Resumen de Conjugated polymer blends for optical gain applications
Conjugated polymers are organic semiconductors which can be processed into thin films following room temperature solution-based methods. Among the wide conjugated polymer family, poly(3-hexylthiophene) (P3HT), has been widely regarded as an archetypical semiconducting electron-donor in bulk heterojunction solar cells, mainly due to its notable exciton and charge transport properties. Conversely, its applications in photonic/lighting devices received little interest owing to its low emission quantum yield in the solid state, related to its large tendency to self-organize into aggregates with highly non-emissive character even upon blending P3HT in a large variety of matrices. Intriguingly, the use of poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), as host matrix for P3HT leads to blend films with efficient red photoluminescence properties. In this thesis, we shed light into the structure-property relationships of these highly promising conjugated polymer blends. We present experimental and theoretical evidences pointing to the formation of F8BT:P3HT complexes involving strong ground state electronic interactions delocalized across adjacent P3HT and F8BT chromophores, accompanied by a large miscibility in blend. Emission in the solid state is entirely ascribed to F8BT:P3HT complexes due to the assistance of a highly efficient Förster resonance energy transfer (FRET) process from F8BT chains to F8BT:P3HT complexes. Concomitantly, blends experience a significant improvement in photoluminescence quantum efficiency (PLQE) with maxima values approaching 25% (almost a seven-fold efficiency enhancement with respect to neat/aggregated P3HT) and exhibit high optical gain properties in the red part of the spectrum. Accordingly, we investigate the application of F8BT:P3HT blends as emissive media in both polymer light-emitting diodes (PLEDs) and optically pumped lasers with distributed feedback (DFB) structures. Our results open up new prospects for improved photonic properties through appropriate control of inter-chain interactions.
