Hal, P.A. van

Hummelen, J.C.

Janssen, R.A.J.

Knol, J.

Langeveld-Voss, B.M.W.

Peeters, E.

English

Covalent oligomer-fullerene donor-acceptor structures can serve as important model systems for plastic PV cells, based on interpenetrating networks of conjugated polymers and fullerene derivatives. Several series of [60]fullerene-oligomer dyads and triads were prepared. Photoinduced electron transfer phenomena were studied using photoinduced absorption spectroscopy. Measurements in solution and in the solid state reveal that the environment plays a crucial role in the formation and lifetime of the photoinduced charged states in these molecules

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   University of GroningenOligomer-fullerene dyads and triads as model compounds for bulk-heterojunction PV cellsJanssen, R.A.J.; Peeters, E.; Langeveld-Voss, B.M.W.; Hal, P.A. van; Knol, J.; Hummelen,J.C.Published in:EPRINTS-BOOK-TITLEIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1999Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Janssen, R. A. J., Peeters, E., Langeveld-Voss, B. M. W., Hal, P. A. V., Knol, J., & Hummelen, J. C. (1999).Oligomer-fullerene dyads and triads as model compounds for bulk-heterojunction PV cells. In EPRINTS-BOOK-TITLE University of Groningen, Stratingh Institute for Chemistry.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 21-01-2023 Oligomer-fullerene dyads and triads as model compounds for bulk-heterojunction PV cells R.A.J. Janssena, E. Peetersa, B.M.W. Langeveld-Vossa, P.A. van Hala, J. Knolb, and J.C. Hummelenb a. Laboratory for Macromolecular and Organic Chemistry, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands and b. Stratingh Institute and MSC, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands; e-mail: j.c.hummelen@chem.rug.nl Abstract. Covalent oligomer-fullerene donor-acceptor structures can serve as important model systems for plastic PV cells, based on interpenetrating networks of conjugated polymers and fullerene derivatives. Several series of [60]fullerene-oligomer dyads and triads were prepared. Photoinduced electron transfer phenomena were studied using photoinduced absorption spectroscopy. Measurements in solution and in the solid state reveal that the environment plays a crucial role in the formation and lifetime of the photoinduced charged states in these molecules. INTRODUCTION Photovoltaic cells, based on composites of conjugated polymers and fullerene derivatives (1,2), are considered as promising devices for solar energy production. At present, the efficiency of such ‘plastic’ PV cells is limited because the devices only produce current when a relatively thin active layer is used. As a consequence, only a small fraction of the incoming light is absorbed (3). The ruling paradigm is that the plastic PV devices suffer from low charge carrier mobility because of the random nature of the interpenetrating networks, formed spontaneously upon spin coating of a co-solution of the two constituents of the active layer. It is expected that structural ordering of the constituents will lead to a substantial increase in charge carrier mobility: the concept of a ‘molecular electronic highway’. At present, little or nothing is known about the relation between the architecture of molecular (multi-)donor-(multi-) acceptor structures on one side and their material properties such as the efficiency of formation, the mobility, and lifetime of the photo-generated charge carriers on the other. A remarkable feature of conjugated polymer/fullerene blends is the fact that an ultra-fast forward electron transfer reaction occurs (< 1 ps), whereas the recombination of photogenerated charges is slow (> ms). The large difference in electron transfer rates can, in principle, be either an intrinsic property of the structures involved or a material property. In order to gain some insight in these matters, we are investigating model systems for such architectures. We use  conjugated  oligomers  as CP486, Electronic Properties of Novel Materials- Science and Technology of Molecular Nanostructures, edited by H. Kuzmany, J. Fink, M. Mehring, and S. Roth © 