Lock-in thermography for investigating solar cells and materials

Abstract

For more than 10 years lock-in thermography (LT) is being used for investigating leakage current phenomena in solar cells. In operation the p-n junction of a solar cell is forward-biased. So-called local shunts are leakage sites which cause losses in efficiency. They can be investigated by applying a pulsed forward-bias to the cell in the dark (typical lock-in frequency 10 Hz), where all local currents become visible as local pulsed heat sources. Such investigations allow the reliable localization of all leakage sites. This is the first step for investigating their physical nature and therefore very helpful for optimizing the efficiency of solar cells. In this contribution the practical realization of LT on solar cells is described and typical results of such investigations are shown. In the last years several special LT techniques have been developed for imaging special physical parameters of solar cells, some of them employing light irradiation. Also non-thermal LT techniques for imaging the minority carrier lifetime in semiconductor wafers have been developed. These special LT techniques are reviewed in the second part of this contribution. Thus, LT meanwhile implies a whole family of techniques for investigating solar cells, solar modules, and materials.