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Thin film chalcogenide kesterites Cu2ZnSnS4 and Cu2ZnSnSe4 (CZTSSe) are promising candidates for the next generation thin film solar cells. They exhibit a high natural abundance of all constituents, a high absorption coefficient and a tunable direct bandgap between 1.0-1.5 eV. A prerequisite for the use of CZTSSe as absorber layers in photovoltaic applications on large scales is a detailed knowledge of the formation reaction. Throughout the literature no consensus is available which describes how a thin film has to be treated in order to form single phase high efficiency material. It is known that above 400°C CZTSSe starts to decompose due to evaporation of Sn(S, Se), i.e. a high vapor pressure material. We will show here that understanding the formation reaction of CZTSSe offers a new route to stop this break down reaction. Absorbers produced by high temperature coevaporation and samples produced by low temperature precursor fabrication followed by annealing in a tube furnace are compared in order to elucidate that in all cases the loss of Sn(S, Se) forms a degraded surface region. This breakdown reaction can be stopped if the heat treatment is performed in a high partial pressure of SnS(e). This improves the solar cell efficiency from 0.02% to 6.1%. The new insights can be used to explain why some of the synthesis routines described in literature yield much better efficiencies than others.