Design, Deposition, Characterization and Modification of Transparent, Conducting Anode Materials for Organic Light-Emitting Diodes

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The electronic properties of various transparent conducting oxide (TCO) surfaces are probed electrochemically via self-assembled monolayers (SAMs). A novel graftable probe molecule having a tethered trichlorosilyl group and a redox-active ferrocenyl functionality (Fc(CH2)4SiCl3) is synthesized for this purpose. On as-received ITO, saturation coverage by a close-packed monolayer having of 6.6 × 10-10 mol/cm2 and an electron-transfer rate of 6.65 s-1 is achieved after 9 hr of chemisorption. CV studies of this redox SAM on five different TCO surfaces reveal that MOCVD-derived CdO exhibits the greatest electro-active coverage and MOCVD-derived ZITO (ZnIn2.0Sn1.5O) exhibits the highest electron transfer rate. Modification of inorganic electrodes has attracted great attention in the quest to optimize organic opto-electronic devices. An air-stable, crosslinkable trimethoxysilane functionalized hole-transporting triarylamine (4,4-bis[(p-trimethoxysilylpropylphenyl) phenylamino]biphenyl, TPD-[Si(OMe)3]2) has been synthesized and self-assembled or spin-coated onto ITO anode surfaces to form monolayers or multilayer siloxane films, respectively. The increase in surface work function and enhanced ITO-HTL contact via robust covalent bonding are expected to facilitate hole injection from the ITO anode, resulting in OLED performance enhancement versus that of device without such interlayers. The air-stable interlayer material developed here is also applicable to large-area coating techniques. Single-walled carbon nanotube (SWNT) films on flexible PET substrates are used as transparent, flexible anodes for organic light-emitting diodes (OLEDs). For polymer-based OLEDs having the carbon nanotube anode, a maximum light output of 3500 cd/m2 and a current efficiency of 1.6 cd/A have been achieved. The device operational lifetime is comparable to that of devices with Sn-doped In2O3 (ITO)/PET anodes. The advantages of this novel type of anode over conventional ITO are discussed. Double-layer transparent conducting thin film structures containing single-walled carbon nanotubes (SW-CNTs) and Sn-doped In2O3 (ITO) layers are deposited on flexible PET substrates by a PDMS (poly-dimethylsiloxane)-based stamp printing method and ion-assisted deposition (IAD), respectively, at room temperature and used as anodes for polymer light-emitting diodes (PLEDs). The overall figure of merit (Φ = T10/Rsheet) of the film with 30 nm CNT and 50 nm IAD-derived ITO is greater than that of the CNT-only film. CNT/ITO-based PLEDs exhibit superior performance to that of the CNT-only control. At last, a novel cyan emitting copolymer poly(2,5-N-n-octyl-3,4-cyclicimidothienyl-co-2',7'-9',9'-di-n- octylfluorene) (C8F8), consisting of electron-deficient cyclicimidothiophene and fluorene moieties, was synthesized and characterized.

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  • 09/01/2018
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