Abstract:We report herein a comparison of the photophysics of a series of polythiophenes with ionization potentials ranging from 4.8 to 5.6 eV as pristine films and when blended with 5 wt% 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]C 61 (PCBM). Three polymers are observed to give amorphous films, attributed to a non-planar geometry of their backbone whilst the other five polymers, including poly(3-hexylthiophene), give more crystalline films. Optical excitation of the pristine films of the amorphous polymers is observed by transient absorption spectroscopy to give rise to polymer triplet formation. For the more crystalline pristine polymers, no triplet formation is observed, but rather a short-lived (~ 100 ns), broad photoinduced absorption feature assigned to polymer polarons. For all polymers, the addition of 5 wt% PCBM resulted in 70 -90% quenching of polymer photoluminescence (PL), indicative of efficient quenching of polythiophene excitons. Remarkably, despite this efficient exciton quenching, the yield of dissociated polymer + and PCBM − polarons, assayed by the appearance of a long-lived, powerlaw decay phase assigned to bimolecular recombination of these polarons, was observed to vary by over two orders of magnitude depending upon the polymer employed. In addition to this power-law decay phase, the blend films exhibited short-lived decays assigned, for the amorphous polymers, to neutral triplet states generated by geminate recombination of bound radical pairs and, for the more crystalline polymers, to the direct observation of the geminate recombination of these bound radical pairs to ground. These observations are discussed in terms of a two-step kinetic model for charge generation in polythiophene/PCBM blend films analogous to that reported to explain the observation of exciplex-like emission in poly(p-phenylenevinylene)-based blend films. Remarkably, we find a excellent correlation between the free energy difference for charge separation (ΔG CS rel ) and yield of the long-lived charge generation yield, with efficient charge generation requiring a much larger ΔG CS rel than that required to achieve efficient PL 3 quenching. We suggest this observation is consistent with a model where the excess thermal energy of the initially formed polarons pairs is necessary to overcome their coulomb binding energy. This observation has important implications for synthetic strategies to optimize organic solar cell performance, as it implies that, at least devices based on polythiophene/PCBM blend films, a large ΔG CS rel (or LUMO level offset) is required to achieve efficient charge dissociation.4

Nanocrystalline, nanoporous metal oxide films are a novel substrate for protein immobilization. Such electrodes allow both electrochemical and spectroelectrochemical studies of the protein redox function and, moreover, provide an attractive approach for the development of both optical and electrochemical biosensors. Previous studies have largely focused on the use of nanocrystalline TiO2 electrodes. In this paper, we extend these studies to nanocrystalline SnO2 electrodes. Immobilization of two proteins, cytochrome c (Cyt-c) and hemoglobin (Hb) is observed to proceed with similar protein loadings for both metal oxides. However, the SnO2 electrode is demonstrated to be preferable to the TiO2 electrode for electrochemical studies of these proteins, as the potential window over which the SnO2 film is conducting is shifted to more positive potentials by 500 mV. Cyclic voltammetry and spectroelectrochemistry are employed to demonstrate that this shift allows the observation of reversible oxidation and reduction of both heme proteins on the SnO 2 electrodes without the use of any electron-transfer promoters or mediators. Electron-transfer rate constants of 1 ( 0.03 and 0.53 ( 0.03 s -1 are determined for the Cyt-c/SnO2 and Hb/SnO2 electrodes, respectively. Finally, we demonstrate that the high protein loading and electrical conductivity of Hb/SnO2 films allow the electrochemical sensing of nitric oxide. Electrochemical sensing is demonstrated, with a limit of detection of 1 µM.

Solar cells based on conjugated polymers are attracting increasing interest due to their potential to enable a renewable energy technology based on simple and low cost manufacture.[1] Devices made from blends of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) have been widely studied and are consistently reported to produce efficiencies over 3%. [2,3] Regioregular P3HT is known to be a good hole transporting polymer with relatively long wavelength absorption, producing films with a highly crystalline morphology and good charge mobility (10 -4 cm 2 V -1 s -1 ). [4,5] According to a model presented by Scharber et al., [6] polymer:PCBM devices could be improved to produce energy conversion efficiencies of up to 11% by replacing P3HT with an electron donating polymer that has better optimised electronic energy levels. In particular, by decreasing the LUMO level of the donor while keeping the HOMO at the same level as P3HT, the reduced band gap would allow a greater light harvesting ability without compromising the open circuit voltage (V oc ).In this paper, we report on a study of a selenium analogue of P3HT, regioregular poly(3-hexylselenophene) (P3HS). The synthesis of this material has recently been reported by Heeney et al. [7] The polymer was shown to have a smaller optical gap (1.6 eV) than regioregular P3HT (1.9 eV) and because both polymers were shown to have the same HOMO level (4.8 eV), this decrease in band gap was assigned to a drop in the P3HS LUMO level. Taking into account the extended absorption range (up to 760 nm) and the similar field-effect transistor (FET) charge mobility to regioregular P3HT, [7] it was anticipated that P3HS might be a good candidate for polymer solar cells. The propensity for P3HT films to form crystalline structures is important for generating the interpenetrating nanomorphology that facilitates charge separation and charge transport in P3HT:PCBM blend devices.[4] X-ray diffraction (XRD) measurements were made of pristine P3HS films, before and after thermal annealing at 150°C, to investigate whether P3HS shares this crystalline nature (Fig. 1a). Since P3HS solutions have a tendency to gel at low temperatures, all P3HS and P3HS blend films reported in this paper were deposited by spin-coating from a hot (80°C) solution on to a substrate of the same temperature. This heating of the substrate and solution allowed for the preparation of much smoother films than if prepared at room temperature. Whilst as spun films showed no XRD peaks, a sharp diffraction peak (diffraction angle 2h = 3.

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In this paper, we report a spectroelectrochemical investigation of proton-coupled electron transfer in flavodoxin D. vulgaris Hildenborough (Fld). Poly-L-lysine is used to promote the binding of Fld to the nanocrystalline, mesoporous SnO(2) electrodes. Two reversible redox couples of the immobilized Fld are observed electrochemically and are assigned by spectroelectrochemistry to the quinone/semiquinone and semiquinone/hydroquinone couples of the protein's flavin mononucleotide (FMN) redox cofactor. Comparison with control data for free FMN indicates no contamination of the Fld data by dissociated FMN. The quinone/semiquinone and semiquinone/hydroquinone midpoint potentials (E(q/sq) and E(sq/hq)) at pH 7 were determined to be -340 and -585 mV vs Ag/AgCl, in good agreement with the literature. E(q/sq) exhibited a pH dependence of 51 mV/pH. The kinetics of these redox couples were studied using cyclic voltammetry, cyclic voltabsorptometry, and chronoabsorptometry. The semiquinone/quinone reoxidation is found to exhibit slow, potential-independent but pH-sensitive kinetics with a reoxidation rate constant varying from 1.56 s(-)(1) at pH 10 to 0.0074 s(-)(1) at pH 5. The slow kinetics are discussed in terms of a simple kinetics model and are assigned to the reoxidation process being rate limited by semiquinone deprotonation. It is proposed that this slow deprotonation step has the physiological benefit of preventing the undesirable loss of reducing equivalents which results from semiquinone oxidation to quinone.

Efficient triplet formation is observed for films of high ionisation potential polythiophenes blended with a fullerene derivative, and assigned to formation via geminate charge recombination of bound radical ion pair states.

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