Improving the performance of low-cost water-based solution-synthesised Cu2ZnSn1-xGex(S,Se)4 absorber thin films by germanium doping

In this work, Cu2ZnSn1-xGexS4 (CZTGS) precursor thin films with varied Ge composition (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) were prepared by using a water-based solution approach. After selenizing at 550 degrees C for 10 min in a graphite box containing selenium powder, the as-fabricated kesterite Cu2ZnSn1-xGex(S,Se)(4) (CZTGSSe) thin films were systematically investigated for phase structure, morphology, valence states of elements, optical properties, and electrical properties. The results showed that the Ge element was successfully incorporated into the host lattice of Cu2ZnSn(S,Se)(4) (CZTSSe), and kesterite CZTGSSe films were formed. As the Ge content increased, the crystal grains in the films grew larger and showed uniform morphology on the sample surfaces. The band gap in the CZTGS(Se) thin films could be engineered in the range of 1.21 eV-2.00 eV by tuning the relative compositions of the Ge elements. Furthermore, it was confirmed that the doping of Ge improved the mobility, resistivity, and photoelectric performance of the films. Finally, the Cu2ZnSn0.8Ge0.2(S,Se)(4) solar cell device with an efficiency of 5.39% was prepared. The results indicate that non-toxic water solution-based CZTGSSe films showed potential for application in solar cells.