Injection-limited and space charge-limited currents in organic semiconductor devices with nanopatterned metal electrodes
Charge injection at metal-organic interfaces often limits the electric current in organic light-emitting diodes without additional injection layers. Integrated nanopatterned electrodes may provide a way to overcome this current injection limit by local field enhancements leading to locally space charge-limited currents. We compare electrical characteristics of planar and nanopatterned hole-only devices based on the charge transport material NPB with different thicknesses in order to investigate the nanopattern's effect on the current limitation mechanism. Integration of a periodic nanograting into the metal electrode yields a current increase of about 1.5-4 times, depending on thickness and operating voltage. To verify the experimental results, we implement a finite element simulation model that solves the coupled Poisson and drift-diffusion equations in a weak form. It includes space charges, drift and diffusion currents, nonlinear mobility, and charge injection at the boundaries. We find in experiment and simulation that the planar devices exhibit injection-limited currents, whereas the currents in the nanopatterned devices are dominated by space charge effects, overcoming the planar injection limit. The simulations show space charge accumulations at the corners of the nanopattern, confirming the idea of locally space charge-limited currents.