Transparent organic LEDs for new lighting applications

Abstract

Organic LEDs (OLEDs) are light sources that do not produce excessive heat and do not contain toxic materials.1 The light emitted from OLEDs is UV-free and can therefore be used to illuminate biological substances without causing harmful effects. OLEDs are ultra-thin (about 150nm), have very low weight, and can be tailored to many formats. A specific type of OLED—a so-called transparent OLED (TOLED)—is bidirectional, i.e., it emits light from its top and bottom surfaces (see Figure 1).2–4 It is vital that all components in a TOLED are transparent so that the simultaneous top and bottom light emission can be achieved. To ensure transparency of TOLEDs, the thickness of their cathodes should not exceed 20nm. Transparent cathodes in TOLEDs usually consist of a multilayer metallic thin film.3, 4 However, limited light transmission through the cathode means that the efficiency of top emission is lower than that of the bottom emission. As such, the asymmetric structure of the light propagation causes different electroluminescence (EL) spectra for the top and bottom emissions from TOLEDs. To address the problem of asymmetry in the EL spectra, we designed a TOLED that uses a capping layer (CL), with a unique optical function.2 The use of the CL does not disturb the electrical characteristics of the device and therefore obviates the need to alter the stack structure. Our CL is an optical dielectric that can enhance the transmittance of the device and the top-emission efficiency. Including the CL on the cathode produces two interfaces, i.e., air/CL and CL/cathode. Due to the complexity of multilayer interference, it is difficult to establish a straightforward optical principle that achieves the EL spectral matching of bottom and top emissions in our TOLED. We achieve high transmittance and high top-emission efficiency by adjusting the CL thickness so that we created Figure 1. A schematic illustration of a typical transparent organic LED (TOLED). The capping layer and transparent cathode are key components in the device.