Efficient Solar Cells from Layered Nanostructures of Donor and Acceptor Conjugated Polymers

Efficient photoinduced charge transfer and separation accompanied by large photovoltaic power conversion efficiencies were observed in layered nanostructures of acceptor poly(benzimidazobenzophenanthroline ladder) (BBL) and donor poly(p-phenylenevinylene) (PPV) or poly(2-methoxy-5(2ʹ-ethyl-hexyloxy)-1,4phenylenevinylene) (MEH-PPV) at practical sunlight intensities. Nearly complete quenching of photoluminescence and large shortening of fluorescence decay lifetimes of donors PPV and MEH-PPV evidenced fast efficient photoinduced charge transfer and separation in PPV/BBL and MEH-PPV/BBL bilayers. ITO/PPV(60 nm)/BBL(60 nm)/Al photovoltaic cells had an external power conversion efficiency of 1.5% and 4.6% under AM1.5 white light illumination intensities of 80 and 1 mW/cm^2, respectively, in ambient air. Under single-wavelength (460 nm) 0.5 mW/cm^2 illumination, the same PPV/BBL diodes had a power conversion efficiency of 5.4% and incident photon-to-current conversion efficiency of 62%. ITO/MEH-PPV(60 nm)/BBL(60 nm)/Al cells had slightly reduced photovoltaic properties, for example 1.1% power conversion efficiency at 80 mW/cm^2 AM1.5 white light illumination, due primarily to the red shift of MEH-PPV absorption band as revealed by the photoaction spectra of the diodes. Studies of donor(PPV)/acceptor polymer diodes with a series of 5 acceptor polymer semiconductors having electron affinities (LUMO levels) of 2.7-4.0 eV showed that high electron affinity was essential to efficient photoinduced charge transfer and large photovoltaic power conversion efficiencies. These results demonstrate that efficient photovoltaic cells can be realized in layered donor/acceptor conjugated polymer semiconductors having complementary electronic and optical properties.