Low light outcoupling efficiency restricts the wide application of organic light-emitting diodes in solid state light market although the internal quantum efficiency of the device could reach near to 100%. In order to improve the output efficiency, different kinds of microlens array on the substrate emission surface were designed and simulated using light tracing method. Simulation results indicate that the microlens array on the substrate could efficiently improve the light output efficiency and an enhancement of 1.8 could be obtained with optimized microlens structure design. The microlens array with semicircle shape using polymer material was fabricated on glass substrate by a facile approach. Finally, the organic device with microlens array substrate was manufactured and the light output of the device with surface microlens structure could increase to 1.64 times comparing with the device without microlens. 1. Introduction Due to high power efficiency, simple fabrication processes, and wide-color covering, organic light-emitting diodes (OLEDs) are the focus of attentions in applications of display and solid state lighting [1, 2]. Because of the difference among the refractive indices of the substrate, anode, organic materials, and the air, two wave guiding phenomena become obvious in the anode/organic layers and substrate/air layers of OLEDs [3]. Only 20% of the total emitted light from the organic layers can escape into the front side of substrate to form useful emission [4, 5]. Therefore the greatest potential for enhancing OLEDs external efficiency is to improve the light outcoupling from substrate. Many different approaches had been done with the aim of optimizing the light outcoupling for OLEDs, including low-index grids [6, 7], periodic corrugated structure [8], Bragg mirrors [9, 10], buckling patterns [11], photonic crystals [12, 13], antireflection coatings [14], and monolayer of SiO2 microparticles [15]. Those methods focused on changing the contact surface of glass substrate with organic device, which were also complicated to achieve them. Different kinds of microlens array on the substrate surface have also been employed in some researches and showed obvious enhancement in device efficiency. The microlens array using prepolymer NOA65 material was fabricated on the substrate which was prepatterned by microcontact printing of hydrophobic self-assembled monolayers, and the light outcoupling efficiency was improved by 24.5% without any apparent color change [16]. Spherical microlens pattern using photoresist material was designed on the
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