Air stable hybrid organic-inorganic light emitting diodes using ZnO as the cathode
Henk J. Bolink, Eugenio Coronado, Diego Repetto, Michele Sessolo
Applied Physics Letters
An air stable hybrid organic-inorganic light emitting device is presented. This architecture makes use of metal oxides as charge injecting materials into the light emitting polymer, avoiding the use of air sensitive cathodes commonly employed in organic light emitting diode manufacturing. We report the application of zinc oxide as a cathode in an organic light emitting device. This electroluminescent device shows high brightness levels reaching 6500 cd/ m 2 at voltages as low as 8 V. Compared
... a conventional device using low workfunction metal cathodes, our device shows a lower turn-on voltage and it can operate in air. Organic light emitting diodes ͑OLEDs͒ are emerging as a next-generation technology for electronic displays and lighting. Apart from optimized device performances, the cost of production should be minimized to ensure the large scale applicability of the OLED technology in display and particularly lighting applications. With this respect it is of particular importance to be able to generate electroluminescence from devices using air stable charge injection interfaces. Such interfaces should not contain reactive metals or air sensitive charge injection layers. Recently, metal oxides have been employed with different characteristics and functions in OLED architecture. 1-4 However, the most appealing examples of the use of this class of compounds are as an alternative to low-workfunction materials commonly employed as cathode. 3, 4 In those cases, titanium dioxide was chosen as the electron injection layer. Zinc oxide ͑ZnO͒ is another promising candidate for optoelectronic applications because of its suitable properties, such as high transparency, good electrical conductivity, tuneable morphology, and large variety of possible nanostructures. 5-7 Könekamp et al. showed an electroluminescent device where ZnO was used as near-UV emitter in combination with poly͑3,4-ethylenedioxythiophene͒ poly͑styrenesulfonate͒ ͑PEDOT:PSS͒ as hole injection material. 8 However, ZnO has not been used as the cathode in OLEDs. Here, we report an air stable device using metal oxides both in electron and hole injection layers. In particular, the hybrid OLED ͑from now on, denominated as HyLED͒ is composed of a yellow-green light emitting polymer ͑LEP͒, poly͑9,9dioctylfluorene-co-benzothiadiazole͒ ͑F8BT͒, sandwiched between a ZnO and a thin molybdenum oxide ͑MoO 3 ͒ layer ͑Fig. 1͒. In this case, the MoO 3 functions as the holeinjecting layer from the top Au electrode to the LEP. 9 It is important to underline that in this device architecture, the indium-tin oxide ͑ITO͒-ZnO electrode is functioning as the cathode and the metal ͑Au͒ as the anode. In this inverted layout with respect to traditional OLEDs, no air sensitive metals or injection layers are employed, generating an, in principle, air stable device. Thin ZnO films have been deposited by spray pyrolysis, which is a commonly used technique for the preparation of compact layers of many metal oxides. 10,11 We followed a procedure similar as reported earlier. 12, 13 Briefly, zinc acetate dihydrate was dissolved in a mixture of ethanol and water ͑3:1͒. Acetic acid was added in order to avoid the formation of a white precipitate ͑zinc hydroxide ͓Zn͑OH͒ 2 ͔͒ and to enhance film deposition. This solution was sprayed onto prepatterned ITO glass plates ͑prior to deposition, the ITOcoated glass substrates were extensively cleaned, using chemical and UV-ozone methods͒ at 400°C on a hot plate and the layers were subsequently annealed in a furnace at 500°C for 12 h. The morphology of the metal oxide layer has been investigated by atomic force microscopy. The surface must be flat in order to prevent shorts circuits between the metal oxide and the gold anode after polymer deposition. The analysis revealed that the surface of the ZnO layer is homogeneous and flat ͑Fig. 2͒, with a low roughness ͑rms of 6 nm͒. Therefore, we can reasonably assume that the oxide film is fully covered by the LEP layer ͑thickness of ϳ55 nm͒. F8BT ͑ADS133YE, American Dye Source, Inc.͒ was spun on substrates from a chlorobenzene solution. Before spin coating the solutions were filtered over a 0.20 m polytetrafluoroethylene filter. After LEP deposition, the thin films were dried at 85°C for 15 min and then transferred to a high vacuum chamber integrated in an inert glove box atmosphere. MoO 3 and Au were thermally evaporated on the polymer layer under a base pressure of 1 ϫ 10 −6 mbar. Thicknesses of the ZnO and spin coated LEP films were determined using an Ambios XP1 profilometer and are depicted in Fig. 1 . The current-voltage ͑J-V͒ and electroluminescence-voltage characteristics were collected using a Keithley 2400 source measurement unit and a Si a͒ Electronic mail: email@example.com FIG. 1. Schematic device layout with indications of layer thicknesses.