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J. Phys. Chem. C 2010, 114, 15116–15120
Photoinduced Charge Carrier Generation in Blends of Poly(Thienothiophene) Derivatives and [6,6]-Phenyl-C61-butyric Acid Methyl Ester: Phase Segregation versus Intercalation Tom J. Savenije,*,† Wojciech J. Grzegorczyk,† Martin Heeney,‡ Steve Tierney,§ Iain McCulloch,‡ and Laurens D. A. Siebbeles† Optoelectronic Materials Section, Department of Chemical Engineering, Delft UniVersity of Technology, 2628 BL Delft, The Netherlands, Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom, and Merck Chemicals Ltd., UniVersity Parkway, Southampton Science Park, Southampton, Hampshire SO16 7QD, United Kingdom ReceiVed: April 13, 2010; ReVised Manuscript ReceiVed: July 7, 2010
The morphology, optical properties, and photoconductance of blends of the poly(thienothiophene) derivatives poly(3,6-dialkylthieno[3,2-b]thiophene-co-bithiophene) (pATBT), poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT), and poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[2,3-b]thiophene) (pBTCT) with [6,6]phenyl-C61-butyric acid methyl ester (PCBM) were studied. After thermal annealing, the pATBT:PCBM blend exhibits formation of phase-segregated polymer and PCBM domains. Annealing of pBTTT:PCBM and pBTCT:PCBM yields a layered structure with PCBM molecules intercalated between layers of π-stacked polymers. In the intercalated systems the photoluminescence is almost completely quenched, in contrast to the phase-segregated pATBT:PCBM blend. The higher degree of exciton quenching in the intercalated systems likely results in a higher initial yield of charges. However, on longer time scales (>10 ns), the microwave photoconductance for the layered systems is lower than for pATBT:PCBM blend systems. This is likely due to restricted motion of charges in intercalated systems, which reduces the yield of free charge carriers or enhances the charge carrier recombination. Introduction Soluble conjugated polymers are intensively investigated for application in molecular optoelectronics as they can be processed by low-cost solution-based techniques such as inkjet printing or spin coating. The possibility to realize efficient light-induced generation of free charges in conjugated polymer:buckminsterfullerene composites is of great promise for use of these materials in so-called bulk heterojunction (BHJ) solar cells.1-5 BHJ solar cells based on poly(3-hexylthiophene) (P3HT) and fullerene derivatives reach photovoltaic power conversion efficiencies close to 5%.6 The high efficiency is in part due to the high charge carrier mobilities of both compounds. Derivatives of polythiophene tend to form crystalline domains consisting of π-stacked conjugated polymer backbones.7,8 Due to the low dielectric constant of these polymers, absorption of a photon leads predominantly to formation of a strongly bound holeelectron pair denoted as exciton and to a lesser extent to free charges.9 In order to convert the exciton efficiently into free charge carriers, the conjugated polymer is blended with an electron acceptor, usually [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). After interfacial electron transfer from the polymer to PCBM, the electron and hole must escape from recombination so that collection by the electrodes can occur. The efficiencies of charge generation and transport are to a large extent determined by the nanomorphology of the blend, as has been shown by many investigations.10-12 In this study we investigate three thiophene copolymers incorporating fused thienothiophene units, in which we sys* Corresponding author: e-mail
[email protected]; fax 0031 15 2787421. † Delft University of Technology. ‡ Imperial College London. § Merck Chemicals Ltd.
Figure 1. X-ray diffraction patterns for films of pure pATBT, pBTTT, and pBTCT and blends with PCBM annealed at 120 °C. For pBTTT-PCBM, the result for annealing at 165 °C is also presented. Molecular structures of the polymers are given on the right.
tematically vary the ionization potential of the polymer. Increasing the ionization potential of the polymeric donor is one strategy to increase the open circuit voltage of the bulk heterojunction cell, since Voc is related to the energy offset between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor, in this case PCBM.13,14 The polymers of interest are shown in Figure 1. In poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT), an unsubstituted thieno[3,2-b]thiophene has been incorporated into the polymer backbone.
