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Modifying the Output Characteristics of an Organic Light‐Emitting Device by Refractive‐Index Modulation
Author(s) -
Höfler T.,
Weinberger M.,
Kern W.,
Rentenberger S.,
Pogantsch A.
Publication year - 2006
Publication title -
advanced functional materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/adfm.200500503
Subject(s) - materials science , optoelectronics , refractive index , oled , high refractive index polymer , optics , active layer , layer (electronics) , grating , nanotechnology , physics , thin film transistor
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 AlQ 3 (tris(8‐hydroxyquinoline) aluminum) as the emitting species. It is assured that only a periodical change of the refractive index (Δ n = 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.