Mechanism of Mobile Charge Carrier Generation in Blends of

Apr 3, 2012 - Despite significant improvement in power conversion efficiencies of bulk-heterojunction solar cells, the mechanism of mobile charge carr...
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Mechanism of Mobile Charge Carrier Generation in Blends of Conjugated Polymers and Fullerenes: Significance of Charge Delocalization and Excess Free Energy D. H. K. Murthy,†,‡ M. Gao,† M. J. W. Vermeulen,† L. D. A. Siebbeles,† and T. J. Savenije*,† †

Optoelectronic Materials Section, Department of Chemical Engineering, Delft University of Technology, 2628 BL Delft, The Netherlands ‡ Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands S Supporting Information *

ABSTRACT: Despite significant improvement in power conversion efficiencies of bulkheterojunction solar cells, the mechanism of mobile charge carrier generation is still under debate. The time-resolved microwave conductivity technique is used to investigate the mobile charge carrier generation in blends of P3HT with monoPCBM and bisPCBM by varying the excitation wavelength from the visible to NIR and the temperature from 88 to 300 K. NIR excitation corresponds to the transition of an electron from the HOMO of the P3HT directly to the LUMO of the fullerene forming the charge transfer band (CT). From the results it is inferred that the binding energy between the electron and hole in the CT state is smaller than thermal energy at 88 K (7.8 meV) that is in large contrast to previously reported values of 0.3 eV. This is ascribed to efficient charge delocalization, which increases the mean distance between the electron and hole at the interface. For P3HT:bisPCBM, the yield of charge carries decreases by a factor of 3 on changing the wavelength from the visible to the NIR. This is attributed to recombination of CT states to triplet level of P3HT. However, as the yields for P3HT:PCBM and P3HT:bisPCBM are comparable on visible excitation, we conclude that for the latter blend formation of mobile charge carrier occurs primarily via a thermally nonrelaxed, hot CT state. This observation indicates that the excess energy involved in the exciton dissociation process is indeed important to avoid recombination to the triplet level and to achieve higher yields of charge carrier generation. On the basis of these findings, we suggest that the excess energy can be small as long as the triplet level of the polymer is located energetically higher than the CT state. This insight is of particular interest for the rational design of novel polymer/fullerene systems to achieve higher power conversion efficiencies.

1. INTRODUCTION Soluble conjugated polymers are intensively investigated for application in low-cost molecular optoelectronics. The possibility to realize efficient photoinduced generation of free charges in blends of conjugated polymers and fullerene derivatives is of great promise for development of bulk heterojunction (BHJ) solar cells. Power conversion efficiencies improve rapidly over the past decade, and values exceeding 8% have been realized.1 Generation of charges in such blends starts by the absorption of a photon by the polymer or the fullerene forming a bound electron/hole pair or exciton. The dissociation of the exciton results in the formation of an electron on the fullerene and a hole on the conjugated polymer.2−8 In case these charges are localized at the interface, the oppositely charged carriers are electrostatically bound which we refer to as charge transfer (CT) states. Typically, the energy of a charge transfer (ECT) state is determined by9 ECT = IPD − EAA − Δ (1)

electrostatic binding energy that decreases with the distance between the charges and the dielectric constant. Generally, Δ is considered to be in the range of 0.1−0.5 eV for low dielectric blends.2,3,10−18 For charge carrier collection the CT states have to overcome the binding energy. Therefore, it is rather intriguing to note that internal quantum efficiencies for BHJ solar cells are approaching unity.19−22 In the next three paragraphs, we discuss briefly several mechanisms that have been developed to explain the formation of mobile charge carriers in low dielectric blends. The current− voltage characteristics of photovoltaic devices based on various polymer:fullerene blends have successfully been described by using the Onsager−Braun model.23 This model involves the escape of thermally equilibrated point charges from geminate recombination by random diffusion and drift in each other’s Coulomb field.9,24 According to this model, changing the temperature or external field strength should have a strong impact on the yield of charges in polymer:fullerene blends. However, the experimentally found independent behavior of

in which IPD is the ionization energy of the electron donor and EAA the electron affinity of the electron acceptor. Note that both the IP and EA values are bulk properties, which may slightly change in a blend. The term Δ represents the

