Warren Duffy

Charge carrier formation in polythiophene/fullerene blend films studied by transient absorption spectroscopy.

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]C61 (PCBM). Three polymers are observed to give amorphous films, attributed to a nonplanar geometry of their backbone while 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 (approximately 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, power-law decay phase assigned to bimolecular recombination of these polarons, was observed to vary by over 2 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 an excellent correlation between the free energy difference for charge separation (deltaG(CS)rel) and yield of the long-lived charge generation, with efficient charge generation requiring a much larger deltaG(CS)rel than that required to achieve efficient PL quenching. We suggest that this observation is consistent with a model where the excess thermal energy of the initially formed polaron pairs is necessary to overcome their Coulombic 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 deltaG(CS)rel (or LUMO level offset) is required to achieve efficient charge dissociation.

Analysis of charge photogeneration as a key determinant of photocurrent density in polymer: fullerene solar cells.

Signifi cant progress has been made in relating the voltage output of organic solar cells to materials’ properties, specifi cally to the energy difference between the donor ionisation potential and acceptor electron affi nity. [ 1–3 ] However, progress in predicting device photocurrent densities on the basis of materials or fi lm properties has proved much more problematic. Signifi cant attention has focused upon enhancing light-harvesting effi ciency by reducing the optical bandgap of the photoactive layer, as discussed in recent reviews. [ 4–6 ] Most models of device effi ciency have typically assumed a unity yield for exciton dissociation into separated charges, requiring only that the donor/acceptor LUMO level offset is greater than 0.3 eV (corresponding to the assumed exciton binding energy). In practice, these models have proved rather poor in predicting the photocurrent densities of real devices, even after processing optimization. [ 7 ] Whilst some materials (e.g. P3HT:PCBM) have indeed achieved photocurrent densities consistent with near unity internal quantum effi ciencies for photocurrent generation, most new materials (with some notable exceptions) evaluated for their performance in organic photovoltaic devices have yielded much lower photocurrent densities, and consequently poor device performance. [ 6 , 7 ] In this paper, we consider the extent to which such variations in photocurrent density can be largely understood in terms of the effi ciency of charge photogeneration. The key processes involved in charge photogeneration in organic bulk heterojunciton solar cells are illustrated in Figure 1 . By ‘charge photogeneration’ we refer to the overall process by which photon absorption leads to the generation of dissociated

Radical ion pair mediated triplet formation in polymer–fullerene blend films

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.

Banana‐shaped dopants for flexoelectric nematic mixtures

The synthesis is reported of new meta‐substituted quaterphenyls and several related dimers as dopants for nematic mixtures with flexoelectric properties. The number and nature of the terminal substituents is varied from apolar with a small dipole moment to polar with a strong dipole moment. The number of methylene units in the flexible aliphatic spacer between the polyfluorinated aromatic groups of the dimers is also varied. Dimers capable of banana‐shaped conformers induce an increase in the flexoelectric coefficients of mixtures containing them and reduce the clearing point to a small extent.

TOF mobility measurements in pristine films of P3HT: control of hole injection and influence of film thickness

Time-of-flight (TOF) photocurrent measurements have been used to study charge transport in films of regioregular poly(3-hexylthiophene) (P3HT). Devices in which the P3HT film had been deposited directly onto an indium tin oxide (ITO) electrode produced high dark currents as a result of hole injection into P3HT from ITO. Photocurrent transients in such devices were disperse. It was found however, that these dark currents could be significantly reduced by inserting a dense TiO2 layer between the ITO and the polymer film. The resulting devices gave non-dispersive transients with hole and electron mobilities in the range of 1 - 2 10-4 cm2 V-1 s-1 at room temperature. The mobility values were observed to be almost independent of film thickness over the range of 350 nm to 4.3 μm. Temperature dependence studies showed a weak dependence on temperature with a low energetic disorder parameter according to analysis using the Gaussian Disorder Model (GDM) of 71 meV.

Rod-shaped dopants for flexoelectric nematic mixtures

We report the synthesis and liquid crystalline behaviour of two series of para-substituted terphenyls as dopants with a rigid rod-like shape, rather than a wedge-, pear- or banana-shape, for guest–host nematic mixtures with flexoelectric properties. One series of liquid crystalline dopants is of low-to-strongly negative dielectric anisotropy and the other is of low-to-strongly positive dielectric anisotropy. The usefulness of apolar and polar rod-like dopants as components of flexoelectric nematic mixtures of positive dielectric anisotropy for use in LCDs is investigated in general and the dependence of the flexoelectric properties of the doped nematic mixtures on the polarity of the dopants is studied in particular. The correlation between the concentration of the dopant and the magnitude of the flexoelastic ratio of several guest–host nematic mixtures is investigated.

The influence of molecular weight on the microstructure and thin film transistor characteristics of pBTTT polymers.

A common strategy to improve the electrical performance of organic field effect transistors is to optimize the charge carrier mobility of the semiconducting thin film. Polymer semiconductor transport properties have shown a dependence on the chain length, due principally to the strong influence of molecular weight on the thin film microstructure. In this work, we report on a study of the influence of increasing molecular weight of poly(2,5-bis(3-docecylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTT-C12) on the polymer bulk thermal properties, thin film microstructure and the electrical performance of thin film field effect transistor devices. Clear differences can be observed within a number average molecular weight range of 8,000 - 18,000 Dalton. A Liquid crystalline phase was only observed at the highest molecular weight, different thin film morphology was observed within the molecular weight range, and the field effect mobility was shown to increase with increasing molecular weight.