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

Abstract

Abstract Studies related to emitting dipole orientation (EDO) have been widely carried out to obtain highly efficient organic light-emitting diodes (OLEDs) in a bottom-emitting structure. Herein, we report on the relation between EDO and the forward luminous efficiency of top-emitting OLEDs (TEOLEDs). The forward emission of OLED device is solely contributed by the horizontal dipole. However, the effect of horizontal and vertical dipole on the various optical modes in OLEDs structure is significantly different, resulting in changing the Purcell factor for the ratio of EDO. Therefore, as horizontally oriented dipole ratio (HODR) is increased, the efficiency is also increased but is not linearly increased. When Ir(ppy)2(acac) with predominantly horizontal EDO (74% HODR) 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 (68% HODR), which is larger than the HODR difference between two dopants.