Highly color-stable solution-processed multilayer WOLEDs for lighting application

Abstract

White organic and especially polymeric light emitting devices (WPLEDs) have received particular attention due to their potential to provide cost-effective and simply manufactured solid-state light sources. The largest acceptable variation of Commission Internationale de L'Eclairage (CIE) coordinates is typically specified as Δx,y < 0.01 for general illumination purposes and even down to Δx,y < 0.005 by the automotive industry. Over the last few years great progress has been made regarding color-stability of OLEDs. In the first publications large color shifts of about Δx,y = 0.2,0.1 were reported. Current publications present devices with CIE variations as small as Δx,y = 0.02,0.02 or better, even for polymeric OLEDs. Here, we present a highly color-stable white fluorescent multilayer OLED consisting of a two-layer (yellow EML/blue EML) stack. The devices show white emission with CIE values of 0.324,0.346. Because of their extremely well-balanced electron and hole distribution, these devices show nearly no change in their CIE values (± 0.009,0.006) between 100 and 10 000 nits. Brightness in that range can be obtained at low voltages (4–8 V), at the same time providing a high efficiency of 6 cd A⁻¹. In addition, due to the broad spectral width of the emission the devices exhibit a color rendering index of 84. This value complies favourably with actual demands for ambient lighting. The extrapolated half-brightness lifetime at an initial brightness of 100 cd m⁻² exceeds 1000 h. All systems include the crosslinking of each layer either photo-chemically or thermally to enable the solution-processed complex multilayer OLED-structures.