J. Phys. Chem. C 2008, 112, 15973–15979
15973
Relationship between Film Morphology, Optical, and Conductive Properties of Poly(thienothiophene): [6,6]-Phenyl C-61-Butyric Acid Methyl Ester Bulk Heterojunctions Wojciech J. Grzegorczyk,† Tom J. Savenije,*,† Martin Heeney,‡ Steve Tierney,| Iain McCulloch,⊥ Svetlana van Bavel,§ and Laurens D. A. Siebbeles† Optoelectronic Materials Section, Faculty of Applied Sciences, DelftChemTech, Delft UniVersity of Technology, 2628 BL Delft, The Netherlands, Department of Materials, Queen Mary UniVersity of London, London, E1 4NS, United Kingdom, Laboratory of Materials and Interface Chemistry (SMG), EindhoVen UniVersity of Technology, Helix STO 2.41, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, Merck Chemicals Ltd., UniVersity Parkway, Southampton Science Park, Southampton, Hampshire SO16 7QD, United Kingdom, Department of Chemistry, Imperial College London, London, SW7 2AZ, United Kingdom ReceiVed: May 20, 2008; ReVised Manuscript ReceiVed: July 18, 2008
The morphology and the optoelectronic properties of films of poly(dialkylthieno[3,2-b]thiophene-cobithiophene) (pDA2T) and blends of this polymer with [6, 6]-phenyl C-61-butyric acid methyl ester (PCBM) is investigated. Upon spin-coating of the blend a thin film is formed containing both PCBM and the polymer in nanocrystalline form. On annealing, phase separation occurs, leading to formation of the PCBM-rich domains embedded in a polymer-rich matrix. The electrodeless time-resolved microwave conductivity technique is used to study the photogeneration of charge carriers and their decay over time. The photoconductance increases dramatically on adding PCBM to the polymer. Annealing of the blend reduces the photogeneration yield of charge carriers, due to the smaller interfacial area between pDA2T and PCBM. The phase separation of the polymer and PCBM after annealing retards recombination of charge carriers, which is beneficial for charge collection in a solar cell. The magnitude of the photoconductance of the pDA2T:PCBM blend is comparable to that for a P3HT:PCBM blend. The above findings, together with the smaller energy loss involved in electron transfer from pDA2T to PCBM, as compared to blends of P3HT and PCBM, make pDA2T a promising material for photovoltaic applications. Introduction Conjugated polymers are promising materials for application as light-absorbing and charge-transporting components in lowcost photovoltaic devices.1 Advantages of conjugated polymers as compared to inorganic semiconductors include cheap and easy production and processing and the high optical absorption. Photoexcitation of a conjugated polymer leads predominantly to formation of bound electron-hole pairs (excitons) and to some extent charge carriers.2-6 To generate free charge carriers the polymer is brought into contact with an electron accepting material. Photogenerated excitons may diffuse through the polymer to reach an interface with the electron acceptor, where excitons can dissociate into free charges.1,5 Recently, it has been suggested that electrons initially generated within the polymer diffuse to the electron acceptor, which leads to suppression of recombination with holes.7 The exciton diffusion length in a conjugated polymer is typically less than 10-20 nm.8-11 Hence, excitons that are created farther away from the interface between the electron donating polymer and the electron acceptor do not contribute to charge generation. Since the light penetration depth in conjugated polymers largely exceeds the exciton diffusion length, solar cells consisting of a flat bilayer configuration of a * To whom correspondence should
[email protected]. † Delft University of Technology. ‡ Queen Mary University of London. | Merck Chemicals Ltd. ⊥ Imperial College London. § Eindhoven University of Technology.
