Influence of Ag-doping on the performance of Cu2ZnSnS4 solar cells

We report an improvement in Cu2ZnSnS4 (CZTS) solar cell efficiency with Ag-doping. The highest efficiency obtained for the fabricated CZTS solar cells was 5.84% and that for Ag-doped (ACZTS) cell efficiency was 6.66% without anti-reflection coating. The precursor film was prepared by co-sputtering of copper, tin sulfide (SnS), and zinc sulfide (ZnS) targets on a molybdenum (Mo) coated glass substrate. For the ACZTS, a thin layer of Ag (∼5 nm) was thermally evaporated on top of the CZTS precursor before annealing in an H2S/N2 environment for crystallization. X-ray diffraction (XRD) patterns of both films showed Kesterite phase. Scanning electron microscopy (SEM) imaging showed that Ag-doping increased grain size and lowered porosity. Energy-dispersive X-ray spectroscopy (EDS) indicated that the ACZTS film was able to prevent some Sn loss compared to bare CZTS. Current Density – Voltage (J-V) measurements showed an improved efficiency for ACZTS was mostly due to increase in short-circuit current density (Jsc), while open circuit voltage (Voc) improved slightly. The solar cell device with Ag-doping also exhibited lower series resistance and higher shunt resistance (Rsh). The lower series resistance is attributed to the suppression of MoS2 formation for ACZTS film, which was confirmed by Raman spectra. The optical band gaps for both CZTS and ACZTS films were measured to be 1.525 eV. Capacitance-Voltage (C-V) measurement showed that built-in voltage (Vbi) for CZTS and ACZTS were 0.75 V and 0.70 V, respectively. The Voc of the ACZTS solar cell being closer to the built-in voltage indicated reduction in defects after Ag-doping.