Realistic evaluation of power losses in solar cells by using thermographic methods

The spatially resolved evaluation of power losses in solar cells is a key issue in identifying technological and material quality problems and realistically judging their influence on solar cell performance. Up to now a spatially resolved image of leakage currents could only be obtained by lock-in thermography measuring the distribution of the dark leakage current in solar cells. However, it is well known from simulations that the dark and illuminated current paths in solar cells may differ considerably. In this article spatially resolved images of the power losses in solar cells measured under realistic operation conditions, that is at the maximum power point and under illumination, are presented. This improvement was made possible by the development of a measurement technique called illuminated lock-in thermography. The underlying physics of this technique are discussed and it is shown that this method gives an image, which is directly proportional to the local power losses in the solar cell. Different loss mechanisms as, e.g., losses in the emitter sheet resistance and due to recombination at grain boundaries could be visualized in two-dimensional images. A comparison and quantification of different loss mechanisms under operation conditions was obtained. It is found that, e.g., the losses due to recombination in the "striation rings" of Czochralski-silicon are clearly underrated as compared to losses via point-like "shunts" if a measurement of the dark characteristics of the cell is used. The comparison of dark and illuminated leakage current images gives laterally resolved experimental evidence for the differences in dark and illuminated current paths in solar cells predicted by device simulations.