This review focuses on the aspects of flexibility, stability and large scale, which are very essential for the future commercialization of PSCs, and many effective materials and fabrication strategies which have been reported to emphasize these three properties of PSCs will be introduced.

The rapid development of the modern electronics gives rise to higher demands of flexible and wearable energy resources. Flexible transparent conducting electrodes (TCEs) are one of the essential components for flexible/wearable thin-film solar cells (SCs). In this regard, commercial indium tin oxide (ITO) on plastics has demonstrated superior optoelectronic performance although some drawbacks, i.e., the low abundance, film brittleness, low infrared transmittance, and poor chemical stability remain. On the other hand, several other transparent conducting oxide (TCO)-free transparent conductive materials, such as carbon nanotubes (CNTs), graphene, metallic nanowires (NWs), and conducting polymers, have experienced a rapid development to address these issues. In this feature article, an overview over the latest development of several flexible TCO-free thin film SCs, i.e., organic solar cells (OSCs), dye-sensitized solar cells (DSSCs), perovskite solar cells (pero-SCs), and fiber/wire-shaped SCs is provided. Three groups of flexible TCO-free thin film solar cells can be categorized according to their configurations: (i) frontside illuminated planar configuration; (ii) back-side illuminated planar configuration, and (iii) fiber-shaped solar cells (FSSCs). The article is focused onflexible TCO-free TCEs, including CNTs, graphene, metallic NW/nanotroughs, metallic grids, conducting polymers, metallic fiber, and carbon based fibers. applications in energy conversion and storage. [8] However, more coverage and concept have been raised and some new type SCs have been developed these years.To systematically summarize different TCO-free TCFs and their applications in thin film SCs, here we attempt to categorize flexible ITO-free thin film SCs into three groups according to their configurations (Figure 1): (i) light is supposed to irradiate from flat photoanodes/TCFs side (front-side illuminated configuration); (ii) photoanode on an opaque substrate and light is supposed to irradiate from a transparent top electrode (back-side illuminated configuration); and (iii) both active layers and counter electrodes (CEs) are fabricated onto cylindrical substrates (flexible fiber-shaped solar cells (FSSCs)). In this review, the latest progress on flexible TCO-free thin film SCs including OSCs, DSSCs, QDSCs, pero-SCs, and fiber/ wire-shaped SCs are presented. Focus was given to their flexible TCO-free TCFs including CNTs, graphene, metallic NW/ nanotroughs, metallic grids, conducting polymers, metallic fiber, and carbon based fibers. The aim of this feature article is to give an overview on the various recent works on flexible ITO-free thin film SCs done by researchers to develop new-style TCEs. The paper will be arranged according to three different configurations as shown in Figure 1. Front-Side Illuminated TCO-Free Flexible Thin Film SCsThese years, most research efforts have been put into designing and fabricating TCO-free flexible thin film SCs with frontside illuminated configuration. A key issue in the fabrication of such SC is s...

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