Paul A. van Hal

Owing to their increased absorption of light in the visible region, [70]fullerene derivatives provide a high photocurrent when incorporated as the electron acceptor in thin-film polymer photovoltaic cells. An incident-photon-to-current efficiency of 66 % is obtained when [70]PCBM (see picture) is used in combination with a poly(p-phenylene vinylene). These high currents originate from an ultrafast charge transfer that occurs upon photoexcitation of either the fullerene or the polymer. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2003/z51647_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

A novel low-bandgap conjugated polymer (PTPTB, Eg = ∼ 1.6 eV), consisting of alternating electron-rich N-dodecyl-2,5-bis(2′-thienyl)pyrrole (TPT) and electron-deficient 2,1,3-benzothiadiazole (B) units, is introduced for thin-film optoelectronic devices working in the near infrared (NIR). Bulk heterojunction photovoltaic cells from solid-state composite films of PTPTB with the soluble fullerene derivative [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as an active layer shows promising power conversion efficiencies up to 1 % under AM1.5 illumination. Furthermore, electroluminescent devices (light-emitting diodes) from thin films of pristine PTPTB show near infrared emission peaking at 800 nm with a turn on voltage below 4 V. The electroluminescence can be significantly enhanced by sensitization of this material with a wide bandgap material such as the poly(p-phenylene vinylene) derivative MDMO-PPV.

Combining conjugated poylmers and TiO 2 in hybrid bulk‐heterojunctions is a promising method for producing novel solar cells. Here TiO 2 is introduced into a poly( p ‐phenylene vinylene) layer. Nanometer‐scale phase separation allows efficient photoinduced charge transfer between the two components. The films can be used as the active layer in a photovoltaic cell and provide external quantum efficiencies of up to 11 % (see Figure).

The synthesis of a homologous series of oligo(p-phenylene vinylene)-fulleropyrrolidines (OPVn-C60, n = 1−4, where n is the number of phenyl rings) is described. The photophysical properties of these donor−acceptor dyads and the corresponding model compounds, α,ω-dimethyl-2,5-bis(2-(S)-methylbutoxy)-1,4-phenylene vinylene oligomers (OPVn, n = 2−4) and N-methylfulleropyrrolidine (MP-C60), are studied as a function of the conjugation length in solvents of different polarity and as thin films. Fast singlet energy transfer occurs after photoexcitation of the OPVn moiety of the dyads toward the fullerene moiety in an apolar solvent. Photoexcitation of the dyads in a polar solvent results in electron transfer for OPV3-C60 and OPV4-C60, and to some extent for OPV2-C60, but not for OPV1-C60. These results are compared to the results obtained for mixtures of OPVn and MP-C60 in the same solvents. The solvent-dependent change in free energy for charge separation of the donor−acceptor systems is calculated from the Weller equation, and the rate constants for energy and electron transfer are derived from the fluorescence lifetime and quenching. The results show that in a polar solvent electron transfer in these dyads is likely to occur via a two-step process, that is, a very fast singlet energy transfer prior to charge separation. In thin solid films of OPV3-C60 and OPV4-C60, a long-lived charge-separated state is formed after photoexcitation. The long lifetime in the film is attributed to the migration of charges to different molecules. A flexible photovoltaic device is prepared from OPV4-C60.