Phase-controllable copper oxides for an efficient anode interfacial layer in organic light-emitting diodes

Abstract

We investigated phase-controllable copper oxide (CuO_x) as a hole injection layer for improving the electrical and optical properties of organic light emitting diodes (OLEDs). The phase of CuO_x is changed from CuO to Cu₂O by elevating the deposition rate during thermal evaporation, and a non-stoichiometric film mixing with two phases occurs in the intermediate condition. In non-stoichiometric films, unbonded oxygen atoms with a large density of gap states are induced near the Fermi level, acting as a hole transport path. Thus, holes can be more easily injected into the highest occupied molecular orbital states of α-NPD, lowering the hole injection barrier significantly. Films of non-stoichiometric CuO_x are inserted at the interface of α-NPD with an Ag electrode, serving as the hole injection layer. Significantly improved current densities and luminance were achieved, reducing the operation voltage at 1 mA cm⁻² from 6.1 V to 3.2 V.