We have investigated how to optimize the efficacy and angle dependence of emission of top-emissive organic light-emitting diodes (OLEDs) based on metal foil substrates with the aim of creating efficient flexible devices for lighting and signage applications. By systematically varying the device architecture we were able to tune the optical microcavity which exists within the device structure and observe the change in performance. We paid particular attention to the effects of the metal foil roughness. We have seen that by changing the layer thickness of the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) [PEDOT:PSS] in the device, and the substrate reflectivity and roughness we can obtain efficacies not too far from those achieved for standard bottom-emissive devices on glass substrates made with the same emitter. The angle dependence of luminance can be tuned from pointing in the forward direction via Lambertian to having a maximum at around 65°, and we have used optical modeling to help us find the optimum device structure. We conclude that (rough) metal foils are a realistic possibility for making flexible OLEDs and have demonstrated large area (up to 12 cm × 12 cm), thin film encapsulated, flexible devices.
We demonstrate the feasibility of white organic light-emitting diodes that exclude the transparent conductor indium-tinoxide.
Instead, a highly conductive Orgacon<sup>TM</sup> PEDOT:PSS material in combination with a metal support structure is
used as transparent anode and hole-injection layer. The PEDOT:PSS exhibits a conductivity of 460±20 S/cm and a work
function of 5.35±0.05 eV. On ITO-free OLEDs on glass with an active area of ~6 cm<sup>2</sup> the inclusion of 120 nm thick
printed metal lines reduces the variation in brightness from 35% to 20%. The ITO-free concept based on PEDOT:PSS
with printed metal structures is scaled up to large flexible OLEDs with a size of 150 cm<sup>2</sup> on a heat-stabilized Teonex®
Polyethylene Naphtalate foil. The voltage distribution across the various electrodes was verified by a finite element
model, allowing a prediction of the OLED brightness and homogeneity over large areas.