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On the interplay of waveguide modes and leaky modes in corrugated OLEDs

Julian Hauss, Tobias Bocksrocker, Boris Riedel, Uli Lemmer, and Martina Gerken

Open Access Open Access

Abstract

Bragg gratings incorporated into organic light-emitting diodes (OLEDs) establish a coupling between waveguide modes and useful light (leaky modes). Here we demonstrate that the net coupling direction depends on the OLED stack design. We fabricated two different device structures with gold Bragg gratings. Angle resolved electroluminescence spectra were recorded. For the first device peaks of enhanced emission due to the Bragg grating are observed corresponding to a net energy transfer in direction of the leaky modes. The second device, on the other hand, exhibits dips in the emission spectrum. This reversed direction of energy transfer from the leaky modes to the waveguide modes is explained considering transfer matrix simulations of modal intensity distributions and device emission simulations. An OLED efficiency enhancement is only achieved, if the waveguide mode extraction is dominant.

©2011 Optical Society of America

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Figures (7)
Fig. 1
Fig. 1 (Color online) Schematic of the reference device and two different devices with anode side gold gratings (dimensions not to scale).

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Fig. 2
Fig. 2 (Color online) Schematic of the measurement setup used for angle resolved electroluminescence emission measurements. For the devices 1 and 2 the spectra were recorded in a plane perpendicular to the grating grooves.

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Fig. 3
Fig. 3 (Color online) Angle resolved electroluminescence emission spectra of (a) the reference device, (b) device 1, and (c) device 2.

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Fig. 4
Fig. 4 (Color online) Electroluminescence emission of (a) device 1, (b) device 2 at a wavelength of 550 nm. (c) Emission of device 1 separated into TE- and TM-polarization.

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Fig. 5
Fig. 5 (Color online) Classification of optical modes in OLEDs in a dispersion diagram ω(kx). The leaky mode continuum of the light that is able to leave the device is located above the air light line. The substrate mode continuum is located between the air light line and the glass light line. Discrete waveguide modes are found below the glass light line. Arrow 1 represents the extraction of a waveguide mode to the leaky modes by Bragg scattering at a grating with reciprocal lattice constant G. Arrow 2 represents the inverse process: Light from the leaky mode region being coupled into the waveguide mode by a Bragg grating with reciprocal lattice constant G.

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Fig. 6
Fig. 6 (a) Comparison of experimental and calculated spectral and angular position of the fundamental TE waveguide mode in device 1. (b) T-Matrix simulation of the normalized electric field intensity of the fundamental TE mode in device 1 at a wavelength of 550 nm. (c) Comparison of experimental and calculated spectral and angular position of the fundamental TE waveguide mode in device 2. (d) T-matrix simulation of the normalized electric field intensity of the fundamental TE mode in device 2 at a wavelength of 550 nm.

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Fig. 7
Fig. 7 (a) Simulated dipole emission as a function of the effective index of refraction n eff at a wavelength of 550 nm. Device 1 exhibits a distinct peak around neff = 1.6 related to the fundamental TE waveguide mode, whereas the coupling to leaky modes (neff < 1) is rather weak. For device 2 the situation is opposite. (b) Scheme of the emitter coupling to leaky modes and waveguide modes including grating coupling via Bragg scattering between modes into both directions. Arrow 1 represents the waveguide mode extraction process described by Eq. (1). Arrow 2 represents the incoupling in waveguide modes described by Eq. (3). In addition the waveguide modes are damped due to the conductive electrodes and leaky modes leave the device towards the far field.

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Equations (4)

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k ′ x = β − G .
θ ( Λ , λ 0 ) = arcsin ( n e f f − λ 0 / Λ ) ,
β = k ′ x = k x + G ,
W T E = W 0 π 2 n o r g k 0 | E ∥ ( z e ) | 2 ∫ | E ∥ ( z ) | 2 d z ,
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