Transparent oxide/metal/oxide trilayer electrode for use in top-emitting organic light-emitting diodes

Abstract

Abstract. The most commonly used transparent electrode, indium-tin oxide (ITO), is costly and requires methods of deposition that are highly destructive to organic materials when it is deposited on top of the organic layers in top-emitting organic light-emitting devices (OLEDs).Here we have employed a trilayer electrode structure consisting of a thin layer of metal sandwiched between two MoO 3 layers, which can be deposited through vacuum thermal evaporation without much damage to the organic active layers. Such MoO 3 /Au/MoO 3 trilayer electrodes have a maximum transmittance of nearly 90% at 600 nm and a sheet resistance of <10 ohms per square ( /sq) with a 10-nm thick Au intermediate layer. Using these trilayers as the top transparent anode, we have fabricated top-emitting OLEDs based on either a fluorescent or phosphorescent emitter, and observed nearly identical emission spectra and similar external quantum efficiencies as compared to the more conventional bottom-emitting OLEDs based on the commercial ITO anode. The power efficiency of the top-emitting devices is 20% to 30% lower than the bottom-emitting devices due to the somewhat inferior charge injection in the topemitting devices. The performance and emission characteristics of these devices indicate that this trilayer structure is a promising candidate as a transparent anode in top-emitting OLEDs.

Transparent electrodes for organic optoelectronic devices: a review

“…[114][115][116][117] In these DMD electrodes, the intermediate thin metal layer provides the electrical conductance for the entire structure, whereas the two dielectric layers improve the overall transparency due to optical interference within the multilayer structure and the surface plasmonic effects at the two metal/dielectric interfaces. 44,[118][119][120] As shown in Figs. 4(b) and 4(c), an MoO x ∕Au∕MoO x structure shows a maximum transmittance of T ¼ 80% at λ ¼ 650 nm (the corresponding reflectance is R ¼ 5%), which is about two times higher than that of the bare Au layer (T ¼ 40% and R ¼ 25%).…”

Enhancing Light Extraction in Top‐Emitting Organic Light‐Emitting Devices Using Molded Transparent Polymer Microlens Arrays

“…OLED Fabrication: Small‐molecule‐based, top‐emitting OLEDs with different color emissions were fabricated using vacuum thermal evaporation in a custom‐made high vacuum evaporator (base pressure = ∼1×10 −7 Torr). The OLEDs generally consist of a 100 nm thick Al cathode on a pre‐cleaned glass substrate, a 1 nm thick interfacial Cs 2 CO 3 layer,34 an organic multilayer stack, and a MoO x (5 nm)/Au (10 nm)/MoO x (40 nm) trilayer transparent anode 26. The organic multilayer stack consists of (in deposition sequence) a 40 nm thick tris[3‐(3‐pyridyl)mesityl]borane (3TPYMB)34 electron transport layer, a 20 nm thick emitting layer (EML), a 10 nm thick 1,1‐bis‐(di‐4‐tolylaminophenyl)cyclohexane (TAPC) hole transport layer, and a 30 nm thick hole injection layer of N , N ′‐diphenyl‐ N , N ″‐bis(3‐methylphenyl)‐[1,1′‐biphenyl]‐4,4′‐diamine) (MeO‐TPD) doped with 2 mol% of tetrafluoro‐tetracyanoquinodimethane (F 4 ‐TCNQ).…”

“…We have previously demonstrated the fabrication of large‐area (several square inches) arrays of close‐packed, hemispherical microlenses from Norland optical adhesives (NOA) using a soft lithography process, and achieved a 50–70% enhancement in η LE when such MLAs are applied to BE‐OLEDs 17. Here we have fabricated a 1 cm 2 area, green‐emitting TE‐OLED based on the structure investigated previously26 with the phosphorescent fac ‐tris‐(phenylpyridine) iridium [Ir(ppy) 3 ]27 as the emitter. Half of the device emitting surface was covered with a MLA ( n lens = 1.56) as schematically illustrated in Figure a.…”

“…ITO commercially pre‐deposited on glass substrates has been commonly used as the transparent conductor for BE‐OLEDs. While methods have been developed to deposit ITO during the rf sputtering process on organic layers without significantly damaging the organics,31, 32 we have used a MoO x /Au/MoO x trilayer structure26 as the transparent electrode for the TE‐OLEDs here, which is fully compatible with organic material deposition in high vacuum. As shown in Figure 3b, the transmittance of such trilayer structure (trilayer 1, with a 10 nm thick intermediate Au layer) is above 80% for 550 nm ≤ λ ≤ 700 nm, approaching that of the commercial ITO film.…”

ITO-free top emitting organic light emitting diodes with enhanced light out-coupling

“…In recent work the use of MAMs electrodes in inverted sub‐module solar cells has been described . However, in the case of OLEDs, MAMs electrodes have only been used in evaporated multi‐layer small molecule OLEDs with emphasis on the stack being a component of the cathode . In our current work, the MAMs has been successfully used as the anode for solution processed single layer polymer top‐emitting OLEDs.…”