The role of Ag in aqueous solution processed (Ag,Cu)2ZnSn(S,Se)4 kesterite solar cells: antisite defect elimination and importance of Na passivation

Abstract

Cu_Zn antisite defects have been regarded as the efficiency limiting factor of kesterite solar cells because they cause band tailing and loss of open circuit voltage (V_OC). To suppress Cu_Zn antisite defects, Ag-alloyed (Ag,Cu)₂ZnSn(S,Se)₄ (ACZTSSe) kesterite has been proposed. Although efficiency enhancement is realized by a limited amount of Ag addition (Ag/(Ag + Cu) ≤ 20%), the antisite defects may not be completely eliminated by the small amount of Ag. Further enhancement by using a high Ag content (Ag/(Ag + Cu) ≥ 20%) has not been achieved yet. In this work, the effects of Ag on the phase stability, crystal structure, defect properties and device performance are investigated. Experimental results demonstrate that the optimized formation temperature of kesterite is reduced with increasing Ag content. We demonstrate that Ag₂ZnSn(S,Se)₄ is ordered kesterite and Cu_Zn antisite defects are effectively eliminated as 35% of Cu is replaced by Ag in ACZTSSe. To further increase the efficiency of the high Ag-content kesterite, 1 at% Na addition to the precursor is critical to passivate the interface as well as grain boundaries and increase the carrier concentration. An efficiency of 10% for ACZTSSe solar cells is reported with Ag/(Ag + Cu) = 35% at a processing temperature as low as 470 °C through an environment-friendly chemical spray pyrolysis process using aqueous solution. This study demonstrates the feasibility of controlling antisite defects and band-gap engineering by a larger amount of Ag substitution in the kesterite system to solve the V_OC deficit problem.