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  • Journal article
    Yan H, Limbu S, Wang X, Nightingale J, Hamilton I, Wade J, Kwon S, Lee K, Kim J-Set al., 2019,

    Efficient charge carrier injection and balance achieved by low electrochemical doping in solution-processed polymer light-emitting diodes

    , Advanced Functional Materials, Vol: 29, Pages: 1-9, ISSN: 1616-301X

    Charge carrier injection and transport in polymer light‐emitting diodes (PLEDs) is strongly limited by the energy level offset at organic/(in)organic interfaces and the mismatch in electron and hole mobilities. Herein, these limitations are overcome via electrochemical doping of a light‐emitting polymer. Less than 1 wt% of doping agent is enough to effectively tune charge injection and balance and hence significantly improve PLED performance. For thick single‐layer (1.2 µm) PLEDs, dramatic reductions in current and luminance turn‐on voltages (VJ = 11.6 V from 20.0 V and VL = 12.7 V from 19.8 V with/without doping) accompanied by reduced efficiency roll‐off are observed. For thinner (<100 nm) PLEDs, electrochemical doping removes a thickness dependence on VJ and VL, enabling homogeneous electroluminescence emission in large‐area doped devices. Such efficient charge injection and balance properties achieved in doped PLEDs are attributed to a strong electrochemical interaction between the polymer and the doping agents, which is probed by in situ electric‐field‐dependent Raman spectroscopy combined with further electrical and energetic analysis. This approach to control charge injection and balance in solution‐processed PLEDs by low electrochemical doping provides a simple yet feasible strategy for developing high‐quality and efficient lighting applications that are fully compatible with printing technologies.

  • Journal article
    Way A, Luke J, Evans AD, Li Z, Kim J-S, Durrant JR, Hin Lee HK, Tsoi WCet al., 2019,

    Fluorine doped tin oxide as an alternative of indium tin oxide for bottom electrode of semi-transparent organic photovoltaic devices

    , AIP Advances, Vol: 9, Pages: 085220-1-085220-5, ISSN: 2158-3226

    Indium tin oxide (ITO) is commonly used as the transparent bottom electrode for organic solar cells. However, it is known that the cost ofthe ITO is quite high due to the indium element, and in some studies ITO coated glass substrate is found to be the most expensive componentof device fabrication. Moreover, indium migration from ITO can cause stability issues in organic solar cells. Nevertheless, the use of ITO asthe bottom electrode is still dominating in the field. Here, we explore the possibility of using fluorine doped tin oxide (FTO) as an alternativeto ITO for the bottom electrode of organic solar cells particularly on semi-transparent cells. We present side-by-side comparisons on theiroptical, morphological and device properties and suggest that FTO could be more suitable than ITO as the bottom electrode for glass substratebased organic photovoltaic devices.

  • Journal article
    Speller EM, Clarke AJ, Luke J, Lee HKH, Durrant JR, Li N, Wang T, Wong HC, Kim J-S, Tsoi WC, Li Zet al., 2019,

    From fullerene acceptors to non-fullerene acceptors: prospects and challenges in the stability of organic solar cells

    , Journal of Materials Chemistry A, ISSN: 2050-7488

    <p>This review highlights the opportunities and challenges in stability of organic solar cells arising from the emergence of non-fullerene acceptors.</p>

  • Journal article
    Cha H, Fish G, Luke J, Alraddadi A, Lee HH, Zhang W, Dong Y, Limbu S, Wadsworth A, Maria IP, Francas L, Sou HL, Du T, Kim J-S, McLachlan MA, McCulloch I, Durrant JRet al., 2019,

    Suppression of Recombination Losses in Polymer:Nonfullerene Acceptor Organic Solar Cells due to Aggregation Dependence of Acceptor Electron Affinity

    , ADVANCED ENERGY MATERIALS, Vol: 9, ISSN: 1614-6832
  • Journal article
    Du T, Xu W, Daboczi M, Kim J, Xu S, Lin C-T, Kang H, Lee K, Heeney MJ, Kim J-S, Durrant JR, McLachlan MAet al., 2019,

    p-Doping of organic hole transport layers in p–i–n perovskite solar cells: correlating open-circuit voltage and photoluminescence quenching

    , Journal of Materials Chemistry A, Vol: 7, Pages: 18971-18979, ISSN: 2050-7488

    Doping is a widely implemented strategy for enhancing the inherent electronic properties of charge transport layers in photovoltaic (PV) devices. Here, in direct contrast to existing understanding, we find that a reduction in p-doping of the organic hole transport layer (HTL) leads to substantial improvements in PV performance in planar p–i–n perovskite solar cells (PSCs), driven by improvements in open circuit voltage (VOC). Employing a range of transient and steady state characterisation tools, we find that the improvements of VOC correlate with reduced surface recombination losses in less p-doped HTLs. A simple device model including screening of bulk electric fields in the perovskite layer is used to explain this observation. In particular, photoluminescence (PL) emission of complete solar cells shows that efficient performance is correlated to a high PL intensity at open circuit and a low PL intensity at short circuit. We conclude that desirable transport layers for p–i–n PSCs should be charge selective contacts with low doping densities.

