Molecular Self-Assembly Routes to Optically Functional Thin Films: Electroluminescent Multilayer Structures
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composed of moleculartobuilding blocks. In the present device contribution, we report the first application oftailored such techniques multilayer electroluminescent fabrication. Covalent self-assembly approaches are distinctly different from either conventional vapor deposition [8,9] or polymer spin-coating [8.9] techniques, and in principle, suggest means to address electrode passivation, balancing electron versus hole injection rates, the unavailability of suitably volatile building blocks, as well as failure modes such as atmospheric degradation, pinholes, layer cracking, layer interdiffusion, layer crystallization, and layer vaporization [8-15].
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Figure 1. Approach to the layer-by-layer assembly ofNLO-active chromophoric multilayers. The steps are: i, coupling layer introduction; ii, chromophore layer introduction; iii, capping layer introduction. EXPERIMENTAL All chlorosilane reagents were synthesized, stored, and manipulated with rigorous exclusion of oxygen and moisture using Schlenk and high vacuum line techniques as well as a Vacuum Atmospheres glove box with an efficient recirculator. All solvents were dried in an appropriate manner and distilled under inert atmosphere. The synthesis of the hole transport layer (HTL), emissive layer (EML), and electron transport layer (ETL) building blocks are outlined in Figures 2-4, respectively. All products were characterized as appropriate by •H and 13C NMR, high resolution mass spectrometry, and elemental analysis. The basic self-assembly techniques and their general temporal characteristics are described elsewhere [4,5,7], as are the procedures for film thickness characterization by scanning ellipsometry [4,7] and specular X-ray reflectivity [4-6]. In the present case, specimens were protected from dust at all times, and specimen transfer from reaction containers was carried out in a class 1000 clean hood. ITO substrates were patterned and cleaned as described previously [16]. The instrumentation for electroluminescence studies is described elsewhere [16]. RESULTS AND DISCUSSION This report addresses preliminary results in the following areas: 1) synthesis of hole transport layer (HTL), emissive layer (EML), and electron transport layer (ETL) building blocks for ultimate incorporation in three-layer devices, 2) self-assembling electrode modification/passivation layers, 3) microstructural/chemical respects of HTL self-assembly, 4) a hybrid self-assembled + vapor deposited device, 5) a totally self-assembled two-layer device.
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Building Block Synthesis Figures 2-4 outline synthetic approaches to reactive precursors for self-assembled layers having hole transport, emissive, and electron transport functions, respectively. The precursor core structures have established efficacies in these functions [8-15]. The chemistry makes heavy use of palladium-catalyzed Stille reactions for allyl group introductio
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