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 hot-electron transistors with metallic base. Additional bias applied to the base electrode permits variable injection efficiency and quantum yield over a wide range.