Effect of hole-transport layer thickness on the performance of organic light-emitting diodes

In this study, we investigated the influence of hole-transport layer thickness (0, 5, 10, 15, 20, 25, 30, 35 and 40 nm), N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-2,2'-dimethylbenzidine (alpha-NPD) on the performance of thermally evaporated multilayer organic light-emitting diodes (OLEDs) with tris(8-hydroxy-quinolinato)aluminium (Alq(3)) as electroluminescent material. A device with 5 nm alpha-NPD thickness showed a current density similar to 30 times higher than without any alpha-NPD layer. However, the current density decreased as the alpha-NPD thickness increased beyond 5 nm. In contrast to the current density, the device efficiency improved as alpha-NPD layer thickness increased to 30 nm, after which a drop in efficiency was observed. The current efficiency of the device with 30 nm alpha-NPD thickness was 10 times higher than the device with 5 nm alpha-NPD thickness. The results indicated that hole-injection in OLEDs was more efficient with thin 5 nm alpha-NPD, while optimal efficiency was at 30 nm of alpha-NPD thickness.