Study of Hole-Injection Assisted Layer in Double-Layer Organic Light-Emitting Diodes by Electric-Field-Induced Optical Second-Harmonic Generation Measurement

Abstract

By using electric-field-induced optical second-harmonic generation (EFISHG) measurement, we studied the physical reason for the decrease in the threshold voltage of indium tin oxide (ITO)/N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD)/tris(8-hydroxy-quinolinato)aluminum(III) (Alq3)/Al diodes with a pentacene layer sandwiched between ITO and α-NPD layers. The amount of charge accumulated at the α-NPD/Alq3 interface was the same as that at ITO/α-NPD/Alq3/Al diodes under an applied DC voltage corresponding to the threshold voltage, although the charge accumulation proceeded faster. Results showed that the pentacene layer assists hole injection and the succeeding hole accumulation at the α-NPD/Alq3 interface, followed by the electroluminescence (EL) emission. The decrease in threshold voltage by inserting a hole-injection assisted layer such as pentacene is one way of improving the EL device performance.