Compared to inorganic photovoltaic devices based on silicon, polymer solar cells (PSCs) show many unique and useful properties such as easy fabrication, low cost and the possibility to be assembled into various flexible products. They have been proposed for the use in a broad spectrum of fields, particularly, electronic skins or integration into clothes that are key to next-generation electronics. For this, PSCs are required to have other desired properties, such as light weight and weavability, for practical fabric applications. However, these cannot be achieved by the generally studied planar PSCs. Learning from textile technology by using chemical fibers as building units, several groups have recently attempted to develop PSCs in a fiber format. These fibershaped PSCs have also been explored for weaving into thin textiles, however, both fiber-shaped and textile-like PSCs were not found to be stretchable, and are likely to get damaged by the generated stresses during movements that often occur in use. Therefore, for practical applications it is not enough for PSCs to be flexible, they also need to be stretchable.