Received: January 20, 2012 Revised: March 28, 2012 Published: April 3, 2012

© 2012 American Chemical Society

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the charge carrier generation with respect to the temperature and electric field is in sharp disagreement with this model.25−29 Moreover, long lifetimes of the CT states up to 7 μs24 in poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are required to model the experimentally observed current voltage plots, which is inconsistent with the found lifetimes of tens of nanoseconds.30 In an alternative model to explain the formation of mobile charges in polymer:fullerene blends, the yield for mobile charge carrier generation was related to the excess energy involved in the exciton dissociation (ΔGCS).31,32 For P3HT:PCBM, ΔGCS amounts to 0.9 eV.33 Increase of ΔGCS values resulted in a higher yield of mobile charge carrier formation. These observations were rationalized in terms of the “hot CT model”, in which a high ΔGCS value provides excess vibrational energy to the formed CT state that is required to overcome the binding energy.34 However, efficient free charge carrier generation was also observed in blend systems for which ΔGCS was only 100 meV,35−37 raising doubts on the role of the excess energy in the dissociation process. In addition, observation of similar internal quantum efficiencies38 on above and below band gap optical excitation suggests that the excess energy is not essential for generating free carriers. In theoretical models, the effect of charge delocalization on the yield of free charge carriers has been taken into consideration.39−41 It was demonstrated that charge delocalization over multiple monomeric units of the conjugated polymer leads to a larger mean distance between the opposite charges at the interface. This reduces the electrostatic binding energy resulting in a small value of Δ. For charge delocalization to occur the blend should contain well-ordered or crystalline domains. The aim of this work is to obtain experimental insight into the validity of the above-discussed mechanisms for P3HT:PCBM and P3HT:bisPCBM blends. To this end, the effect of the excess free energy for exciton dissociation and of the temperature on the quantum yields will be investigated. The blend will be photoexcited at 500 nm (visible) or at 830 nm (NIR). Optical excitation at the latter wavelength corresponds to the transition of an electron from the HOMO of the P3HT directly to the LUMO of PCBM(CT band).42,43 As depicted in Figure 1, on 500 nm excitation charge carrier generation can occur either via “the hot CT state” (process 4) or via the thermally relaxed CT state (processes 3 and 5). Importantly, on excitation at 830 nm charge carrier formation is only possible via the relaxed CT state (process 5). For the energy level corresponding to the mobile uncorrelated charges, a value of 1.2 eV versus the ground state of the molecules is reported for P3HT:PCBM blends.44 For the P3HT:bisPCBM blends this value is ∼0.2 eV higher than P3HT:PCBM owing to smaller electron affinity for bisPCBM.45 Note that the triplet energy level of P3HT is located at ∼1.4 eV.14 To investigate the formation of mobile charge carriers, the time-resolved microwave conductivity (TRMC) technique is employed. In using this technique, the change in conductance of the film, induced by a short laser pulse, is recorded on a nanosecond time scale without applying external electrodes. In addition, photoconductance traces are measured as a function of wavelength yielding an action spectrum comparable with an external quantum efficiency (EQE) spectrum. These measurements allowed us to deduce the energy of the CT states from the onset in the formation of mobile charge carriers. The effect of varying the temperature and of the excitation wavelength on

Figure 1. Schematic energy level diagram including the main photophysical processes involved in charge carrier photogeneration for P3HT:PCBM. Process 1: photoexcitation; 2: exciton dissociation; 3: thermal relaxation; 4: dissociation of the hot CT state into mobile carriers; 5: dissociation of the thermally relaxed CT state into mobile carriers. For P3HT:bisPCBM only the energetic levels of the CT states are indicated. This energetic distribution is attributed to the different electron affinities for various bisPCBM isomers. Note that the triplet energy level for P3HT is located approximately at 1.4 eV close to the CT states of P3HT:bisPCBM.

the yield of charge carriers is investigated. The results enable us to provide insight on the mechanism of mobile charge carrier generation.