  • Journal article
    Wade J, Pugh H, Nightingale J, Kim J, Williams STet al., 2019,

    Colour in bivalve shells: Using resonance Raman spectroscopy to compare pigments at different phylogenetic levels

    , Journal of Raman Spectroscopy, Pages: 1-10, ISSN: 0377-0486

    Several studies have suggested that shell colour may be phylogenetically distributed within the phylum Mollusca, but this pattern is confounded by our ignorance of the homology of colour and lack of understanding about the identity of most molluscan pigments. We use resonance Raman spectroscopy to address this problem by examining bivalve pigments producing a range of colours and compare spectra from taxa at different phylogenetic levels. The spectra of most shell pigments exhibited a skeletal signature typical of partially methylated polyenes, possibly modified carotenoids, with the strongest peaks occurring between 1,501–1,540 cm−1 and 1,117–1,144 cm−1 due to the C═C (ν1) and C–C (ν2) stretching modes, respectively. Neither pigment class nor mineral structure differentiated Imparidentia and Pteriomorphia. Spectral acquisitions for purple pigments for two species of Asaphis suggest that identical or nearly identical pigments are shared within this genus, and some red pigments from distantly related species have similar spectra. Conversely, two species with brown shells have distinctly different pigments, highlighting the difficulty in determining the homology of colour even within a single class of pigments. Curiously, we were unable to detect any Raman activity for green‐coloured shell or pigment peaks for the yellow area of Codakia paytenorum, suggesting that these colours are due to structural elements or a pigment that is quite different from those observed in other taxa examined to date. Our results are consistent with the idea that classes of pigments are evolutionarily ancient but heritable modifications may be specific to clades.

  • Journal article
    Jayaram AK, Pitsalidis C, Tan E, Moysidou C-M, De Voider MFL, Kim J-S, Owens RMet al., 2019,

    3D Hybrid Scaffolds Based on PEDOT:PSS/MWCNT Composites

    , FRONTIERS IN CHEMISTRY, Vol: 7, ISSN: 2296-2646
  • Journal article
    Wang Y, Daboczi M, Mesa CA, Ratnasingham SR, Kim JS, Durrant JR, Dunn S, Yan H, Briscoe J, Wang Y, Daboczi M, Mesa CA, Ratnasingham SR, Kim J-S, Durrant JR, Dunn S, Yan H, Briscoe Jet al., 2019,

    Bi₂Fe₄O₉ thin films as novel visible-light-active photoanodes for solar water splitting

    , Journal of Materials Chemistry A, Vol: 7, Pages: 9537-9541, ISSN: 2050-7496

    We report the chemical solution deposition (CSD) of a phase-pure Bi2Fe4O9 thin film for use as a photoanode in photoelectrochemical (PEC) water splitting. The energy levels of Bi2Fe4O9 films have been measured and n-type characteristics have been confirmed. With band gaps determined as 2.05 eV (indirect) and 2.80 eV (direct) and valence and conduction bands straddling the water oxidation and reduction potentials, this material is highly promising as a photocatalyst for solar water splitting. The photocurrent of a planar photoanode reached 0.1 mA cm−2 at 1.23 VNHE under AM1.5G illumination. The addition of H2O2 as a hole scavenger increased the photocurrent to 0.25 mA cm−2, indicating hole injection is one limiting factor to the performance. The performance was enhanced by nearly 5-fold when the Bi2Fe4O9 photoanode is coupled to a Co–Pi surface co-catalyst. The photoanode also shows excellent stability with no change in photocurrent over three hours of continuous illumination. These results indicate that this material represents a promising addition to the growing selection of low-cost, stable photocatalysts for use in solar water splitting.

  • Journal article
    Luke J, Speller EM, Wadsworth A, Wyatt MF, Dmiitrov S, Lee HKH, Li Z, Tsoi WC, McCulloch I, Bagnis D, Durrant JR, Kim J-Set al., 2019,

    Twist and degrade – Impact of molecular structure on the photostability of non-fullerene acceptors and their photovoltaic blends

    , Advanced Energy Materials, Vol: 9, Pages: 1-14, ISSN: 1614-6832

    Non-fullerene acceptors (NFAs) dominate organic photovoltaic (OPV) research due to their promising efficiencies and stabilities. However, there is very little investigation into the molecular processes of degradation, which is critical to guiding design of novel NFAs for long-lived, commercially viable OPVs. Here we investigate the important role of molecular structure and conformation on NFA photostability in air by comparing structurally similar but conformationally different promising NFAs; planar O-IDTBR and non-planar O-IDFBR. We identify a three-phase degradation process: (i) initial photo-induced conformational change (i.e. torsion about the Core-BT dihedral), induced by non-covalent interactions with environmental molecules, (ii) followed by photo-oxidation and fragmentation, leading to chromophore bleaching and degradation product formation, and (iii) finally complete chromophore bleaching.Initial conformational change is a critical prerequisite for further degradation, providing fundamental understanding of the relative stability of IDTBR and IDFBR, where the alreadytwisted IDFBR is more prone to degradation. When blended with the donor polymer P3HT, both NFAs exhibit improved photostability whilst the photostability of the polymer itself is significantly reduced by the more miscible twisted NFA. Our findings elucidate the important role of NFA molecular structure on photostability of OPV systems, and provide vital insights into molecular design rules for intrinsically photostable NFAs.

