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  • Journal article
    Nielsen CB, Holliday S, Chen H, Cryer S, Mcculloch Iet al., 2015,

    Non-fullerene electron acceptors for use in organic solar cells

    , Accounts of Chemical Research, Vol: 48, Pages: 2803-2812, ISSN: 1520-4898

    The active layer in a solution processed organic photovoltaic device comprises a light absorbing electron donor semiconductor, typically a polymer, and an electron accepting fullerene acceptor. Although there has been huge effort targeted to optimize the absorbing, energetic, and transport properties of the donor material, fullerenes remain as the exclusive electron acceptor in all high performance devices. Very recently, some new non-fullerene acceptors have been demonstrated to outperform fullerenes in comparative devices. This Account describes this progress, discussing molecular design considerations and the structure–property relationships that are emerging.The motivation to replace fullerene acceptors stems from their synthetic inflexibility, leading to constraints in manipulating frontier energy levels, as well as poor absorption in the solar spectrum range, and an inherent tendency to undergo postfabrication crystallization, resulting in device instability. New acceptors have to address these limitations, providing tunable absorption with high extinction coefficients, thus contributing to device photocurrent. The ability to vary and optimize the lowest unoccupied molecular orbital (LUMO) energy level for a specific donor polymer is also an important requirement, ensuring minimal energy loss on electron transfer and as high an internal voltage as possible. Initially perylene diimide acceptors were evaluated as promising acceptor materials. These electron deficient aromatic molecules can exhibit good electron transport, facilitated by close packed herringbone crystal motifs, and their energy levels can be synthetically tuned. The principal drawback of this class of materials, their tendency to crystallize on too large a length scale for an optimal heterojunction nanostructure, has been shown to be overcome through introduction of conformation twisting through steric effects. This has been primarily achieved by coupling two units together, forming dimers
  • Journal article
    Baran D, Vezie MS, Gasparini N, Deledalle F, Yao J, Schroeder BC, Bronstein H, Ameri T, Kirchartz T, McCulloch I, Nelson J, Brabec CJet al., 2015,

    Role of Polymer Fractionation in Energetic Losses and Charge Carrier Lifetimes of Polymer: Fullerene Solar Cells

    , JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 119, Pages: 19668-19673, ISSN: 1932-7447
  • Journal article
    Yue W, Ashraf RS, Nielsen C, Collado-Fregoso E, Niazi M, Yousaf S, Kirkus M, Chen H-Y, Amassian A, Durrant JAMES, McCulloch IAINet al., 2015,

    A Thieno[3,2-b][1]benzothiophene Isoindigo Building Block for Additive- and Annealing-Free High-Performance Polymer Solar Cells

    , Advanced Materials, Vol: 27, Pages: 4702-4707, ISSN: 1521-4095

    A novel photoactive polymer with two different molecular weights is reported, based on a new building block: thieno[3,2-b][1]benzothiophene isoindigo. Due to the improved crystallinity, optimal blend morphology, and higher charge mobility, solar-cell devices of the high-molecular-weight polymer exhibit a superior performance, affording efficiencies of 9.1% without the need for additives, annealing, or additional extraction layers during device fabrication.
  • Journal article
    Andernach RE, Rossbauer S, Ashraf RS, Faber H, Anthopoulos TD, McCulloch I, Heeney M, Bronstein HAet al., 2015,

    Conjugated Polymer-Porphyrin Complexes for Organic Electronics

    , CHEMPHYSCHEM, Vol: 16, Pages: 1223-1230, ISSN: 1439-4235
  • Journal article
    Rumer JW, McCulloch I, 2015,

    Organic photovoltaics: Crosslinking for optimal morphology and stability

    , Materials Today, Vol: 18, Pages: 425-435, ISSN: 1873-4103
  • Journal article
    Nielsen CB, White AJP, McCulloch I, 2015,

    Effect of fluorination of 2,1,3-benzothiadiazole

    , Journal of Organic Chemistry, Vol: 80, Pages: 5045-5048, ISSN: 1520-6904
  • Journal article
    Dimitrov SD, Wheeler S, Niedzialek D, Schroeder BC, Utzat H, Frost JM, Yao J, Gillett A, Tuladhar PS, McCulloch I, Nelson J, Durrant JRet al., 2015,

    Polaron pair mediated triplet generation in polymer/fullerene blends

    , Nature Communications, Vol: 6, ISSN: 2041-1723

    Electron spin is a key consideration for the function of organic semiconductors in light-emitting diodes and solar cells, as well as spintronic applications relying on organic magnetoresistance. A mechanism for triplet excited state generation in such systems is by recombination of electron-hole pairs. However, the exact charge recombination mechanism, whether geminate or nongeminate and whether it involves spin-state mixing is not well understood. In this work, the dynamics of free charge separation competing with recombination to polymer triplet states is studied in two closely related polymer-fullerene blends with differing polymer fluorination and photovoltaic performance. Using time-resolved laser spectroscopic techniques and quantum chemical calculations, we show that lower charge separation in the fluorinated system is associated with the formation of bound electron-hole pairs, which undergo spin-state mixing on the nanosecond timescale and subsequent geminate recombination to triplet excitons. We find that these bound electron-hole pairs can be dissociated by electric fields.
  • Journal article
    Nielsen CB, Ashraf RS, Treat ND, Schroeder BC, Donaghey JE, White AJP, Stingelin N, McCulloch Iet al., 2015,

