Inorganic, organic and perovskite solar cells have been fabricated on polymer substrates including polyethylene terephthalate and polyethylene naphthalate which have high transmittance (>85%), low permeability to water, and can bend up to 1°mm radius ( Elli, 1986 Zardetto et al., 2011 Kaltenbrunner et al., 2012 Kaltenbrunner et al., 2015 Poorkazem et al., 2015 Cui et al., 2019). For instance, substrates that maximize transparency while having high bending radius, high elastic modulus, low roughness, as well as chemical stability and adequate thermomechanical properties for process compatibility, can become game changers for photovoltaic applications. Different challenges that need to be addressed by substrates are dependent on the application and the type of device that is fabricated on top. By using paper strips instead of the rigid microscope slides, they fabricated flexible CdS thin-film transistors that could bend up to a 1/16 inch radius without damage ( Brody, 1984). Perhaps the first demonstrations of vacuum deposited semiconductor materials onto flexible substrates were performed at Westinghouse in the 1960s. In this article the status, key challenges and opportunities for the field of next-generation flexible devices are elaborated in terms of materials, fabrication and specific applications. This represents a huge opportunity for scientific research and development to rapidly and considerably advance this area ( Figure 2). While early progress and many important innovations have already been achieved, the field of flexible electronics has many challenges before it becomes part of our daily life. From bending and rolling for easier handling of large area photovoltaics, to conforming onto irregular shapes, folding, twisting, stretching, and deforming required for devices in electronic skin, all while maintaining device performance and reliability. The definition of flexibility differs from application to application. While conventional electronics are very capable of these functions, flexible electronics are intended to expand the mechanical features to adhere to novel form factors through hybrid strategies, or as standalone solutions where the application does not require high computation power, intended to be highly robust to deformation, low cost, thin, or disposable.

These next-generation flexible electronics open up a wide range of exciting new applications such as flexible lighting and display technologies for consumer electronics, architecture, and textiles, wearables with sensors that help monitor our health and habits, implantable electronics for improved medical imaging and diagnostics, as well as extending the functionality of robots and unmanned aircraft through lightweight and conformable energy harvesting devices and sensors. This new progress is now enabling devices which can conform to complex and dynamic surfaces, such as those found in biological systems and bioinspired soft robotics. Likewise, biocompatibility and biodegradability has been achieved through polymers that do not cause adverse effect to the body and can be broken down into smaller constituent pieces after utilization ( Bettinger and Bao, 2010 Irimia-Vladu et al., 2010 Liu H. This interest has also extended to not only flexibility but also properties like stretchability and healability which can be achieved by utilizing elastomeric substrates with strong molecular interactions ( Oh et al., 2016 Kang et al., 2018). Presently there is great interest in new materials and fabrication techniques which allow for high-performance scalable electronic devices to be manufactured directly onto flexible substrates.

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