Study of Electrical Conduction Mechanism of Organic Double-Layer Diode Using Electric Field Induced Optical Second Harmonic Generation Measurement
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.