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Enhancing light extraction efficiency of polymer light-emitting diodes with a 12-fold photonic quasi crystal.

This work demonstrates the enhancement of light extraction of polymer light-emitting diodes (PLEDs) by incorporating a 12-fold photonic quasi crystal (PQC) in the device structure. Multi-exposure two-beam interference technique combined with inductively coupled plasma etching was employed to pattern a 12-fold PQC structure on the ITO film on a glass substrate of the diode. The air-hole coverage (AHC) and etching depth dependences of the light emitting performance of the 12-fold PQC patterned PLEDs were investigated. For AHC within the range between 6.4% and 32.3%, a nearly constant enhancement of the luminance efficiency of the PQC PLEDs was observed. On the other hand, the light emitting performance of the PQC PLEDs is very sensitive to the etching depth. The photoluminescence intensity of the PQC PLEDs increases monotonically with the etching depth. In contrast, the electro luminance efficiency shows a non-monotonic dependence on etching depth with a maximum occurring at 55 nm etching depth. The maximum improvement of luminance efficiency of the 12-fold PQC PLEDs reaches nearly 95% compared with an un-patterned PLED at an injection current of 110 mA.