Publications

Journal article

Westacott P, Treat ND, Martin J, Bannock JH, de Mello JC, Chabinyc M, Sieval AB, Michels JJ, Stingelin Net al., 2016,

Origin of fullerene-induced vitrification of fullerene: donor polymer photovoltaic blends and its impact on solar cell performance

, Journal of Materials Chemistry A, Vol: 5, Pages: 2689-2700, ISSN: 2050-7488

Organic solar cell blends comprised of an electron donating polymer and electron accepting fullerene typically form upon solution casting a thin-film structure made up of a complex mixture of phases. These phases can vary greatly in: composition, order and thermodynamic stability; and they are dramatically influenced by the processing history. Understanding the processes that govern the formation of these phases and their subsequent effect on the efficiency of photo-generating and extracting charge carriers is of utmost importance to enable rational design and processing of these blends. Here we show that the vitrifying effect of three fullerene derivatives ([60]PCBM, bis[60]PCBM, and [60]ICBA) on the prototypical donor polymer (rr-P3HT) can dominate microstructure formation of fullerene/donor polymer blends cast from solution. Using a dynamic crystallization model based on an amalgamation of Flory–Huggins and Lauritzen–Hoffman theory coupled to solvent evaporation we demonstrate that this vitrification, which can result in a large fraction of highly intermixed amorphous solid solution of the fullerene and the polymer, is due to kinetic and thermodynamic reasons. The former is partly determined by the glass transition temperature of the individual components while donor polymer:fullerene miscibility, strongly influenced by the chemical nature of the donor and the fullerene and leading to thermodynamic mixing, dictates the second phenomena. We show that our approximate dynamic crystallization model assists understanding the different solid-state structure formation of rr-P3HT:fullerene blends. Due to the generality of the assumptions used, our model should be widely applicable and assist to capture the influence of the different vitrification mechanisms also of other photovoltaic blends, including the high-efficiency systems based on the strongly aggregating PCE11 (PffBT4T-2OD), which also feature clear signs of vitirfication upon blending with, e.g., [60

Journal article

Bannock JH, Xu W, Baïssas T, Heeney M, de Mello JCet al., 2016,

Rapid flow-based synthesis of poly(3-hexylthiophene) using 2-methyltetrahydrofuran as a bio-derived reaction solvent

, European Polymer Journal, Vol: 80, Pages: 240-246, ISSN: 1873-1945

We report the synthesis of poly(3-hexylthiophene) (P3HT) by Grignard metathesis (GRIM) polymerization using the bio-derived ‘green’ solvent 2-methyltetrahydrofuran (2-MeTHF). Using a standard flask-based reaction, the molecular weight distribution, regioregularity and product yield were found to be similar to those obtained under equivalent conditions using tetrahydrofuran (THF) as a reaction solvent. The synthesis was subsequently adapted to a novel “tube-in-shell” droplet-based flow reactor, using a newly developed high-solubility catalyst derived from nickel(II) bromide ethylene glycol dimethyl ether complex (Ni(dme)Br2) and 1,3-bis(diphenylphosphino)propane (dppp). Use of the new catalyst together with an increased reaction temperature of 65 oC (enabled by the higher boiling point of 2-MeTHF) resulted in an approximate four-fold increase in reaction rate compared to a standard THF- based synthesis at 55 oC, with full conversion reached within one minute. The purified flow- synthesized polymer had an Mw of 46 kg mol-1, a low PDI of 1.4, and a regioregularity of 93 %, indicating the suitability of flow-based GRIM polymerization in 2-MeTHF for the high- throughput synthesis of high quality P3HT.

Journal article

Bannock JH, Treat N, Chabinyc M, Stingelin N, Heeney M, de Mello JCet al., 2016,

The influence of polymer purification on the efficiency of poly(3-hexylthiophene):fullerene organic solar cells

, Scientific Reports, Vol: 6, ISSN: 2045-2322

We report the influence of different polymer purification procedures on the photovoltaic performance ofbulk heterojunction solar cells formed from binary blends of poly(3-hexylthiophene) (P3HT) andfullerenes. Selective Soxhlet extractions and metal scavenging agents were used to remove residualmonomer, magnesium salt by-products and catalyst from high-weight P3HT (Mw 121 kg/mol, PDI 1.8,RR 99%) synthesised by the Grignard metathesis (GRIM) polymerization route. Using phenyl-C61-butyric acid methyl ester (PC60BM) as an electron acceptor, we observed an increase in average powerconversion efficiency from 2.3 to 4.8 % in going from crude to fully purified material. Using indene-C60bisadduct (IC60BA) in place of PC60BM, we observed a further increase to an average value of 6.6 %,which is high for a bulk heterojunction formed from a binary blend of P3HT and C60 fullerene derivatives.

