Non-linear relation between emissive dipole orientation and forward luminous efficiency of top-emitting organic light-emitting diodes

We report on the effect of emitting dipole orientation (EDO) on the improvement of the optical characteristics of top-emitting OLEDs. When Ir(ppy)2(acac) with predominantly horizontal EDO was applied to top-emitting OLEDs, the device efficiency in the normal direction was improved by 26%, compared with OLEDs based on Ir(ppy)3 with isotropic EDO. Angular emission distribution is determined dominantly by the horizontal dipole and thus, the full width at half maximum (FWHM) of photoluminescence (PL) spectrum is more important than EDO. As FWHM of PL of Ir(ppy)2(acac) is narrower than that of Ir(ppy)3, the change of CIE coordinates of top-emitting OLEDs based on Ir(ppy)2(acac) is (0.033, 0.021), which is less than that of top-emitting OLEDs based on Ir(ppy)3, i.e., (0.042, 0.021).183