To further improve the efficacy of hybrid WOLEDs, light extraction is very necessary because a large number of generated photons (70–80%) are trapped inside the device [71], [72] and [73]. It should be pointed out that the light extraction for white emission should give consideration to the optical performances as CIE and CRI. Reineke and co-workers have reported a good light out-coupling structure based on a hemisphere for WOLEDs [9]. Using this technique, a hybrid WOLED with 63.6 lm/W at 1967 cd/m2 was achieved by Loebl et al. [74], and another one with the efficacy up to 90.5 lm/W (improved by a factor of 2.9) at 1000 cd/m2 was achieved by Rosenow et al. [75].
Another challenge for commercial applications is scaling hybrid WOLEDs to large areas. The efficacy, lifetime and color uniformity for large-area devices are quite different from those of devices with small areas (a few square millimeters, for example). The key issue is to ensure that the square resistance of both electrodes in the large devices is low enough to avoid the increase of operation voltage and the heterogeneity of current across the active area. Encouragingly, successful techniques, such as introducing a metal grid on top of ITO and employing silver nanowire material as transparent electrode, have been used to improve the performance of large-area OLEDs. Fig. 11 shows the prototypes of a hybrid WOLED fabricated in our lab with the metal grid consisting of Mo/Al/Mo. The panel size is 7 cm × 7 cm.
To further improve the efficacy of hybrid WOLEDs, light extraction is very necessary because a large number of generated photons (70–80%) are trapped inside the device [71], [72] and [73]. It should be pointed out that the light extraction for white emission should give consideration to the optical performances as CIE and CRI. Reineke and co-workers have reported a good light out-coupling structure based on a hemisphere for WOLEDs [9]. Using this technique, a hybrid WOLED with 63.6 lm/W at 1967 cd/m2 was achieved by Loebl et al. [74], and another one with the efficacy up to 90.5 lm/W (improved by a factor of 2.9) at 1000 cd/m2 was achieved by Rosenow et al. [75].
Another challenge for commercial applications is scaling hybrid WOLEDs to large areas. The efficacy, lifetime and color uniformity for large-area devices are quite different from those of devices with small areas (a few square millimeters, for example). The key issue is to ensure that the square resistance of both electrodes in the large devices is low enough to avoid the increase of operation voltage and the heterogeneity of current across the active area. Encouragingly, successful techniques, such as introducing a metal grid on top of ITO and employing silver nanowire material as transparent electrode, have been used to improve the performance of large-area OLEDs. Fig. 11 shows the prototypes of a hybrid WOLED fabricated in our lab with the metal grid consisting of Mo/Al/Mo. The panel size is 7 cm × 7 cm.
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