A simple method for Ge incorporation to enhance performance of low temperature and non-vacuum based CZTSSe solar cells

•Selenization treatment of CZTS is done at low temperature: 450 °C.•Chlorine based compounds are successful for Na, K and Ge inclusion.•Ge with Se generates GeSe, Ge inclusion limits Se incorporation in CZTS.•Ge-devices show a higher carrier concentration and lower compensating defects.•Ge-CZTSSe based devices have an almost 25% increase in efficiency.