  • Journal article
    Speller EM, Clarke AJ, Aristidou N, Wyatt MF, Francàs L, Fish G, Cha H, Lee HKH, Luke J, Wadsworth A, Evans AD, McCulloch I, Kim JS, Haque SA, Durrant JR, Dimitrov SD, Tsoi WC, Li Zet al., 2019,

    Toward improved environmental stability of polymer:fullerene and polymer:non-fullerene organic solar cells: a common energetic origin of light and oxygen induced degradation

    , ACS Energy Letters, Vol: 4, Pages: 846-852, ISSN: 2380-8195

    With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation.

  • Journal article
    He Q, Shahid M, Panidi J, Marsh AV, Huang W, Daboczi M, Kim J-S, Fei Z, Anthopoulos TD, Heeney Met al., 2019,

    A versatile star-shaped organic semiconductor based on benzodithiophene and diketopyrrolopyrrole

    , Journal of Materials Chemistry C, ISSN: 2050-7526

    We report the synthesis of a new star-shaped π-conjugated oligomer, BDT(DPP)4, containing a benzodithiophene core and four diketopyrrolopyrrole arms. The thermal, electrochemical and optical properties are characterized and the results complemented by computational studies. The utility of the molecule is demonstrated in both solar cell and field-effect transistor devices. In the former, BDT(DPP)4 displays low efficiency when used as an acceptor in blends with poly(3-hexylthiophene) but exhibits promising performance as a donor, in blends with either a fullerene or a non-fullerene acceptor. In field-effect transistors BDT(DPP)4 exhibits typical p-type transistor behavior, which is in accordance with its better donor performance in solar cell devices.

  • Journal article
    Goudarzi H, Limbu S, Cabanillas-González J, Zenonos VM, Kim J-S, Keivanidis PEet al., 2019,

    Impact of molecular conformation on triplet-fusion induced photon energy up-conversion in the absence of exothermic triplet energy transfer

    , Journal of Materials Chemistry C, Vol: 7, Pages: 3634-3643, ISSN: 2050-7526

    The use of photon energy up-converted luminescence driven by triplet-exciton annihilation reactions (TTA-UC) is increasingly gaining attention for developing next-generation light-management, and wavelength-shifting technologies. Here we present a spectroscopic study for elucidating the photophysical mechanism that operates in an unusual TTA-UC model system comprising the blue-light emitting poly(fluorene-2-octyl) (PFO) activator mixed with the green-light absorbing (2,3,7,8,12,13,17,18-octaethyl-porphyrinato) PtII (PtOEP) metalo-organic complex. The unconventional character of the PFO:PtOEP composite manifests in the fact that no exothermic triplet energy transfer (TET) is possible between triplet-excited PtOEP and PFO. Yet green-to-blue TTA-UC luminescence of PFO is obtained even when PtOEP is selectively photoexcited by pulsed laser intensities as low as 2.5 mW cm−2. Continuous-wave photo-induced absorption spectroscopy verifies that no energy transfer from triplet-excited PtOEP to the triplet level of PFO takes place, pointing to triplet–triplet annihilation (TTA) events in the PtOEP phase as the origin of the observed TTA-UC PL signal. In the PFO:PtOEP composite, the PtOEP component holds a dual role of annihilator/sensitizer; photon energy storage in PtOEP is enabled via TTA when triplet exciton diffusion coefficient values of DPtOEP = 4.1 × 10−9 cm2 s−1 are reached. With a simple yet powerful solution processing protocol, and by combining Raman and time-gate photoluminescence (PL) spectroscopy we demonstrate that the brightness of the produced TTA-UC luminescence depends on the molecular conformation of the PFO activator. A four-fold increase in the TTA-UC luminescence intensity is registered in the time-integrated and time-gated PL spectra, when the PFO matrix is arrested in its planar β-phase molecular conformation. Further enhancement of the TTA-UC PL signal is achieved when temperature lowers from 290 K down to 100 K. Th

  • Journal article
    Kim J-S, 2019,

    Impact of initial bulk-heterojunction morphology on operational stability of polymer:fullerene photovoltaic cells

