Physical characterization of Ag:WO3 cermet films used as top electrode for stable and high contrast organic light-emitting diodes

Abstract

A low reflectance green top-emitting OLED using a cermet Ag:WO3 thin film as a top cathode has been fabricated and fully characterized. The cermet film has been made by co-evaporating Ag and WO3 materials. Depending on the Ag concentration in the WO3 matrix, the optical and electrical properties vary a lot, as this will be investigated by optical simulations as well as by studying single film properties deposited on silicon and glass wafers. A plasmonic absorption of Ag occurs for metal concentrations around 60–70%, which confirms the “nano” nature of the cermet material, with Ag nanocrystals embedded in an amorphous WO3 oxide matrix (also confirmed by TEM). Using an optimal (optical, electrical) Ag:WO3 (70%) cermet composition as a top electrode in the OLED allows to strongly reduce the device mean reflectance in the visible while its contrast ratio becomes far better (maximum + 76% at 1000 lux) than a reference green device (with Ag as top electrode instead of Ag:WO3). The device with the cermet shows very good lifetimes when driven at constant current drive: an extrapolated lifetime of 90 000 h at 100 cd/m2 has been estimated, based on accelerating ageing tests done for 1000 cd/m2 to 20 kcd/m2 initial luminance values.