Investigation of the impact and mechanisms of changes in the process evaporation rate in high efficiency exciplex-based OLEDs

In this paper, exciplex-based OLEDs with the ITO/ m-MTDATA (30 nm)/ m-MTDATA:Bphen (40 nm, 1:1, x Å/s)/ Bphen (30 nm)/ LiF (1 nm)/ Al (80 nm) structural configuration are fabricated with different process evaporation rates. Since exciplexes are formed by combining m-MTDATA and Bphen materials, we maintained the volume ratio of these two materials at 1:1 in the emission layer to that these two different electrical types of organic molecules were evenly mixed. AFM observations, capacitance-voltage measurements, and photoluminescence spectra used to investigate the surface morphology and the exciton generation ability, and a comparison of the number of photons in the emission layer with various evaporation rates was conducted. Finally, the maximum OLED efficiency of 61.0 cd/A was obtained with minimum emission layer surface roughness and the maximum capacitance subtraction value (the difference in the maximum capacitance with the initial capacitance without an externally applied voltage) when the emission layer evaporation rate was 0.8 Å/s. This value was increased 4.1 times over the results in the literature. The detailed mechanisms are also investigated in this paper.