We present a comparative study of the photovoltaic properties of an alternating polyfluorene co-polymer (F8TBT: poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2’,2’’-diyl) blended with two different electron acceptors, namely CdSe nanoparticles and PCBM (PCBM = (6,6)-phenyl C61-butyric acid methyl ester). Such photovoltaic devices take advantage of both the high electron mobilities of inorganic semiconductors/fullerenes and the solution processing possibilities of polymer semiconductors.
We employed transient absorption laser spectroscopy to resolve the cascade of excitonic and polaronic intermediate states participating in the photovoltaic processes. This study was complemented by steady-state photoluminescence quenching and quantum efficiency measurements to probe excited-state charge transfer, and photoinduced absorption measurements to identify long-lived charged species. We were able to correlate the phototophysical measurements with well-defined and controllable morphological parameters and ultimately with the photovoltaic performance..
This work demonstrates that polyfluorenes are versatile polymers for photovoltaic applications and points to how further improvements might be made. Initial photovoltaic devices based on these two blend systems, show a spectral response extending to 600 nm and solar power conversion efficiency of 1.7% under Air Mass 1.5 Global (AM1.5G) conditions.