Towards an improved Laplacian-based photoluminescence image evaluation method

Solar cells with J01 shunts were modelled realistically as a 2D circuit.Conventional evaluation of simulated PL images reveals wrong J01 results.Two alternative PL evaluation methods are tested for validity.The Laplacian-based method appears promising and is further improved.Preliminary results are not perfect yet, the method has to be improved further.

In a previous contribution it was found by evaluating 2-dimensional device simulation-based photoluminescence (PL) images of solar cells that the previous methods of PL image evaluation give good results for imaging the local series resistance, but lead to an erroneous distribution of an inhomogeneous saturation current density J01. This is caused by the model of independent diodes used in these evaluation methods, which does not take into account the distributed character of the series resistance and horizontal balancing currents occurring in inhomogeneous solar cells. In this contribution it is checked whether two alternative PL evaluation methods, which were already published but did not become popular yet, may overcome this problem. These are the differential luminescence imaging technique proposed by Rau et al. and the Laplacian-based method proposed by Glatthaar et al. By simulating PL images of a model device based on a 2-dimensional equivalent circuit with a known distribution of J01 and applying these PL evaluation methods, retrieved images of J01 are obtained and compared with the input J01 distribution. It is found that the differential luminescence imaging technique, at least in its presently used form, leads to wrong results, as the traditional PL evaluation techniques do. The Laplacian-based method, on the other hand, indeed has the potential to image J01 correctly. However, until now this method lacks in sensitivity. In this contribution some improvements are proposed, which may enhance the sensitivity of the Laplacian-based method. One important improvement is to enable a scaling measurement at a high instead of a low current resp. illumination intensity. Thereby the signal-to-noise ratio of the scaling measurement, which until now limited the accuracy of the whole PL evaluation, may be improved. Another proposal is that the actual PL measurement is performed under open circuit condition and not under current extraction. First experimental results show the effectiveness of this new method, but limitations still remain.