Revealing the Interplay between Charge Transport, Luminescence Efficiency, and Morphology in Organic Light-Emitting Diode Blends

Phosphorescent emissive materials in organic light-emitting diodes (OLEDs) manufactured using evaporation are usually blended with host materials at a concentration of 3-15 wt% to avoid concentration quenching of the luminescence. Here, experimental measurements of hole mobility and photoluminescence are related to the atomic level morphology of films created using atomistic nonequilibrium molecular dynamics simulations mimicking the evaporation process with similar guest concentrations as those used in operational test devices. For blends of fac-tris[2-phenylpyridinato-C2,N]iridium(III) [Ir(ppy)(3)] in tris(4-carbazoyl-9-ylphenyl)amine (TCTA), it is found that clustering of the Ir(ppy)(3) (surface of the molecules within approximate to 0.4 nm) in the simulated films is directly relatable to the experimentally-measured hole mobility. Films containing 1-10 wt% of Ir(ppy)(3) in TCTA have a mobility of up to two orders of magnitude lower (approximate to 10(-6) cm(2) V-1 s(-1)) than the neat TCTA film, which is consistent with the Ir(ppy)(3) molecules acting as hole traps due to their smaller ionization potential. Comparison of the simulated film morphologies with the measured photoluminescence properties shows that for luminescence quenching to occur, the Ir(ppy)(3) molecules have to have their ligands partially overlapping. Thus, the results show that the effect of guest interactions on charge transport and luminescence are markedly different for OLED light-emitting layers.