Ag-alloyed Cu₂ZnSn(S,Se)₄ kesterite semiconductors have been attracting considerable attention recently because doping with Ag can suppress the Cu_Zn related anti-site defects, which is the main factor that limits the efficiency of kesterite solar cells. Although the performance of the thin film and the device is enhanced (for example, better crystallinity, fewer deep level defects and longer lifetime of the minority carrier) by doping a small amount of Ag in the CZTS thin film, the density of the defects remains high, and the band alignment of CdS/CZTS is not fully optimized, and the interface carrier concentration is greatly decreased. These phenomena will limit any further improvement in efficiency. In this work, to further increase the performance of the device, the interface of CdS/CZTS for an Ag-alloyed CZTS thin film solar cell was doped with In or Al. Then the properties of the thin film, interface band structure and device performance were investigated. After doping the interface with In or Al, the concentrations of the n-type and p-type carriers at the CdS/CAZTS heterojunction were boosted, and the interface band alignment was optimized, resulting in over 11% efficiency for an Ag-alloyed CZTS (with Al interface doping) thin film solar cell. Moreover, low-cost Al-doped ZnO (AZO) was used to replace indium tin oxide (ITO) as the front electrode after optimization, and more than 10% efficiency of CAZTS device without In was obtained. This study demonstrates the feasibility of controlling the doping of the interface to improve the interface performance of a heterojunction by using a simple spin coating process, and also provided a suitable experimental basis for future low-cost thin film solar cells.