Charge transport in a π-conjugated polymer: Generalized Langevin equation analysis

The complex dielectric constants of polymer light-emitting diodes using poly @2-methoxy-5-~2’-ethylhexyloxy!-1,4-phenylenevinylene# were measured, and the generalized Langevin equation was used to analyze the dielectric behavior in the frequency domain. We also proposed appropriate fitting functions for the voltage dependence of the fitting parameters. We confirmed that the generalized Langevin equation offers a very good approach to analyze and understand the transport properties of charge carriers in a p-conjugated polymer. A typical polymer light-emitting diode ~LED! consists of a thin layer of conjugated polymer sandwiched between two electrodes on top of a glass substrate. Under forward dc bias, electrons and holes are injected from the cathode and the anode, respectively, into the polymer. The injected charge carriers move through the polymer due to the applied external electric field over a certain distance until recombination takes place. Thus the operation of the polymer LED could be considered in three processes: injection, transport, and recombination of the charges. 1 To improve the performance of polymer LED’s, it is vital to understand which mechanism~s! control~s! the current density-voltage ( J-V) characteristics of a given device structure. 2 Transport at dc or at very low frequencies requires a kind of percolating network of transition, in which the weakest links ~intrachain connection! determine the magnitude of the conductivity. At highfrequencies, the charge carriers become localized in small regions of low-energy barriers. In this context, studying the conductivity in the frequency domain is essential due to its spectroscopic character. In practice also, for the pulse operation of the LED, we have to understand the current-voltage ( I-V) characteristics in the frequency domain, i.e., the relaxation process should be studied. Recently, ionic motions in amorphous LiNbO3 ,KNbO3, and PbTiO3 were described by using a modified generalized Langevin equation. 3 In this article, we analyzed the frequency-dependent dielectric constant of an indium tin oxide ~ITO!-poly@2-methoxy-5-~2’-ethyl-hexyloxy !-1,4phenylenevinylene #~ MEH-PPV!-aluminum ~Al! device using the generalized Langevin equation. Based on the results of this analysis, the conduction mechanism of the charge carrier is discussed. Polymer films were obtained by spin coating the filtered polymer solution onto the substrates with an ITO electrode. Then the polymer films on the ITO/glass substrates were subjected to a heat treatment in a vacuum oven for an hour. Aluminum was vapor deposited as the cathode at a working pressure below 4310 26 Torr, yielding an active size of 5-mm diameter. The I-V characteristics and the impedances of the samples were measured by a source-measurement unit ~Keythley model 236! and impedance gain/phase analyzer~SI1260, Solatron!, respectively. Figure 1 shows the I-V characteristics of the ITO/MEHPPV/Al device. At low voltages, up to around 1 V, the current is Ohmic with a slope equal to one, while at higher voltages, the current is bulk limited with slopes .3.3. Figs. 2~a! and 2~b! show the frequency-dependent real and imaginary part of the dielectric constants of the ITO/MEH-PPV/Al device, respectively, calculated from the impedance data. As shown in Fig. 2~a!, the real part of the capacitance does not change up to around 10 5 Hz and relaxes around 10 6 Hz. However, a strong frequency-dependent behavior in the imaginary part of the dielectric constants, i.e., low-frequency dispersion, is observed as seen in Fig. 2~b!. This indicates that the effect on the conduction of the remaining ions used during the polymerization process of MEH-PPV is negligible, and that only the hole is a major charge carrier. 4