2, The Research Institute for Molecular Electronic Devices, Osaka Prefecture University, Sakai, , Japan
Determination of transport properties of charge carriers gives important information for designing organic semiconductor devices such as organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). In this presentation, we studied the carrier transport properties -electron and hole drift mobilities, bimolecular recombination coefficients, and localized-state distributions- in working OLEDs in terms of impedance spectroscopy (IS). IS has been a powerful tool to study the carrier transport properties of thin-film devices with thicknesses of ~100 nm, typical active layer thickness of working OLEDs and OPVs.
The OLED configuration we studied was aluminum doped zinc oxide: AZO (150 nm)/polyethyleneimine: PEI/poly(9,9-dioctylfluore-ne-alt-benzothiadiazole): F8BT (300 nm)/ molybdenum oxide: MoO3 (10 nm)/Al (50 nm). IS measurement was carried out using a Solartron 1260 impedance analyzer with a 1296 dielectric interface in the frequency sweep range from 1 Hz to 1 MHz.
The transport properties that we found in the OLEDs are as follows: the electron and hole mobilities of the OLEDs at 300 K are 10-3 cm2V-1s-1 and 10-5 cm2V-1s-1, respectively, and are consistent with those of F8BT in literature. The bimolecular recombination coefficients are determined from the imaginary part of complex impedance spectra above the luminescence turn-on voltage. The bimolecular recombination coefficients of the OLEDs are 10-13-10-12 cm3s-1, which is 10-3-10-2 times lower than the Langevin recombination coefficient. The localized-tail-state distributions from the conduction band and the valence band mobility edges of F8BT are determined from the electric field dependences of the activation energy of the mobilities. Both localized-tail-state distributions are well described by Gaussian distributions.
Information concerning the carrier transport properties of F8BT is essential for the design of organic devices, for the analysis of degradation processes, and for the understanding of device physics. For instance, we showed that strong thickness dependence of the current efficiency of F8BT OLEDs is well reproduced by a device simulation (Silvaco, Atlas) using the transport properties obtained in this study as inputs for the simulation.