Charge Injection into Cathode-Doped Amorphous Organic Semiconductors

We analyze electron injection at a wide variety of metal-organic semiconductor interfaces, and find remarkably universal characteristics at low temperature. The current voltage characteristics at T = 10 K follow power-law behavior, $J \quad \sim \quad V^{m}$, where $m $=~(20+/-1) for over 15 combinations of the metals Li, Mg, Al, Ag and Au and the organic semiconductors Alq$_{3}$, BCP, CBP, TAZ, and CuPC. The material independence of injection characteristics at low temperature is attributed to the effect of interface roughness on the image potential. We develop an analytic model to explain charge injection into organic semiconductors at disordered interfaces.