Improvement of Performance of CZTSSe Solar Cells by the Synergistic Effect of Back Contact Modification and Ag Doping

To improve the performance of Cu₂ZnSn(S,Se)₄ solar cells, a strategy is proposed to improve the quality of absorber and back interface simultaneously by substituting V-doped Mo (Mo:V) for a conventional Mo back electrode and incorporating Ag into the Cu₂ZnSn(S,Se)₄ (ACZTSSe) absorber in this work. Since p⁺-type V-doped MoSe₂ (MoSe₂:V) is formed in the site between the absorber and Mo:V during selenization, the conventional Mo/n-MoSe₂ back contact is modified to Mo:V/p⁺-MoSe₂:V, a back surface passivation field (BSPF) is established at the back interface, the band bending of MoSe₂:V is downward and that of bottom of the absorber is upward. Further investigation reveals that the back contact modification and Ag doping have a synergistic effect on inhibiting carrier recombination, decreasing series resistance and increasing shunt resistance, thereby leading to the PCE of device without antireflection coating increased from 8.61 to 10.98%, which is larger than the sum of increase in PCE induced by Ag doping alone (8.61 to 9.66%) and back contact modification alone (8.61 to 9.63%). It is demonstrated that the synergistic effect stems mainly from the strengthened BSPF and the further reduced back contact barrier height. The former is due to the increased difference in work function (W_F) between MoSe₂:V and absorber induced by the reduced W_F of the absorber after Ag doping and the raised W_F of MoSe₂:V after V doping. The latter is due to the downshifted valence band maximum of absorber after Ag doping. This work highlights the synergistic effect of back contact modification and Ag doping on improving the performance of CZTSSe solar cells and also provides an effective way to suppress carrier recombination.