Photophysics of TADF Emitters and their application in OLEDs

This thesis presents the photophysical characterization and device performance of a series of small molecules and copolymers showing thermally activated delayed fluorescence (TADF). This triplet harvesting mechanism allows triplet states to contribute to the light generation process, thus increasing the efficiency of organic light emitting diodes (OLEDs). TADF based OLEDs with internal quantum efficiency (IQE) close to 100% have already been demonstrated. However, many aspects of the mechanism still remain unclear and need to be tackled in order to design novel and more efficient TADF molecules, with emission in different regions of the spectrum. In order to maximize the TADF mechanism in different organic systems, the interplay between the charge transfer state and local triplet excited states are studied in detail and it was found that the mixing of CT and local triplet states is essential to achieve efficient reverse intersystem crossing (RISC). The consequence of introducing bulky side groups on the D unit was also studied and the luminescence from these molecules varies from efficient TADF to strong phosphorescence at room temperature. Remarkably, in clear contrast with the donor substituted molecules, their acceptor substituted analogues are strong TADF emitters. Furthermore, the fine-tuning of TADF efficiency in copolymers was explored by using spacer groups in a range of polymeric structures, overcoming the theoretical IQE limit for pure fluorescent compounds in solution-processed OLEDs. Moreover, the contribution of TADF emission in Cu-complexes showing aggregated induced emission was studied to probe the effects of vibrations on the luminescence quenching in these complexes.