A promising next-generation application of organic solar cells (OSCs) :
Intrinsically stretchable OSCs (IS-OSCs) for the power source of wearable electronic devices
Solar energy is one of the largest alternative energy sources that can replace existing fossil fuel. Accordingly, OSCs are one of the greatest candidates to substitute the silicon-based inorganic solar cells, with its advantages such as low production cost and easy processability. In particular, the organic materials used for OSCs provide strong mechanical durability and unique stretchability. These characteristics make OSCs very promising power sources for next-generation portable & wearable electronic devices. To realize that, our group has been developing IS-OSCs. We have replaced all the layers of rigid OSCs (the electrodes, interlayers, and active layers) with highly stretchable materials. In the same time, we also have suggested many effective strategies to achieve both superior photovoltaic performances as well as high stretchability in IS-OSCs. We anticipate that the development of IS-OSCs will absolutely contribute to better human life by accelerating the advance in internet-of-things (IoT) technologies.
Molecular engineering to enhance photovoltaic performance, mechanical robustness, and thermal/photo-stability of OSCs
For the commercialization of OSCs, high photovoltaic performance, mechanical robustness, and long-term stability should be simultaneously ensured. However, there exists a trade-off relationship between these properties, and therefore, we have been dedicated to develop photoactive materials to address the limitation. First, we have developed oligomerized small molecule acceptors (OSMAs) such as dimers and trimers to ensure sufficiently large molecular sizes and controlled molecular weight dispersity of 1. These desirable properties of OSMAs lead to superior thermal/photo-stability of over 10,000 hours by suppressing molecular diffusion and high power conversion efficiency of over 19% achieved by efficient molecular packing. Second, we have designed and synthesized efficient and stretchable conjugated polymers by integrating both rigid and soft blocks into the single polymer chain. This strategy enables suppressing phase separation between rigid/soft blocks and overcoming the trade-off between charge mobility and stretchability. Our work opens a promising avenue to realize efficient and stable OSCs through innovative molecular designs.
Eco-friendly processed organic electronics :
Developing green-solvent-compatible and electroactive materials and exploring natural processing solvent
Given the increasing need for environmentally conscious manufacturing of optoelectronic devices, developing eco-friendly processing methods suitable for commercialization has been a significant focus on recent investigations. Conventional non-halogenated aromatic organic solvents (e.g., toluene, xylene, anisole) still pose environmental and health risks. More importantly, their use is strictly forbidden within the semiconductor industry. Therefore, we newly developed water/alcohol soluble conjugated polymers for organic electronics incorporating oligo-ethylene glycol (OEG) side chains. Intriguingly, the addition of a common anti-solvent, water, to ethanol is found to remarkably improve the solubility of OEG side chain-based electroactive materials, leading to the successful fabrication of eco-OSCs with high hole mobility and a power conversion efficiency of 2.5%. In addition, the eco-OSCs with water/ethanol processing exhibit high stability under ambient conditions, showing the significant potential of this new process for industrial applications. Additionally, we are pioneering the use of natural solvents derived from renewable bio-sources (e.g., leaves, lemon peel, etc.) to not only reduce reliance on toxic chemicals but also contribute to the decrease of waste, such as discarded orange peels in large quantities annually, promoting sustainability in electronic device manufacturing.
Development of ion-electron coupled bioelectronics :
Organic electrochemical transistors (OECTs)
OECTs have drawn great attention as biocompatible electronics due to their diverse applications such as transduction/amplification of ionic-electronic signals and the detection of ions and molecules. These unique roles of OECTs are enabled by their operation principles, which involve ion injection into the active channel by the gate bias and the electrochemical doping of the active channel by electrolyte ions for charge compensation. To this end, active semiconductors in OECTs need to ensure both ion and electron transport capabilities and these materials are classified as organic mixed ionic-electronic conductors (OMIECs). We have aimed to maximize the steady-state performance of OECTs by the molecular design and film morphology control of OMIECs and achieved a record-high device performance with a µC* figure-of-merit of over 700 F V-1 cm-1 s-1. In addition, we have investigated underlying physics of OECT devices, while demonstrating the applicability of OECTs for sensors and neuromorphic devices. We envision that a comprehensive understanding ranging from fundamental principles to material design and device fabrication will further advance the frontiers of the OECT field.