    , Advanced Materials Interfaces, Vol: 6, ISSN: 2196-7350

    Controlling initial bulk-heterojunction (BHJ) morphology is critical for device performance of organic photovoltaic (OPV) cells. However, its impact on performance, specifically long-term operational stability is still poorly understood. This is mainly due to limitations in direct measurements enabling in-situ monitoring of devices at a molecular level. Here, we utilize thermal annealing preconditioning step to tune initial morphology of model polymer:fullerene BHJ OPV devices and molecular resonant vibrational spectroscopy to identify in-situ degradation pathways. We report direct spectroscopic evidence for molecular-scale phase segregation temperature (TPS) which critically determines a boundary in high efficiency and long operational stability. Under operation, initially well-mixed blend morphology (no annealing) shows interface instability related to the hole-extracting PEDOT:PSS layer via de-doping. Likewise, initially phase-segregatedmorphology at a molecular level (annealed above TPS) shows instability in the photoactive layer via continuous phase segregation between polymer and fullerenes in macroscales, coupled with further fullerene photodegradation. Our results confirm that a thermal annealing preconditioning step is essential to stabilize the BHJ morphology; in particular annealing below TPS is critical for improved operational stability whilst maintaining high efficiency.

  • Journal article
    Dash BP, Hamilton I, Tate DJ, Crossley D, Kim J-S, Ingleson MJ, Turner Met al., 2019,

    Benzoselenadiazole and benzotriazole directed electrophilic C-H borylation of conjugated donor-acceptor materials

    , Journal of Materials Chemistry C, Vol: 7, Pages: 718-724, ISSN: 2050-7526

    Benzoselenadiazole and benzotriazole directed electrophilic borylation using BCl3 results in the C–H functionalization of an adjacent aromatic unit and produces fused boracycles. Subsequent arylation at boron afforded air and moisture stable products displaying large bathochromic shifts and significantly reduced LUMO energy levels. OLEDs fabricated containing borylated benzoselenadiazole derivatives showed emission centered at 723 nm in the near infra-red region of the spectrum.

  • Journal article
    Newman MJ, Speller EM, Barbe J, Luke J, Li M, Li Z, Wang Z-K, Jain SM, Kim J-S, Lee HKH, Tsoi WCet al., 2018,

    Photo-stability study of a solution-processed small molecule solar cell system: correlation between molecular conformation and degradation

    , Science and Technology of Advanced Materials, Vol: 19, Pages: 194-202, ISSN: 1468-6996

    Solution-processed organic small molecule solar cells (SMSCs) have achieved efficiency over 11%. However, very few studies have focused on their stability under illumination and the origin of the degradation during the so-called burn-in period. Here, we studied the burn-in period of a solution-processed SMSC using benzodithiophene terthiophene rhodamine:[6,6]-phenyl C71 butyric acid methyl ester (BTR:PC71BM) with increasing solvent vapour annealing time applied to the active layer, controlling the crystallisation of the BTR phase. We find that the burn-in behaviour is strongly correlated to the crystallinity of BTR. To look at the possible degradation mechanisms, we studied the fresh and photo-aged blend films with grazing incidence X-ray diffraction, UV–vis absorbance, Raman spectroscopy and photoluminescence (PL) spectroscopy. Although the crystallinity of BTR affects the performance drop during the burn-in period, the degradation is found not to originate from the crystallinity changes of the BTR phase, but correlates with changes in molecular conformation – rotation of the thiophene side chains, as resolved by Raman spectroscopy which could be correlated to slight photobleaching and changes in PL spectra.

  • Journal article
    Keivanidis PE, Khan JI, Katzenmeier L, Kan Z, Limbu S, Constantinou MK, Lariou E, Constantinides G, Hayes SC, Kim J-S, Laquai Fet al., 2018,

    Impact of structural polymorphs on charge collection and non-geminate recombination in organic photovoltaic devices

    , The Journal of Physical Chemistry C, Vol: 122, Pages: 29141-29149, ISSN: 1932-7447

    The formation of different types of structural polymorphs of poly(3-hexyl-thiophene) (P3HT) affects the performance of organic photovoltaic (OPV) devices that use thermally-annealed P3HT:PCBM[60] blend films as photoactive layer. Here it is demonstrated that, when densely-packed and non-densely packed P3HT polymorphs co-exist in the P3HT:PCBM[60] layer, non-geminate charge recombination is fast; however, in a device non-geminate recombination is effectively overruled by efficient and fast charge carrier extraction. In stark contrast, when only a less-densely packed P3HT polymorph is present in the blend, non-geminate charge recombination losses are less pronounced, and the charge carrier extraction efficiency is lower. The antagonistic non-geminate charge recombination and charge carrier extraction processes in these systems are monitored by time-delayed-collection field (TDCF) and ultrafast transient absorption (TA) experiments. Furthermore, resonance Raman spectroscopy reveals that in the absence of the densely-packed P3HT polymorph, the energetic disorder present in the P3HT:PCBM[60] blend is higher. High-resolution atomic force microscopy imaging further identifies pronounced differences in the layer morphology when the polymorph distribution varies between unimodal and bimodal. These results indicate that less-densely packed P3HT polymorphs increase disorder and impede charge collection, leading to a reduction of the device fill factor.