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conjugated polymer and an electron acceptor exhibit low power conversion efficiency. This issue can be overcome by blending the electron donor and acceptor to form a so-called bulk heterojunction (BHJ).1 In such a nanoscale interpenetrating network of donor and acceptor components, the average distance between the site of light absorption and the exciton dissociation interface is smaller than the exciton diffusion length. In this way most excitons will encounter a donor-acceptor interface. For charge carrier collection at the electrodes, percolation pathways for both hole and electron transport must exist in the bulk heterojunction. This can be realized by proper tuning of the molecular structure of the materials and the conditions used for spin-coating and processing. A variety of solar cells with different combinations of conjugated polymers and electron acceptors has been studied, with the highest power conversion efficiencies near 5% for BHJs of regioregular poly(3-hexylhiophene) (P3HT) and [6,6]-phenyl C-61-butyric acid methyl ester (PCBM).1,12-18 It has been argued that an efficiency close to 10% can be achieved by optimization of material properties and cell architecture.1,19-21 Reducing the polymer band gap enhances absorption of solar light and can theoretically lead to a device efficiency exceeding 6%.20,21 Even if one assumes that the polymer band gap is constant, the efficiency can still be further improved by a reduction of the energy loss involved in electron transfer from the polymer to PCBM. For P3HT the electron transfer step occurs at the expense of about 1.1 eV. By reduction to 0.5 eV, which is expected to be sufficient for efficient charge transfer, theoretically enhances the cell efficiency to 8%. Lowering the LUMO while maintaining a constant bandgap leads to an increase in
10.1021/jp8044548 CCC: $40.75 2008 American Chemical Society Published on Web 09/19/2008
15974 J. Phys. Chem. C, Vol. 112, No. 41, 2008 the ionization potential of the polymer, which leads to enhancement of the open circuit voltage.21,22 Finally, realization of balanced electron and hole transport by enhancement of the hole mobility in the polymer improves the charge carrier collection efficiency and may lead to cells with an efficiency approaching 11%.20 To improve the hole mobility in polymer materials, research on self-organizing, liquid-crystalline conjugated (poly)thiophenes has been intensive.23-26 Recently, several groups have synthesized novel polythienothiophenes, designed to self-assemble into ordered domains on crystallization from the liquid-crystalline phase.27-34 These polymers contain the conjugated comonomer thieno[3,2-b]thiophene, which increases torsional rigidity of the polymer backbone. This supports the formation of highly ordered crystalline domains, which leads to an enhanced mobility of the charge carriers.28,30,34,35 In addition, the ionization potential of these polymers is approximately 0.3 eV higher than that of P3HT. These two aspects make bulk heterojunctions of these polymers with PCBM of interest for solar cells with improved charge transport and increased open circuit voltage. Very recently short circuit currents have been reported on nonoptimized solar cells based on pDA2T and PCBM.34 The present paper aims to provide insight into the relationship between the morphological, optical, and conductive properties of a thin pure layer of poly(2,5-bis(2-thienyl)-3,6-dihexadecylthieno[3,2-b]thiophene) (pDA2T) and of a blend layer consisting of this polymer and PCBM (pDA2T:PCBM) in a 1:1 weight ratio. The chemical structure of the polymer is presented in Figure 3. The preparation of similar polymers but with differing side chain lengths has been previously reported.31,36,37 Interestingly there appears to be a significant effect of polymer molecular weight on the thermal behavior of the polymer. Thus the polymer with pentadecyl sidechains and a Mn of 12400 gmol-1 is reported to exhibit semicrystalline behavior with a melting temperature of 421 K,31 whereas the higher molecular weight polymer investigated in this study (Mn 36000 gmol-1) exhibits thermotropic liquid crystalline behavior with a phase transition at 321 K and a melt at 533 K upon heating.38 The differences are probably related to the different aspect ratios for the different molecular weights, although the differing methods of preparation may also play a role. The layer morphology was characterized using transmission electron microscopy (TEM), electron diffraction (ED), and X-ray diffraction (XRD). The photoconductive properties of these layers were measured using the time-resolved microwave conductivity (TRMC) technique. The morphology and conductive properties of pDA2T:PCBM blends were compared with those of P3HT: PCBM blends. It is inferred that the new polythienothiophene forms highly ordered domains within the pDA2T:PCBM blend, which is favorable for charge transport. The photoconductivity of the pDA2T:PCBM blend is higher than that of a P3HT:PCBM blend, despite the fact that the driving force for electron transfer in the pDA2T:PCBM blend is 0.3 eV less than in a P3HT: PCBM blend. These findings make this material attractive for application in solar cells with improved charge transport and higher open circuit voltage. Experimental Section pDA2T was synthesized by the Stille polymerization of 5,5′bis(trimethylstannyl)-2,2′-bithiophene with 2,5-dibromo-3,6dihexadecylthieno[3,2-b]thiophene under microwave heating as previously reported.36,39 The polymer has a number average molecular weight (Mn) of 36300 gmol-1 with a polydispersity
Grzegorczyk et al. of 2.4. The molecular weight was determined by gel permeation chromatography (GPC) in chlorobenzene at 333 K against polystyrene standards. Electronic grade regioregular P3HT (Rieke) was used as received. Solutions of pDA2T, pDA2T: PCBM, P3HT, and P3HT:PCBM containing 8 mg/mL polymer were prepared by dissolving the compounds in a 1:1 weight ratio in 1,2-dichlorobenzene. The solutions were prepared by overnight stirring and subsequent centrifuging at 4000 RPM for 5 min to remove undissolved residues. Afterward, the films were spin-coated in a glovebox filled with nitrogen with an oxygen concentration
