To fabricate high quality CZTSe film with low Cu content (Cu/(Zn+Sn)<0.8) is a challenge.In this work,liquid SnSe2 phase formed during selenization is demonstrated to facilitate the growth of large and compact Cu2ZnSnSe4 (CZTSe) grains.The formation of SnSe2 can be tailored by adjusting the Se vapour property during selenization.At the beginning of selenization process,the formation of SnSe2 phase is suppressed to prevent the film from forming over compact surface,thus facilitating the diffusion of Se to the bottom and Zn to the surface.While during subsequent high temperature selenization process,excessive macromolecular Se vapour is supplied to promote the formation of liquid SnSe2 phase to assist the growth of large CZTSe grains.Benefiting from the optimized two step selenization process,high crystalline CZTSe film with low Cu content (Cu/(Zn+Sn)=0.75) is obtained.Further study of the effect of Cu content on the morphology and photovoltaic performance of corresponding CZTSe solar cells has shown that the roughness of CZTSe absorber film increases with the decrease of Cu content,and the textured surface morphology is achieved with the ratio of Cu/(Zn+Sn) as low as 0.75.As a consequence,the reflection loss of CZTSe solar ceils reduces dramatically and the short circuit current density of the cells improve from 34.7 mA/cm2 for Cu/(Zn+Sn)=0.88 to 38.5 mA/cm2 for Cu/(Zn+Sn)=0.75.In addition,the CZTSe solar cells with low Cu content show longer minority carrier life time and higher open circuit voltage than the high Cu content devices.A champion performance CZTSe solar cell with 10.4% efficiency is fabricated with Cu/(Zn+Sn)=0.75 in the CZTSe film without anti-reflection coating.