Measurement Of Solar Cell AC Parameters Using Impedance Spectroscopy
Photovoltaic (PV) conversion of solar energy appears to be one of the most promising ways of meeting the increasing future energy demand. In space, photovoltaic power source is the only alternative. The demand for higher power has necessitated the use of high speed switching charge controller and power conditioner. To design an efficient and reliable switching charge controller, the static (I-V) and dynamic (AC) characteristics of a solar cell need to be understood. The AC parameters of a solar cell can be measured either by Frequency Domain technique or by Time Domain technique. In frequency domain technique, a small signal is applied about the operating point and the AC parameters are measured. Hence, in the frequency domain technique the steady state values of AC parameters at a particular operating condition are measured. In time domain technique, a transient measurement is made where the cell voltage varies from short-circuit to open circuit or vice versa. Hence, this technique gives only the time constant of a solar cell. The impedance spectroscopy is a frequency domain technique widely used in electro chemistry to study battery characteristics. In the present investigation, the impedance spectroscopy is proposed for measuring the AC parameters of solar cells. An experimental set-up has been developed to measure the solar cell AC parameters. The AC parameters of Silicon (BSR and BSFR) solar cells and GaAs/Ge solar cells are measured using impedance spectroscopy (IS). The cell capacitance, the parallel resistance and the series resistance are measured and compared. GaAs/Ge solar cell has shown only transition Capacitance throughout its operating range while silicon (BSR and BSFR) solar cells exhibited both transition and diffusion capacitances. Theoretical and experimental values of the cell parallel resistance are compared and are in good agreement. While the diode factor in silicon solar cell varies from 2 to 1, where as in GaAs/Ge solar cell it varies from 4 to 2 to 1. Measurements conducted using open circuit voltage buildup (time domain technique) on silicon BSR solar cell shows that the collected data can be used for the restricted purpose of measuring cell transient response. The dime domain technique could not estimate the solar cell. It may be noted that the impedance spectroscopy assumes piece-wise linearity of the solar cell characteristics, lending itself for easy measurement and modeling. This assumption is valid as the signal amplitude is less than thermal voltage (VT). Since, the parameters are measured under steady state, the values are more stable and accurate. An attempt has also been made to correlate the measured AC parameters with the requirements of switching charge controllers. These correlations can be used to design the switching controllers for device rating, circuit stability and other aspects.