Further Boosting Solar Cell Performance via Bandgap-Graded Ag Doping in Cu2ZnSn(S,Se)4 Solar Cells Compared to Uniform Ag Doping

Cu2ZnSn(S,Se)4 (CZTSSe) is a hopeful substitution to commercialized Cu(In,Ga)Se2 (CIGSe) devices with similar structure and optoelectronic properties and has advantages of nontoxicity, low cost, and abundant reserves. However, the traditional flat bandgap structure of the CZTSSe absorber layer does not efficiently enhance the collection of photogenerated electrons and decrease recombination. Graded bandgap engineering toward the interfaces of CIGSe solar cells is the key to realize high-efficiency devices. In this study, we obtained (Cu1-xAgx)2ZnSn(S,Se)4 (CAZTSSe) absorber layers with high-concentration Ag doping at both ends of the absorption layer and undoped or low-concentration Ag doping in the middle part through a new miscible layered precursor method. This bandgap structure suppressed CuZn defects, delayed Fermi level pinning near the CZTSSe/CdS interface, sustained good electrical conductivity and light absorption in the middle of the absorption layer, improved the conversion efficiency of incident light, and inhibited recombination of carriers toward the Mo back electrode. In addition, we also compared the performance of undoped, uniformly Ag-doped, and V-type Ag-doped CZTSSe devices to acquire a deeper understanding of the reasons for the enhanced performance. It can be found that compared with undoping, the open-circuit voltage (Voc) of the best devices with uniform doping (x = 15%) increased from 379 to 386 mV, the fill factor (FF) increased from 44.70 to 54.14%, and the photoelectric conversion efficiency (PCE) increased from 4.63 to 6.21%. More surprisingly, the Voc of the optimal CAZTSSe devices (sample D) with Ag-graded doping was increased to 413 mV and the FF was increased to 59.63%. It also achieved an impressive PCE of 7.35%. The above results prove the importance of tuning Ag gradient doping of CZTSSe films for improving solar cell performance.