2. EXPERIMENTAL SECTION Electronic grade regioregular poly(3-hexylthiophene) was purchased from Rieke. The fullerene derivative, [6,6]-phenylC61-butyric acid methyl ester, was purchased from Solenne. Both compounds were used without further purification. The materials were dissolved in 1,2-dichlorobenzene at a total concentration of 30 mg/mL (1:1 weight ratio). The films were deposited onto precleaned quartz substrates (ESCO Products) by spin-coating. Samples were annealed at 393 K for 20 min in the nitrogen atmosphere of a glovebox with oxygen and water concentrations of less than 2 ppm. The thickness of the films was found to be ∼300 nm. The optical attenuation spectrum (FA) of the blend layer corresponds to the fraction of incident photons that is absorbed by the sample. This was obtained according to ⎛ I + IR ⎞ FA = 1 − ⎜ T ⎟ ⎝ I0 ⎠

(2)

where IT and IR transmission and reflection, respectively, recorded by a Perkin-Elmer Lambda 900 Vis/NIR spectrophotometer equipped with an integrating sphere. I0 is the incident light intensity. Time-resolved microwave photoconductance (TRMC) measurements were carried out using a custom-made liquid nitrogen cooled microwave cavity with a resonance frequency at ca. 8.45 GHz. Samples were photoexcited with a 3 ns laser pulse from an optical parametric oscillator pumped at 355 nm with the third harmonic of a Q-switched Nd:YAG laser (Vibrant II, Opotek). Photogeneration of mobile charge carriers in the sample leads to an increase of the conductance, ΔG(t), and consequently to an enhanced absorption of microwave power by the sample. The time-dependent change of the conductance is obtained from the normalized change in microwave power (ΔP(t)/P) reflected from the cavity according to ΔP(t ) = −K ΔG(t ) P 9215

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influence the dynamics of photogeneration and recombination of the mobile carriers. From the maximum change in conductance the product of the quantum yield per absorbed photon (φ) and the sum of the charge carrier mobilities (∑μ) is calculated (eq 4). Figure 3

The geometrical dimensions of the cavity and dielectric properties of the media in the microwave cavity determine the sensitivity factor, K. From the maximum change in the conductance (ΔGmax) and the incident light (I0), the parameter φ∑μ denoting the product of the charge carrier generation yield (φ) per absorbed photon and the sum of the electron and hole mobilities (∑μ) can be calculated using φ∑μ =

ΔGmax I0βeFA

(4)

where β is the ratio between the broad and narrow inner dimensions of the waveguide, e is the elementary charge, and FA (eq 2) is the fraction of light absorbed by the sample. For more experimental details see refs 46 and 47.

3. RESULTS AND DISCUSSION In the first part of this section, we address the mechanism for the generation mobile charge carriers in P3HT:PCBM blends. In the second part results on P3HT:bisPCBM blends are presented and discussed. Figure 2 shows the microwave photoconductance traces obtained on excitation of the P3HT:PCBM blend at 500 nm

Figure 2. Photoconductance traces for P3HT:PCBM blends obtained on excitation at 500 nm (solid lines) and at 830 nm (dotted lines) in a log/log representation. Note that, due to very weak optical absorption of the CT states, the fluence at 830 nm is increased to obtain a similar conductance trace as on excitation at 500 nm.

Figure 3. Product of the quantum yield per absorbed photon (φ) and the sum of the charge carrier mobilities (∑μ) versus incident intensity normalized to the optical absorption at 500 and 830 nm for blends of P3HT:PCBM (A) and P3HT:bisPCBM (B).

due to the formation of mobile charge carriers as reported previously.48 The relatively slow rise of the trace is in agreement with the 3 ns width of the laser pulse and 18 ns response time of the microwave cavity, resulting in a maximum of the photoconductance at about 60 ns. As no electrodes are involved in TRMC measurements, the conductance slowly decays on a time scale of tens microseconds due to charge recombination.49 Photoexcitation at 830 nm resulted in similar photoconductance traces provided that the laser intensity was increased by more than 2 orders of magnitude. Note that the photoconductance at 830 nm excitation cannot be due to charge carriers generated by hole transfer from PCBM to P3HT, since at this wavelength the optical absorption for PCBM is negligible.42,43,50 The similar photoconductance traces unambiguously show that formation of mobile charge carriers by excitation of the thermally relaxed CT state is feasible (process 5, Figure 1). This similarity shows that the excess energy of 0.9 eV involved in 500 nm excitation does not

