Polarization Conversion in Surface‐Plasmon‐Coupled Emission from Organic Light‐Emitting Diodes Using Spontaneously Formed Buckles

Abstract

A surface plasmon (SP) polariton is an electromagnetic wave propagating along the interface between a dielectric and a metal, and its electromagnetic fi eld exponentially decays into the surrounding media. Because the wavevector of the SP mode is larger than that of a photon of the same frequency in vacuum, the SP mode on a fl at surface is nonradiative and its energy dissipates as heat in the metal layer. Organic light-emitting diodes (OLEDs) consisting of organic layers and metallic electrodes inevitably accompany the SP mode and the power loss to the SP mode signifi cantly limits the device effi ciency, particularly in small-moleculebased OLEDs. [ 1 , 2 ] Although wavelength-scale periodic gratings have been introduced in OLED structures to convert the dissipated energy to the SP mode into useful light, reported structures are effective only at a specifi c wavelength and angle, satisfying the Bragg diffraction condition. [1][2][3][4][5][6] In our previous study, we demonstrated that quasiperiodic buckling structures with broad distribution and directional randomness can effectively enhance the light-extraction effi ciency without introducing spectral changes and directionality by outcoupling the waveguide modes. [ 7 ] In that study, however, we could not differentiate the outcoupling of transverse electric (TE) mode from that of the SP mode (transverse magnetic (TM) mode) by buckles because of the broad periodicity of the buckling structure and the similar propagation vectors of the TE and SP modes. In this study, we report that a buckling structure is remarkably effective at outcoupling the SP mode over all emission wavelengths and angles through an OLED structure with a thin indium-tin oxide (ITO) layer, by which any other waveguide modes are suppressed and only the SP mode is excited. Interestingly, we found that the diffraction of the SP mode by buckles produces TE-polarized light in addition to TM-polarized light, which indicates polarization conversion from TM (SP) to TE mode.The existence of waveguide modes in OLEDs is strongly dependent on the thickness and refractive indices of device layers. In an OLED structure illustrated in Figure 1 a, we used a thin ITO layer of 40-nm thickness to minimize the TE waveguide mode and excite only one TM 0 waveguide mode that is a surface plasmon (SP) mode at the interface between the organic and cathode layers. The waveguide modes were characterized by calculating their in-plane propagation vectors for the emission wavelength using a transfer-matrix method. One weak TE 0 mode (blue line) that is leaky to resin or glass mode above ∼ 500 nm (below a dotted line in Figure 1 b), and one strong SP mode (red line) existing over entire emission wavelength from 450-750 nm were found in the OLED structure (Figure 1 b). The SP mode with a large in-plane propagation wave vector k SP can fall into the escape cone by the diffraction grating with a wave vector k G ,where k 0 denotes the absolute value (wavenumber) of the wavevector in free space, k | | its in-plane component, θ ...

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