1999 American Institute of Physics 1-56396-900-9/99/$15.00 155  model compounds for the polymer. These oligomers have been synthesized, functionalized, and coupled to C60 in such a way that well-defined donor-acceptor dyad or triad molecules are obtained that can be studied both as individual molecules in solution and as (ordered) assemblies in the solid state. Plastic PV cells are usually made using either polythiophenes or poly[p-phenylenevinylene]s. Therefore, model systems were constructed that contain oligomer parts of these two types of polymers. RESULTS AND DISCUSSION Two types of oligothiophene-fullerene compounds were synthesized using the well-established Prato reaction (4). First, a simple dyad containing a terthiophene donor moiety and one fullerene acceptor moiety was prepared. Terthiophene monoaldehyde (1b), N-methyl glycine, and C60 were heated in toluene at reflux temperature during 16 hrs to yield a mixture of C60, the desired mono-fulleropyrrolidine 2 and some higher adducts (Figure 1, left). The monoadduct was isolated in 47% yield using standard column chromatography. Dyad 2 was soluble enough in the standard organic solvents to allow for full characterization. FIGURE 1.  Synthesis of oligothiophene-fullerene dyads. Step i: DMF, POCl3; step ii: N-methylglycine, C60,  toluene, ∆. Similarly, triad 4 was prepared by refluxing a chlorobenzene solution of the highly soluble sexithiophene α,ω-bisaldehyde 3b, N-methyl glycine, and C60 during 16 hrs (Figure 1, right). Since the starting oligomer material consisted of a mixture of three isomers and because the Prato reaction yields chiral products in the case of aldehyde starting material, triad 4 is a mixture of (maximally) 10 regio- and stereoisomers. Nevertheless, molecular models indicate that in all of these isomers the donor-acceptor  configuration  is  very  similar.  1H-NMR  spectroscopy  indicates  the 156  presence of (at least) 2 isomers. In HPLC analysis, 3 peaks (~ 1:2:1) were observed. These peaks most likely represent the three groups of isomers with different types of arrangements of the solubilizing chains on the sexithiophene unit. Triad 4, was further characterized by MALDI-TOF MS and UV-vis spectroscopy. Second, a series of oligo[p-phenylenevinylene] containing donor-acceptor dyads was prepared. The homologous series of compounds 6-9 was prepared in 44, 47, 48 and 40 % yield, respectively, from the corresponding oligomer ω-aldehydes, again using the above mentioned Prato method (Figure 2). The monoadducts 6-9 (each as a mixture of 2 diastereoisomers) are all highly soluble compounds that were fully characterized by MALDI-TOF MS, 1H- and 13C-NMR, UV-vis and IR spectroscopy. FIGURE 2. Homologous series oligo[p-phenylenevinylene]-fullerene dyads We now turn to the photoinduced absorption measurements. Bis(dodecyl)-sexithiophene 3a (6T) and N-methylfulleropyrrolidine 5 (4) were used as reference compounds. Photoinduced absorption (PIA) spectra were recorded between 0.25 and 3.5 eV by exciting with a mechanically modulated Ar-ion laser (458 nm, 50 mW, 275 Hz) pump beam. The resulting change in transmission of the probe light through the sample (∆T) was monitored with a phase sensitive lock-in amplifier after dispersion by a triple grating monochromator and detection, using Si, InGaAs, and cooled InSb detectors. Oxygen-free solutions were studied at room temperature. Thin films were held at 80 K using an Oxford Optistat continuous flow cryostat. When benzonitrile solutions of 3a or 5 are studied with PIA spectroscopy, long-lived photo-excitations are formed in both cases (Figure 3a and 3b). For 3a, the triplet state exhibits a narrow triplet-triplet absorption band at 1.78 eV with a vibrational shoulder at higher energy (5). The lifetime of the 6T triplet state is 155 µs. In addition, 3 shows photoluminescence (PL) at 2.40 eV with a shoulder at 2.25 eV. For 5, the PIA spectrum also shows the formation of a triplet state with characteristic bands at 1.78 eV and 1.52 eV. The lifetime of this triplet state is 220 µs. At 1.26 eV a small band of the radical anion of 5  can  be  seen.  