Charge injection into cathode-doped amorphous organic semiconductors

The current-voltage characteristics of electron injection from metals into organic semiconductors often follows a power law, $J\sim V^m$. At low temperatures, we observe that the power-law slope $m=(20\pm 1)$ is conserved for many combinations of cathode metals and small molecular weight organic semiconductors. The material independence of injection characteristics at low temperature is attributed to the effect of interface roughness on the image potential. At high temperatures, different cathode materials affect injection primarily by varying the density of free charge in the organic semiconductor. A simple analytic model describes the current-versus-voltage characteristics of these doped interfaces over a wide range of temperatures.