compares the φ∑μ values for excitation at 500 nm and at 830 nm for blends of P3HT with PCBM and bisPCBM. For P3HT:PCBM (see Figure 3A), φ∑μ values for excitation at 500 nm using low fluences amount to 0.045 cm2/(V s), comparable with previous reported values.48,49 At higher fluences a reduction of the φ∑μ values is observed that has been attributed to higher order recombination processes.51 For photoexcitation at 830 nm the same maximum φ∑μ values are observed. Note that at 830 nm excitation φ∑μ values do not show a decrease on higher laser fluences. This is attributed to the homogeneous excitation profile across the blend layer due to the very low absorption coefficient of the CT states. In contrast, the absorption coefficient at 500 nm is ∼3 orders of magnitude higher,38,42 resulting in a short penetration depth of the incident light yielding a high charge carrier density. Therefore, higher order recombination processes can be seen at 9216

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are very similar. Figure 4 collects the φ∑μ values for P3HT:PCBM blends for both wavelengths. As has been demonstrated previously for excitation in the visible, the yield for charge carrier generation is not affected in the temperature range from 88 K to room temperature.26 Interestingly, for excitation at 830 nm similar φ∑μ values are found. Hence the excess free energy involved in the exciton dissociation process does not influence the yield of charges even at 88 K at which temperature electrons have about 1/3 of the thermal energy at room temperature. This result is in contrast to the notion that electron mobility should exceed to that of hole mobility at least by 2 orders of magnitude for efficient dissociation of CT states into mobile charge carriers.9,37 From the results discussed above we conclude that for P3HT:PCBM φ is independent of the excitation wavelength and of the temperature ranging between 88 and 300 K. These two aspects can only be explained by assuming that the electrostatic binding energy between the opposite charges at the interface is small, i.e., smaller than thermal energy at 88 K. This small binding energy of the CT state (Δ) can be understood by the presence of significant delocalization of a single or of both charges comprising the CT state. This notion is in accordance with the conclusion drawn from the theoretical modeling by Deibel et al.39 and Nenashev et al.41 Therefore, the classical description of a CT state formed by the two opposite point charges at close distance with a significantly larger binding energy (>kBT) is not pertinent to refer to the delocalized state formed on NIR excitation of a P3HT:PCBM blend. This charge delocalization is not considered in the Braun−Onsager model and application of this model is hence seems less appropriate. In order to confirm the small value of Δ corresponding to the delocalized CT states in P3HT:PCBM blends, the onset of the optical absorption of this CT state should coincide with the onset of the formation of mobile carriers. Figure 5 shows the

higher fluences explaining the decrease in φ∑μ values. Nevertheless, at low fluences higher order recombination processes are virtually absent and hence φ∑μ values obtained on excitation at 830 and 500 nm can be compared. Most importantly since ∑μ is independent of the excitation wavelength, we conclude that the quantum yield, φ, of mobile charge carriers is equal on excitation at 500 and 830 nm, in agreement with the work of Lee et al.38 This result clearly suggests that the excess energy involved in excitation of the polymer (500 nm) does not enhance the yield of charge carriers as compared to excitation of the CT band. Previous work has shown that at room temperature the mobility of the electrons exceeds that of the holes by approximately a factor of 3.26,49 Therefore, at room temperature the photoconductance traces are dominated by the contribution of the electrons.26 However, at 88 K the electrons on PCBM are virtually immobile. To investigate the effect of the temperature on the charge carrier formation mechanism, photoconductance traces were recorded at 88 K (see inset Figure 4). The decay of the traces becomes substantially slower in comparison with the traces recorded at room temperature. However, the traces obtained for excitation at 500 and 830 nm

