Higher Light Extraction Efficiency in Organic Light-Emitting Devices by Employing 2D Periodic Corrugation

Photons trapped in the form of waveguide (WG) modes associated with the organic–organic interface and in the form of surface plasmon polariton (SPP) modes associated with the metallic electrode–organic interface result in a large energy loss in organic light-emitting devices (OLEDs). Introducing gratings onto the metallic electrode is especially crucial for recovering the power lost to the associated SPP modes. In our research, we demonstrate the efficient outcoupling of SPP modes in TE mode by two-dimensional (2D) grating, which cannot excited in one-dimensional (1D) grating OLED. This causes a 62.5% increase in efficiency from 2D grating OLED than 1D grating OLED. The efficient outcoupling of the WG and SPP modes is verified by the numerical simulation of both the emission spectra and the field distribution.

Realization of Efficient Phosphorescent Organic Light-Emitting Devices Using Exciplex-Type Co-Host

High-performance phosphorescent organic light-emitting devices with an exciplex-type co-host were fabricated. The co-host is constituted by 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene, and 4,4,4-tris (N-carbazolyl) triphenylamine, and has obvious virtues in constructing efficient devices because of the thermally activated delayed fluorescence (TADF) resulting from a reverse intersystem crossing (RISC) process. The highest external quantum efficiency and luminance are 14.60% and 100,900 cd/m2 for the optimal co-host device. For comparison, 9.22% and 25,450 cd/m2 are obtained for a device employing 4,4,4-tris (N-carbazolyl) triphenylamine as a single-host. Moreover, the efficiency roll-off is notably alleviated for the co-host device, indicated by much higher critical current density of 327.8 mA/cm2, compared to 120.8 mA/cm2 for the single-host device. The alleviation of excitons quenching resulting from the captured holes and electrons, together with highly sufficient energy transfer between the co-host and phosphorescent dopant account for the obvious boost in device performances.

Hole Injection Role of p-Type Conjugated Polymer Nanolayers in Phosphorescent Organic Light-Emitting Devices

Here, we report the hole injection role of p-type conjugated polymer layer in phosphorescent organic light-emitting devices (OLEDs). Poly(3-hexylthiophene) (P3HT) nanolayers (thickness = ~1 nm thick), which were subjected to thermal annealing at 140 °C by varying annealing time, were inserted between indium tin oxide (ITO) anodes and hole transport layers (N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine, NPB). The 1 nm-thick P3HT layers showed very weak absorption in the visible light range of 500~650 nm. The device results disclosed that the presence of P3HT layers were just able to improve the charge injection of OLEDs leading to an enhanced luminance irrespective of thermal annealing condition. The highest luminance and efficiency were achieved for the OLEDs with the P3HT layers annealed at 140 °C for 10 min. Further annealing for 30 min resulted in turn-down of device performances. The emission color was almost unchanged by the presence of P3HT layers even though the color coordinates were marginally fluctuated according to the annealing time. The present result delivers the possibility to use p-type conjugated polymers (i.e., P3HT) as a hole injection layer in OLEDs.