Multi-layer contacts for organic light-emitting diodes with enhanced injection efficiency
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Multi-layer contacts for organic light-emitting diodes with enhanced injection efficiency Ludmila Bakueva, Sergei Musikhin, Edward H. Sargent, and Alexander Shik Department of Electrical & Computer Engineering, University of Toronto, 10 King’s College Rd., Toronto M5S 3G4, Canada ABSTRACT A low level of electron injection is one of the major obstacles to achieving high-efficiency organic light-emitting diodes. With the goal of improving injection characteristics, we fabricated and investigated herein multi-layer contacts which included a tunnel-transparent dielectric layer of nanometer thickness. Polymer layers were prepared by the spin-coating method and dielectric and metallic contact layers grown by vacuum deposition. The voltage drop at this layer shifts the metal Fermi level relative to the polymer molecular orbitals responsible for the carrier transport, increasing the injection efficiency. The introduction of a suitably chosen dielectric layer results in an increase in the injection efficiency by up to a factor of several tens. Further sophistication of the injecting contacts consists in creating and additional intermediate thin metallic layer playing the role of the third, base electrode, similar to hotelectron transistors with metallic base. Additional bias applied to the base electrode permits variable injection efficiency and quantum yield over a wide range. INTRODUCTION The efficiency of organic light-emitting diodes (OLEDs) is determined in significant part by the efficiency of electron and hole injection into polymer layers. Contact materials for effective hole injection should have a high work function. Indium-tin oxide (ITO) is the most popular material for this purpose and serves simultaneously as a transparent window for the radiation output. For electron injection, a low-work-function contact material is required. However, in such materials as Na, Li, Ca, a low work function is accompanied by high chemical activity, resulting in fast oxidation of the contact. This drawback can be partially neutralized by using more chemically inert materials with an additional tunnel- transparent dielectric layer between contact and polymer. The voltage drop at this layer shifts the metal Fermi level relative to the polymer molecular orbitals [1-3]. Though this tunnel layer may cause some decrease in the current density, exponential growth of the number of injected electrons with applied bias compensates this decrease at a rate sufficient to result in an eventual increase of injection. A similar layer can be created in a hole-injecting contact. Further increase of injection may be achieved by introducing an additional thin metallic layer between polymer and dielectric playing the role of a third electrode. Its function is similar to the base of a hot-electron transistor [4]. Additional bias applied to the base electrode causes electron tunneling from the external metal layer (emitter) through dielectric to the base layer (Figure 1). If the base thickness is less than the electron mean free path (on the order of 10
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