NPB-DPA, full name N,N'-Bis[4-(diphenylamino)phenyl]-N,N'-di(1-naphthyl)benzidine, is a derivative of triarylamines. It has a skeleton of two joined triarylamines with two diphenylamine as pending units. It is electron-rich and commonly used as a hole-injection layer (HIL) material in OLED devices.
NPB is normally used as a hole-transporting layer (HTL) material. In comparison, NPB-DPA is inserted between an ITO anode and a HTL to effectively modify the cathode surface and facilitate the collection of the holes to the electrode.
Like NPB-DPA, NPB's sister compound NPNPB is also commonly used as the buffer layer between a HTL and an anode to lower the barrier of the hole injections.
* Measurable with an optical spectrometer