Solution-Processed Ultra-Flexible/Stretchable Organic Photovoltaics for Wearable Applications

Organic photovoltaics (OPVs) have garnered attention as promising advanced wearable power generators for on-skin electronics, owing to recent improved high power conversion efficiency (PCE) exceeding 20% and their flexibility/stretchability. The large-scale production of such flexible OPVs and other optoelectronic devices can be achieved via all-solution processing without requiring vacuum processes. The usage of ultra-thin substrate films having thickness of 1 μm can enable minimized wearing load and increased power per device weight. Additionally, to secure the structural and functional integrity of the epidermal power generators, sufficient mechanical robustness must be ensured for withstanding the repeated tensile strains imposed on the devices by continuous body movements, thereby permitting long-term reliable operation. To meet this requirements, intrinsically stretchable OPVs are important hot topic recently.<br/>Here we show our recent progress of solution-processed ultra-flexible/stretchable OPVs and their potential applications for wearables. More specifically, we are currently focusing on interface engineering to improve the performance and stability of ultra-thin organic solar cells. By improving the adhesion of interfaces between functional layers, we could improve both mechanical robustness [1]and waterproof properties [2]. We control the wetting conditions of solution processed layers to make perfect interfaces of organic photoactive layers and liquid metal interface, enabling reasonable performance of OPVs and other optical devices [3]. Furthermore, we have succeeded in developing intrinsically stretchable OPVs using strain redistribution technology [4]. An additive to PEDOT:PSS transparent electrodes improved both crack onset train value and adhesiveness to the stretchable substrates. Thanks to these performances, the strain to the active layer could be redistributed to the beneath PEDOT:PSS layers, enhancing COS values of photoactive layers.<br/>Using such OPVs as power sources, we have developed some applications for wearables. The one example is wearable bio-signal monitoring devices [3,5], and the other is rechargeable system of cyborg insects where a living insect is used as soft-robots for doing rescue tasks during disasters [6].<br/>[1] B. Du et al., <i>ACS Appl. Mater. Interfaces</i>, 15, 14624 (2023).<br/>[2] S. Xiong et al., <i>Nat. Commun</i>., 15, 681 (2024).<br/>[3] L. Sung et al., <i>Sci. Adv</i>., 10, eadk9460 (2024).<br/>[4] J. Wang et al., <i>Nat. Commun</i>. 15, 4902 (2024).<br/>[5] S. Park et al., <i>Nature</i>, 561, 516 (2018).<br/>[6] Y. Kakei et al.,<i> npj Flex. Electron.</i>, 6, 78 (2022).