In recent years, highly flexible solar cells have been gathering great interest as a power source for operating wearable and/or on-skin electronic devices which are necessary technologies for the Internet of things (IoT) society. By adapting such flexible solar cells, wearable and on-skin devices can be free from the troubles of replacement of batteries and contact problems of external wirings. In this talk, our approaches for ultra-thin organic solar cells will be introduced with three important keywords, namely 1. high power conversion efficiency (PCE) , 2. strethchability/flexibility, and 3. robustness against the environmental conditions. We succeeded to fabricate organic solar cells with total thickness of 3 μm and achieved excellent PCE up to 10.5% for free-standing ultra-thin solar cells. There are trade-off relations between ultimate thinness and environmental stabilities such as air, water, and heating. We tackled to solve these problem and developed technologies to improve thermal stability and water stability of ultra-thin organic solar cells. Especially, high thermal stability of more than 100 degrees Celsius allows hot-melt adhesion process onto textiles, which enabled wearable power source systems. Additionally, we developed self-powered on-skin sensor systems by integrating our solar cells with ultra-thin organic electrochemical transistors and monitored heart-beat rate with such integrated devices.