Tuning the band gap of CdS in CZTS/CdS solar cells

The n-type semiconductor, cadmium sulfide (CdS), is commonly used as buffer material in thin-film photovoltaics, such as Cu(In,Ga)Se2 and Cu2ZnSn(S,Se)4.In this work, we present band gap tunable CdS thin films and their applications in Cu2ZnSnS4 (CZTS) solar cells. CdS is deposited by chemical bath deposition (CBD), and we investigate six different deposition temperatures (ranging from 60oC to 80oC) with at least four different deposition times at each temperature. Thinner CdS films are achieved at higher deposition temperatures, and we show that the absorption in CdS increases with CdS film thickness. We demonstrate that the band gap of CdS can be tuned between 2.3 eV and 2.4 eV by changing the CdS film thickness between 100 nm and 40 nm, respectively. This variation in band gap could be explained by quantum confinement as many films made by CBD contain small grains. The change in band gap could also be caused by several factors, including: (i) lattice strain and structural parameters, (ii) changes in carrier concentration, (iii) the presence of impurities, or (iv) deviations from stoichiometry. For sputtered CZTS devices, we find that the device performance increases with CdS deposition temperature and with thinner films, i.e. with a lower absorption in the CdS film as well as a higher CdS band gap. This is caused by improvements in JSC and VOC, while the FF remains constant. The samples are characterized by SEM, Raman, XRD, UV-Vis, J-V, EQE, and depth-profile XPS and UPS.