With the emergence of low-power electronic devices such as wireless sensor nodes, the Internet of Things (IoT) is rapidly developing and spreading. While the IoT has significant potential to benefit our lives, commerce, and industry, the use of large number of wireless or portable devices would need to be powered by distributed energy sources at the lowest possible cost, especially in regions that are limited to the range of a few milliwatts. However, conventional batteries, such as coin cells, require periodic replacement and maintenance, thus spurring the demand for emerging energy-harvesting systems that utilize ambient energy to semipermanently operate indoor electronic devices.

 Thus, various methodologies have been proposed for harvesting energy from local environments, including light, heat, movement/vibration, and electromagnetic waves. Of these technologies, photovoltaic (PV) energy conversion has shown great potential because of its higher energy density and output voltages. Thus, recent developments of PV devices not only target conventional solar cells under 1-sun illumination (i.e., 100 mW cm⁻²) but also dim-light indoor applications (with a factor 100–1000 lower incident power). As sunlight is not available always and at all locations, artificial indoor lighting sources, such as white light-emitting diodes (LEDs) and fluorescence lamps, can steadily supply small amounts of energy for powering sensors and electronic components inside buildings. Unlike the AM 1.5G solar spectrum, typical indoor illumination spectra are limited to the visible wavelength region, as they are optimized for the human eye. Here, eco-friendly organic photovoltaics (OPVs)^9,10 serve to be a promising indoor energy-harvesting technology, which offer various inherent advantages such as lightweight, mechanical flexibility, solution processability, and cost-effective large-area manufacturing capability. Additionally, the characteristics of OPV materials such as a high absorption coefficient and a tunable absorption range are well suited for indoor applications when compared to robust inorganic silicon devices. Recently, some research groups, including us, have investigated whether OPVs can demonstrate indoor performance superior to that of silicon solar cells, and the results revealed that OPVs indeed show higher power conversion efficiencies (PCEs) under white LED lighting conditions. It is necessary to further optimize OPV materials and devices, and thereby improve their PCEs for future practical applications. Nevertheless, thus far, only a few OPV materials including representative semiconducting polymers and low-bandgap small molecules have been tested for their potential in terms of organic energy-harvesting devices; thus, a systematic study on indoor PV characteristics and mechanism is still lacking.

In this presentation, we propose some small-molecule based light absorbers and evaluate their OPV performances using a binary bulk-heterojunction (BHJ) system with fullerene acceptor under LED illumination with different illuminances (200–10000 lx). Furthermore, we demonstrated flexible energy-harvesting modules of ~10 cm² under dimly lit conditions of 200 lx. Our research paves the way for practical indoor applications of solution-processed OPVs.