Ag alloying for modifications of carrier density and defects in Zn-rich (Ag,Cu)2ZnSnSe4 thin film solar cells

Abstract The Ag alloying Cu2ZnSnSe4 absorber has profound influences on the material properties including the carrier density, defects, grain structure and morphology, etc. It is found that Ag alloying could effective decrease the carrier density of Ag alloyed (Ag,Cu)2ZnSnSe4 (ACZTSe) absorber and thus increase the depletion region width (DRW) of corresponding kesterite solar cells. At the same time, the CuZn related defects are decreased and consequently the band tailing and possible Fermi level pinning could be supressed. In addition, further adjusting [Zn]/[Sn] ratios in Zn-rich ACZTSe could be helpful to the performance of solar cells due to a fine modification of the carrier density and defects. After optimizing of chemical compositions by combining both modifications, the Ag alloyed Zn-rich ACZTSe based kesterite thin film solar cells have achieved the highest efficiency of 8.68%, in which the Ag alloying content with respected to [Ag]/([Ag]+[Cu]) and the [Zn]/[Sn] are 10% and 1.14, respectively.