Organic Solar Cells
Solar flux is unlimited and environmentally friendly energy resource evaluated as a potential primary energy source for the future. Commercialized silicon solar cells satisfy the high efficiencies but encounter the practical problems because they need costly processes to make, require large space to install and are heavy to bring. Thus, organic solar cells is expected to be an alternative technology suitable for portable devices, building integrated photovoltaics and flexible electronic devices because they can be fabricated using high-throughput roll-to-roll printing processes on the flexible substrates. For commercialization, the development of high efficiency organic materials is considered to be the most important issue and thus we are focusing on developing novel organic electron donating, accepting and transporting materials satisfying industrial demand.
1. Electron donor
For decades, the structural design and construction of electron donor has been systemically studied for developing high performance photovoltaic materials, and several fundamental criteria for electron donating polymers have been discovered. The alternation of electron-rich (D) and electron-deficient (A) monomers along the polymeric backbone is the most successful way to build a low bandgap polymer for efficient solar energy absorption and to form enough internal polarization for better charge carrier separation. We are trying to find out new synthetic strategy to improve the photovoltaic performance or to overcome the drawback of existing organic materials.
2. Electron acceptor
Currently, most popular bulk heterojunction (BHJ) solar cells require the use of fullerene derivatives as the electron acceptor for facile photo-induced electron transfer from electron rich low band gap polymer donors. The fullerene derivatives, phenyl-C61(or 71)-butyric acid methyl ester (PC61BM and PC71BM), are almost exclusively used as acceptors due to their high electron affinity and electron mobilities. However, these fullerenes are rather expensive and exhibit limited absorption in longer wavelength region of the solar spectrum and thermal instability in the morphology of blend films. Thus, we are developing new electron accepting materials to replace the fullerene acceptors on the basis of superior performance.
Printed electronic sensors have gained much attention as inexpensive and fully flexible devices because they can be fabricated using the cost-saving roll-to-roll printing technique and room-temperature processes on flexible plastic or paper substrates. Photoelectric sensors have been extensively studied and many progresses have made for the research of organic photodetectors including molecular engineering and interface engineering. However, most of these studies are focused on the optimization of the device structures, rather than the materials. Thus, we try to suggest a general synthetic guidance to develop new organic photodetecting materials.
The current interest in wearable device technology is promoting the development of organic thermoelectric devices that can utilize residual body heat as the power. The thermoelectric performance of conjugated polymers has been advancing rapidly. Crispin et al. reported that poly(3,4-ethylenedioxythiophene):tosylate (PEDOT:Tos) films exhibited PFs up to 324 µW m-1K-2 at room temperature, and Pipe et al. reported that PEDOT:poly(styrenesulfonate) (PSS) films exhibited PFs up to 469 µW m-1K-2 at room temperature. However, research efforts on thermoelectric conjugated polymers have been mainly focused on PEDOT, and only a few other polymer systems showed promise in terms of thermoelectric application. We are interested in development of high PF conjugated materials to surpass the PEDOT-based materials.