The roles of injection and mobility in organic light emitting diodes

Numerical methods have been used to solve the bipolar current problem for a single emitting layer between electrodes with explicit injection characteristics. We consider ohmic and tunneling contacts at the anode and/or cathode for various ratios of hole to electron mobility. Diffusion is included and found to have minimal effect on the recombination efficiency. The recombination profile is dictated mainly by the ratio of mobilities, less so by contacts. Maximum efficiency is obtained for two ohmic contacts. When the injection is imbalanced, higher efficiencies are achieved when the majority carrier has the lower mobility. At sufficiently high voltages, the current tends towards balance, allowing for maximum efficiency.