How to characterize capacitance of organic optoelectronic devices accurately

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/Alq3/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 Alq3 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.

Direct Digital Frequency Control Based on the Phase Step Change Characteristic between Signals

We present a new digital phase lock technology to achieve the frequency control and transformation through high precision multi-cycle group synchronization between signals without the frequency transformation circuit. In the case of digital sampling, the passing zero point of the phase of the controlled signal has the phase step characteristic, the phase step of the passing zero point is monotonic continuous with high resolution in the phase lock process, and using the border effect of digital fuzzy area, the gate can synchronize with the two signals, the quantization error is reduced. This technique is quite different from the existing methods of frequency transformation and frequency synthesis, the phase change characteristic between the periodic signals with different nominal is used. The phase change has the periodic phenomenon, and it has the high resolution step value. With the application of the physical law, the noise is reduced because of simplifying frequency transformation circuits, and the phase is locked with high precision. The regular phase change between frequency signals is only used for frequency measurement, and the change has evident randomness, but this randomness is greatly reduced in frequency control, and the certainty of the process result is clear. The experiment shows that the short term frequency stability can reach 10-12/s orders of magnitude.