Micro-spherically textured organic light emitting diodes: A simple way towards highly increased light extraction
Introduction
Organic light emitting diodes (OLEDs) are already commercially available in display applications and they are also promising candidates for general lighting. OLEDs have been of great interest in research over the last years [1], [2], [3], [4] and highly efficient devices have been reported [5], [6], [7], [22]. The progress made in materials development [8], [9] has to be accompanied by new device architectures to overcome the high optical losses in an OLED. Both, the organic layers as well as the transparent conductive oxides used as electrodes in an OLED possess high refractive indices (typically 1.7–2.1 [10]). Hence, an OLED forms a slab waveguide and about 50% of the generated light is coupled to waveguide modes and surface plasmon polaritons. Another ∼30% of the generated photons are coupled into substrate modes [11], [12]. These optical losses are to date the highest loss channel in OLEDs. Different approaches have been reported to enhance the outcoupling efficiency. While coupling out substrate modes can be relatively simply achieved by the use of microstructured [13] or roughened substrate/air interfaces [14], coupling out waveguide modes is more challenging. To scatter guided modes, micro- or nanostructures underneath [15], on top [16] or implemented in the anode [17] have been reported. Other approaches such as the use of stochastic scattering via nanoparticles have been shown [18]. Many of the reported methods to enhance the outcoupling in OLEDs suffer from disadvantages, such as complicated fabrication [16], change of the electrical behavior [19] or angular spectral dependencies [20].
Here we report an elegant way to significantly enhance the efficiency of white organic light emitting diodes. By building the OLED not on a flat substrate, but on a monolayer of SiO2 microparticles, we texture the OLED spherically. This leads not only to a larger area on the same footprint but also to an enhanced outcoupling due to the curved waveguide. This method enhances the device efficiency up to a factor of ∼3.7 without changing the electrical behavior of the OLEDs. This indicates that this method addresses both optical loss channels: waveguide and substrate modes.
Section snippets
Spherical monolayer fabrication
For the fabrication of the spherical textured substrate we started with 25 mm × 25 mm glass substrates, which were subsequently cleaned with acetone and isopropyl alcohol in an ultrasonic bath for 10 min each. After cleaning, we treated the substrate with oxygen plasma for 2 min. We used monodisperse SiO2–microparticles (purchased from Microparticles GmbH) dispersed in H2O at a weight ratio of 5% and a diameter of 1.55 μm as forming spheres. We dispensed 75 μl of this solution equally distributed on
Measurements and results
The devices were characterized in an integrating sphere, which was connected with a multimode fiber to a spectrometer (Acton Research Corporation SpectraPro-300i) with an attached ICCD-camera (Princeton Instruments PiMax:512). The I–V characteristic was obtained with a source-measure unit (Keithley SMU 236), which also served as the power supply of the OLEDs. To obtain the emissive characteristic of the OLEDs the devices were operated at a constant current and rotated with respect to a fixed
Conclusion
We fabricated OLEDs on a monolayer of SiO2–microparticles resulting in spherically textured devices. These OLEDs show an improvement of the luminous efficacy by a factor of up to 3.7 without changing the I–V characteristic of the OLEDs. We attributed these effects to an enhanced area on the same footprint and to an enhanced outcoupling of waveguide and substrate modes. Goniometric measurements show, that the emissive characteristics of the devices remain unchanged.
Acknowledgements
The authors would like to thank Merck OLED Materials GmbH for material support. T.B. and F.M.-F. acknowledge generous support by the Karlsruhe School of Optics and Photonics (KSOP). The authors would like to thank Patrice Brenner from the Center of Functional Nanostructures (CFN) for the support with the FIB.
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