  • Journal article
    Nightingale J, Wade J, Moia D, Nelson J, Kim J-Set al., 2018,

    Impact of molecular order on polaron formation in conjugated polymers

    , The Journal of Physical Chemistry C, Vol: 122, Pages: 29129-29140, ISSN: 1932-7447

    The nature of polaron formation has profound implications on the transport of charge carriers in conjugated polymers, but still remains poorly understood. Here we develop in situ electrochemical resonant Raman spectroscopy, a powerful structural probe that allows direct observation of polaron formation. We report that polaron formation in ordered poly(3-hexyl)thiophene (P3HT) polymer domains (crystalline phase) results in less pronounced changes in molecular conformation, indicating smaller lattice relaxation, compared to polarons generated in disordered polymer domains (amorphous phase) for which we observe large molecular conformational changes. These conformational changes are directly related to the effective conjugation length of the polymer. Furthermore, we elucidate how blending the P3HT polymer with phenyl C-61 butyric acid methyl ester (PCBM) affects polaron formation in the polymer. We find that blending disturbs polymer crystallinity, reducing the density of polarons that can form upon charge injection at the same potential, whilst the lost capacity is partly restored during post-deposition thermal annealing. Our study provides direct spectroscopic evidence for a lower degree of lattice reorganisation in crystalline (and therefore more planarised) polymers than in conformationally disordered polymers. This observation is consistent with higher charge carrier mobility and better device performance commonly found in crystalline polymer materials.

  • Journal article
    Lee S, Kim DB, Hamilton I, Daboczi M, Nam YS, Lee BR, Zhao B, Jang CH, Friend RH, Kim J-S, Song MHet al., 2018,

    Control of interface defects for efficient and stable quasi-2D Perovskite light-emitting diodes using nickel oxide hole injection layer

    , Advanced Science, Vol: 5, ISSN: 2198-3844

    Metal halide perovskites (MHPs) have emerged as promising materials for light‐emitting diodes owing to their narrow emission spectrum and wide range of color tunability. However, the low exciton binding energy in MHPs leads to a competition between the trap‐mediated nonradiative recombination and the bimolecular radiative recombination. Here, efficient and stable green emissive perovskite light‐emitting diodes (PeLEDs) with an external quantum efficiency of 14.6% are demonstrated through compositional, dimensional, and interfacial modulations of MHPs. The interfacial energetics and optoelectronic properties of the perovskite layer grown on a nickel oxide (NiOx) and poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate hole injection interfaces are investigated. The better interface formed between the NiOx/perovskite layers in terms of lower density of traps/defects, as well as more balanced charge carriers in the perovskite layer leading to high recombination yield of carriers are the main reasons for significantly improved device efficiency, photostability of perovskite, and operational stability of PeLEDs.

  • Journal article
    Kim H, Lee G, Becker S, Kim J, Kim H, Hwang Bet al., 2018,

    Novel patterning of flexible and transparent Ag nanowire electrodes using oxygen plasma treatment

    , Journal of Materials Chemistry C, Vol: 6, Pages: 9394-9398, ISSN: 2050-7526

    We report a novel patterning method using oxygen plasma treatment for flexible and transparent Ag nanowire electrodes. Using a dry film photoresist as a solid-state film-type photoresist, Ag nanowires were selectively oxidized under oxygen plasma treatment. Microstructural analysis revealed that the Ag nanowires were fully oxidized after 30 s of oxygen plasma treatment, which was also reflected in the changes in the optoelectronic properties of the Ag nanowires. The fully oxidized Ag nanowires could be completely dissolved in NH3 solution (aq.), without using a toxic etchant to form sharp patterns of Ag nanowire electrodes. To further confirm the applicability of the patterning technique demonstrated here in electronic devices, MoS2 thin-film transistors (TFTs) with patterned Ag-nanowire source/drain (S/D) electrodes were fabricated and they showed similar performances to typical MoS2 TFTs with thin-film-type Ti/Au S/D electrodes.

  • Journal article
    Heeney MJ, Creamer A, Wood C, Howes P, Casey A, Cong S, Marsh A, Godin R, Panidi J, Anthopoulos T, Burgess C, Wu T, Fei Z, McLachlan M, Stevens Met al., 2018,

    Post-polymerisation functionalisation of conjugated polymer backbones and its application in multi-functional emissive nanoparticles

    , Nature Communications, Vol: 9, ISSN: 2041-1723

    Backbone functionalisation of conjugated polymers is crucial to their performance in many applications, from electronic displays to nanoparticle biosensors, yet there are limited approaches to introduce functionality. To address this challenge we have developed a method for the direct modification of the aromatic backbone of a conjugated polymer, post-polymerisation. This is achieved via a quantitative nucleophilic aromatic substitution (SNAr) reaction on a range of fluorinated electron deficient comonomers. The method allows for facile tuning of the physical and optoelectronic properties within a batch of consistent molecular weight and dispersity. It also enables the introduction of multiple different functional groups onto the polymer backbone in a controlled manner. To demonstrate the versatility of this reaction, we designed and synthesised a range of emissive poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) based polymers for the creation of mono and multifunctional semiconducting polymer nanoparticles (SPNs) capable of two orthogonal bioconjugation reactions on the same surface.

