Passivation of grain boundaries (GBs) and interfaces to suppress recombination and to improve minority carrier lifetime (MCLT) is essential for the functionality of devices based on polycrystalline materials. Improvement of MCLT is believed to be a very promising way to bring CdTe solar cells to the next efficiency level. However, which parameters significantly affect MCLT is not well understood. Here, high-efficiency CdTe solar cells in an unconventional inverted structure are used to approach this issue. Advanced characterization tools such as secondary ion mass spectroscopy 3D chemical imaging, atom probe tomography, and X-ray photoelectron spectroscopy are used to detect small amounts of impurities at GBs and are synergetically used together with time resolved photoluminescence measurements to correlate impurity distribution with electronic properties in CdTe solar cells. MCLT increases by an order of magnitude upon sulfur diffusion along GBs of the CdTe layer, which can occur by an elemental exchange with oxygen. Chlorine segregates at GBs and at the CdS/CdTe interface and bonding to cadmium and tellurium is indicated. CdTe solar cells in the inverted structure are presented with a certified efficiency of 13.5%. The results give guidance to further improve the performance of CdTe solar cells.