Lateral and Vertical Phase Separation Control of Thin-film Structures for Photovoltaics
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Lateral and Vertical Phase Separation Control of Thin-film Structures for Photovoltaics A. C. Arias, J. D. MacKenzie, N. Corcoran and R. H. Friend Cavendish Laboratory, Madingley Road, Cambridge, CB3 0HE, UK ABSTRACT
Investigations on microscopic and photovoltaic properties of polyfluorene blends are presented here. The length scale of lateral phase separation is manipulated by control of solvent evaporation conditions. Photoluminescence efficiency measurements show that charge transfer is more effective in blends phase separated on the nanometer scale. Vertically segregated structures are obtained by a combination of solution viscosity and spin coating conditions. The external quantum efficiency of photovoltaic devices fabricated with vertically segregated blend is found to be 4 times higher than that of devices made with laterally segregated blends. INTRODUCTION
The first problem that has to be considered with the use of organic molecular semiconductors is that the excited states produced by photon absorption are usually excitons that have relatively high binding energies and do not dissociate to give electrons and holes [1-3]. It is known that exciton dissociation is efficient at interfaces formed between materials with different electron affinities and ionization potentials, where the electron is accepted by the material with larger electron affinity and the hole by the material with lower ionization potential [4]. Although the absorption coefficient for organic semiconductors is very high (> 105 cm-1), the absorption depth, which is wavelength dependent, is usually greater than the diffusion range of the excitons (10 nm) created by the absorption process. Thus, only a fraction of the excitons generated in the film are able to find the interface at which dissociation can occur [1]. A strategy to improve performance is to use high purity, highly crystalline molecular semiconductors, which can show greater diffusion ranges for excitons [5]. A different approach is to arrange a structure in which there is a very large surface area, so that all absorbing regions lie close to an interface at which ionization can occur. This approach was demonstrated by O’Regan and Grätzel [6], who used a sintered electrode of TiO2 onto which they surface-adsorbed a layer of an organic dye (ruthenium bipyridinium complex). This principle has been applied to the polymers by Halls [7] et al and Yu and Heeger [8]. Similar structures using derivatised fullerene as the electron-accepting component have also been reported [9, 10]. It is known that the low entropy of mixing prevents homogeneous mixing of polymers on a molecular level, and they tend to phase separate into different domains [11-15]. If the diffusivity of the polymers is high enough and if given enough time, the phase separation can go through several stages of coarsening [12, 14]. In equilibrium, the lowest energy state is generally attained when the two components separate to form two bulk domains. In general, the faster the solvent evaporates, the less time the polymer has
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