Tregnago, G; (2015) Photophysics and applications of organic semiconductors. Doctoral thesis , UCL (University College London).

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Abstract

The work presented in this thesis is motivated by the great commercial impact of organic semiconductors especially in optoelectronics. In particular, we focus our attention on some of the current challenges in organic light-emitting diodes from the point of view of the photophysical properties of materials (i.e. via steady-state and time-resolved photoluminescence characterization) and the device physics. In view of the interest in near-infrared emission, we propose two ways to obtain emission at long wavelength. Firstly, we report a new molecular design of one of the currently best performing polymer for near-infrared light-emitting diodes. We investigate the substitution of sulphur with selenium and find that it is more effective both in terms of photo- and electro-luminescence efficiency than by exploiting higher sulphur-chromophore loadings, while achieving a more important red-shift. Secondly, we explore the tuning of the energy gap through a careful choice of the relative positions of the frontier levels of two organic semiconductors. Following this strategy, we obtain a nearly pure near-infrared electroluminescence with essentially no emission from the single components at any operational voltage. We believe that results obtained are a valuable feedback as they suggest materials design criteria. Given to the potential of phosphorescent materials for obtaining high efficiency OLEDs, we also consider the investigation of a novel emitter based on a wide-gap host co-polymerized with a low-gap phosphorescent emitter for efficient energy transfer. Finally, we report a detailed investigation of the photoluminescence emission at low temperature of the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), widely employed as an electron acceptor in organic solar cells. Owing to the availability of solvent-free single-crystals, whose growth has been recently reported by our group, we are able to investigate PCBM optical properties without solvent dependence. Our attempt is to provide significant information on the ordering and relative importance of the relevant excited states in PCBM.

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