Very bright and efficient ITO-free narrow-spectrum micro-cavity top-emitting organic light-emitting diodes with low operation voltage

In this work, the efficacy of the micro-cavity effect in achieving high-performance top-emitting organic light-emitting diodes (TE-OLEDs) and adjusting the emission spectrum was experimentally confirmed. By optimizing the electrode thickness, cavity length and emission angle one by one, highly efficient green TE-OLEDs were obtained. A strong dependence of electroluminescence (EL) performances and spectral characteristics on the cavity length was observed. Interestingly, these devices displayed a significant angular dependence of the EL spectrum. Ultimately, a tremendous improvement (nearly 47%) in current efficiency was realized in the case of TE-OLEDs (502 nm) compared with bottom-emitting organic light-emitting diodes (BE-OLEDs) (507 nm). In particular, the device with the 516 nm emission peak exhibited external quantum efficiency, current efficiency and power efficiency as high as 47.4%, 168.59 cd A(-1) and 186.38 lm W-1, respectively. In addition, TE-OLEDs displayed a narrower emission spectrum, and thus improved color purity. By controlling the Ag layer thickness, a minimum full width at half maximum (FWHM) of 18 nm was realized.