Journal article

Hu H, Zhao K, Fernandes N, Boufflet P, Bannock JH, Yu L, de Mello JC, Stingelin N, Heeney M, Giannelise EP, Amassian Aet al., 2015,

Entanglements in marginal solutions: a means of tuning pre-aggregation of conjugated polymers with positive implications for charge transport

, Journal of Materials Chemistry C, Vol: 3, Pages: 7394-7404, ISSN: 2050-7534

The solution-processing of conjugated polymers, just like commodity polymers, is subject to solvent and molecular weight-dependent solubility, interactions and chain entanglements within the polymer, all of which can influence the crystallization and microstructure development in semi-crystalline polymers and consequently affect charge transport and optoelectronic properties. Disentanglement of polymer chains in marginal solvents was reported to work via ultrasonication, facilitating the formation of photophysically ordered polymer aggregates. In this contribution, we explore how a wide range of technologically relevant solvents and formulations commonly used in organic electronics influence chain entanglement and the aggregation behaviour of P3HT using a combination of rheological and spectrophotometric measurements. The specific viscosity of the solution offers an excellent indication of the degree of entanglements in the solution, which is found to be related to the solubility of P3HT in a given solvent. Moreover, deliberately disentangling the solution in the presence of solvophobic driving forces, leads consistently to formation of photophysically visible aggregates which is indicative of local and perhaps long range order in the solute. We show for a broad range of solvents and molecular weights that disentanglement ultimately leads to significant ordering of the polymer in the solid state and a commensurate increase in charge transport properties. In doing so we demonstrate a remarkable ability to tune the microstructure which has important implications for transport properties. We discuss its potential implications in the context of organic electronics and photovoltaics.

Journal article

Buchaca-Domingo E, Vandewal K, Fei Z, Watkins SE, Scholes FH, Bannock JH, de Mello JC, Richter LJ, DeLongchamp DM, Amassian A, Heeney M, Salleo A, Stingelin Net al., 2015,

Direct correlation of charge transfer absorption with molecular donor: acceptor interfacial area via photothermal deflection spectroscopy

, Journal of the American Chemical Society, Vol: 137, Pages: 5256-5259, ISSN: 0002-7863

Here we show that the charge transfer (CT) absorption signal in bulk-heterojunction solar cell blends, measured by photothermal deflection spectroscopy, is directly proportional to the density of molecular donor:acceptor interfaces. Since the optical transitions from the ground state to the interfacial CT state are weakly allowed at photon energies below the optical gap of both the donor and acceptor, we can exploit the use of this sensitive linear absorption spectroscopy for such quantification. Moreover, we determine the absolute molar extinction coefficient of the CT transition for an archetypical polymer:fullerene interface. The latter is ∼100 times lower than the extinction coefficient of the donor chromophore involved, allowing us to experimentally estimate the transition dipole moment as 0.3 D and the electronic coupling between the ground and CT states to be on the order of 30 meV.

Journal article

Hellmann C, Treat ND, Scaccabarozzi AD, Hollis JR, Fleischli FD, Bannock JH, de Mello J, Michels JJ, Kim J-S, Stingelin Net al., 2015,

Solution Processing of Polymer Semiconductor: Insulator Blends-Tailored Optical Properties Through Liquid-Liquid Phase Separation Control

, JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS, Vol: 53, Pages: 304-310, ISSN: 0887-6266

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Citations: 27

Journal article

Dattani R, Telling MTF, Lopez CG, Krishnadasan SH, Bannock JH, Terry AE, de Mello JC, Cabral JT, Nedoma AJet al., 2015,

