Impedance spectroscopy of OLEDs as a tool for estimating mobility and the concentration of charge carriers in transport layers

The article presents a novel impedance spectroscopy based technique designed to characterise organic light emitting diodes (OLED) during operation. A number of devices based on NPB (N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine) and TPBi (2,2',2 ''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) conducting layers, CBP (4,4'-bis(N-carbazolyl)-1,1'-biphenyl) as a host and BPTZ-DBTO2 (3,6-bis(10H-phenothiazine-10-yl)-9H-fluorene-[dibenzothiophene-S, S-dioxide]) as an emitter were taken as an example to validate the method. Using equivalent electrical circuits, values of capacitance and conductivity of both hole-(HTL) and electron-transport (ETL) layers were determined and analysed separately. Those parameters, as well as current-voltage characteristics, were used to calculate (i) mobility (mu, cm(2) V-1 s(-1)) of both types of charge carriers, (ii) charge density (n, cm(-3)) of the charge carriers, and (iii) electric field intensity (E, V m(-1)) in both transport layers as functions of the applied voltage.