  • Journal article
    Hamilton I, Chander N, Cheetham NJ, Suh M, Dyson M, Wang X-H, Stavrinou PN, Cass M, Bradley DDC, Kim J-Set al., 2018,

    Controlling molecular conformation for highly efficient and stable deep-blue copolymer light-emitting diodes

    , ACS Applied Materials and Interfaces, Vol: 10, Pages: 11070-11082, ISSN: 1944-8244

    We report a novel approach to the achievement of deep-blue, high-efficiency, and long-lived solution processed polymer light-emitting diodes (PLEDs) via a simple molecular-level conformation change whereby we introduce rigid β-phase segments into a 95% fluorene - 5% arylamine copolymer emission layer (EML). The arylamine moieties at low density act as efficient exciton formation sites in PLEDs whilst the conformational change alters the nature of the dominant luminescence from a broad, charge-transfer like emission to a significantly blue-shifted and highly vibronically structured, excitonic emission. As a consequence, we observe a significant improvement in Commission International de L'Eclairage (CIE) (x, y) co-ordinates from (0.149, 0.175) to (0.145, 0.123) whilst maintaining high efficiency and improving stability. We achieve peak luminous efficiency, η = 3.60 cd/A and luminous power efficiency, ηw = 2.44 lm/W; values that represent state of the art performance for single copolymer deep-blue PLEDs. These values are five-fold better than for otherwise-equivalent, β-phase poly(9,9-dioctylfluorene) (PFO) EML PLEDs (0.70 cd/A and 0.38 lm/W). This report represents the first demonstration of the use of molecular conformation as a vector to control the optoelectronic properties of a fluorene copolymer; previous examples have been confined to homopolymers.

  • Journal article
    Wade J, Wood S, Collado-Fregoso E, Heeney M, Durrant J, Kim J-Set al., 2017,

    Impact of Fullerene Intercalation on Structural and Thermal Properties of Organic Photovoltaic Blends

    , JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 121, Pages: 20976-20985, ISSN: 1932-7447

    The performance of organic photovoltaic blend devices is critically dependent on the polymer:fullerene interface. These interfaces are expected to impact the structural and thermal properties of the polymer with regards to the conjugated backbone planarity and transition temperatures during annealing/cooling processes. Here, we report the impact of fullerene intercalation on structural and thermal properties of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT), a highly stable material known to exhibit liquid crystalline behavior. We undertake a detailed systematic study of the extent of intercalation in the PBTTT:fullerene blend, considering the use of four different fullerene derivatives and also varying the loading ratios. Resonant Raman spectroscopy allows direct observation of the interface morphology in situ during controlled heating and cooling. We find that small fullerene molecules readily intercalate into PBTTT crystallites, resulting in a planarization of the polymer backbone, but high fullerene loading ratios or larger fullerenes result in nonintercalated domains. During cooling from melt, nonintercalated blend films are found to return to their original morphology and reproduce all thermal transitions on cooling with minimal hysteresis. Intercalated blend films show significant hysteresis on cooling due to the crystallized fullerene attempting to reintercalate. The strongest hysteresis is for intercalated blend films with excess fullerene loading ratio, which form a distinct nanoribbon morphology and exhibit a reduced geminate recombination rate. These results reveal that careful consideration should be taken during device fabrication, as postdeposition thermal treatments significantly impact the charge generation and recombination dynamics.

  • Journal article
    Cha H, Wu J, Wadsworth A, Nagitta J, Limbu S, Pont S, Li Z, Searle J, Wyatt MF, Baran D, Kim J-S, McCulloch I, Durrant JRet al., 2017,

    An efficient, "burn in" free organic solar cell employing a nonfullerene electron acceptor

    , Advanced Materials, Vol: 29, ISSN: 0935-9648

    A comparison of the efficiency, stability, and photophysics of organic solar cells employing poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3'″-di(2-octyldodecyl)-2,2';5',2″;5″,2'″-quaterthiophen-5,5'″-diyl)] (PffBT4T-2OD) as a donor polymer blended with either the nonfullerene acceptor EH-IDTBR or the fullerene derivative, [6,6]-phenyl C71 butyric acid methyl ester (PC71 BM) as electron acceptors is reported. Inverted PffBT4T-2OD:EH-IDTBR blend solar cell fabricated without any processing additive achieves power conversion efficiencies (PCEs) of 9.5 ± 0.2%. The devices exhibit a high open circuit voltage of 1.08 ± 0.01 V, attributed to the high lowest unoccupied molecular orbital (LUMO) level of EH-IDTBR. Photoluminescence quenching and transient absorption data are employed to elucidate the ultrafast kinetics and efficiencies of charge separation in both blends, with PffBT4T-2OD exciton diffusion kinetics within polymer domains, and geminate recombination losses following exciton separation being identified as key factors determining the efficiency of photocurrent generation. Remarkably, while encapsulated PffBT4T-2OD:PC71 BM solar cells show significant efficiency loss under simulated solar irradiation ("burn in" degradation) due to the trap-assisted recombination through increased photoinduced trap states, PffBT4T-2OD:EH-IDTBR solar cell shows negligible burn in efficiency loss. Furthermore, PffBT4T-2OD:EH-IDTBR solar cells are found to be substantially more stable under 85 °C thermal stress than PffBT4T-2OD:PC71 BM devices.

