Fused Dithienogermolodithiophene Low Band Gap Polymers for High-Performance Organic Solar Cells without Processing Additives

Abstract

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...

Materials Design Considerations for Charge Generation in Organic Solar Cells

“…The results of many of these studies have been published previously. 7,37,40,56,[78][79][80][81][82][83][84][85][86][87][88] This large body of data has allowed us to identify statistically significant empirical materials correlations which can be difficult to observe from smaller data sets. In this perspective we aim to summarise some of the key findings of these studies.…”

“…These advances in efficiency have largely been achieved by the synthesis and testing of new materials and device architectures, including in particular new photoactive layer materials whose energy levels have been modulated to achieve higher output voltages and / or improved photocurrent generation due to lower optical bandgaps. 1,2,4,7,9 However, at present our ability to predict device function from materials structure remains relatively limited. As a consequence, the vast majority of new materials tested have not performed better than the on-going state of the art, typically achieving significantly lower device performance than that predicted by simple, energy level based, device models.…”

Fused Ring Cyclopentadithienothiophenes as Novel Building Blocks for High Field Effect Mobility Conjugated Polymers

“…(3,3′-Dibromo-2,2′-bithieno [3,2-b]thiene-5,5′-diyl)bis(trimethylsilane) (1) was synthesized following a similar method in previous reports. 24 4,9-Dibromonaphtho[1,2-c:5,6-c]bis [1,2,5]thiadiazole was purchased from Lumtec. Flash chromatography (FC) was carried out on silica gel (Merck Kieselgel 60 grade 40−63 μm F254) using flash techniques.…”

Conjugated Polymer Photovoltaic Materials

“…Followed by PDTS-TPD and PDTG-TPD, dithienogermolodithiophene (DTTG) was also incorporated as building blocks in TPD-based polymers because of the potentials of extended conjugation length and improved coplanarity. Very recently, Heeney et al [154] reported the first synthesis of a novel ladder-type fused ring donor, DTG, in which two thieno [3,2-b]thiophene units are held coplanar by a bridging dialkyl germanium (Ge). Polymerization of DTTG with TPD afforded a polymer, PDTTG-TPD, with an optical band gap of 1.75 eV combined with a HOMO level of −5.68 eV.…”