By using electric field induced optical second harmonic generation (EFISHG) and current voltage (I–V) measurements, we studied the electrical transport mechanism of organic doublelayer diodes with a structure of Au/N, N′-di-[(1-naphthyl)-N, N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine
(α-NPD)/poly(methyl methacrylate) (PMMA)/indium zinc oxide (IZO). Here the α-NPD is a carrier transport layer and the PMMA is an electrical insulating layer. The current level was very low, but the I–V characteristics showed a rectifying behavior.
The EFISHG measurement selectively and directly probed the electric field across the α-NPD layer, and showed that the electric field across the α-NPD layer is completely relaxed owing to the charge accumulation at the α-NPD/PMMA interface in the region
V>0, whereas the carrier accumulation was not significant in the region V<0. On the basis of these experimental results, we proposed a model of the rectification. Further, by coupling the I–V characteristics with the EFISHG measurement, the I–V
characteristics of the diodes were well converted into the current-electric field (I–E) characteristics of the α-NPD layer and the PMMA layer. The I–E characteristics suggested the Schottky-type conduction governs the carrier transport.
We conclude that the I–V measurement coupled with the EFISHG measurement is very useful to study carrier transport mechanism of the organic double-layer diodes.