  • Journal article
    Jeon T, Kim SJ, Yoon J, Byun J, Hong HR, Lee TW, Kim JS, Shin B, Kim SOet al., 2017,

    Hybrid Perovskites: Effective Crystal Growth for Optoelectronic Applications

    , Advanced Energy Materials, Vol: 7, ISSN: 1614-6832

    Outstanding material properties of organic-inorganic hybrid perovskites have triggered a new insight into the next-generation solar cells. Beyond solar cells, a wide range of controllable properties of hybrid perovskites, particularly depending on crystal growth conditions, enables versatile high-performance optoelectronic devices such as light-emitting diodes, photodetectors, and lasers. This article highlights recent progress in the crystallization strategies of organic–inorganic hybrid perovskites for use as effective light harvesters or light emitters. Fundamental background on perovskite crystalline structures and relevant optoelectronic properties such as optical band-gap, electron-hole behavior, and energy band alignment are given. A detailed overview of the effective crystallization methods for perovskites, including thermal treatment, additives, solvent mediator, laser irradiation, nanostructure, and crystal dimensionalityis reported offering a comprehensive correlation among perovskite processing conditions, crystalline morphology, and relevant device performance. Finally, future research directions to overcome current practical bottlenecks and move towards reliable high performance perovskite optoelectronic applications are proposed.

  • Journal article
    Razzell-Hollis J, Fleischli F, Jahnke AA, Stingelin N, Seferos DS, Kim J-Set al., 2017,

    Effects of side-chain length and shape on polytellurophene molecular order and blend morphology

    , Journal of Physical Chemistry C, Vol: 121, Pages: 2088-2098, ISSN: 1932-7447

    We investigate the molecular order and thin film morphology of the conjugated polymer polytellurophene, in order to understand how the tellurium atom and the choice of side-chain influence the conjugated polymer’s backbone planarity and performance in organic transistors. We find that poly(3hexyltellurophene) (P3HTe) continues the trend from polythiophene (P3HT) to polyselenophene (P3HS): substitution with Tellurium leads to a more planar backbone, evident from the shifts of the C═C vibrational peak to lower wavenumbers (∼1389 cm–1) and a smaller optical band gap (∼1.4 eV). Resonant Raman spectroscopy revealed that molecular order was highly dependent on the structure of the P3ATe alkyl side-chain: a longer chains introduces kinetic hindrance, reducing the fraction of ordered phase obtained at room temperature, while a branched side-chain introduces steric hindrance, with intrinsic disorder present even when deposited at higher temperatures. When blended with the insulator HDPE, all three polymers exhibit little additional disorder and instead form phase-separated networks of high molecular order that are beneficial to percolated charge transport in transistors. We find that molecular order, as measured by Raman, correlates well with reported transistor mobilities and provides a greater understanding of the structure–property relationships that determine the performance of these novel organometallic polymers in electronic devices.

  • Journal article
    Wood S, Hollis JR, Kim J-S, 2017,

    Raman spectroscopy as an advanced structural nanoprobe for conjugated molecular semiconductors

    , Journal of Physics D: Applied Physics, Vol: 50, ISSN: 0022-3727

    Raman spectroscopy has emerged as a powerful and important characterisation tool for probing molecular semiconducting materials. The useful optoelectronic properties of these materials arise from the delocalised π-electron density in the conjugated core of the molecule, which also results in large Raman scattering cross-sections and a strong coupling between its electronic states and vibrational modes. For this reason, Raman spectroscopy offers a unique insight into the properties of molecular semiconductors, including: chemical structure, molecular conformation, molecular orientation, and fundamental photo- and electro-chemical processes—all of which are critically important to the performance of a wide range of optical and electronic organic semiconductor devices. Experimentally, Raman spectroscopy is non-intrusive, non-destructive, and requires no special sample preparation, and so is suitable for a wide range of in situ measurements, which are particularly relevant to issues of thermal and photochemical stability. Here we review the development of the family of Raman spectroscopic techniques, which have been applied to the study of conjugated molecular semiconductors. We consider the suitability of each technique for particular circumstances, and the unique insights it can offer, with a particular focus on the significance of these measurements for the continuing development of stable, high performance organic electronic devices.

