Enhanced efficiency of multilayer organic light-emitting diodes with a low-refractive index hole-transport layer: An effect of improved outcoupling?

The authors report on an internal device modification for multilayerorganic light-emitting diodes(OLEDs) with enhanced efficiency that promises high compatibility with conventional manufacturing processes. By copolymerization of a hole-conducting monomer with a compound possessing a lower refractive index, a hole-transport layer with reduced optical density but slightly reduced hole-transport properties is formed. MultilayerOLEDs based on this reduced-index layer show a 25% increased efficiency compared to reference devices. The results are compared to optical simulations of the dipole emission from thin organic films. It is found that the efficiency improvement is only to some extent due to enhanced outcoupling resulting directly from the reduced refractive index but primarily due to a change of the width of the emission zone

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

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