Mixtures of conjugated polymers and fullerenes command considerable attention for application in organic solar cells. To increase their efficiency, the design of new materials that absorb at longer wavelengths is of substantial interest. We have prepared such low band gap polymers using the donor-acceptor route, which is based on the concept that the interaction between alternating donors and acceptors results in a compressed band gap. Furthermore, for application in photovoltaic devices, sufficient polymer solubility is required. We have prepared four low band gap conjugated polymers, with a bis(1-cyano-2-thienylvinylene)phenylene base structure, and achieved an excellent solubility by the introduction of long alkoxy and alkyl side chains. The polymers were synthesized via an oxidative polymerization. Their electronic properties were determined from electrochemical and optical measurements, which confirm that they indeed have a low band gap. In the blend of such a low band gap polymer with PCBM, evidence for efficient charge transfer was obtained from PL and EPR measurements. However, bulk heterostructure solar cells made of such blends display only low efficiencies, which is attributed to low charge carrier mobilities.

It has been demonstrated that the dithiocarbamate precursor route is a suitable pathway towards poly(thienylene vinylene) (PTV)-type low-band-gap polymers. Two particular dithiocarbamate precursor polymers and the corresponding conjugated polymers, poly(3,4-diphenyl-2,5-thienylene vinylene) and poly(3,4-bis(4-butylphenyl)-2,5-thienylene vinylene), have been studied. The introduction of butyl side chains in the latter leads to excellent solubility in common organic solvents. Both polymers have been prepared in a straightforward manner and in good yield. The thermal conversion of the precursor polymers into the conjugated structure was studied with in situ FT-IR and UV−vis spectroscopy. Also, with the latter technique, the band gap was determined and the thermochromic effect was studied and compared with the unsubstituted PTV. The HOMO and LUMO levels of the polymers were determined from UV−vis and electrochemical measurements.

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Efficient charge transfer between a newly synthesized low band gap (LBG) conjugated polymer and the C60 derivative PCBM is demonstrated. Spectral and time-resolved photoluminescence (PL) measurements show strong quenching and charge separation in the picosecond time range. The charge transfer is further confirmed by light-induced electron paramagnetic resonance (EPR) measurements of both the positive polarons and the fullerene radicals, which can be resolved in high-frequency EPR. The potential for the LBG polymer to act as acceptor was examined in blends with the para-phenylene-vinylene polymer MDMO-PPV. No charge transfer, but instead energy transfer from MDMO-PPV to LBG occurs in these blends, as shown by PL spectroscopy.