DOI: 10.1002/adfm.200700686 Inverted Solution Processable OLEDs Using a Metal Oxide as an Electron Injection Contact ** By Henk J. Bolink,* Eugenio Coronado , Diego Repetto, Michele Sessolo, Eva M. Barea, Juan Bisquert, Germà Garcia-Belmonte, Jan Prochazka and Ladislav Kavan 1. Introduction Interest in organic light-emitting diodes (OLEDs) for use in display and lighting applications is increasing. This is mainly because of reports of new breakthroughs in device efficiencies, lifetimes, and achievable colors, including white. [1] However, multilayer devices are needed to obtain these high perfor- mance levels and, additionally, the devices need to be rigor- ously encapsulated because of the reactive cathodes or electron injection layers that are used. To enter successfully the general lighting market, OLEDs require a strong reduction in the cost of the devices as well as high performance levels. In this re- spect, it is of particular importance to be able to generate elec- troluminescence from devices using air-stable charge-injection interfaces. Although some examples exist, these devices rely on the presence of ionic charges to generate a dipole across the metal–light-emitting-layer interface and their reported life- times are low. [2–4] Metal oxides are, in principle, promising can- didates that may lead to good charge injection as they combine properties such as high transparency, low resistance, and air stability. Recently, there have been reports about the use of metal oxides as charge injection layers. They range from ultra- thin layers on the anode side to nanostructured layers on the cathode side of the devices. [5–9] The use of a hole-blocking met- al-oxide material on the anode side modifies the device effi- ciency by adjusting the charge balance in the device. [6] A more beneficial use of the metal-oxide layer is as an alternative cath- ode material. The use of an unreactive metal oxide as the cath- ode is appealing as this would allow the preparation of OLEDs requiring no, or only simple, encapsulation. This would signifi- cantly reduce costs and, therefore, increase the feasibility of the use of OLEDs in display and especially lighting applica- tions. Recently, Morii et. al. showed that it is possible to generate electroluminescence from a device that uses TiO 2 as the cath- ode. [7] They reached brightness levels of 700 cd m –2 at a driving voltage of 6 V. The current to light efficiency, however,was of the order of 0.1 cd A –1 because of the high current densities flowing through the device. The mechanism of operation of the OLEDs using a simple undoped metal oxide such as titanium dioxide as the cathode is intriguing. An energetic barrier exists for the injection of elec- trons from the conduction band of the metal oxide to the low- est unoccupied molecular orbital (LUMO) of the light-emitting material (Fig. 1). LUMO energies for most light-emitting poly- mers (LEPs), as well as for electron transporting materials used in multilayer OLEDs, range from –2.5 to –3.0 eV. This mis- Adv. Funct. Mater. 2008, 18, 145–150 © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 145 – [*] Dr. H. J. Bolink, Prof. E. Coronado, Dr. D. Repetto, M. Sessolo Instituto de Ciencia Molecular, Universidad de Valencia PO Box 22085, E-46071 Valencia, (Spain) E-mail: henk.bolink@uv.es Dr. E. M. Barea, Prof. J. Bisquert, Dr. G. Garcia-Belmonte Departament de Física, Universitat Jaume I 12071 Castelló, (Spain) J. Prochazka, Prof. L. Kavan J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic 182 23 Prague 8 (Czech Republic) [**] This work has been supported by the European Union (HETERO- MOLMAT, STRP 516982) the Spanish Ministry of Education and Science (MEC) (MAT2007-61584, CSD2007-00010 and MAT2004- 05168) the Czech Ministry of Education (grant no. LC-510), and the Generalitat Valenciana. H.J.B. acknowledges the support of the Pro- gram “Ramon y Cajal” of the MEC. A new type of bottom-emission electroluminescent device is described in which a metal oxide is used as the electron-injecting contact. The preparation of such a device is simple. It consists of the deposition of a thin layer of a metal oxide on top of an indium tin oxide covered glass substrate, followed by the solution processing of the light-emitting layer and subsequently the deposition of a high-workfunction (air-stable) metal anode. This architecture allows for a low-cost electroluminescent device because no rigorous encapsulation is required. Electroluminescence with a high brightness reaching 5700 cd m –2 is observed at voltages as low as 8 V, demonstrating the potential of this new approach to organic light-emitting diode (OLED) devices. Unfortunately the device efficiency is rather low because of the high current density flowing through the device. We show that the device only operates after the insertion of an additional hole-injection layer in between the light-emitting polymer (LEP) and the metal anode. A simple model that explains the experimental results and provides avenues for further optimization of these devices is described. It is based on the idea that the barrier for electron injection is lowered by the formation of a space– charge field over the metal-oxide–LEP interface due to the build up of holes in the LEP layer close to this interface. FULL PAPER