Series resistance effects on solar cell measurements

Abstract

Current-voltage characteristics of photovoltaic solar energy converter cells are obtainable by three methods, which yield different results due to the effects of the cell internal series resistance. The three resultant characteristics are: (1) the photovoltaic output characteristic, (2) the p- n junction characteristic, and (3) the rectifier forward characteristic. Choice of the proper method is necessary for obtaining the correct information for the individual application. Most frequently used, e.g. for the determination of solar converter performance, is the photovoltaic output characteristic. A quick way is described for deriving such a characteristic for any light level from a corresponding characteristic obtained at a different light level. This method involves two translations of the coordinate system and requires only the knowledge of the series resistance and the difference in light intensities or short circuit currents. An inversion of this method permits an easy determination of the series resistance, involving measurements at two arbitrary light levels of unknown magnitude. The effects of series resistance consist at high light levels in a flattening of the photovoltaic output characteristic and a related drop in the maximum power point voltage. The resultant decrease in efficiency has to be overcome by series resistance reduction for solar cell applications with optical concentrators or for space missions in closer sun-proximity. In cell portions progressively further distant from the contact strip increasing cell voltages develop, approaching open circuit condition at very high light intensities. This yields a reduction of current contribution from those portions of the cell and a deviation from the normal proportionality between short circuit current and light intensity. The direct measurability of the p- n junction characteristic at high current densities without series resistance effects by the second method provides a powerful tool to the device development engineer, besides yielding a second method for the determination of the series resistance. Results from the application of this method indicate that, in the current density range as used in solar energy conversion, the silicon solar cell characteristic is much more closely described by the diffusion theory for p- n junctions than was previously believed.