Flexible energy devices are the building blocks for next-generation wearable electronics. Flexible energy devices are expected to have multiple functions, such as energy conversion from light to electricity and vice versa, energy generation from triboelectric, energy storage and so on. These functions can be efficiently realized by solar cells, light-emitting diodes (LEDs), triboelectric nanogenerators (TENG), batteries and supercapacitors, etc. The flexible energy devices can be integrated into flexible, wearable, and/or portable platforms to enable wide application prospects in the fields of information, energy, medical care, national defense, etc. However, flexible energy devices face more challenges when compared to their rigid counterparts, which requires more breakthroughs and research efforts on fabrication techniques, materials innovation, novel structure designs, and deep physical understandings.



This special issue assembles 8 review articles providing a comprehensive summary of the latest development on flexible energy devices based on various technologies and functional semiconductor materials. Briefly, Dong and Wang have comprehensively reviewed and discussed the challenges of TENG-based self-charging power textiles that can become the future energy autonomy power sources^[1]. The self-charging power textiles hybridizing fiber/fabric shaped batteries/supercapacitors are summarized from the aspect of textile structural designs. Ma et al. reviewed the flexible energy devices for wearable bioelectronics^[2]. These flexible energy devices can be constructed in flexible platforms as the stable power supply for smart sensing, personalized healthcare, and robotics applications. More importantly, they summarized the existing challenges for flexible energy devices: mechanical stability, energy storage capacity for miniaturized devices, and biocompatibility. More specifically, Li et al. reviewed the recent advances in zinc ion batteries (ZIBs)^[3]. Because of the intrinsic safety, abundant storage, and low cost, the rechargeable aqueous ZIBs have become one of the most promising portable candidates for wearable energy storage devices with medical and primary safety. They also summarized the advances in overcoming the most important challenges for ZIBs, namely the rigid current collector and/or metal anode, easily detached cathode materials, and the relatively narrow voltage window of flexible electrolytes. This timely review makes the ZIBs one step closer to commercial applications.



Zhu et al. delivered a timely review on the progress of efficient flexible solar cells made of nanostructures^[4]. The nanostructures enhance the mechanical robustness of flexible solar cell devices while optimizing the light-trapping capabilities. Kong et al. provided a focused discussion on the flexible perovskite solar cells (FPSCs)^[5]. Considering the power conversion efficiency (PCE) of FPSCs has already achieved 21.1%, their thorough review on the development of flexible substrates, carrier transport layers, perovskite films, and electrodes are highly appropriate and enlightening. Moreover, they also pointed out the future work on FPSCs can be focused on the directions, such as large-area solar cells modules, environmental stability, and mechanical stability. Tang et al. reviewed the development of FPSCs from three aspects, namely the small-area flexible devices, roll-to-roll (R2R) processed devices with large scale, and emerging flexible cells with deformability and stretchability^[6]. These deformable and lightweight FPSCs can be applied in wearable and portable electronics as the power supply. Their review shows a pathway to the high possibility of commercialization of FPSCs for self-powered systems. On the other hand, Ye et al. reviewed the recent advances on the asymmetric non-fullerene small molecule acceptors for polymer solar cells^[7]. The advantages of asymmetric small molecules were discussed in three aspects: charge transport, molecular energy level, and active layer accumulation morphology. They also pointed out that the complicated synthesis of asymmetric small molecule acceptors (A-NF-SMAs) is still a critical problem that scientists need to address in the future.



Besides the solar cells devices which convert light energy to electrical energy, this special issue also includes the review on blue perovskite LEDs for next-generation displays from Yoon et al., as one kind of efficient energy conversion devices^[8]. They summarized the three types of perovskite structures: perovskite nanocrystals (PeNCs), 2-dimensional (2D) and quasi-2D perovskites, and bulk (3D) perovskites. Moreover, they brought researchers’ attention to the remaining challenges of blue perovskite LEDs, namely the quantum efficiency of the emissive layer, the operational stability, and color stability, by solving which people can further boost the development of next-generation displays.



We sincerely hope this special issue could provide the meaningful and profound review and perspective on the field of flexible energy devices. We would like to thank all the authors for their outstanding contributions to this special issue. We are also grateful to the editorial and production staff of the Journal of Semiconductors for their kind assistance.