Joule heat-induced breakdown of organic thin-film devices under pulse operation

We investigated the influence of the substrate's thermal conductivities (k) and the widths of the electrical pulses (τ_pulse) on the maximum current densities (J_max) 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 J_max. For a long τ_pulse of 5 μs, J_max 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 τ_pulse. Operation of devices with high-k sapphire substrates for a short τ_pulse of 70 ns resulted in further relaxation of ΔT, leading to an increase of J_max to around 5 kA/cm². Interestingly, for such a short τ_pulse, devices with high-k sapphire and low-k plastic substrates showed similar J_max 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.