Rigid fused perylene diimide (PDI) dimers bridged with heterocycles exhibit superior photovoltaic performance compared to their unfused semiflexible analogues. Changing the chalcogen atoms in the aromatic bridges gradually increases the twist angles between the two PDI planes, leading to a varied morphology in which the one bridged by thiophene achieves a balance and shows the best efficiency of 6.72%.

A key challenge to the commercialization of organic bulk heterojunction solar cells is the achievement of morphological stability, particularly under thermal stress conditions. Here we show that a low-level light exposure processing step during fabrication of blend polymer:PC 60 BM solar cells can result in a 10-fold increase in device thermal stability and, under certain conditions, enhanced device performance. The enhanced stability is linked to the light-induced oligomerization of PC 60 BM that effectively hinders their diffusion and crystallization in the blend. We thus suggest that light processing may be a promising, general and cost-effective strategy to optimize fullerene-based solar cell performance. The low level of light exposure required suggests not only that this may be an easily implementable strategy to enhance performance, but also that light-induced PC 60 BM oligomerization may have inadvertently influenced previous studies of organic solar cell device behaviour.

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...

Although the performance of organic thin-film transistors (OTFTs) has increased significantly over the last decade, it still remains challenging to fabricate large area arrays of transistors with good device-to-device parameter uniformity.[1] Approaches based on the use of solution processable organic semiconductors are attractive, both in terms of throughput and the ability to pattern the semiconductor directly onto a range of substrate materials such as plastic. In terms of organic semiconductors, both soluble small molecules and polymers are viable materials classes with the former often being advantageous in terms of the higher charge carrier mobilities that can be obtained in OTFTs. Despite this significant advantage, however, the processing of highly crystalline small molecules into uniform thin films over large area substrates is often much more challenging than polymers, especially with respect to

Minimization of open-circuit-voltage ( V) loss is required to transcend the efficiency limitations on the performance of organic photovoltaics (OPV). We study charge recombination in an OPV blend comprising a polymer donor with a small molecule nonfullerene acceptor that exhibits both high photovoltaic internal quantum efficiency and relatively high external electroluminescence quantum efficiency. Notably, this donor/acceptor blend, consisting of the donor polymer commonly referred to as PCE10 with a pseudoplanar small molecule acceptor (referred to as FIDTT-2PDI) exhibits relatively bright delayed photoluminescence on the microsecond time scale beyond that observed in the neat material. We study the photoluminescence decay kinetics of the blend in detail and conclude that this long-lived photoluminescence arises from radiative nongeminate recombination of charge carriers, which we propose occurs via a donor/acceptor CT state located close in energy to the singlet state of the polymer donor. Additionally, crystallographic and spectroscopic studies point toward low subgap disorder, which could be beneficial for low radiative and nonradiative losses. These results provide an important demonstration of photoluminescence due to nongeminate charge recombination in an efficient OPV blend, a key step in identifying new OPV materials and materials-screening criteria if OPV is to approach the theoretical limits to efficiency.