Rapid precipitation: An alternative to solvent casting for organic solar cells

, Chemphyschem, Vol: 16, Pages: 1231-1238, ISSN: 1439-7641

Rapid precipitation, immersion of a liquid formulation into a nonsolvent, is compared with drop casting for fabricating organic solar cells. Blends comprising poly-3-hexylthiophene (P3HT), phenyl-C61-butyric acid methyl ester (PCBM), and chlorobenzene were processed into bulk samples by using two distinct routes: rapid precipitation and drop casting. The resulting structure, phases, and crystallinity were analyzed by using small-angle neutron scattering, X-ray diffraction, differential scanning calorimetry, and muon spin resonance. Rapid precipitation was found to induce a finely structured phase separation between PCBM and P3HT, with 65 wt % crystallinity in the P3HT phase. In contrast, solvent casting resulted in a mixed PCBM/P3HT phase with only 43 wt % P3HT crystallinity. The structural advantages conferred by rapid precipitation were shown to persist following intense thermal treatments.

Journal article

Dattani R, Bannock JH, Fei Z, MacKenzie RCI, Guilbert AAY, Vezie MS, Nelson J, de Mello JC, Heeney M, Cabral JT, Nedoma AJet al., 2014,

A general mechanism for controlling thin film structures in all-conjugated block copolymer: fullerene blends

, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 2, Pages: 14711-14719, ISSN: 2050-7488

Journal article

Bannock JH, Krishnadasan SH, Heeney M, de Mello JCet al., 2014,

A gentle introduction to the noble art of flow chemistry

, MATERIALS HORIZONS, Vol: 1, Pages: 373-378, ISSN: 2051-6347

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Citations: 35

Journal article

Beesley DJ, Semple J, Krishnan Jagadamma L, Amassian A, McLachlan MA, Anthopoulos TD, DeMello JCet al., 2014,

Sub-15-nm patterning of asymmetric metal electrodes and devices by adhesion lithography

, Nat Commun, Vol: 5

Coplanar electrodes formed from asymmetric metals separated on the nanometre length scale are essential elements of nanoscale photonic and electronic devices. Existing fabrication methods typically involve electron-beam lithography—a technique that enables high fidelity patterning but suffers from significant limitations in terms of low throughput, poor scalability to large areas and restrictive choice of substrate and electrode materials. Here, we describe a versatile method for the rapid fabrication of asymmetric nanogap electrodes that exploits the ability of selected self-assembled monolayers to attach conformally to a prepatterned metal layer and thereby weaken adhesion to a subsequently deposited metal film. The method may be carried out under ambient conditions using simple equipment and a minimum of processing steps, enabling the rapid fabrication of nanogap electrodes and optoelectronic devices with aspect ratios in excess of 100,000.

Journal article

Bannock JH, Al-Hashimi M, Krishnadasan SH, Halls JJM, Heeney M, de Mello JCet al., 2014,

Controlled synthesis of conjugated random copolymers in a droplet-based microreactor

, Materials Horizons

Journal article

Xia R, Leem D-S, Kirchartz T, Spencer S, Murphy C, He Z, Wu H, Su S, Cao Y, Kim JS, deMello JC, Bradley DDC, Nelson Jet al., 2013,

Investigation of a Conjugated Polyelectrolyte Interlayer for Inverted Polymer:Fullerene Solar Cells

, ADVANCED ENERGY MATERIALS, Vol: 3, Pages: 718-723, ISSN: 1614-6832

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Citations: 89

Journal article

Nightingale AM, Bannock JH, Krishnadasan SH, O'Mahoney FTF, Haque SA, Sloan J, Drury C, McIntyre R, de Mello JCet al., 2013,

Large-scale synthesis of nanocrystals in a multichannel droplet reactor

, Journal of Materials Chemistry A

Journal article

Westacott P, Tumbleston JR, Shoaee S, Fearn S, Bannock JH, Gilchrist JB, Heutz S, DeMello J, Heeney M, Ade H, Durrant J, McPhail DS, Stingelin Net al., 2013,

On the role of intermixed phases in organic photovoltaic blends

, Energy & Environmental Science, ISSN: 1754-5692

Journal article

Bannock JH, Krishnadasan SH, Nightingale AM, Yau CP, Khaw K, Burkitt D, Halls JJM, Heeney H, de Mello JCet al., 2012,

