Modifying the Output Characteristics of an Organic Light‐Emitting Device by Refractive‐Index Modulation

Abstract

In order to modify the output characteristics of organic light‐emitting devices (OLEDs), the optical properties of an active layer within the device are patterned without introducing any thickness modulation. For this purpose a new conjugated copolymer, which serves as a hole‐transporting material and at the same time can be index patterned using UV techniques, is synthesized. Poly(VC‐co‐VBT) (VC: N‐vinylcarbazole; VBT: 4‐vinylbenzyl thiocyanate) is prepared by free‐radical copolymerization of VC and VBT. The material contains photoreactive thiocyanate groups that enable altering of the material's refractive index under UV illumination. This copolymer is employed as a patternable hole‐transporting layer in multilayer OLEDs. Refractive‐index gratings in poly(VC‐co‐VBT) are inscribed using a holographic setup based upon a Lloyd mirror configuration. The fourth harmonic of a Nd:YAG (YAG: yttrium aluminum garnet) laser (266 nm) serves as the UV source. In this way 1D photonic structures are integrated in an OLED containing AlQ3 (tris(8‐hydroxyquinoline) aluminum) as the emitting species. It is assured that only a periodical change of the refractive index (Δn = 0.006 at λ = 540 nm) is generated in the active material but no surface‐relief gratings are generated. The patterned devices show more forward‐directed out‐coupling behavior than unstructured devices (increase in luminosity by a factor of five for a perpendicular viewing direction). This effect is most likely due to Bragg scattering. For these multilayer structures, optimum outcoupling was observed for grating periods Λ ∼ 390 nm.

Acetylenic Polymers: Syntheses, Structures, and Functions

“…In the advanced optoelectronic systems, the light refractivities of the optical materials should ideally undergo large changes when internally modulated or externally perturbed because the photonic functions are often realized by the differences in the RI values of the components in the devices. − There has been limited room to manipulate the refractivity of conventional condensation and vinyl polymers because of the limited structural tunability of their electronically saturated backbones. For this reason, acetylenic polymers with electronically conjugated backbones are promising materials because they are composed of unsaturated structural units with mobile electrons that can be modulated by molecular engineering or external stimuli.…”

Emission Characteristics of Organic Light-Emitting Diodes and Organic Thin-Films with Planar and Corrugated Structures

“…The pitch is around 4 μm which is limited by the microcontact printing process. Höfler et al in 2006 showed the possibility of creating planar surfaces with refractive index change, rather than the corrugated structure which increases the difficulties in fabrication [ 72 ]. In this study, a five-fold improvement was achieved for the normal direction.…”

Horizontally Orientated Sticklike Emitters: Enhancement of Intrinsic Out-Coupling Factor and Electroluminescence Performance

“…However, another factor, out-coupling coefficient (η out ), is also critical to the final external quantum efficiency (EQE). The η out is strongly dependent on extrinsic situations such as light-extraction techniques on substrate and status of electrodes. − Meanwhile, it is also affected by some intrinsic elements, for instance, refraction coefficient and thickness of each functional layer . Generally, the η out for bottom emission OLEDs without any extra light-extraction enhancement structures was estimated to be 20–30%. , Recently, a number of study works discovered that some emitters tend to be anisotropic in device circumstances and cause their transition dipole moment horizontal to the plane of substrate in a high degree. − Such orientations of dipole are very helpful in increasing the intrinsic out-coupling factor and leading to enhanced device performance.…”

“…24−26 Meanwhile, it is also affected by some intrinsic elements, for instance, refraction coefficient and thickness of each functional layer. 27 Generally, the η out for bottom emission OLEDs without any extra light-extraction enhancement structures was estimated to be 20−30%. 28,29 Recently, a number of study works discovered that some emitters tend to be anisotropic in device circumstances and cause their transition dipole moment horizontal to the plane of substrate in a high degree.…”