Thermal emission, electroluminescence

Therefore, by applying electric and thermal treatments simultaneously, homogeneous and enhanced EL emission was obtained from the active area of the devices with high reproducibility. Moreover, the efficiency of the devices was also observed to improve. As a result, an ionic f-i-n PHOLED with a peak external quantum efficiency of 8.6 % was achieved in the sample device. On the basis of these results, it is demonstrated that simultaneous annealing can lead to more efficient electroluminescence through increased and balanced carrier injection. This improvement can be attributed to the excellent balancing of holes and electrons. 

We have shown an extreme enhancement of the exciplex emission in bilayer EL as compared to PL and the appearance of weak exciton EL only through thermal activation from the exciplex at higher temperatures. At low temperatures, the exciton contribution is frozen out completely and only exciplex electroluminescence is seen. This demonstrates unambiguously that the only source of bulk excitons during electrical excitation is endothermic energy transfer from exciplex states that are generated via barrier-free electron-hole capture and confirms the work presented in Section 2.2.1 that was based on room-temperature emission from polymer blend LEDs and time-resolved PL. 

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