Improving the outcoupling efficiency of white organic light-emitting devices based on a gradient refractive index substrate

Abstract

For use in lighting applications, white organic light-emitting devices (WOLEDs) must operate at higher biases to ensure an ample flux. However, stressed operation voltages often result in poor performance and limited device lifetime. This could be addressed by modifying the inherent optical properties of OLEDs. This study proposes a gradient refractive index (GRIN) substrate to adjust the ratio of the light-waveguided modes as well as the radiation mode. An embedded nanocomposite film consisting of titanium dioxide (TiO2) nanoparticles (NPs) was inserted between ITO and glass to create an internal light-extraction structure (IES). The high refractive index of TiO2 is essential for increasing the refractive index of the photoresist film and thus diminishing the total internal reflection between the interfaces. In addition, the silicon dioxide NPs mixed poly(dimethylsiloxane) was used to form an external light-extraction structure (EES). The refractive indices of the IES and EES were adjusted to form a GRIN substrate. Compared with a control device, this sophisticated substrate produced a 1.6 fold efficiency improvement. Furthermore, the experiment results indicates that the size of NPs in the nanocomposite layer affects the efficiency enhancement of OLEDs with different emission colors.

Gradient refractive index structure of phosphor‐in‐glass coating for packaging of white LEDs

“…In Figure A, the GRIN structure consists of several layers with the refractive index (n) decreasing along the normal direction. Due to the mass difference in the refractive index, the nongradient structure produces total reflection at the coating‐air interface . Therefore, the GRIN structure can reduce the difference in the refractive index.…”

Photon management in solution-processed organic light-emitting diodes: a review of light outcoupling micro- and nanostructures

“…In this context, scattering layers based on nanoparticles are industrially relevant candidates that can improve the outcoupling of substrate modes when deposited onto the substrate, 50 as well as internal modes (including waveguide and SPP modes provided that their spatial mode overlap with the scattering film is high enough 51 ) if they are inserted within the OLED thin film stack. [52][53][54][55] In general, the nanoparticles' concentration should be tuned to avoid backward scattering. However, in the case of translucent/bidirectional OLEDs, this effect can be exploited on purpose to simultaneously improve light extraction in both the forward and backward directions, as demonstrated by Chang et al 56 In Sec.…”