**2.2 Transition metal dichalcogenides**

Although graphene has various excellent properties, due to zero-bandgap, work-function, and toxic nature, the research on new atomically thin 2d materials gained attention. These necessities have been fulfilled by TMDs. These2d materials attracted more attention as they have grown on a flexible surface and can be bears the stress and deformation [32–34]. Generally, TMDs are formulized as MX2where M expresses the transition metal from group IV-VIII, (M = Ti, Zr, Hf, V, Nb, Cr Ta, Mo, W, etc.) and X is a chalcogen atom (X = S, Se, Te) [35–36]. TMDs have opened the new pipeline of research as having tunable bandgap (1–2 eV) and explore an excellent picture of electrical, optical, and mechanical properties [37–39]. Various combinations of TMDs such as MoS2, CrS2, WS2, TiS2, MoSe2, CrSe2, WSe2, TiSe2 etc. found in metallic, semiconductor and insulator phase [40]. TMDs are a collection of big crystal family, found in different phases such as 1 T, 2H, and 3R., having two-third materials with layered structure [41]. In particular, MoS2 shows mechanically 30% more strength than steel and can be ruptured after warping 1%. It generates the most distensible and strongest semiconducting materials [36, 42]. Counter electrodes manufactured by platinum (Pt) were replaced by MoS2 in photovoltaic devices [43].

Typically, the synthesis approaches like exfoliation, hydrothermal, CVD, molecular beam epitaxy (MBE), and atomic layer deposition (ALD) are used to prepare the desired size of TMDs [44–48].

**Figure 1.** *Schematic illustration of exotic properties of 2d materials used for solar cell devices.*