Figure 5. Intensity normalized photoconductance action spectra for blends of P3HT with mono- and bisPCBM. The inset shows the optical attenuation spectra (FA) for P3HT:PCBM blends in the NIR. Figure 4. φ∑μ values versus reciprocal temperature for blends of P3HT with PCBM and bisPCBM at 500 and 830 nm excitation. The straight lines are results from fits of the thermal activation energy, Ea, using the Arrhenius behavior φ∑μ ∼ exp(−Ea/kBT). For both blends, two distinct Ea values were determined: 75 meV above 220 K and 10 meV below 120 K at both the excitation wavelengths. These Ea values are attributed to thermally activated charge transport of electrons in fullerene domains at T > 220 K52 and to holes in P3HT at T ≤ 100 K.26,49 Inset: normalized photoconductance traces for P3HT:PCBM blends at 500 and 830 nm at a temperature of 88 K.

intensity normalized maximum photoconductance as a function of wavelength for blends of P3HT with PCBM and bisPCBM. The photoconductance action spectrum is reminiscent to an EQE spectrum of a device recorded near open-circuit conditions. The onset of mobile carrier formation for P3HT:PCBM at ∼1000 nm (1.2 eV) is in agreement with EQE measurements in previous reports.38,53 The strong increase in the signal below 800 nm is due to optical absorption 9217

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than P3HT:PCBM excited at 830 nm. This is in agreement with an increase in the triplet yield on using higher adducts of fullerene with polyfluorene polymers.60 In addition, Liedtke et al.61 reported the generation of triplet excitons for P3HT:endohedral fullerene blends as compared to P3HT:PCBM, where the CT state in the former blend is above the triplet level of P3HT. As discussed above for both blends studied in this work similar φ∑μ values are observed for excitation in the visible. At 500 nm excitation two routes for charge carrier formation are possible: 1vis ⇒ 2 ⇒ 4 or 1vis ⇒ 2 ⇒ 3 ⇒ 5 (see Figure 1). As the latter route would result in a reduced yield in P3HT:bisPCBM blends due to the fact that recombination to the triplet can occur, we conclude that for this blend charge carrier formation occurs primarily via the hot CT state (1vis ⇒ 2 ⇒ 4). For the P3HT:PCBM we cannot deduce if charge carrier formation occurs via the hot CT state, since the energy level of the CT state is located below the P3HT triplet level. Hence, recombination from the relaxed CT state to the triplet state is energetically unfavorable. From this study it is inferred that for bulk heterojunction solar cells the driving force for exciton discociation can be small as long as the triplet level is located energetically higher than the CT state.

