Charge carrier photogeneration and transport properties of a novel low-bandgap conjugated polymer for organic photovoltaics

The overall power conversion efficiency of organic solar cells depends on many factors, some of which such as photon absorption, charge carrier photogeneration, separation and transport are intrinsic properties of the active material. The use of low-bandgap conjugated polymers in polymer/fullerene bulk heterojunctions improves the spectral overlap between the polymer absorption and the solar irradiance spectrum, and is therefore a promising route toward increased light harvesting and higher power conversion efficiency of polymer photovoltaics. We present our studies on the optical and charge transport properties of a novel low-bandgap conjugated polymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)], PCPDTBT, with an optical energy gap of Eg=1.46 eV. The combination of steady-state and transient photoconductivity with photoinduced absorption measurements has allowed us to investigate the charge carrier photogeneration and charge transport mechanisms in pristine PCPDTBT and PCPDTBT:PCBM interpenetrating networks, and to compare them to the P3HT and P3HT:PCBM model systems. The picture of the photophysics of PCPDTBT:PCBM emerging from these studies is very similar to that of P3HT:PCBM blends. We discuss the potential of PCPDTBT as a new material for high efficiency polymer solar cells.