New insights into organic light emitting devices: photophysics, recombination, and out coupling

Abstract

Display technology is expected to have a market industry value greater than 70 billion US dollars by 2026. Although organic light emitting devices have found their way into the display industry, there is still a need for higher device efficiencies, lower cost materials, and easier, scalable production methods. This drives the need to gain a deeper understanding of organic semiconductors, which can pave the way to these goals. As such, there is undeniable demand for new classes of semiconductor materials, ideally with a high photoluminescence quantum yield, balanced charge transport, and high light out-coupling through dipole engineering.This thesis describes a body of work which specifically addresses the task of independent control over luminescent, transport and processing properties of organic semiconducting materials with dendritic structures. A family of Ir(III) complexes is introduced and studied including dendrimers; poly(dendrimers); and co-polymers. In the first instance, photo-physical and electrical properties of materials are described, before building on this knowledge to develop efficient organic light emitting diodes. The dipole orientation was furthermore studied in these materials as an intrinsic property of with a view to achieving higher out coupling. Finally, moving away from a material-centric approaches and dendritic design, the charge transport and emissive properties of organic semiconductors were simultaneously studied in a heterostructure light-emitting field effect transistor by cryogenic techniques.The main findings from this research were as follows: I) the photophysical properties improved by increasing the number of dendron branches in the dendritic structure. This delivers extra insulating space between the chromophore cores which leads to less concentration quenching; II) combining dendrimers with a polymer backbone was beneficial not only toward improving the film quality but also providing heteroleptic structures which are more likely to contain horizontally oriented emissive dipoles; III) the results of temperature-dependent measurements demonstrated that, as the device was cooled down, the intrinsic hole mobility followed an Arrhenius response with the overall EQE increases.