**1. Introduction**

Nanofabrication means the manufacturing techniques of material or structures with critical dimensions in range of one to few hundreds of nanometers. These techniques realizes exceptionally small, features, structures, devices and systems those have applications in numerous fields of basic and applied sciences. It is comparatively a new class of manufacturing that signifies recent areas of sciences as well as creates new markets. Unlike conventional fabrication approaches, research in nanofabrication is multidisciplinary and needs combined work crosswise conventional fields. In nanofabrication, the final product is based on nanoscale materials, such as powders or fluids, and the components are realized either in "bottom up" or "top down" fashion, using various nanotechnologies. Similar to other fields, the applications of nanofabrication approaches are enormous in optoelectronic devices, [1] for instance, solar cells, [2] smart windows, [3] light-emitting diodes, [4] displays, [5] transparent sensors, [6] and touchscreens. [7] Transparent electrodes (TEs) are the key components in such optoelectronic devices. In addition to high optical transmittance and low sheet resistance [8] required for traditional TEs, next-generation

soft optoelectronic devices also need decent mechanical deformability [1, 9] in TEs. Currently, the most utilized TEs are based on vacuum-processed TCOs, comprising fluorine-doped tin oxide and indium tin oxide (ITO). [10, 11] Although TCOs based TEs have demonstrated the required optoelectronic performance, several limitations, such as low abundance, [12] film brittleness, [13] low infrared transparency, [14] and failure during high temperature sintering, undermine their appropriateness for utilization in the future soft optoelectronic systems. Thus, researchers have developed novel TE materials and vacuum-free approaches for its fabrication to substitute the TCOs. [15, 16]

Novel intrinsically transparent materials including graphene, [17] carbon nanotubes (CNTs), [18] and conducting polymers [19, 20] have been explored to replace the TCOs. Besides, other promising class of soft TEs designed from metals are widely employed due to their excellent electrical, optical, and mechanical performance. This typically include metal NWs networks [21, 22] and systematic metal meshes, [23–28] and ultra-thin metal films. [29–31] In addition to the advancement of new materials for soft TEs, plenty of research is performed on the development of vacuum-free technologies for the low-cost fabrication of soft TEs. The list of these techniques is mainly consists of spin coating, [32] spray deposition, [33] inkjet printing, [34] screen printing, [35] transfer printing, [36] and slot-die coating. [37]

There have been several reviews published over the years, aiming at soft TEs from applications perspective. [1, 11, 38] However, few of them focuses on the soft TEs from the fabrication perspective. In this chapter, latest review of the vacuum-free fabricated TEs for emerging soft electronic devices is presented. The chapter begins with the discussion of key properties of TEs for soft electronics (sections 2). We then introduce the TE materials including metals, carbon materials, and transparent conducting polymers (section 3,4). Finally in section 5, the recent progress on vacuum-free methods that are typically employed for the realization of TEs, discussing their merits and demerits. We hope this chapter will enlighten the readers about the emergent soft TEs to better design and fabricate low-cost soft electronics devices.