Its  formation  is  attributed  to  the 157  presence of a small impurity of isocyanobenzene (PhNC) in benzonitrile (PhCN) which acts as a donor to the  triplet  state  of  5.  This  is  a  bimolecular  reaction  and FIGURE 3. PIA and PL spectra of (a) 3a in benzonitrile; (b) 5 in benzonitrile; (c) 3a and 5 (1:1 molar ratio) in benzonitrile; (d) 4 in benzonitrile; (e) 4 in toluene; (f) a thin film of 4 on quartz. Spectra a-e were recorded at 295 K, spectrum f was recorded at 80 K. The PL curves shown in spectra band d-f are mainly due to scattering of the excitation laser beam the lifetime of the anions formed is long because charge recombination is hampered by follow-up reactions of the isocyano-benzene cation radical, acting as an sacrificial electron donor. When 3a and 5 are dissolved in benzonitrile in a 1:1 ratio, we find the clear characteristics of the 6T radical cation with bands at 0.87 and 1.60 eV in solution and a band of the radical anion of 5 at 1.24 eV (Figure 3c) (6). The fact that under these conditions the fluorescence of 3a is not quenched demonstrates that electron transfer does not occur from the S1-state of 3a which is formed preferentially at first. Rather electron transfer occurs from the T1 triplet state of 3a which is formed via intersystem crossing from S1. The lifetime of the 6T radical cation is on the order of 200-400 µs. The determination of the lifetime of the radical anion of 5 is hampered by 158  the isocyanobenzene impurity which remains to act as electron donor to the triplet state of 5. When 3a and 5 are photoexcited in an apolar solvent such as toluene, instead of the highly polar benzonitrile, only triplet states are formed. Photoexcitation of the 1:1 mixture in toluene results in triplet energy transfer from 6T(T1) to 5 rather than electron transfer, similar to previous observations for mixtures of 6T and C60 (6). The reason for a preference for energy transfer in toluene is that the charge-separated state is less stabilized in apolar solvents. Photoexcitation of a benzonitrile solution of triad 4 results neither in a detectable PIA signal nor in a detectable PL signal (only scattering of pump laser) (Figure 3d). Apparently, the S1-state of the 6T moiety is completely quenched. This behavior can be rationalized by assuming that in benzonitrile a fast forward electron transfer reaction occurs resulting in an intramolecularly charge-separated state. The fact that this charge-separated state is not detected with the PIA technique is explained by assuming that the lifetime is less than about 10 µs, below the detection limit of our near-steady state PIA spectrometer. When the solvent is changed to toluene (which is less polar) the PL is not restored but a clear PIA signal similar to the triplet state of 4 is observed with a lifetime of about 250 µs (Figure 3e). This indicates that in toluene an intramolecular energy transfer occurs in which the singlet excitation energy is transferred to the C60fragment. This singlet C60 photoexcitation subsequently intersystem crosses to a C60triplet state. When triad 4 is cast from solution on quartz glass a solid film is obtained. PIA spectroscopy of this thin film at 80 K shows that a long-lived charge-separated state is formed (Figure 3f). Characteristic absorption bands of the radical cation of 6T at 0.72 and 1.49 eV and of the radical anion of C60 at 1.24 eV appear in the spectrum. The PIA bands increase with the square root of the excitation intensity, indicative of bimolecular decay. The frequency dependence follows a power law: ∆T ∝ ω-p, with p = 0.12-0.17. Such a power-law behavior is often observed for a distribution of lifetimes. We propose that such a long-lived charge-separated state can only exist when the hole and electron are localized on different molecules. The experiments of 4in benzonitrile show that intramolecular charge recombination occurs with a time constant that must be less than 10 µs. In solution, a fast intramolecular photoinduced electron transfer reaction may occur in 4, depending on the Coulombic stabilization of the charge-separated state by the medium. The charge transfer in solution is governed by the Weller equation (7). Intramolecular charge