Recently, Cu-based quaternary chalcogenides, namely Cu 2 BCD 4 (B = Zn, Cd, Hg, Ba, C = Sn, Ge, D = S, Se), have emerged as accompanying various functional properties and explored for multiple applications. [1,2] For instance, similar to chalcopyrite, Cu 2 ZnSn(S/Se) 4 has been well studied and experimentally materialized as an absorber layer for single/multiple junction and flexible solar cells [3][4][5][6] and various efforts are made to enhance its performance. [3,[7][8][9] The alloys with composition Cu 2 ZnSn(Se x S 1Àx ) 4 and different anion and cation substitution of Cu 2 ZnSnS 4 have also been studied in detail, [6][7][8][9][10][11][12] and found to possess the energy bandgap ranging from 1.5 to 1 eV, allowing them to be used for solar cells as well as thermoelectric and optoelectronic materials. [10] However, it is noticed that the similar structural environment and small size mismatch in Cu 1þ and Zn 2þ ions favors Cu Zn deep antisite defect formation, resulting in a high voltage deficit compared to the existing copper indium gallium selenide based solar cells. [1,11,13] Though, the voltage deficit problem can be addressed by replacing Zn with another group II element like Ca, Sr, Ba, Cd, and Hg, providing a significant size mismatch with Cu. [14] Considering these facts, another compound named Cu 2 HgSn(Se/S) 4 is also investigated in parallel, which exists in the stannite phase in contrast to the most favorable kesterite phase for the Cu 2 ZnSnS 4 system. This material has relatively larger Hg cations, which prevents Cu Hg deep antisite defect in the compound. The beauty of this material is that its optimal bandgap of 1.33 eV lies in the visible range, making it a suitable solar cell material; [15] however, the risk of toxicity is involved with this compound which needs to be mitigated. Further, its selenium counterpart, that is, Cu 2 HgSnSe 4 with 0.8 eV bandgap, provides hope in realizing it as an active layer for demanding infrared photodetectors; however, its experimental realization is still to be done. In addition, the bandgap engineering with a change in its composition, that is, Cu 2 HgSn(Se x S 1Àx ) 4 , can also serve as an advantage for multiple optoelectronics applications. Therefore, we need to understand the variation in parameters like bandgap, effective masses, dielectric constant, etc. We find no report so far in the literature addressing the optoelectronic properties of Cu 2 HgSn(Se x S 1Àx ) 4 for the entire x (0 ≤ x ≤ 1) range, except the parent members, that is, x = 0 and 1. Additionally, in the era of flexible technology, thin film having good elastic strength is required for fabricating devices like solar cells. [16] Thereby, it becomes essential to determine the mechanical strength and quantify the ductileness of material for its exploration over flexible substrates. The alloy Cu 2 HgSn(Se x S 1Àx ) 4 is less explored in this context. However, its Zn counterpart Cu 2 ZnSn(Se x S 1Àx ) 4 has been investigated in detail. [17] In this work, we present the physical properties of C...
