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According to simulated calculations, in our bottom emitting OLED devices, the outcoupling efficiency (ηOUT) is limited to only 17% of light escaping into free space due to light lost as substrate guided (22%), organic waveguided (12%) and evanescent modes (31%) and the air/substrate, substrate/organic and organic/metal interfaces respectively [1]. To extract substrate guided modes, our previous work showed that OLEDs fabricated over sample glass patterned substrates, can simultaneously enhance the ηOUT and reduce view angle dependency [1]. This represents an improvement on past research efforts, wherein some outcoupling structures tended to induce strong viewing angle dependence [2] and those intended suppress this, had no effect on ηOUT [3]. Previous research for extracting organic waveguides, however, may involve complex nm-scale fabrication techniques resulting in 20-25% enhancement in ηOUT [4-5]. In this work, we demonstrate that a novel pattern of ITO-coated glass substrate can extract up to 30.8% of organic waveguided modes, by micrometer-scale patterning of both the ITO and glass beneath the ITO.
Using the simulation programme, Setfos, existing patterns at the air/substrate interface were first replicated and then manually optimized in terms of depth, diameter, edge-to-edge distance, the shape of holes and their packing arrangement. A combination of the most optimized dimensions then resulted in a maximum enhancement in ηOUT by 30.36%. Devices fabricated with similar pattern dimensions at the air/substrate interface (albeit slight differences) showed a 27.3% enhancement as a result of extracted substrate guided modes. The substrate/organic side of the device was also patterned with similar dimensions as mentioned above and resulted in a 30.8% enhancement of ηOUT due to extracted organic waveguided modes being redirected as leaky modes, wherein light is then able to escape into the forward viewing direction.
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