Synergistic effect of Mn on bandgap fluctuations and surface electrical characteristics in Ag-based Cu2ZnSn(S,Se)4 solar cells

Abstract

In Cu₂ZnSn(S,Se)₄ photovoltaic devices, the exceptionally high density of 2Cu_Zn + Sn_Zn donor defects may induce bandgap fluctuations and thus limit V_oc and PCE enhancement. We have previously reported that the partial substitution of Ag for Cu in Cu₂ZnSn(S,Se)₄ provides a promising way to effectively suppress Cu_Zn defects and considerably enhance the cell performance for Cu₂ZnSn(S,Se)₄ photovoltaic devices. However, the bandgap fluctuations and detrimental band tailing fundamentally still limit cell performance. On the basis of a Ag-substituted CZTSSe system, in this paper, we demonstrate that partially substituting Zn with Mn could be a feasible tactic to pronouncedly decrease the bandgap fluctuations and prevent detrimental band tailing. Our in-depth investigation reveals that Mn substitution could increase depletion width, decrease the defect densities of the Cu_Zn acceptor and 2Cu_Zn + Sn_Zn donor, and enable grain boundary (GB) inversion in a (Cu,Ag)₂ZnSn(S,Se)₄-based cell. As a result, an impressive PCE of 11.81% was achieved when the Mn substitution level was 10%, with a V_oc of 478 mV, a J_sc of 37.89 mA cm⁻², and an FF of 65.23%. This is so far the highest efficiency among Mn-substituted CZTSSe-based photovoltaic devices. By thoroughly understanding these photoelectric properties of the CZTSSe-based photovoltaic devices, we believe that the breakthrough in device efficiency will come soon through our continuous efforts.