ABSTRACT:We report the synthesis of a novel ladder-type fused ring donor, dithienogermolodithiophene, in which two thieno [3,2-b]thiophene units are held co-planar by a bridging dialkyl germanium. Polymerisation of this extended monomer with N-octylthienopyrrolodione by Stille polycondensation afforded a polymer, pDTTG-TPD, with an optical band gap of 1.75 eV combined with a high ionization potential. Bulk heterojunction solar cells based upon pDTTG-TPD:PC71BM blends afforded efficiencies up to 7.2% without the need for thermal annealing or processing additives.There has been significant recent progress in the development of conjugated polymers for use in organic field effect transistors and bulk heterojunction (BHJ) polymer solar cells. 1 One promising class of polymers for these applications are the so-called ladder polymers 2 , in which linked aromatic units such as thiophene or benzene are forced to be coplanar and fully conjugated by the use of bridging heteroatoms. 3 The enforced co-planarity reduces rotational disorder thereby lowering reorganization energy and potentially increasing charge carrier mobility. 4 The bridging atoms also serve as a point of attachment for the necessary solubilizing groups needed to ensure processable materials.Within the class of donor-acceptor ladder polymers, bridged bithiophenes have proven to be a particularly useful building block. For example donor-acceptor type copolymers of cyclopentadithiophene (a C bridge) with 2,1,3-benzothiadiazole have exhibited FET mobilities up to 3.3 cm 2 V -1 s -1 when substituted with long hexadecyl sidechains. 5 The incorporation of bulky 2-ethylhexyl sidechains affords a more amorphous polymer, which nevertheless showing promising BHJ efficiencies of 5.5% when processed from solutions with high boiling additives. 6 Changing the bridging heteroatom from C to Si (dithienosilole) or Ge (dithienogermole) for analogous benzothiadiazole co-polymers enhances crystallinity, leading to improved charge transport and a reduction in bimolecular recombination. 7 The improvement in crystallinity has been rationalised on the basis of the longer C-Si/Ge bond compared to the C-C bond, which alters the geometry of the fused heterocycle facilitating enhanced intramolecular interactions. 8 In addition the replacement of the C bridge with Si or Ge alters the electronic energy levels of the resultant polymers, generally resulting in a lowering of both the HOMO and LUMO. This has been rationalized by interaction σ* orbital of the silylene/germylene fragment with the π* orbital of the aromatic system. Based upon the promising performance of these bridged dithiophene monomers, we were interested to further extend the conjugation length of the monomer and improve its coplanarity by the incorporation of fused thieno[3,2-]thiophene (TT) instead of thiophene. 10 Thieno[3,2-b]thiophene has been widely utilized as a co-monomer in a variety of high performing polymers, where it has been shown to promote intrachain packing and improve charge carrier mobility. 11 In add...

We present the synthesis and characterization of two novel thiazole-containing conjugated polymers (PTTTz and PTTz) that are isostructural to poly(3-hexylthiophene) (P3HT). The novel materials demonstrate optical and morphological properties almost identical to those of P3HT but with HOMO and LUMO levels that are up to 0.45 eV deeper. An intramolecular planarizing nitrogen−sulfur nonbonding interaction is observed, and its magnitude and origin are discussed. Both materials demonstrate significantly greater open circuit voltages than P3HT in bulk heterojunction solar cells. PTTTz is shown to be an extremely versatile donor polymer that can be used with a wide variety of fullerene acceptors with device efficiencies of up to 4.5%. It is anticipated that this material could be used as a high-open circuit voltage alternative to P3HT in organic solar cells.

The performance of bulk heterojunction solar cells based on a novel donor polymer DPP-TT-T was optimised by tuning molecular weight and thermal annealing.

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