(Invited) Performance Improvement and Potential Applications of Ultrathin Organic Solar Cells

Abstract

Flexible and thin photovoltaics can supply a variety of applications such as power sources for wearable electronics and soft robots. We reduce the total thickness of such organic photovoltaics down to several μm, which enables extreme bendability and lightweight properties. The important parameters include high efficiency and stability in ambient air conditions. Here we show our recent progress with ultra-thin organic solar cells and potential applications for wearable electronics and soft robots.The current best power conversion efficiency (PCE) for ultrathin organic solar cells is 15.8% reported by ourselves1). We have solved the specific issues behind the usage of ultrathin substrate films to achieve almost comparable PCE with those using normal flexible organic solar cells. One specific technology is the usage of polymer films having good thermal stability and smooth surface roughness2). Another important technology is the development of stable charge carrier injection layers and interfaces at the active layer. The state-of-the-art active layer materials such as PM6:Y6 is known not to be stable in ambient air and other environmental stresses. We verified the degradation occurred by the diffusion of Zn atoms from the conventional electron-transport layer (ETL) of ZnO obtained from the sol-gel process to the active layer, and solved it by replacing the ETL with more stable organic/inorganic hybrid material1,3). Such improved ETL and interface achieved highly balanced performance and stability, enabling good PCE on ultrathin substrates.We improved both PCE and environmental stabilities simultaneously. The stabilities include mechanical robustness3), water-proof property4), thermal stability2), and storage stability in ambient air5). Material development and process engineering enable such improved stabilities. In addition, such environmental stability can offer another practical usage. Our ultrathin solar cells have excellent thermal stability up to 120 ℃. This is capable of the hot-melt process which is a mature technology for the apparel industry. We can adhere our ultrathin organic solar cells directly onto textiles with such hot-melt films without any degraded performances. Apart from the improvement of the performance of ultrathin organic solar cells, we are focusing on how to implement such flexible devices onto three-dimensional surfaces5).We demonstrated some potential applications using our ultrathin organic solar cells used as a power source for wearable sensor applications7,8). We fabricated ultrathin organic solar cell modules and sensors. The sensor was powered by the solar cells under light illumination and monitored electrocardiogram signals with a high signal-to-noise ratio. In addition to this, we also demonstrated rechargeable cyborg insects9). In this research, we adhered our ultrathin organic solar cells onto a living insect abdomen and demonstrated a recharging wireless locomotion control system.We are focusing on both performance improvements of ultrathin organic solar cells and the integration of these cells to supply power to wearable electronics. By accelerating system-level integration research in the next decade to meet the required specifications of appropriate target applications, ultrathin organic solar cells will solve the problems associated with the supply of power for various electronics and will contribute to the overall development of society.1) S. Xiong et al., Adv. Sci. 2022, 9, 2105288.2) X. Xu et al., Proc. Natl. Acad. Sci. 2018, 115, 4589.3) F. Qin et al., Nat. Commun. 2020, 11, 4508 .4) H. Jinno et al., Nat. Energy 2017, 2, 780.5) Z. Jiang et al., Proc. Natl. Acad. Sci. 2020, 117, 6391.6) S. I. Rich, S. Lee, K. Fukuda, and T. Someya, 2022, 34, 2106683.7) S. Park et al., Nature 2018, 561, 516.8) H. Jinno et al., Nat. Commun. 2021, 12, 2234.9) Y. Kakei et al., npj Flex. Electron. 2022, 6, 78. Figure 1