Solution-processed optoelectronic devices, including organic light-emitting diodes (OLEDs) and organic photodetectors (OPDs), have the promising potential to produce low-cost, flexible, and large-area functional devices via printing technologies. To achieve efficient and stable devices with low driving voltages in OLEDs, hole/electron injection and transport, as well as balanced hole/electron carriers, are important parameters. This thesis presents several efficient approaches to enhance charge injection and balance by either doping or inserting interlayers and explores the origin of charge injection mechanisms.
Firstly, hole injection improvement by electrochemical doping process is discussed. An electrochemical doping process (p-type doping) is proved to strongly reduce hole injection barrier, enhance hole injection and mobility, thereby lowering OLED turn-on voltages with reduced efficiency roll-off.
Secondly, controlling dipole strength and energetics of conjugated polyelectrolytes as electron injection layers in OLEDs has achieved more balanced charge carrier density and hole blocking property with improved device efficiency, which gives a new synthesis route for developing highly efficient CPE injection materials.
Thirdly, controlling dark current and response time of OPD via non-fullerene acceptor (NFAs) is discussed. The nano-scale aggregated morphology of trap-free NFA blends can not only maintain charge generation, but also improve charge mobility to reduce the response time of OPDs.
Finally, dynamic molecular conformation-controlled energy transfer from locally excited states to either charge transfer state (CT) or β phase in light-emitting polymers is discussed. The formation of the β-phase was a dynamic and reversible process. Spontaneous and reversible CT on-off emission with temperature dependence was observed due to dynamic molecular conformation activated by thermal energy to align transition dipole and enhance energy transfer from glassy to CT.
Overall, these findings of this thesis provide important insights into the mechanism of charge carrier injection and balance in organic optoelectronic devices that can guide for improving device performance.