Two-dimensional TiO₂ inverse opal with a closed top surface structure for enhanced light extraction from polymer light-emitting diodes.

Light-emitting diodes (LEDs), including organic LEDs and polymer LEDs, have been studied intensively for use in displays and interior lighting due to their promising advantages of high effi ciency, high contrast ratio, long lifetime and low power consumption. [ 1 , 2 ] However, in a conventionally structured LED, less than 20% of the generated light is emitted as useful radiation. [ 3 , 4 ] Not surprisingly, the improvement of light extraction effi ciency is currently considered to be one of the most important issues for LED research. A variety of strategies have been suggested towards this end, including modifi ed substrates, microlenses, silica microspheres, silica aerogels [ 5–8 ] and twodimensional (2D) photonic crystals (PhCs). [ 9–11 ]