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

Abstract

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 ≈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 (≈10−6 cm2 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.

Improved Efficiency and Stability of Blue Phosphorescent Organic Light Emitting Diodes by Enhanced Orientation of Homoleptic Cyclometalated Ir(III) Complexes

“…Jong Soo Kim Energy level engineering based on simple device models has been playing an essential role in device optimization; however, recent emphasis on the atomistic understanding of EML morphologies should be noted to exploit its full potential. Molecular arrangement determines crucial macroscopic properties including charge mobility, [19] molecular aggregation, [20][21][22][23][24][25] and emission dipole orientation (EDO). [26][27][28][29][30] Particularly, recent studies on the relationship between host-dopant interactions and EDO have opened new possibilities on material development [31][32][33][34] by demonstrating the definite improvement of the external quantum efficiency (EQE).…”

Rivers of Light—Ternary Exciplex Blends for High Efficiency Solution‐Processed Red Phosphorescent Organic Light Emitting Diodes

“…Before fabricating the OLEDs we measured the hole mobilities of the different films using Metal‐Insulator‐Semiconductor Charge Extraction by Linear Increasing Voltage (MIS‐CELIV) [ 38 ] with the transients shown in Figure S3 in the Supporting Information and the results summarised in Table 1 . The hole mobility of neat films of the unannealed and annealed m‐MTDATA were around twice that of the Ir(tD C pq) 3 , with thermal annealing not causing any significant change in either case.…”

“…Normally blending a light‐emitting guest into a host leads to a significant reduction in charge mobility due to the guest acting as a charge trap, even at low concentrations. [ 38–39 ] However, for the blend films containing 2 wt% of the Ir(tD C pq) 3 guest there was little change in the hole mobility observed for the unannealed or annealed films relative to the m‐MTDATA:OXD‐7 blends. However, at higher concentrations there was a decrease in the hole mobility with increasing Ir(tD C pq) 3 for the annealed films.…”

Effect of PEDOT:PSS on the performance of solution-processed blue phosphorescent organic light-emitting diodes with an exciplex host

“…The electron [(8.0 AE 1.2) Â 10 À8 cm 2 V À1 s À1 ] and hole [(4.9 AE 0.8) Â 10 À7 cm 2 V À1 s À1 ] mobilities of the emissive layer containing 20 wt% of Ir(DTri) 3 were lower than those of the TCTA:POPH blend, which is consistent with the Ir(DTri) 3 trapping the charges. 31,32 Exciplex organic light-emitting diodes…”