Photoluminescence image evaluation of solar cells based on implied voltage distribution

Abstract

All previous methods for quantitatively evaluating photoluminescence (PL) images of solar cells assumed a laterally constant short circuit current density Jsc. Moreover, they had to subtract a Jsc PL image from all other PL images for considering the diffusion-limited carriers. Here a more realistic PL evaluation method is introduced, which is based on a recently published alternative model of the illuminated solar cell. In this model an analytic expression is derived by considering the illuminated current as a diffusion process between bulk and the pn-junction and linking the implied voltage in the bulk with the local pn-junction voltage under illumination. This model does not assume a laterally constant Jsc but a constant light absorption rate, and it leads to a prediction of the Jsc distribution solely based on PL imaging results. Moreover, it regards the shadowing of the cell by the busbars and grid lines. This model is applied to the quantitative evaluation of PL images of an industrial multicrystalline silicon solar cell. The resulting series resistance and saturation current density images are compared with that of an established PL evaluation method, and the resulting distribution of Jsc is compared with LBIC results. The results of the new method appear slightly more realistic than that of the old one, since they consider the inhomogeneity of Jsc.