Weak charge-transfer complexes based on conjugated polymers for plastic solar cells

Abstract

As known for short conjugated molecules, the absorption band of a weak (Mulliken) charge-transfer complex (CTC) can be tuned in the region where both the donor and acceptor are transparent. This can be used for tailoring low-band gap absorbing materials to match the solar spectrum. We studied donor/acceptor blends of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with 2,4,7-trinitrofluorenone or 2,6-dinitroantraquinone by a number of optical and photoelectric methods. From optical and vibrational IR absorption spectra, we found that the donor/acceptor blends demonstrate noticeable ground-state interaction indicating CTC formation. The CTC is characterized by a long absorption tail extending to the near-IR region. The blends also show strong quenching of photoluminescence (PL) and a near-IR photoinduced absorption (PIA) band, indicating donor/acceptor charge-transfer. Essentially, this band appears for both visible and IR pump wavelengths. In addition, to probe the charge photogeneration mechanisms, a magnetic field spin effect technique was applied. This technique shows that the charge photogeneration mechanism is essentially different in the blends compared to pristine MEH-PPV. Furthermore, photoconductivity in the blends strongly increases compared to pristine MEH-PPV and the photocurrent action spectrum closely follows to the extended absorption tail. We discuss photoinduced charge-transfer and relaxation of photoexcited CTC as possible mechanisms of mobile charge generation.