Influence of Zn excess on compositional, structural and vibrational properties of Cu2ZnSn0.5Ge0.5Se4 thin films and their effect on solar cell efficiency

•CZTGSe thin films grown by co-evaporation and thermal treatment at T < 500 °C.•CZTGSe thin films with Ge/(Ge + Sn) around 0.5 with Eg near 1.3 eV.•The absence of secondary phases results in larger grain size and compact structure.•ZnSe secondary phase at the surface and in the bulk of CZTGSe for >14 at % Zn.•ZnSe secondary phase led to a decreased device performance, from 6.4% to 4.2%.