Enhanced light extraction for red organic light emitting diodes via PDMS convex lens based on breath figure method

To inhibit the light loss of substrate mode at the substrate/air interface in organic light emitting diodes (OLEDs), regular porous polystyrene (PS) template was fabricated via the "Breath figure" method. The structure of porous mold was optimized by adjusting the PS solution concentration from 10 to 60 mg/ml and relative humidity from 60% to 80%. The optimal porous PS mold was achieved at 50 mg/ml PS solution under 70% relative humidity. Then, elastic micro-lens arrays based on transparent elastomer polydimethylsiloxane (PDMS) were obtained via a two-step transferring process from the porous PS mold. The projection experiment exhibits excellent focusing effect of the PDMS convex lens relative to the planar PDMS film. These PDMS micro-lens films were in situ fabricated onto the external face of the red OLED device to extract substrate mode light. By using the PDMS micro-lens array with a height of 1.2 mu m and a diameter of 6.7 mu m, the OLED device demonstrates an enhancement of 32.7%, 35.6%, 46.5%, and 27.6% for maximum luminance, current efficiency, power efficiency, and external quantum efficiency, respectively, without changing its color purity. The device with PDMS micro-lens exhibits a higher intensity than an ideal Lambertian emission pattern, especially at viewing angles from 30 degrees to 60 degrees. It suggests that the introduction of external micro-lens structure significantly improves the light outcoupling efficiency.