Organic light-emitting diodes with 30% external quantum efficiency based on a horizontally oriented emitter

Phosphorescent organic light-emitting diodes (OLEDs) with ultimate efficiency in terms of the external quantum effi ciency (EQE), driving voltage, and effi ciency roll-off are reported, making use of an exciplex-forming co-host. This exciplex-forming co-host system enables effi cient singlet and triplet energy transfers from the host exciplex to the phosphorescent dopant because the singlet and triplet energies of the exciplex are almost identical. In addition, the system has low probability of direct trapping of charges at the dopant molecules and no charge-injection barrier from the charge-transport layers to the emitting layer. By combining all these factors, the OLEDs achieve a low turn-on voltage of 2.4 V, a very high EQE of 29.1% and a very high power effi ciency of 124 lm W − 1 . In addition, the OLEDs achieve an extremely low effi ciency roll-off. The EQE of the optimized OLED is maintained at more than 27.8%, up to 10 000 cd m − 2 .

Ancillary ligands in heteroleptic iridium complexes significantly influence the orientation of the transition dipole moments. Ir(ppy)3, a homoleptic iridium complex, exhibits isotropic dipole orientation, whereas the heteroleptic Ir complexes of Ir(ppy)2tmd show a highly preferred dipole orientation (78%) in the horizontal direction. In addition, we demonstrate an unprecedented highly efficient green OLED exhibiting an EQE of 32.3% and a power efficiency of 142.5 lm/W by using an emitter with high quantum yield and horizontally oriented dipoles.

Extremely high light out-coupling efficiency from a transparent organic light-emitting diode integrated with microstructures on both sides of the device is reported. The metal free device offers dramatically reduced surface plasmonic and intrinsic absorption losses. Moreover, high refractive index micropatterns with optimal light extraction condition are fabricated based on the well-matched analysis of optical simulations.

The bulk‐ionized photoconductivity of C60 is reported as an origin of the bias‐dependent linear change of the photocurrent in copper phthalocyanine (CuPc)/C60 planar heterojunction solar cells, based on the observation of the variation of the bias‐dependent photocurrent on excitation wavelengths and the thickness‐dependent photocurrent of the C60 layer. A theoretical model, which is a combination of the Braun‐Onsager model for the dissociation of excitons at the donor/acceptor interface and the Onsager model for the bulk ionization of excitons in the C60 layer, describes the bias‐dependent photocurrent in the devices very well. The bulk‐ionized photoconductivity of C60 must generally contribute to the photocurrent in organic photovoltaics, since fullerene and fullerene derivatives are widely used in these devices.