**4.1 Carbon materials**

*Graphene:* Graphene efficiently conducts electricity and heat, is stronger than steel (~200 times), and is nearly transparent. [101] Due to these unique characteristics, it has been suggested as a substitute soft TE material. Over the years, various vacuum-free approaches are established to produce thin films of graphene on soft substrate materials. [102–104] Recently, significant advancement has been made to enhance the optoelectronic properties of the graphene based TEs. Large-area graphene film was made-up on copper catalyst (~30 inches diagonal size), which was then accurately transferred to the target soft substrate using transfer printing technique. [105] In an ideal world, graphene has massive capability and is currently offering the assurance of being the vital transparent material for soft TEs. However as a matter of fact, uniform ultra-thin films of graphene are exceedingly challenging and are costly to produce. In addition, optoelectronic performances of graphene based TEs reduce quickly, due to the wrinkles/folds and crystallographic defects formed in these ultra-thin films during mechanical deformation. [105, 106]

*Carbon Nanotubes:* Similar to graphene, CNT is one of the hardest materials recognized. Due to its decent electronic and mechanical characteristics, CNTs are productively employed as TE material in soft electronic devices. [107–110]

Numerous vacuum-free approaches, for example, spin coating [111] and transfer printing, [112] are developed to produce CNTs based soft TEs. While, CNTs based TEs have attractive characteristics, such as higher optical transmittance and superior mechanical deformation capability, these have typically poor electrical conductivity. This limitation makes CNTs less suitable for large-area commercial soft electronics.

#### **4.2 Transparent conducting polymers**

*PEDOT:PSS:* Few transparent polymers, having intrinsically poor electrical conductivity, are transformed into conducting polymers via addition of conductive dopants into their iterating chains. Poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is one of the classic model of such conducting polymers. In PEDOT: PSS unit chain, PEDOT acts as the conducting polymer, while the PSS plays the role of a dopant, enhancing its electrical conductance via significantly increasing the charge carriers. Since, PEDOT:PSS has no visible absorptive resonances, therefore it is routinely used as TEs in small scale soft electronic devices. Yet, a number of concerns, for example, instable molecular structure and high water solubility have limited the use of PEDOT:PSS in largescale soft electronics. [113, 114]

*Other Conducting Polymers:* Besides PEDOT:PSS, other conducting polymers comprising poly(p-phenylene-vinylene) (PPV), polyaniline (PANI), polyfuran (PF), polypyrrole (PPy), are utilized as TE materials for several soft electronic devices, due to their decent electrical and optical conductivity. [115, 116]

As discussed above, each class of soft TEs offers unique set of favorable properties, and also has some disadvantages. Researchers have combined different classes of TEs into a single electrode structure to fabricate hybrid soft TEs. The objectives of developing this new class TEs are: (1) take advantage of the benefits offered by individual electrode. (2) overcome those challenges associated with the electrode once employed individually. **Table 3** summarizes, the optical and electrical performance, and applications of hybrid soft TEs published in recent literature.