  • Journal article
    Yu JC, Kim DW, Kim DB, Jung ED, Lee K-S, Lee S, Di Nuzzo D, Kim J-S, Song MHet al., 2017,

    Effect of the solvent used for fabrication of perovskite films by solvent dropping on performance of perovskite light-emitting diodes

    , Nanoscale, Vol: 9, Pages: 2088-2094, ISSN: 2040-3364

    Organic–inorganic hybrid perovskites have emerged as a next-generation candidate for light-emitting device applications due to their excellent optical and electrical properties with narrow band emission compared to organic emitters. The morphological control of perovskite films with full surface coverage and few defect sites is essential for achieving highly efficient perovskite light-emitting diodes (PeLEDs). Here, we obtain a highly uniform perovskite film with a remarkably reduced number of defect sites in a perovskite crystal using chlorobenzene dropping. This effort leads to the enhanced performance of PeLEDs with a CH3NH3PbBr3 film using chlorobenzene dropping with a maximum luminance of 14 460 cd m−2 (at 3.8 V) and a maximum external quantum efficiency (EQE) of 0.71% (at 2.8 V). This research confirms that the role of the solvent in the solvent dropping method is to fabricate a dense and uniform perovskite film and to passivate the defect sites of the perovskite crystal films.

  • Journal article
    Razzell-Hollis J, Thiburce Q, Tsoi WC, Kim J-Set al., 2016,

    Interfacial chemical composition and molecular order in organic photovoltaic blend thin films probed by surface-enhanced Raman spectroscopy

    , ACS Applied Materials and Interfaces, Vol: 8, Pages: 31469-31481, ISSN: 1944-8244

    Organic electronic devices invariably involve transfer of charge carriers between the organic layer and at least one metal electrode, and they are sensitive to the local properties of the organic film at those interfaces. Here, we demonstrate a new approach for using an advanced technique called surface-enhanced raman spectroscopy (SERS) to quantitatively probe interfacial properties relevant to charge injection/extraction. Exploiting the evanescent electric field generated by a ∼7 nm thick layer of evaporated silver, Raman scattering from nearby molecules is enhanced by factors of 10–1000× and limited by a distance dependence with a measured decay length of only 7.6 nm. When applied to the study of an all-polymer 1:1 blend of P3HT and F8TBT used in organic solar cells, we find that the as-cast film is morphologically suited to charge extraction in inverted devices, with a top (anode) interface very rich in hole-transporting P3HT (74.5%) and a bottom (cathode) interface slightly rich in electron-transporting F8TBT (55%). While conventional, uninverted P3HT:F8TBT devices are reported to perform poorly compared to inverted devices, their efficiency can be improved by thermal annealing but only after evaporation of a metallic top electrode. This is explained by changes in composition at the top interface: annealing prior to silver evaporation leads to a greater P3HT concentration at the top interface to 83.3%, exaggerating the original distribution that favored inverted devices, while postevaporation annealing increases the concentration of F8TBT at the top interface to 34.8%, aiding the extraction of electrons in a conventional device. By nondestructively probing buried interfaces, SERS is a powerful tool for understanding the performance of organic electronic devices.

  • Journal article
    Boufflet P, Wood S, Wade J, Fei Z, Kim JS, Heeney Met al., 2016,

    Comparing blends and blocks: Synthesis of partially fluorinated diblock polythiophene copolymers to investigate the thermal stability of optical and morphological properties

    , Beilstein Journal of Organic Chemistry, Vol: 12, Pages: 2150-2163, ISSN: 1860-5397

    The microstructure of the active blend layer has been shown to be a critically important factor in the performance of organic solar devices. Block copolymers provide a potentially interesting avenue for controlling this active layer microstructure in solar cell blends. Here we explore the impact of backbone fluorination in block copolymers of poly(3-octyl-4-fluorothiophene)s and poly(3-octylthiophene) (F-P3OT-b-P3OT). Two block co-polymers with varying block lengths were prepared via sequential monomer addition under Kumada catalyst transfer polymerisation (KCTP) conditions. We compare the behavior of the block copolymer to that of the corresponding homopolymer blends. In both types of system, we find the fluorinated segments tend to dominate the UV–visible absorption and molecular vibrational spectral features, as well as the thermal behavior. In the block copolymer case, non-fluorinated segments appear to slightly frustrate the aggregation of the more fluorinated block. However, in situ temperature dependent Raman spectroscopy shows that the intramolecular order is more thermally stable in the block copolymer than in the corresponding blend, suggesting that such materials may be interesting for enhanced thermal stability of organic photovoltaic active layers based on similar systems.

  • Journal article
    Wood S, Kim J-H, Wade J, Park JB, Hwang D-H, Kim J-Set al., 2016,

    Systematic control of heteroatoms in donor-acceptor copolymers and its effects on molecular conformation and photovoltaic performance

    , JOURNAL OF MATERIALS CHEMISTRY C, Vol: 4, Pages: 7966-7978, ISSN: 2050-7526

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