Silver and sodium incorporation into wide bandgap CZTS absorbers on transparent back electrodes and their application in kesterite/c-silicon tandem solar cells: Experiments and simulations

In recent years, kesterite solar cells have emerged as a promising candidate for integration as top subcells in tandem structures with silicon solar cells. Nevertheless, the production of high-quality kesterite absorbers on transparent substrates has remained a significant challenge. In this study, the combination of silver (Ag) alloying and sodium (Na) doping was employed to enhance the properties of copper zinc tin sulfide (CZTS) absorbers developed on FTO (fluorine-doped tin oxide) back electrodes using the sol–gel method. X-ray diffraction demonstrated a notable enhancement in the crystallinity and phase composition of the kesterite materials with the introduction of silver. Furthermore, Raman spectroscopy indicated a more organized matrix with a considerable reduction in Cu/Zn disorder and CuZn defect density in the kesterite materials following the incorporation of silver. Optical analysis exhibited a slight increase in the kesterite optical bandgap from 1.53 to 1.57 eV due to the presence of silver atoms. A finite-difference time-domain (FDTD) optical simulation was conducted using realistic optical inputs to calculate the transmitted light from the FTO/CZTS/CdS/ZnO/ITO solar structure. Subsequently, the performance of a well-established c-Si bottom subcell (with state-of-the-art efficiency) under calculated transmission was evaluated using the Solar Cell Capacitance Simulator (SCAPS-1D) for electrical simulation. The simulated tandem device achieved an efficiency of 14.5 %, which is lower than that of a crystalline silicon (c-Si) solar cell under AM1.5 due to lower transmittance. This resulted in only 6 % efficiency from c-Si in the tandem configuration, in addition to the lower electrical performance of the kesterite top subcell with only 7.8 % efficiency. These findings suggest that incorporating silver represents a promising approach to enhancing the properties of kesterite materials on transparent back electrodes. However, further optical improvements are necessary to fully realize the potential of kesterite materials for tandem applications.