J. Phys. Chem. C 2009, 113, 14533–14539
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The Effect of Ionization Potential and Film Morphology on Exciplex Formation and Charge Generation in Blends of Polyfluorene Polymers and Silole Derivatives Clare Dyer-Smith,†,‡ Jessica J. Benson-Smith,§ Donal D. C. Bradley,† Hideyuki Murata,| William J. Mitchell,⊥ Sean E. Shaheen,# Saif A. Haque,‡ and Jenny Nelson*,† Department of Physics, Imperial College London, London SW7 2BW, U.K., Department of Chemistry, Imperial College London, London SW7 2AZ, U.K., Plextronics Inc., 2180 William Pitt Way, Pittsburgh, PennsylVania 15238, Department of Physical Materials Science - Composite Materials, School of Materials Science, Japan AdVanced Institute for Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292 Japan, Department of Physics and Astronomy, UniVersity of DenVer, DenVer, Colorado 80208, and Merck Chemicals Limited, Southampton, Hampshire, U.K. SO16 7QD ReceiVed: March 5, 2009; ReVised Manuscript ReceiVed: May 7, 2009
Long-lived emission from an exciplex state is observed in a series of blend films of polyfluorene-triarylamine copolymers with silole derivatives. The energy of the exciplex emission is found to correlate with the energy offset between the frontier orbitals of donor and acceptor, whereas the yield of the exciplex is more strongly influenced by blend film morphology. Charge separation occurs in these films, and the external quantum efficiency of photovoltaic devices is found to depend upon both the energetics and the morphology of the blend film. Introduction Organic solar cells are a promising alternative to silicon-based technologies due to the potential for low-cost solution processing and large area deposition methods. Power conversion efficiencies approaching 5% have been achieved with polymer-fullerene blend solar cells utilizing the bulk heterojunction concept whereby an electron donor and acceptor are spin-cast from a single solvent to form a blend with a high interfacial surface area. The most widely studied polymer-small molecule bulk heterojunction cells utilize poly-3-hexylthiophene (P3HT) as the light absorbing, electron donating component and [6,6]-phenylC61-butyric acid methyl ester (PCBM) as the electron acceptor.1 The efficiency of organic solar cells can be reduced by, among other things, the efficiency of charge separation in the device, and so it is important to understand this process. Charge separation within organic photovoltaic devices is based upon the photoinduced electron transfer from a donor to a neighboring acceptor molecule. Early studies showed that the process can occur on a time scale of less than 60 ps and that the charge-separated state is metastable.2 Brabec et al.3 showed that the electron transfer occurred on the order of 50 fs and concluded that there was no ground-state interaction in a blend of MDMO-PPV and PCBM, indicating that resonant interaction between LUMO states in the fullerene and the polymer π* orbitals was involved in the transfer (rather than a direct chargetransfer absorption). Halls et al.4 showed that photoinduced electron transfer between two polymers is also possible. In many early studies, the photoinduced electron transfer was viewed as a one-step process from a molecule in its singlet excited state to a neutral molecule in which no intermediate state is involved. * Corresponding author. E-mail:
[email protected]. † Department of Physics, Imperial College London. ‡ Department of Chemistry, Imperial College London. § Plextronics Inc. | Japan Advanced Institute for Science and Technology. ⊥ University of Denver. # Merck Chemicals Limited.
Recently, a number of studies have demonstrated the formation of charge-transfer excited states (often termed exciplexes) in polymer/fullerene and polymer/polymer blend films, which may suggest an intermediate step in charge separation.5-15 However, there remains a lack of consensus over the role of exciplexes or charge-transfer excited states in charge separation. Morteani et al. have presented several spectroscopic studies, characterizing the excited states formed between a polyfluorene-triarylamine polymer as donor and F8BT as electron acceptor, and observe a red-shifted long-lived emission that they assign to an exciplexsa complex formed between the excited state of the donor or acceptor and the ground state of the other component.5-8 More recently, Huang et al. have studied these systems using DFT and find that the degree of charge separation in the excited state for parallel oligomers of donor and acceptor depends upon the relative displacement along the polymer backbone of the BT unit in the acceptor and the triarylamine segment in the donor.10 They find that the relative probability of forming the exciplex or the charge-separated state (“polaron pair”) also varies depending on the energy separation of the donor HOMO and acceptor LUMO and the energies of these states relative to the energy of the polymer exciton. A detailed study on a polyfluorene/PCBM blend has recently been reported by Veldman et al.,11 and they also observe spectroscopic features that they attribute to “charge-transfer (CT) states”. In this work, measurements under large applied bias show quenching of the emission from CT excited states by an electric field. The electric field did not quench the singlet excited state, suggesting that charge separation in devices based on this system proceeds via the CT state and not by direct dissociation of the exciton. In addition, time-dependent measurements showed that the quenching of CT luminescence under an applied bias originates from a decrease in the decay time rather than in the initial yield of CT states (similar to the effect observed by Offermans et al. in their study on PPV derivatives15). This provides support for a charge-separation mechanism based upon the dissociation of charge-transfer states. The authors find that
