Abstract The preparation process for CZTSe absorbers at UOL includes an independent tuning of the starting and ending composition of the layer in terms of Cu/Sn ratio. This is realized by a composition shift during the high temperature step of the annealing/selenization by Sn incorporation / Sn loss from the layer. In this work, we present a correlation between the precursor/absorber composition and the resulting device parameters. In brief, different starting compositions have a fundamental influence on the alloy structure of the precursor and hence can influence the dominating reaction pathway. For the Cu rich case the dominating alloy is Cu3Sn which moves towards Cu6Sn5 + Cu3Sn for lower Cu/Sn ratios. For high added Sn amounts the fraction of non-alloyed elemental Sn in the precursor layer rises. These differences can influence the reaction pathway and the resulting defect landscape, leading to a significant influence on final device properties. A series of precursors with different Cu/Sn ratios were processed in identical annealing conditions ending up with an absorber composition in the usual composition region, favourable for high efficiency devices. The resulting device properties such as PCE, EU, VOC-def and long-term device stability (after 1000h accelerated aging at 85°C) were found to correlate with the starting composition. Preliminary results from low-temperature PL analysis indicate an influence on activation energies of defects.