dependent fit results for the triplet exciton lifetime at temperatures between 10 and 290 K. The data were normalized to compare the different temperatures. The dashed black line has a slope of -1/2, characteristic of a bimolecular recombination process.

Triplet exciton generation rate in pristine DCV4T-Me To ensure that the generation of triplet excitons from intermolecular intersystem crossing does not depend on temperature, we investigate the triplet exciton generation rate, measured with PIA, in the pristine DCV4T-Me layer. The effective generation rate, geff , is extracted from fits to the modulation frequency dependent measurements equivalent to the treatment of the blend layer in the main paper and excited at the same laser intensity. The obtained value is approximately constant over the whole accessible wavelength range from 10 to 200 K (see Fig. S2). At higher temperatures, the signal is too small to apply a decent fitting procedure and extract reasonable values.

Efficient PL quenching in the DCV4T-Me:C60 blend layer Like generally observed in donor/acceptor (D/A) blends for organic solar cells, the donor photoluminescence (PL) is strongly quenched when it is blended with C60 due to efficient charge transfer from the donor to the acceptor. The normalized PL of a pristine layer compared to a 1:1 mixed 27 ACS Paragon Plus Environment


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Figure S2: Temperature dependent effective triplet generation rate, geff , in the pristine DCV4TMe layer with identical fitting and measurement conditions like those applied for the blend layer evaluation in the paper. layer is shown in Fig. S3. Compared to the neat layer, the blend layer PL is quenched by a factor of 50. Additionally, the spectrum is slightly changed. The shoulder at 660 nm and the peak at 720 nm could originate from C60 , additional contribution may also originate from luminescent charge transfer (CT) exciton recombination. The absolute value of pure donor PL quenching can, however, not fully be elucidated from this measurement. The measurements were performed with a Edinburgh Instruments FSP920 fluorescence spectrometer. Both samples were excited with a xenon arc lamp at 550 nm with comparable intensity. The absorption strength of both samples at the excitation wavelength is similar, thus the absolute numbers can directly be compared.

Temperature dependent photoluminescence The temperature dependence of the PL was measured using the PIA setup. The blend layer PL spectra at various temperatures, presented in Fig. S4, are therefore not corrected for the spectral 28 ACS Paragon Plus Environment

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Figure S3: Normalized PL spectra of a 30 nm DCV4T-Me layer (solid line) and a 60 nm 1:1 blend layer (mixed by volume) (dashed line). transfer function of the measurement setup and are not meant to be compared to Fig. S3. The step at 830 nm is due to a grating change in the monochromator. Due to efficient quenching, the signal is quite small and as the setup is not optimized for PL measurements, the noise is high. Nevertheless, Fig. S4 shows that the residual luminescence in the blend layer further decreases from 10 to 200 K, and then stays approximately constant until 290 K. This indicates that all singlet excitons can reach the D/A interface and undergo a CT. The remaining signal may therefore originate from luminescent CT state recombination, which can, however, not be fully confirmed without further measurements.



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Figure S4: Temperature dependent measurements of the residual blend layer luminescence (the IP component of the lock-in signal), using the PIA setup, which is not optimized for PL measurements. The neat layer PL, measured at 10 K, is shown for comparison (black dashed line, divided by a factor of 15 to match the scale). The spectra are not corrected for the setup’s spectral transfer function.


Temperature Activation of the Photoinduced Charge Carrier

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