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We investigated the influence of the substrate's thermal conductivities (k) and the widths of the electrical pulses (τpulse) on the maximum current densities (Jmax) in organic thin-film devices. We also estimated the temperature rise (ΔT) inside devices under the pulse operation using numerical calculations to interpret the observed differences in Jmax. For a long τpulse of 5 μs, Jmax is higher for devices with high-k sapphire substrates (around 1.2 kA/cm2) than devices with low-k plastic substrates (around 0.4 kA/cm2). This is because high-k sapphire substrates can work as heat sinks to relax ΔT for such a long τpulse. Operation of devices with high-k sapphire substrates for a short τpulse of 70 ns resulted in further relaxation of ΔT, leading to an increase of Jmax to around 5 kA/cm2. Interestingly, for such a short τpulse, devices with high-k sapphire and low-k plastic substrates showed similar Jmax and ΔT values, the reason for which may be that it is difficult for the generated Joule heat to travel to the substrate across a low-k organic layer within this short time.We investigated the influence of the substrate's thermal conductivities (k) and the widths of the electrical pulses (τpulse) on the maximum current densities (Jmax) in organic thin-film devices. We also estimated the temperature rise (ΔT) inside devices under the pulse operation using numerical calculations to interpret the observed differences in Jmax. For a long τpulse of 5 μs, Jmax is higher for devices with high-k sapphire substrates (around 1.2 kA/cm2) than devices with low-k plastic substrates (around 0.4 kA/cm2). This is because high-k sapphire substrates can work as heat sinks to relax ΔT for such a long τpulse. Operation of devices with high-k sapphire substrates for a short τpulse of 70 ns resulted in further relaxation of ΔT, leading to an increase of Jmax to around 5 kA/cm2. Interestingly, for such a short τpulse, devices with high-k sapphire and low-k plastic substrates showed similar Jmax and ΔT values, the reason for which may be that it is difficult for the generated Joule heat to travel t...

We developed organic light-emitting diodes (OLEDs) with nanopatterned current flow regions using electron-beam lithography with the aim of suppressing singlet–polaron annihilation (SPA). Nanopatterns composed of lines and circles were used in the current flow regions of nano-line and nano-dot OLEDs, respectively. Excitons partially escape from the current flow regions where SPA takes place. As such, current densities where external quantum efficiencies were half of their initial values (J0) increased as line width and circle diameter were decreased to close to the exciton diffusion length. Circles were more efficient at enhancing exciton escape and increasing J0 than lines. The J0 increase in the nano-dot OLEDs containing nanopatterned circles with a diameter of 50 nm was approximately 41-fold that of a conventional OLED with a current flow region of 4 mm2. The dependence of J0 on the size and shape of the nanopatterns was well explained by an SPA model that considered exciton diffusion. Nanopatterning of...

Using e-beam nanolithography, the current injection/transport area in organic light-emitting diodes (OLEDs) was confined into a narrow linear structure with a minimum width of 50 nm. This caused suppression of Joule heating and partial separation of polarons and excitons, so the charge density where the electroluminescent efficiency decays to the half of the initial value (J0) was significantly improved. A device with a narrow current injection width of 50 nm exhibited a J0 that was almost two orders of magnitude higher compared with that of the unpatterned OLED.