Impedance spectroscopy has been used to investigate various electrochemical devices and electrode reactions. The principle of impedance spectroscopy is explained in this article. The relationships between simple circuits and the impedance spectra are revealed using Nyquist and bode plots. Impedance spectra of an electrode/electrolyte interface are obtained as a semicircle on the Nyquist plot. The equivalent circuit for this interface involves the charge transfer resistance and the interfacial capacitance in parallel, and comparison of this circuit with the impedance spectra provides the structural and physical information concerning the interface.
Impedance spectroscopy is a powerful tool for the characterization of organic devices such as organic light-emitting diodes and organic solar cells. Theoretical background for the determination of charge carrier mobilities and localized-state distributions from the impedance spectra of the organic devices is described. The applicability of impedance spectroscopy is demonstrated in polyfluorene-based organic light-emitting diodes.
Carrier behaviors in organic field effect transistors (OFETs) have been analyzed based on dielectric physics. Maxwell-Wagner model analysis accounts for carrier accumulation and transport in OFETs, and electric field-induced optical second harmonic generation method (EFISHG) visualizes dynamic carrier motion by probing electric field generated from carriers injected from electrode. Impedance spectroscopy (IS) and charge modulation spectroscopy (CMS) are also available for probing carriers in OFETs, and effective for modeling carrier behaviors in OFETs.
We present the impedance analysis of multi-layered organic electronic devices. We have developed a new analysis technique, Dynamic Modulus Plot (DMP), which is the combination of C-V measurement and Cole-Cole plot. It is applied to analyze the anomaly of color index change against the current density of a stacked Red/Green phosphorescent Organic Light Emitting Diode (OLED). DMP shows that electrons accumulate in the green emitting layer below the turn-on voltage. The anomaly of the color index is attributed to the quench of the triplet excitons by the electrons accumulated in the green emitting layer. The color index anomaly becomes less when the accumulation is reduced by optimizing the host material of the green emitting layer. We demonstrate that DMP is a useful method to investigate the layer-to-layer carrier dynamics of separated function devices.
AC impedance spectroscopy is a powerful and convenient technique to investigate the elemental reaction at electrode | electrolyte interface in rechargeable lithium ion battery. This paper describes fitting procedure of impedance plots and assignment of each separated resistance to elemental reaction. In particular, the elemental reaction occurred at A-site deficient perovskite, La1/3NbO3, electrode is focused in this paper, since no significant side-reaction was observed due to mild voltage range from 2 to 1 V vs. Li+/Li. The results indicated two elemental reactions at the interface of electrode | electrolyte corresponding to higher and lower frequency region in AC impedance spectra, and attempts has been made to be described in terms of an ‘adatom model’.
Open-circuit voltage (VOC) plays a key role of determining the power conversion efficiency for solar cells, as well as short-circuit current. However, the origins of VOC have still remained unsolved for organic solar cells. Although VOC has been reported to depend on the electronic states at the Donor/Acceptor (D/A) interface such as the energy difference (ΔEHL) between HOMO (the highest occupied molecular orbital) of donor and LUMO (the lowest unoccupied molecular orbital) of acceptor, some organic solar cells did not exhibit such the dependence. This is because previous discussion has been based on the D/A interfacial electronic states under “dark condition”. Recently, we have performed in situ impedance spectroscopy to examine the correlation between VOC and the electronic states (built-in potential: Vbi) in the vicinity of the D/A interface upon photo-irradiation, and found an excellent agreement between VOC and the sum of Vbi estimated from capacitance-voltage (C-V) characteristics.
Thermal nano-imprinting is a useful method for making nano-patterns onto various polymer surfaces. The clarification of the relationship between polymer properties and moldability of nano-patterns is very important in order to form the nano-patterns with very high precision and without defects. So far, reports on the moldability of nano-patterns have discussed in terms of bulk polymer properties. The objective of this research is to characterize the relationship between a surface glass transition temperature and moldability of nano-patterns by thermal nano-imprinting. The surface glass transition temperature of a polymer in nano-patterns was estimated by a model measuring method. By the scanning probe microscope measurement, it was supposed that the glass transition temperature in a nano-pattern is lower than that of a bulk polymer and declines as the pattern size becomes fine. As a result of nano-imprinting, to keep a surface glass transition temperature higher than a mold releasing temperature was required in order to make nano-patterns with very high precision and without defects.