    2,1,3-Benzothiadiazole-5,6-Dicarboxylic Imide - A Versatile Building Block for Additive-and Annealing-Free Processing of Organic Solar Cells with Effi ciencies Exceeding 8%

    , ADVANCED MATERIALS, Vol: 27, Pages: 948-953, ISSN: 0935-9648
  • Journal article
    Ashraf RS, Meager I, Nikolka M, Kirkus M, Planells M, Schroeder BC, Holliday S, Hurhangee M, Nielsen CB, Sirringhaus H, McCulloch Iet al., 2015,

    Chalcogenophene Comonomer Comparison in Small Band Gap Diketopyrrolopyrrole-Based Conjugated Polymers for High-Performing Field-Effect Transistors and Organic Solar Cells

    , JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 137, Pages: 1314-1321, ISSN: 0002-7863
  • Journal article
    Holliday S, Ashraf RS, Nielsen CB, Kirkus M, Roehr JA, Tan C-H, Collado-Fregoso E, Knall A-C, Durrant JR, Nelson J, McCulloch Iet al., 2015,

    A rhodanine flanked nonfullerene acceptor for solution-processed organic photovoltaics

    , Journal of the American Chemical Society, Vol: 137, Pages: 898-904, ISSN: 0002-7863

    A novel small molecule, FBR, bearing 3-ethylrhodanine flanking groups was synthesized as a nonfullerene electron acceptor for solution-processed bulk heterojunction organic photovoltaics (OPV). A straightforward synthesis route was employed, offering the potential for large scale preparation of this material. Inverted OPV devices employing poly(3-hexylthiophene) (P3HT) as the donor polymer and FBR as the acceptor gave power conversion efficiencies (PCE) up to 4.1%. Transient and steady state optical spectroscopies indicated efficient, ultrafast charge generation and efficient photocurrent generation from both donor and acceptor. Ultrafast transient absorption spectroscopy was used to investigate polaron generation efficiency as well as recombination dynamics. It was determined that the P3HT:FBR blend is highly intermixed, leading to increased charge generation relative to comparative devices with P3HT:PC60BM, but also faster recombination due to a nonideal morphology in which, in contrast to P3HT:PC60BM devices, the acceptor does not aggregate enough to create appropriate percolation pathways that prevent fast nongeminate recombination. Despite this nonoptimal morphology the P3HT:FBR devices exhibit better performance than P3HT:PC60BM devices, used as control, demonstrating that this acceptor shows great promise for further optimization.
  • Journal article
    Qiu W, Paetzold UW, Gehlhaar R, Smirnov V, Boyen H-G, Tait JG, Conings B, Zhang W, Nielsen CB, McCulloch I, Froyen L, Heremans P, Cheyns Det al., 2015,

    An electron beam evaporated TiO<sub>2</sub> layer for high efficiency planar perovskite solar cells on flexible polyethylene terephthalate substrates

    , JOURNAL OF MATERIALS CHEMISTRY A, Vol: 3, Pages: 22824-22829, ISSN: 2050-7488
  • Journal article
    Venkateshvaran D, Nikolka M, Sadhanala A, Lemaur V, Zelazny M, Kepa M, Hurhangee M, Kronemeijer AJ, Pecunia V, Nasrallah I, Romanov I, Broch K, McCulloch I, Emin D, Olivier Y, Cornil J, Beljonne D, Sirringhaus Het al., 2014,

    Approaching disorder-free transport in high-mobility conjugated polymers

    , NATURE, Vol: 515, Pages: 384-388, ISSN: 0028-0836
  • Journal article
    Schroeder BC, Li Z, Brady MA, Faria GC, Ashraf RS, Takacs CJ, Cowart JS, Duong DT, Chiu KH, Tan C-H, Cabral JT, Salleo A, Chabinyc ML, Durrant JR, McCulloch Iet al., 2014,

    Enhancing fullerene-based solar cell lifetimes by addition of a fullerene dumbbell

    , Angewandte Chemie-International Edition, Vol: 53, Pages: 12870-12875, ISSN: 1521-3773

    Cost-effective, solution-processable organic photovoltaics (OPV) present an interesting alternative to inorganic silicon-based solar cells. However, one of the major remaining challenges of OPV devices is their lack of long-term operational stability, especially at elevated temperatures. The synthesis of a fullerene dumbbell and its use as an additive in the active layer of a PCDTBT:PCBM-based OPV device is reported. The addition of only 20 % of this novel fullerene not only leads to improved device efficiencies, but more importantly also to a dramatic increase in morphological stability under simulated operating conditions. Dynamic secondary ion mass spectrometry (DSIMS) and TEM are used, amongst other techniques, to elucidate the origins of the improved morphological stability.
  • Journal article
    McCulloch I, Planells M, Schroeder BC, 2014,

    Effect of Chalcogen Atom Substitution on the Optoelectronic Properties in Cyclopentadithiophene Polymers

    , Macromolecules, ISSN: 0024-9297
  • Journal article
    Rumer JW, Schroeder BC, Nielsen CB, Ashraf RS, Beatrup D, Bronstein H, Cryer SJ, Donaghey JE, Holliday S, Hurhangee M, James DI, Lim S, Meager I, Zhang W, McCulloch Iet al., 2014,

    Bis-lactam-based donor polymers for organic solar cells: Evolution by design

    , Thin Solid Films, Vol: 560, Pages: 82-85, ISSN: 0040-6090

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