Device optimization, solution processible small molecules

Due to the easy processing, thin films of solution processable polymer-small molecule composites are more appealing for the realization of integrated circuits on large area by printing techniques. As for the coevaporated systems, the composition and microstructure of the film will affect the performances of the devices. For the polymer-small molecule blends, these characteristics can be tuned by the choice of solvents and spin coating parameters [52], until the optimal conditions are reached for balanced bipolar transport. 

The overall performance of OLEDs can be dramatically improved by the use of dye molecules. These small molecules efficiently trap electrons and holes (ie carbanions and carbocations) and have high radiative recombination rates relative to non-radiative decay processes. One can quickly see why this would be effective in a device as the dye molecule can be optimized for recombination, while the HTL and ETL layers can be optimized for carrier transport, carrier injection and chemical stability without having to worry about radiative recombination. From the discussion in Section 9.2, it is known that many molecules exhibit low radiative recombination rates due to the symmetries of their HOMO and LUMO orbitals, resulting in disallowed optical recombination pathways. The solution to the problem is to dissolve dye molecules optimized for emission in one or more of the transport layers. 

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