Conjugated polymer blends: linking film morphology to performance of light emitting diodes and photodiodes

Blending is a technique known in polymer technology that takes advantage of the processibility of polymers to produce new solid materials or composites with specific structural and physical properties, distinct from the ones of their components. In thin films of polymer blends interesting morphologies are formed because of phase separation. For conjugated polymers, i.e. solution-processible semiconductors, blending also opens a way to optimize the performance of opto-electronic devices, bringing about technological benefits. It is therefore crucial to achieve understanding of the effect film morphology has on the device performance, and, ultimately, to achieve control over the phase separation in a blend, so that structures can be designed that yield the desired device performance. Light-emitting diodes (LEDs) made of polymer blends have shown strongly enhanced electroluminescence (EL) efficiencies, as compared to pure homopolymers. Colour conversion, white light emission, polarized light emission, emission line narrowing, and voltage-tunable colours are other effects that have been observed in blends containing light-emitting polymers. Although the enhanced EL efficiency is attributed to Forster-type energy transfer in numerous reports, the exciton dynamics behind this effect is not well understood. Here we review the formation and morphology of thin films of conjugated polymer blends, as well as modern microscopic and spectroscopic techniques to study them. Furthermore, we attempt to link the film morphology to the electronic performance of electroluminescent and photovoltaic devices and discuss energy and charge transfer phenomena at the interfaces. We also report some new results, specifically for polyfluorene blends in LEDs.