Self-Assembly Processes for Organic Led Transport Layers and Electrode Passivation
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times from dust by conducting sample transfers in a class 1000 clean hood. ITO substrates were patterned and cleaned as described previously[3, 18]. The instrumentation for electroluminescence studies is described elsewhere[3]. RESULTS AND DISCUSSION We report here two separate techniques for the modification of vacuum-deposited OLEDs. One approach consists of the deposition of a OLED hole transport layer (HTL) in which triarylamine core structures with reactive trichlorosilane fuinctionalities undergo rapid crosslinking and densification upon spin-coating of the material in ambient atmosphere. Following a thermal cure, this process provides a robust, adherent, essentially pinhole-free charge transporting network embedded in a siloxane matrix. These HTL structures are then incorporated into a two-layer hybrid OLED by vacuum deposition of an emissive ETL [12]. The second method of OLED modification discussed here involves the use of similar self-limiting chemisorptive siloxane self-assembly techniques[13, 15, 16, 18, 19] to introduce a thin, conformal, microstructurally/electronically well-defined, nearly pinhole-free dielectric layer between the ITO anode and the HTL of a two-layer OLED device. We show that the introduction of this layer can effect significant enhancements in maximum external quantum and luminous efficiencies. OLED Hole Transport Layers via Spin-Coating/Siloxane Condensation In order to form a heavily crosslinked glassy HTL, a solution of trichlorosilane functionalized triarylamine I was spin-coated from hydrocarbon solutions in air. In the presence of moisture, I readily undergoes hydrolysis and subsequent -=Si-OH+ HO-Si= condensation[2022] which, upon thermal curing (120 'C in air), produces a smooth, hard, thermally stable, and adherent, glassy matrix (II) as seen in Figure 1. By varying the concentrations of the spincoating solutions, the thickness of the films can be controlled (100-700 A) as assayed by ellipsometry on duplicate samples cast on (100) Si wafers. AFM studies of these spin-coated films on clean ITO (rms roughness -30 A) show smooth (rms roughness -15 A), conformal films with no obvious cracks or pinholes. The reduction in the roughness of the substrate after deposition of the HTL suggests that the spin-coating of the cross-linked II-HTL layer effects planarization of the surface. .0 0-Si
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Figure 1. Conversion of trichlorosilane-functionalized triarylamine building block I to a siloxane crosslinked hole transport layer (HTL) I1. 460
In order to study the extent to which the HTL precursor I undergoes hydrolysis, a cast film was mechanically detached from a glass substrate. Elemental analysis of this removed
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Figure 2. Thermogravimetric analysis under Ar of a II sample obtained from a spin-coated, cured film. The temperature ramp is 15°C/min material revealed Cl levels below the detection limit, suggesting essentially complete hydro
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