of the PCBM. The inset of Figure 5 shows the optical attenuation spectrum (eq 2) of the P3HT:PCBM blend. Notably, the optical onset coincides with the onset in charge carrier formation, which again suggests a value of Δ close to zero. Recently, Davis et al.44 studied the interfacial energy level alignment of P3HT:PCBM blends using photoelectron spectroscopy. For the energy difference IP(P3HT) − EA(PCBM) a value of 1.2 eV was found, which interestingly matches closely with the onset found in this work, if Δ is close to zero. From the results discussed above it is inferred that in P3HT:PCBM blends on exciton dissociation at the interface prompt delocalization of the oppositely charged carrier occurs. This explains why the formation of mobile charge carriers is independent of temperature and of wavelength. In the next part results for P3HT:bisPCBM blends are presented and discussed. On using bisPCBM as the electron acceptor, the onset of the photoconductance is blue-shifted by ∼100 nm as compared to P3HT:PCBM (see Figure 5). This shift correlates to the smaller electron affinity of bisPCBM as compared with PCBM.54 Concomitantly, the driving force for exciton dissociation for P3HT:bisPCBM is approximately reduced by 0.2 eV. Figure 3B depicts the φ∑μ values on excitation at 500 and 830 nm of this blend. Comparable φ∑μ values for P3HT:PCBM and P3HT:bisPCBM are observed on excitation at 500 nm at low intensities (compare φ∑μ values between Figures 3A and 3B). This is surprising since the DC electron mobility reported for bisPCBM is an order of magnitude smaller than for PCBM.54,55 To understand the similarity between the φ∑μ values additional XRD and luminescence measurements were carried out. For both blend layers identical XRD patterns were recorded, indicating that the lamellar structure and domain size of the P3HT is not changed on replacing PCBM with bisPCBM (see Supporting Information, Figure S1). Moreover, comparable photoluminescence quenching (data not shown) is observed in P3HT:PCBM and P3HT:bisPCBM blends in agreement with recent reports.56 These two observations indicate a similar nanomorphology in both blends and hence in comparable values of φ. This is in line with the similar charge carrier yields reported for P3HT:PCBM and P3HT:bisPCBM blends as found with transient absorption spectroscopy.57 As φ is not affected by replacing PCBM with bisPCBM and similar φ∑μ values are observed for both blends on excitation at 500 nm (Figures 3A and 3B), it is evident that the local electron mobility in bisPCBM is close to that of PCBM. In order to elucidate whether this assumption is reasonable, the effect of temperature on the photoconductance was investigated. Similar activation energies (see Figure 4) were found from the Arrhenius plots of both blends, indicating similar temperature activated electron mobilities for both fullerene derivatives. Comparable short circuit currents for corresponding photovoltaic devices are consistent with this finding.45,57−59 In contrast to the P3HT:PCBM blend, for P3HT:bisPCBM the φ∑μ values on excitation at 830 nm are a factor of 3 lower as compared to excitation at 500 nm (Figure 3B). Since ∑μ does not depend on excitation wavelength, the observed change in φ∑μ values can only be attributed to a reduction in φ. As depicted in Figure 1, the energy levels of the CT states in P3HT:bisPCBM are close to the triplet level of the P3HT.14 If the initially formed singlet CT state undergoes a spin flip, it can recombine to the triplet level of P3HT. Because of this competitive pathway, excitation of P3HT:bisPCBM at 830 nm results in a lower quantum yield of charges for P3HT:bisPCBM

4. CONCLUSIONS In this work we have investigated which mechanistic model is most appropriate to describe the charge carrier photogeneration in blends of P3HT with PCBM or bisPCBM using the time-resolved microwave conductivity technique (TRMC). We found that for P3HT:PCBM blends the quantum yields for mobile charge generation on excitation in the visible and NIR are very similar. In addition, it is found that the yield is independent of temperature ranging between 88 and 300 K for both wavelengths. This result shows that excess energy does not affect the yield of charges even at 88 K for P3HT:PCBM. These observations are explained by substantial reduction in the electrostatic binding energy between opposite charges, resulting in a Δ value close to zero, i.e., smaller than thermal energy at 88 K. This is attributed to extensive charge delocalization of a single or of both charges, which increases the mean distance between the electron and hole at the interface between the P3HT and PCBM. For charge delocalization to occur it is expected that at least one of the constituents of the blend should contain crystalline domains. Despite a reduction in the driving force for exciton dissociation for P3HT:bisPCBM, a similar quantum yield of charge carriers is found as for P3HT:PCBM on excitation in the visible. However on NIR excitation of P3HT:bisPCBM, the yield is decreased by a factor of 3 as compared to excitation in the visible. As the energy levels of the CT states in P3HT:bisPCBM are close to the triplet level of the P3HT, a CT state can recombine to the triplet level of P3HT after a spin flip. Because of this competitive pathway, NIR excitation of P3HT:bisPCBM results in a lower yield than excitation of P3HT:PCBM. However, as the charge carrier generation yield for P3HT:PCBM and for P3HT:bisPCBM are identical on excitation in the visible, we conclude that for the latter blend charge carrier formation occurs primarily via the hot CT state. For photovoltaic applications it is important to realize that photoexcitation to the CT state does not significantly contribute to the optical absorption of the blend. The TRMC results imply that in case the driving force for exciton dissociation in a particular polymer/fullerene couple is small, 9218

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the triplet level of the polymer should be located above the energy level of the CT state. However, for a polymer/fullerene couple with a large driving force this seems less important.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns of P3HT:PCBM and P3HT:bisPCBM blends. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work of DHKM forms the research program of the Dutch Polymer Institute (DPI, project #681). We acknowledge Ben Norder from the department of Chemical Engineering, Delft University of Technology, for assistance with XRD measurements.

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