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.

Modeling and visualization of carrier motion in organic films by optical second harmonic generation and Maxwell-displacement current

“…In more detail, the EL intensity gradually decays with the increase of the frequency of the applied ac square voltage, and reaches a minimum at the frequency corresponding to the carrier transit time of Alq3 layer. Interestingly, the EL intensity again increased by the application of further higher frequency ac square voltages, relying on the dc component of the ac square voltages [138][139][140]. Results suggest the presence of two EL modes, where the space charge field caused by the interface charge makes a significant contribution.…”

Probing and modeling of carrier motion in organic devices by electric-field-induced optical second-harmonic generation

“…This model well accounts for two EL modes generated in these sandwich-type EL devices, depending on the frequency of the applied square voltage. [76][77][78][79][80] As mentioned above, we can monitor dynamical carrier behaviors in charging and discharging processes in EL devices by EFISHG measurement. Furthermore, note that EL has also received much attention as one of the prebreakdown phenomena of insulators in the field of electrical insulation engineering for the past 10 years, before the pioneering OLED work by Tang and Van Slyke, 72) where the main research interest is to detect very weak EL signals for the diagnostics of insulators.…”

Influence of Pentacene Interface Layer in ITO/-NPD/Alq3/Al Organic Light Emitting Diodes by Time-Resolved Electric-Field-Induced Optical Second-Harmonic Generation Measurement

“…In previous work we employed EFI-SHG measurement and observed the decrease in operating voltage of OLED with an interface layer. 16 Nevertheless, we still have many debates on the energy-structure of OLEDs under operational conditions; how external applied voltage is distributed in OLEDs, how interface layer is acting as hole assisted layer and so on. This is possibly due to the difficulty in the direct probing of electric field distributions and carrier motions.…”