10.1021/jp1033068 2010 American Chemical Society Published on Web 08/18/2010
Photoinduced Charge Carrier Generation We have previously demonstrated that this results in an increase in ionization potential of approximately 0.25 eV in comparison to P3HT.15-18 In addition, the reduction in side-chain density caused by incorporation of thieno[3,2-b]thiophene allows adjacent polymer chains to self-organize by interdigitation of their side chains, resulting in highly ordered thin films.16-19 Poly(3,6dialkylthieno[3,2-b]thiophene-co-bithiophene) (pATBT) is a regioisomer of pBTTT in which the solubilizing side chains have been moved from the bithiophene monomer to the thieno[3,2-b]thiophene unit.20 This results in an additional increase in the measured ionization potential of 0.05 eV over pBTTT, most likely as a result of increased torsional freedom in the unalkylated bithiophene unit, which reduces π-π overlap. Poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[2,3-b]thiophene) (pBTCT) is a copolymer with thieno[2,3-b]thiophene. This monomer has a central cross-conjugated double bond, which means that full delocalization along the polymer backbone is not possible, and the polymer has a relatively high ionization potential of 5.3 eV and exhibits air-stable field-effect transistor (FET) performance.21,22 All polymers exhibit good FET charge carrier mobilites, with typical mobilities up to 0.6 cm2/(V s) for pBTTT, 0.2 cm2/(V s) for pATBT, and 0.03 cm2/(V s) for pBTCT in bottom-gate, bottom-contact 10 µm channel devices.20 Both pBTTT and pATBT display thermotropic liquid crystalline behavior, with the first thermal transition ascribed to sidechain melting, affording a smecticlike phase.23 The temperature of this melt upon heating for the hexadecyl side-chain derivatives used here is approximately 140 °C for pBTTT and 50 °C for pATBT.20 The lower temperature for the pATBT may be due to the cooperative nature of side-chain movement when they are attached to the rigid thieno[3,2-b]thiophene, as opposed to the bithiophene in which they can move independently. We note that lower molecular weight samples of pATBT than those investigated here do not demonstrate liquid crystallinity,24 most likely as a result of their reduced aspect ratio. pBTCT does not exhibit liquid crystalline behavior, possibly due to the less linear polymer backbone and more irregular side-chain spacing caused. As has been reported very recently, pBTTT forms bimolecular crystals if blended with PC[71]BM in 1:1 weight ratio.25,26 X-ray diffraction (XRD) was used to demonstrate that fullerene molecules are intercalated between layers of π-stacked polymer backbones. It has been proposed that whether these highly ordered structures are formed depends on the free volume between the side chains of the conjugated polymer.25 This class of polymer has also been studied in combination with PCBM as the photoactive layer in solar cells.27-29 This work aims at relating the morphology, optical properties, and time-dependent photoconductance of blends of pATBT, pBTTT, or pBTCT as electron donor with PCBM as electron acceptor. The present study focuses only on blend samples with a 1:1 weight ratio to avoid the occurrence of both phase segregation and intercalation in one sample, as has been shown to occur in the case of a blend of a pBTTT analogue and PC[71]BM.25 The morphology of thin films of the pure polymer and blends with PCBM was investigated with XRD. The effect of the blend morphology on photogeneration of charge carriers and their decay was studied by the time-resolved microwave conductance technique (TRMC). This technique allows determination of the photogeneration efficiency and the mobility of charge carriers without the necessity to apply electrodes. Experimental Section Polymers were synthesized as previously reported.15,20 Molecular weight determinations were carried out in chlorobenzene
J. Phys. Chem. C, Vol. 114, No. 35, 2010 15117 at 60 °C on an Agilent 1100 series HPLC using two Polymer Laboratories mixed B columns in series and were were calibrated against narrow weight PL polystyrene calibration standards. The molecular weights were as follows: pBTTT-C16 had Mn 24 800 g/mol, Mw 42 600 g/mol; pATBT-C16 had Mn 36 300 g/mol, Mw 86 600 g/mol; and pCTBT-C12 had Mn 22 600 g/mol, Mw 39 300 g/mol. Solutions of the pure polymers and of mixtures of polymer-PCBM containing 8 mg/mL polymer were prepared by dissolving the compounds in 1:1 weight ratio in 1,2-dichlorobenzene (ODCB) and stirring overnight at a temperature of 50 °C. The films were spin-coated in a glovebox filled with nitrogen with an oxygen concentration
