DETERMINATION OF THE SOLAR CELL EQUATION PARAMETERS: NEW METHODS, EXISTING METHODS, ANALYSIS AND COMPARISON.

Abstract

Presented here is an analysis of some of the existing methods used for the determination of the series resistance of solar cells which is one of the key parameters in solar cell fabrication and technology together with the diode ideality factor and the reverse saturation current. These methods are based on the network analysis of the single-exponential lumped constant parameters model which has been accepted as being operationally sufficient to describe the current-voltage characteristics of the solar cell. The methods analyzed in this study are divided into two main groups. Methods using two I-V characteristics and methods using a single I-V output curve. For comparison purposes, all methods are applied first using data extracted from existing I-V curves and then using in-lab measurements of a commercial solar cell. It is demonstrated that the determination of the series resistance of solar cells using two I-V characteristics has several advantages over methods using a single I-V output curve. It becomes evident that methods which use a single I-V output curve are only accurate for cells operating under very high illumination conditions. At normal intensities, however, such methods result in erroneous R(s) values. This is due to the assumption of a constant diode ideality factor along the entire I-V output curve used in the derivation of these methods. It is shown that this assumption is inaccurate at normal intensity levels and can be appropriate only under very high illuminations. Three new methods are proposed in this study. One of the methods presents a new approach in determining the solar cell equation parameters. The new approach relies upon treating the diode ideality factor of the solar cell as a variable that is a function of both the terminal current and the light intensity level. The method uses two I-V output curves at different illumination levels in determining all solar cell parameters: The series resistance, the diode ideality factor and the reverse saturation current. Although somehow tedious, the new approach shows that, for accurate modeling of solar cells and prediction of illuminated characteristics at different light levels based on the single-exponential model, the diode ideality factor should be treated as a variable while the series resistance is held constant. Comparison between all methods is presented and a reasonable judgement and recommendations concerning the best method to be used are given.