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Spontaneous buckling in flexible organic light-emitting devices for enhanced light extraction

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Abstract

We herein present the results of a study of the direct fabrication of buckled patterns in flexible organic light-emitting devices (FOLEDs) that had a conducting polymer anode on a polyethersulfone substrate. These patterns were produced spontaneously by the thermal deposition of an aluminum cathode on an electroluminescent (EL) composite layer. The polymer used for the anode was modified poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) and the EL layer was composed of a solution-processable small molecular composite including phosphorescent Iridium complex mixed with a poly(vinylcarbazole) host. It is shown that FOLEDs produced with buckled patterns can exhibit a luminance as high as ca. 14,900 cd/m2 with a peak efficiency of 50.5 cd/A. The patterned structure formed by the buckling of the EL layer allows FOLEDs to be produced with a high peak external quantum efficiency of 15% with an increase in light extraction by a factor of ca. 3.1. These results show that spontaneous buckling yields patterned structures that offer considerable promise for the production of high performance, reproducible and reliable FOLEDs.

©2011 Optical Society of America

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Figures (7)

Fig. 1
Fig. 1 Schematic diagram of the fabrication process for a buckled FOLED with a PEDOT:PSS anode on a flexible PES substrate. The buckled EL layer was spontaneously formed by a thermal expansion technique.
Fig. 2
Fig. 2 (a) Transmittance spectra of PEDOT:PSS anodes on a flexible PES substrate (solid curve) and a glass substrate (dotted curve). (b) Photograph of the PEDOT:PSS layers on PES (left) and glass (right) substrates.
Fig. 3
Fig. 3 SEM analyses of buckling patterns. Upper panels: topographic images of buckled structures formed in the FOLED. Lower panels: cross-sectional views of the FOLED with buckling on PES substrate (left) and the reference OLED without buckling on glass substrate (right).
Fig. 4
Fig. 4 (a) Photograph of an operating FOLED with a buckled surface on a PES substrate at 10 V. (b) J-V (red) and L-V (blue) characteristics and (c) current efficiency (red) and power efficiency (blue) for the sample FOLED (solid curves) and the reference OLED (dotted curves).
Fig. 5
Fig. 5 (a) Normalized viewing angle dependence of EL intensity for the sample FOLED (circles) and the reference OLED (squares). The dotted line represents the Lambertian emission pattern. (b) Normalized EL spectra measured at the surface normal direction for the sample FOLED (blue) and reference OLED (red).
Fig. 6
Fig. 6 (a) EQE as a function of input power for the sample FOLED (blue) and reference OLED (red). (b) Spectral ratio of output radiant power measured by using an integrated sphere for the sample FOLED to that for the reference OLED at a fixed input power (222 mW/cm2). Inset shows the total EL spectral outputs of the radiant power from the devices by using the integrated sphere.
Fig. 7
Fig. 7 The current efficiencies (a) and power efficiencies (b) of the sample FOLED and reference OLED as functions of output luminance.
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