The kesterite structure of Cu2ZnSn(S,Se)4 (CZTSSe) quaternary system has a narrow single-phase stability region based on first-principle calculations,which easily leads to the formation of undesirable defects and second phases.Therefore,the growth of high-quality absorber layers is crucial for efficient CZTSSe solar cells.Generally,CZTSSe absorber layers can be prepared by annealing the deposited Cu2ZnSnS4 compounds or CuZnSn metal precursor films under Se atmosphere.However,this high-temperature annealing process usually results in the loss of Sn,which hinders the control of the film composition,film uniformity and thus the efficiency of solar cell.Here,we control the loss of Sn elements and the growth of films by adjusting the selenization pressure according to the kinetic process of the selenization reaction: Cu2ZnSnS4(s)(=)Cu2S(s)+ZnS(s)+SnS(g)+1/2S2(g).[1]We found the rapid growth of the film under low pressure leads to excessive local stresses making the absorber layer separate from the Mo substrate,which will hinder carrier transport in the back surface,while detrimental deep defects will be formed due to the Sn loss.At high selenization pressure,the Sn loss is suppressed,but the formed Sn(S,Se)2 secondary phase plays the role of a shunt path in the film,which deteriorates the FF of the device.Finally,a 10.28 % efficient CZTSSe solar cell is fabricated.This study provides guidance for the preparation of high-quality absorber layers.