Renewable energy generation/storage devices are promising solutions to address the current energy dilemma and global environmental degradation. Solar energy harvesting, especially photovoltaic (PV) systems, are attracting ever-increasing attention because of the abundance of renewable solar energy. However, the unstable and intermittent nature of sunshine requires the integration between PVs and energy storage systems, which has pro. Renewable energy generation/storage devices are promising solutions to address the current energy dilemma and global environmental degradation. Solar energy harvesting, especially photovoltaic (PV) systems, are attracting ever-increasing attention because of the abundance of renewable solar energy. However, the unstable and intermittent nature of sunshine requires the integration between PVs and energy storage systems, which has promoted the development of solar-rechargeable energy systems (SESs). Flexibility and portability are two key features that need to be considered when designing next-generation wearable and portable energy devices, especially for SESs. In this review, we firstly focus on the recent development of flexible SESs based on a range of PV systems aiming at revealing the limitations of state-of-the-art technologies. Then, we discuss the fundamental principles which determine the key performances of flexible SESs, such as energy efficiency, matching between PVs and ESSs, and flexibility issues. Finally, we provide the current challenges and outlook for the future of flexible SESs.••Flexible solar-rechargeable systemPhotovoltaicEnergy storageSupercapacitorsBatteriesConverted efficiencyThe continuously growing energy consumption, rapidly diminishing fossil fuels, and ever-increasing concern for global climate deterioration have continuously stimulated the research of renewable energy conversion and storage systems [,,, ]. In the last few decades, researchers have made much progress in high-performance renewable energy harvesting and energy storage systems, including photovoltaic (PV) cells, fuel cells, metal-ion batteries, supercapacitors and other energy systems. As an individual energy system cannot viably be operated as a constant power source, combining renewable energy harvesting with energy storage systems has been recognized as a desirable approach to address the above issues. Amongst these technologies, Solar-rechargeable Energy Systems (SESs), in which PVs and Energy Storage Systems (ESSs) are integrated for solar energy conversion and storage respectively (Fig. 1), has been demonstrated as one of the most promising self-powered energy sources, mostly due to the worldwide abundance of the solar resource. Recently, SESs were widely studied and applied in smart-grids, house energy suppliers, regular and electric vehicles, consumer electronics, and, more importantly, in portable and wearable devices [2,9].For the wide-spread application of SESs, flexibility and portability are two key features that need to be considered. While the conventional crystalline sili. There exists a far greater number of energy harvesting systems than storage systems. Furthermore, the energy storage system is dependent on the energy harvesting system because the amount and rate of energy harvested determines the amount and rate of storage required (Fig. 1b). These two factors combined means the SESs are mainly defined by the energy harvesting system, and thus we have organised the review by classifying SESs in this way.For the wide-spread application of solar cells, flexibility and portability are two key features that need to be considered. The flexible solar cells can not only be applied to portable or wearable devices (Fig. 1c), but also reduce the cost of transportation and installation of solar panels. During recent decades, the sector is moving away from heavy, fragile glass-coated silicon panels to become more focussed on thin-film technologies. Although their efficiencies are still relatively low (less than 20%), thin-film photovoltaics (PVs) are cheaper than previous polysilicon-based panels and thus have fewer obstacles to enter the photovoltaic energy market.Previous sections discussed the recent progress on the investigation of various flexible SESs. However, to achieve high-performance flexible SESs, there are several critical aspects which need to be considered. Flexible SESs are not only synergetic systems, but also need to meet various flexibility requirements. We presented the significant paramet.