recombination in the photoexcited arrays occurs on short time scales (probably  << 10 µs). This demonstrates that the long lifetime of the charge-separated states in solid blends of 6T and fullerene derivatives (8) or solid films of the C60-6T-C60 array is a material rather than a molecular property. In solvents where electron transfer does not occur, we invariably observe the spectral characteristics of the triplet-state of the fullerene moiety, even when the oligothiophenes are preferentially excited (6). In combination with the absence of oligothiophene fluorescence it can be concluded that in these cases an efficient energy transfer occurs from the excited oligothiophene to the fullerene. Solutions of dyad 2 in toluene and in benzonitrile show similar photoinduced absorption spectra. No signatures of charge separated states  were  found.  Instead,  the 159  typical absorption spectrum, corresponding to triplet C60, is observed. This indicates that in both solvents either highly efficient intramolecular energy transfer occurs from the terthienyl moiety to the fullerene part of the molecule or direct excitation of the fullerene moiety takes place (at 458 nm, 5 has a higher absorption coefficient than terthienyl). The oligo[p-phenylenevinylene]-fullerene dyads 6-9 are currently being investigated using photoinduced absorption spectroscopy in solutions and as solid films. Preliminary results show that the behavior of 8 and 9 parallels that of triad 4. For 8 and 9 the photoluminescence is strongly quenched under all conditions. Photoexcitation of 8 and 9 in toluene gives a C60 triplet state, while in benzonitrile a fast forward and fast backward electron transfer reaction occurs. In thin films of 8 and 9 a long-lived charge separated state is observed with the hole localized on the oligo[p-phenylenevinylene] fragment and the electron on the fullerene moiety. In conclusion, it has been demonstrated that compounds 4, 8, and 9 behave photophysically in such a way that they can serve as model compounds for conjugated polymer / fullerene blends. The formation and the lifetimes of photoinduced charge separated states in oligomer-fullerene compounds 4, 8, and 9 depend fully on the chemical environment of the molecules. Long lifetimes of charge separated states are only observed in the solid state. Hence, stabilization of the charge-separated state is a material property of these compounds. Based on our experiments, we propose that a rapid diffusion mechanism for a substantial fraction of the photo-generated charges to the corresponding units of neighboring molecules is operating in the solid state. ACKNOWLEDGMENTS We thank the Netherlands Organization for Energy and the Environment (Novem) for financial support (Novem 146.120-008.1). EP was supported by the Netherlands Foundation for Chemical Research (NCO) with financial aid from the Netherlands Technology Foundation (STW, 349-3562). PvH was supported by the Ministries of Economic affairs, of the Environment, and of Education (E.E.T. K97115) REFERENCES 1. Sariciftci, N. S., Smilowitz, L., Heeger, A.J., Wudl F., Science 258, 1474-1476 (1992). 2. Yu, G., Gao, Y., Hummelen, J.C., Wudl, F., Heeger, A.J., Science 270, 1789-1791 (1995) 3. Brabec, C.J., Padinger, F., Dyakonov, V., Hummelen, J.C., Janssen, R.A.J., Sariciftci, N.S., Electronic   Properties of  Novel Materials, World Scientific Publishing, 1998, pp 519-522 4. Maggini, M., Scorrano, G., Prato, M., J.Am. Chem. Soc. 115, 9798-9799 (1993) 5. Janssen, R.A.J., Smilowitz, L., Sariciftci, N. S., Moses, D., J. Chem. Phys. 101, 1787-1798 (1994). 6. Janssen, R.A.J., Moses, D., Sariciftci, N. S., J. Chem. Phys. 101, 9519-9527 (1994). 7. Weller, A., Z. Phys. Chem. Neue Folge 133, 93-98 (1982) 8.  Janssen, R.A.J., Christiaans, M.P.T., Pakbaz, K., Moses, D., Hummelen, J.C., Sariciftci, N.S., J. Chem. Phys. 102, 2628-2635 (1995). 160 

Oligomer-fullerene dyads and triads as model compounds for bulk-heterojunction PV cells

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Oligomer-fullerene dyads and triads as model compounds for bulk-heterojunction PV cells

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