[en] A simple analytical model has been developed to explain the shape of low-energy spectra taken with pn-CCDs, which are developed as imaging spectroscopic detectors for ESA's X-ray observatory satellite X-Ray Multimirror Mission (XMM). The model has been tested on experimental data taken with test devices at the German synchrotron Berliner Elektronen Speicherring fuer Synchrotronstrahlung (BESSY-I). It can be used to generate the response matrix function for pn-EPIC (European Photon Imaging Camera), one of the three X-ray cameras onboard XMM. First results in applying the model to polychromatic spectra are shown