How to characterize capacitance of organic optoelectronic devices accurately

Abstract

The selection of circuit model (i.e., parallel or series model) is critical when using a capacitance-frequency and capacitance-voltage technique to probe properties of organic materials and physical processes of organic optoelectronic devices. In the present work, capacitances of ITO/Alq 3 /Al and ITO/CuPc/Al are characterized by series and parallel model, respectively. It is found that the large series resistance comes from the ITO electrode and results in the inapplicability of the parallel model to measuring the capacitances of organic devices at high frequencies. An equivalent circuit model with consideration of the parasitical inductance of cables is constructed to derive the capacitance, and actual capacitance-frequency spectra of Alq 3 and CuPc devices are obtained. Further investigation of temperature-dependent capacitance-frequency and capacitance-voltage characteristics indicates that CuPc and Al form the Schottky contact, the density and ionization energy of impurities in CuPc are obtained. Moreover, more practical guidelines for accurate capacitance measurement are introduced instead of the impedance magnitude, which will be very helpful for the organic community to investigate capacitance-related characteristics when dealing with various organic optoelectronic devices.

Accurate capacitance–voltage characterization of organic thin films with current injection*

“…[22,23] However, rather than the direct obtainment of capacitance, the magnitude and phase of impedance are firstly measured with an LCR meter, subsequently the capacitance of device is calculated according to the circuit model selected. [24,25] Therefore, of upmost importance in maintaining the reliable and accurate capacitance measurement is the selection of a proper circuit model. The series model (consisting of a capacitance C and a parasitic resistance R S in series, highlighted by the blue box in Fig.…”

“…1(a)) are two typical equivalent circuit models in capacitance measurement, [24][25][26][27] between which, the parallel model is more commonly employed in organic thin films since the series resistance R S is negligible compared with the parallel resistance R P (sufficiently high) and the capacitive impedance in a usual measuring frequency range from 10 Hz to 100 kHz. [24,25] However, when the measuring frequency is high enough (exceeding 100 kHz), the capacitive impedance decreases reciprocally with the increase of measurement frequency and drops down to hundreds of Ω or less, the parallel resistance R P becomes negligible while the existence of series resistance R S (with a typical value approaching to 100 Ω in organic devices) [25] can significantly influence the measured capacitance according to the parallel model. At the moment, series model is more accurate approximation at high frequency when R S is a considerable voltage divider, as Carr et al have already discussed.…”