**6. Conclusions**

*Nanofibers - Synthesis, Properties and Applications*

electronic devices. [15, 60, 69, 70]

**5.6 Slot-die coating**

the graphene was initially covered by a PMMA thin film. Next, the underneath Cu film was removed by an etching step in FeCl3. The graphene film covered by PMMA was then lifted-off either using a PDMS stamp for transfer, or directly picked up using the target substrate itself. [131] To enhance the throughput and production speed, transfer printing has been integrated with R2R process for the fabrication of large-area graphene (30-in) soft TEs. [105] Despite such potential, wet transfer has a limitation for the fabrication of top graphene-based TEs for the soft thin-film devices as the functional materials used in these devices are sensitive to moisture. To overcome this a dry transfer approach is developed, where a the film is directly coated on the PDMS stamps before transfer. [132] Besides graphene, other major transfer printable material for soft TEs is the metal nanowire/ mesh films. These films typically have weak adhesion with the transfer substrates. This poor adhesion between the transfer substrates and metal films makes it easier to lift these films up with the PDMS, or another sticky polymeric stamp/target substrate. [60, 69] The high optical transmittance and superior conductivity of fabricated soft TEs using transfer printing ensure the high performance of soft

It is an effective process for printing one-dimensional structures. Slot-die coating is typically integrated with the R2R system for rapid production of soft electronic devices. As shown in **Figure 7a** and **b**, the solution is pushed out of the slot-die using a pneumatic scheme, and the solution is printed laterally in the

*(a) Schematic illustration of the slot-die coating technique. Reproduced with permission from Ref. [37] (b) photograph of the slot-die coating system. Reproduced with permission from Ref. [133] (c) photographs of the large-area soft OCSs having PEDOT:PSS TEs fabricated by slot-die coating. Reproduced with permission from Ref. [134] (d) photographs of the OCSs with high geometric fill factor. The employed PEDOT:PSS TEs are* 

*fabricated by slot-die coating. Reproduced with permission from Ref. [135].*

**292**

**Figure 7.**

Recent progress of the development of vacuum-free TEs for soft electronics has been promising. This chapter presents a detailed overview on the latest advances of the vacuum-free soft TEs, comprising the introduction of electrode materials classes, the optical, electrical, mechanical, and surface features of the soft TEs. The chapter summarizes the vacuum-free techniques for the fabrication of soft TEs. Regardless of all the shortcomings discussed, we are optimistic that the vacuum-free TEs be going to play vital roles in soft electronic industries in the future.
