Small band gap copolymers based on furan and diketopyrrolopyrrole for field-effect transistors and photovoltaic cells Bijleveld, Johan C.; Karsten, Bram P.; Mathijssen, Simon G.J.; Wienk, Martijn M.; Leeuw, Dago M. de; Janssen, René A.J.

In the search of new electron acceptor, n-type materials for organic solar cells that combine a strong absorption over a broad range with good electrical characteristics, the use of diketopyrrolopyrrole (DPP) derivatives with low reduction potentials is explored. A series of small molecule DPP derivatives is presented and the compounds are tested as electron acceptors in combination with poly(3-hexylthiophene) (P3HT) as the donor material. Working photovoltaic devices are obtained that show a photoresponse in the wavelength region where the DPP molecules absorb. The best device shows a power conversion efficiency of 0.31% in simulated solar light, with a photon-to-electron conversion efficiency of ∼10% up to 700 nm. The efficiency seems to be limited by the coarse morphology of the blend.

The synthesis and the optical and electrochemical properties of thiophene end capped oligo(2,3-alkylthieno[3,4- b]pyrazine)s are presented. The optical absorption rapidly shifts to lower energies with increasing chain length, caused in almost equal amounts by a rise of the HOMO and a lowering of the LUMO levels. The optical band gap of the polymer is estimated to be 1.13 +/- 0.07 eV. Extrapolated redox potentials indicate that the polymer is a small band gap p-type material.

Diketopyrrolopyrrole‐based polymers have shown good performance in polymer solar cells. Monodisperse short oligomers, monomer up to tetramer, of one of these polymers, pBBTDPP1, have been prepared and the chain length dependence of their optical and electrochemical properties has been investigated. The optical and electrochemical data obtained in solution lead to the conclusion that conjugation in this system is very limited. The most probable reason for this is a twist in the backbone, caused by steric hindrance of the dodecyl side chains with the sulfur atoms. In thin films, the chain length dependence of the optical properties is much stronger, due to planarization of the oligomer chains upon aggregation. magnified image

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