10.1021/jp9020307 CCC: $40.75 2009 American Chemical Society Published on Web 07/15/2009
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J. Phys. Chem. C, Vol. 113, No. 32, 2009
both CT states and free charges are stabilized by PCBM, possibly due to its higher dielectric constant. Interestingly, morphology is found to have an influence on charge generation, with the process being more efficient in a moderately phaseseparated blend; this is attributed to the presence of high mobility and high dielectric constant PCBM clusters that allow charges to move away from the interface more readily.11 Yin et al., in a study on PPV derivatives, suggest that the exciplex is a loss pathway for geminate charge pairs13 because annealing the devices led to an increase in efficiency and a decrease in the exciplex emission. However, Chasteen et al. observed that exciplex emission is more intense in the most efficient devices14 and suggest that the exciplex, therefore, promotes charge separation, either by direct dissociation of the exciplex into charges or by offering a source for regenerating the MEH-PPV exciton (essentially prolonging the lifetime of this state). In a recent study by Benson-Smith et al.,16 emission from an exciplex in a blend of TFMO and a silole-based electron acceptor was found to depend on morphology. A kinetic model based upon exciton trapping at interfaces and back transfer from exciplex to exciton was proposed, with the amount of back transfer being found to increase with increasing phase separation. In this work, we report spectroscopic and device studies of a series of polyfluorene-triarylamine copolymers with two silole acceptors: 2,5-bis(2,2-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PyPySPyPy, referred to herein as “silole”) and a fluorinated version, 2,5-bis(5,5-fluoro-2,2-bipyridin-6-yl)1,1-dimethyl-3,4-diphenylsilacyclopentadiene (“F-silole”), in which exciplex emission is observed. PyPySPyPy has previously been shown to have electron mobilities greater than that of Alq3 (around 2 × 10-4 cm2/(V · s))17 and so is of interest for organic device applications, particularly LEDs.18 The chemical structure, ionization potential, and electron affinity of the materials studied here are given in Table 1. Experimental Methods Blend films (average thickness ) 80 nm for TFMO-based films, 145 nm for other arylamines) of the polymers poly(9,9dioctylfluorene-co-N-(4-butylphenyl) diphenylamine) (TFB, Sumitomo Chemical), poly(9,9-dioctylfluorene-co-N-(4-methoxyphenyl) diphenylamine (TFMO, Sumitomo Chemical), and poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,Nphenyl-1,4-phenylenediamine) (PFB, Dow Chemical) together with either 2,5-bis-(2,2-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (“silole”) or 2,5-bis-(5,5-fluoro-2,2-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (“Fsilole”) were spin-coated from 20 mg/mL chlorobenzene solution (Analar, >99.5%) at a rate of 1200-1500 rpm for 25 s (50 µL droplet volume) onto Spectrosil B glass substrates (Kaypul Optics Ltd.) cleaned by sonication in acetone (Chromanorm, >99.8%) and propan-2-ol (HiPerSolv, >99.8%). For device measurements, films of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) were spin-coated onto ITO substrates. The active layer was deposited under the same spin-coating conditions as those of the films on glass, and 100 nm thick Al electrodes were evaporated on top under a vacuum of 6 × 10-6 bar. Measurements of pristine silole were made on blend films of silole and polystyrene to prevent crystallization in the film and to reproduce more accurately the morphology of the silole when blended with the polyfluorene polymers. Polystyrene was used because it is transparent at the wavelengths used to excite the samples, ensuring that there is no interference with excitation and emission.19 Absorption spectra were measured using a Jasco V560 UV-vis spectrophotometer (long wavelength measurements on
Dyer-Smith et al. solutions used a Shimadzu UV-1601 spectrophotometer or a PerkinElmer Lambda 950 spectrophotometer for wavelengths beyond 1100 nm). Steady-state emission spectra were recorded using a Fluoromax-3 spectrofluorimeter, exciting at the wavelength of maximum absorption (379, 388, 376, 393, and 384 nm for silole, F-silole, TFB, TFMO, and PFB, respectively). Time-resolved photoluminescence was measured by timeresolved single-photon counting using a diode laser (λ ) 404 nm, full width at half-maximum ∼230 ps), IBH photon detection module, and IBH Datastation hub (all Jobin-Yvon). Decay kinetics were measured at two wavelengths for each samplesthe wavelength of the pristine polymer exciton (not shown) and the maximum wavelength for exciplex emission. Lifetime analysis was performed using IBH data analysis software. Absorption and PL spectra were recorded in air (there was negligible change in spectral shape when the samples were held in a N2-filled cuvette). Ionization potentials (IPs) were determined by cyclic voltammetry (CV), and electron affinities (EA) were obtained by CV in the case of PFB and by subtracting the optical gap of the material from the IP in the case of silole, F-silole, TFMO, and TFB with no allowance for exciton binding energy.20,21 The optical gap is obtained by measuring the onset wavelength of absorption in the UV-visible absorption spectrum (taken from the crossing point of the absorption and PL spectra). Transient absorption spectroscopy was performed under nitrogen in a sealed quartz cuvette. A 100 W tungsten lamp was used as the continuous probe source (detection performed using Si and InGaAs detectors, in the 500-1000 and 1000-1700 nm wavelength ranges, respectively, coupled to a Costronics amplifier with high- and low-pass filters), and a 400 nm dye laser pumped by a GL-3300 N2 laser (Photon Technology International) was operated with a pulse width of