Continuous Synthesis of Device-Grade Semiconducting Polymers in Droplet-Based Microreactors

, Advanced Functional Materials

A method is reported for the controlled synthesis of device-grade semiconducting polymers, utilizing a droplet-based microfluidic reactor. Using poly(3-hexylthiophene) (P3HT) as a test material, the reactor is shown to provide a controlled and stable environment for polymer synthesis, enabling control of molecular weight via tuning of flow conditions, reagent composition or temperature. Molecular weights of up to 92 000 Da are readily attainable, without leakage or reactor fouling. The method avoids the usual deterioration in materials quality that occurs when conventional batch syntheses are scaled from the sub-gram level to higher quantities, with a prototype five-channel reactor producing material of consistent molecular weight distribution and high regioregularity (>98%) at a rate of ≈60 g/day. The droplet-synthesized P3HT compares favorably with commercial material in terms of absorption spectrum, polydispersity, regioregularity, and crystallinity, yielding power conversion efficiencies of up to 4% in bulk heterojunction solar cells with [6,6]-phenyl-C61-butyric acid methyl ester.

Journal article

Faist MA, Kirchartz T, Gong W, Ashraf RS, McCulloch I, de Mello JC, Ekins-Daukes NJ, Bradley DDC, Nelson Jet al., 2012,

Competition between the Charge Transfer State and the Singlet States of Donor or Acceptor Limiting the Efficiency in Polymer:Fullerene Solar Cells

, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 134, Pages: 685-692, ISSN: 0002-7863

Journal article

Kim S, Wang X, Yim JH, Tsoi WC, Kim J-S, Lee S, deMello JCet al., 2012,

Efficient organic solar cells based on spray-patterned single wall carbon nanotube electrodes

, JOURNAL OF PHOTONICS FOR ENERGY, Vol: 2, ISSN: 1947-7988

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Citations: 9

Journal article

Kumar K, Nightingale AM, Krishnadasan SH, Kamaly N, Wylenzinska-Arridge M, Zeissler K, Branford WR, Ware E, deMello AJ, deMello JCet al., 2012,

Direct synthesis of dextran-coated superparamagnetic iron oxide nanoparticles in a capillary-based droplet reactor

, JOURNAL OF MATERIALS CHEMISTRY, Vol: 22, Pages: 4704-4708, ISSN: 0959-9428

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Citations: 101

Journal article

Leem D-S, Edwards A, Faist M, Nelson J, Bradley DDC, de Mello JCet al., 2011,

Efficient Organic Solar Cells with Solution-Processed Silver Nanowire Electrodes

, ADVANCED MATERIALS, Vol: 23, Pages: 4371-+, ISSN: 0935-9648

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Citations: 494

Journal article

Bronstein H, Leem DS, Hamilton R, Woebkenberg P, King S, Zhang W, Ashraf RS, Heeney M, Anthopoulos TD, de Mello J, McCulloch Iet al., 2011,

Indacenodithiophene-<i>co</i>-benzothiadiazole Copolymers for High Performance Solar Cells or Transistors via Alkyl Chain Optimization

, MACROMOLECULES, Vol: 44, Pages: 6649-6652, ISSN: 0024-9297

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Citations: 158

Journal article

Woebkenberg PH, Eda G, Leem D-S, de Mello JC, Bradley DDC, Chhowalla M, Anthopoulos TDet al., 2011,

Reduced Graphene Oxide Electrodes for Large Area Organic Electronics

, ADVANCED MATERIALS, Vol: 23, Pages: 1558-1562, ISSN: 0935-9648

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Citations: 89

Journal article

George WN, Giles M, McCulloch I, Steinke JHG, deMello JCet al., 2011,

Efficient Quenching of a Guanidinium-Containing Fluorescence Sensor

, CHEMPHYSCHEM, Vol: 12, Pages: 765-768, ISSN: 1439-4235

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Citations: 6

Journal article

Ashraf RS, Chen ZY, Leem DS, Bronstein H, Zhang WM, Schroeder B, Geerts Y, Smith J, Watkins S, Anthopoulos TD, Sirringhaus H, de Mello JC, Heeney M, McCulloch Iet al., 2011,