Enhanced performance of OLED based on molecular orientation of emission layer by optimized substrate temperature

Horizontally oriented transition dipole moment (TDM) of the molecule in emission layers (EMLs) can boost out-coupling efficiency, resulting in higher performance organic light-emitting diodes (OLEDs). As known, the substrate temperature (T_s) has a significant effect on the TDM of molecules. In this paper, the doped EMLs deposited with different T_s ranging from 30 to 100 °C were prepared and the corresponding photoluminescence quantum yield (PLQY) and TDM were measured. In addition, these EMLs with different T_s were employed to the OLED devices. The best luminous efficiency (51.4 cd/A), power efficiency (68.9 lm/W) and external quantum efficiency (EQE) (19.5%) were obtained in the OLED device where EMLs were deposited at 60 °C. However, the anisotropy factor Θ at this temperature is not the best, which is inconsistent with the common rules that high Θ causes high EQE. This phenomenon was ascribed to the in-situ annealing process for the hole transport layer (HTL) during the deposition of EMLs with different T_s. The synergy of PLQY and TDM orientation of EMLs, the hole mobility of hole only devices (HODs) contribute to the high performance of OLED device at 60 °C.