Tuning Bandgap of p-Type Cu₂Zn(Sn, Ge)(S, Se)₄ Semiconductor Thin Films via Aqueous Polymer-Assisted Deposition

Bandgap engineering of kesterite Cu₂Zn(Sn, Ge)(S, Se)₄ with well-controlled stoichiometric composition plays a critical role in sustainable inorganic photovoltaics. Herein, a cost-effective and reproducible aqueous solution-based polymer-assisted deposition approach is developed to grow p-type Cu₂Zn(Sn, Ge)(S, Se)₄ thin films with tunable bandgap. The bandgap of Cu₂Zn(Sn, Ge)(S, Se)₄ thin films can be tuned within the range 1.05-1.95 eV using the aqueous polymer-assisted deposition by accurately controlling the elemental compositions. One of the as-grown Cu₂Zn(Sn, Ge)(S, Se)₄ thin films exhibits a hall coefficient of +137 cm³/C. The resistivity, concentration and carrier mobility of the Cu₂ZnSn(S, Se)₄ thin film are 3.17 ohm·cm, 4.5 × 10¹⁶ cm^-3, and 43 cm²/(V·S) at room temperature, respectively. Moreover, the Cu₂ZnSn(S, Se)₄ thin film when used as an active layer in a solar cell leads to a power conversion efficiency of 3.55%. The facile growth of Cu₂Zn(Sn, Ge)(S, Se)₄ thin films in an aqueous system, instead of organic solvents, provides great promise as an environmental-friendly platform to fabricate a variety of single/multi metal chalcogenides for the thin film industry and solution-processed photovoltaic devices.