Exciton, excimer, exciplex : study of triplet state harvesting in organic molecules for organic light-emitting diode applications

Triplet state harvesting is an important issue in the area of organic electronics, including organic light emitting diode (OLED) technology that has already entered the global market. In the aim to achieve efficient light-emitting diodes the photophysical properties of OLED emitters need to be understood in great detail. This work is devoted to triplet state harvesting in OLEDs. In this work a set of triplet-harvesting systems comprising exciton, excimer or exciplex emitters are characterized and used to fabricate prototype devices. The first system is based on metal-free emitters, using acridone or phenothiazine, which show thermally activated delayed fluorescence (TADF) or room-temperature phosphorescence (RTP). The competition between the rate of deactivation pathways affecting the triplet state and the reverse intersystem crossing (RISC) rate determine whether these molecules emit through TADF or RTP. The second system explores the effects of different substitution patterns on the properties of excitonic tetradentate ONNO Pt(II) complexes, and their performance in OLEDs, revealing a complicated host-to-guest energy transfer mechanism in doped films. The third work in this thesis explores the properties of newly synthesized Pt(II) metal complexes that have been found to efficiently form photoluminescent excimers and have strong potential to be used in solution-processed OLED devices. The photophysical characterisation of these complexes doped in film has revealed co-existence of excimer and aggregate emissions. Finally, the last two works in this thesis are focused on small molecule and polymer-based exciplex blends that exhibit efficient TADF emissions and can be used to fabricate solution-processed or vacuum-deposited OLEDs. The photophysics of these exciplex systems is characterised in-depth and undoubtedly demonstrates that local triplet states are not involved in the RISC process in this blends, which is in clear contrast with most exciton small molecule TADF systems. Furthermore, in this work a clear rationale for the observation of emission decaying in power law regimes is obtained for the first time.