Prof. Carlos Silva

MoSE 3201A
Thursday, November 12, 2015 -
4:00pm to 5:00pm

Are excitation spectral lineshapes in semicrystalline polymeric semiconductors dominated by two-dimensional excitonic coupling or chromophore energetic disorder?

The optical properties of semicrystalline polymeric semiconductors are governed fundamentally by the interplay of electronic interactions occurring within a given polymer chain and those occurring between chains that constitute crystalline motifs. The competition between through-bond (intrachain) and through-space (interchain) electronic coupling determines two-dimensional spatial coherence of excitons. The balance of intra- and interchain excitonic coupling depends very sensitively on solid-state microstructure of the polymer film (e.g. polycrystalline, semicrystalline with amorphous domains, etc.). Regioregular poly(3-hexylthiophene) (P3HT) has emerged as a model material to study the dependence of excitonic properties on microstructure because its photoluminescence (PL) spectral lineshape reveals intricate information on the magnitude of excitonic coupling, the extent of energetic disorder, and on the extent to which the disordered energy landscape is correlated. Single-chain spectroscopic measurements at low temperature, however, have suggested that spectral lineshapes in the solid state may not be due to photophysical aggregate effects, but rather a manifestation of a large inhomogeneous distribution of emitter energies that is determined by chain conformation. In this presentation, I discuss implementation of coherent two-dimensional PL excitation spectroscopy in order to address this issue. This is an ultrafast probe belonging to the family of 2D optical spectroscopies that allows measurement of correlations between quantum transitions induced by the electromagnetic field in the excitation spectrum, detected via the time- and spectrally-integrated PL intensity. Using this technique, we identify cross peaks between 0–0 and 0–1 excitation peaks, and we measure their time evolution, which we interpret within the context of a hybrid HJ aggregate model. By measurement of the homogeneous linewidth in diverse polymer microstructures, we address the nature of optical transitions within such hybrid aggregate model. These dynamics depend strongly on sample processing, and we discuss the relationship between microstructure, steady-state absorption and PL spectral lineshape, and 2D coherent PL excitation spectral lineshape.

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