Polymers for organic photovoltaics
Solar cells based on organic semiconductor materials which can produce electricity when irradiated are referred to as organic photovoltaics. Conjugated polymers are used as absorber materials in organic photovoltaic cells. One concept is the "bulk-heterojunction" solar cell: In a blend system both the light-absorbing conjugated polymer and a fullerene derivative (PCBM) are mixed for the very fast charge transfer. Poly (3-hexylthiophene) (P3HT) is used as a standard absorber material in such cells. Here, the HOMO and LUMO energy levels of P3HT are not optimally adjusted to the solar spectrum for the applied acceptor (PCBM) and the band gap. The aim is to achieve a defined reduction in the band gap, as well as a decrease of the energy gap between donor-LUMO and the acceptor-LUMO. Therefore, we focus on developing polymers with tailored HOMO and LUMO energy levels. Such polymers can be achieved e.g. by incorporation of alternating electron-donor and electron-acceptor moieties in the polymer backbone. Thus, band gaps between 1.8 and 1.3 eV can be achieved.