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  • 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
  • Journal article
    Perevedentsev A, Chander N, Kim J-S, Bradley DDCet al., 2016,

    Spectroscopic properties of poly(9,9-dioctylfluorene) thin films possessing varied fractions of -phase chain segments: enhanced photoluminescence efficiency via conformation structuring

    , Journal of Polymer Science Part B-Polymer Physics, Vol: 54, Pages: 1995-2006, ISSN: 1099-0488

    Poly(9,9-dioctylfluorene) (PFO) is a widely studied blue-emitting conjugated polymer, the optoelectronic properties of which are strongly affected by the presence of a well-defined chain-extended “β-phase” conformational isomer. In this study, optical and Raman spectroscopy are used to systematically investigate the properties of PFO thin films featuring a varied fraction of β-phase chain segments. Results show that the photoluminescence quantum efficiency (PLQE) of PFO films is highly sensitive to both the β-phase fraction and the method by which it was induced. Notably, a PLQE of ∼69% is measured for PFO films possessing a ∼6% β-phase fraction induced by immersion in solvent/nonsolvent mixtures; this value is substantially higher than the average PLQE of ∼55% recorded for other β-phase films. Furthermore, a linear relationship is observed between the intensity ratios of selected Raman peaks and the β-phase fraction determined by commonly used absorption calibrations, suggesting that Raman spectroscopy can be used as an alternative means to quantify the β-phase fraction. As a specific example, spatial Raman mapping is used to image a mm-scale β-phase stripe patterned in a glassy PFO film, with the extracted β-phase fraction showing excellent agreement with the results of optical spectroscopy.

  • Journal article
    Razzell-Hollis J, Limbu S, Kim J-S, 2016,

    Spectroscopic investigations of three-phase morphology evolution in polymer: fullerene solar cell blends

    , Journal of Physical Chemistry C, Vol: 120, Pages: 10806-10814, ISSN: 1932-7447

    Nanoscale morphology is critical to determining the device efficiency of bulk heterojunction organic solar cells, and the ideal structure is often described as a three-phase network with one well-mixed phase for efficient charge separation and two purer phases for efficient charge transport. In order to understand such nanoscale morphology, we have performed detailed spectroscopic investigations and identified the three-phase morphology evolution in one of the classic blend systems, P3HT:PCBM. The impact of different phases on polymer molecular (chain conformational) order and blend thermal and optical properties were monitored in situ using resonant Raman, absorption, and photoluminescence spectroscopy techniques. Semicrystalline P3HT was found to accommodate up to ∼25% PCBM (by weight) in its amorphous phase, with very little impact on either polymer molecular order or aggregation. Higher concentrations of PCBM resulted in a greater proportion of amorphous mixed phase and reduced polymer molecular order and aggregation. On the other hand, the formation of crystalline purer phases via phase separation was evident during in situ thermal annealing, revealing a consistent glass transition temperature (Tg) of ∼50 °C in blends with up to 50% wt PCBM. This indicates similar local chemical compositions in the amorphous mixed phase present in blends despite different overall blend ratios. A much higher Tg (80–100 °C) was observed for blends with >50% wt PCBM, indicating a stronger impact of PCBM on P3HT molecular order and thermal properties, requiring a higher annealing temperature to ensure formation of the preferred three-phase morphology.

  • Journal article
    Bogatko SA, Haynes PD, Sathian J, Wade J, Kim J-S, Tan K-J, Breeze J, Salvadori E, Horsfield AP, Oxborrow Met al., 2016,

    Molecular design of a room-temperature maser

    , The Journal of Physical Chemistry C, Vol: 120, Pages: 8251-8260, ISSN: 1932-7447
  • Journal article
    Kang C-M, Wade J, Yun S, Lim J, Cho H, Roh J, Lee H, Nam S, Bradley DDC, Kim J-S, Lee Cet al., 2016,

    1 GHz Pentacene Diode Rectifiers Enabled by Controlled Film Deposition on SAM-Treated Au Anodes

  • Journal article
    Suh M, Bailey J, Kim SW, Kim K, Yun DJ, Jung Y, Hamilton I, Chander N, Wang X, Bradley DD, Jeon DY, Kim JSet al., 2015,

    High-Efficiency Polymer LEDs with Fast Response Times Fabricated via Selection of Electron-Injecting Conjugated Polyelectrolyte Backbone Structure.

    , ACS Applied Materials & Interfaces, Vol: 7, Pages: 26566-26571, ISSN: 1944-8244

    Imidazolium ionic side-group-containing fluorene-based conjugated polyelectrolytes (CPEs) with different π-conjugated structures, poly[(9,9-bis(8'-(3″-methyl-1″-imidazolium)octyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] dibromide (F8im-Br) and poly[(9,9-bis(8'-(3″-methyl-1″-imidazolium)octyl)-2,7-fluorene)-alt-(benzo(2,1,3)thiadiazol-4,8-diyl) dibromide (F8imBT-Br), are synthesized and utilized as an electron injection layer (EIL) in green-emitting F8BT polymer light-emitting diodes (PLEDs). Both CPE EIL devices significantly outperform Ca cathode devices; 17.9 cd A(-1) (at 3.8 V) and 16.6 lm W(-1) (at 3.0 V) for F8imBT-Br devices, 11.1 cd A(-1) (at 4.2 V) and 9.1 lm W(-1) (at 3.4 V) for F8im-Br devices, and 7.2 cd A(-1) (at 3.6 V) and 7.0 lm W(-1) (at 3.0 V) for Ca devices. Importantly, unlike the F8im-Br EIL devices, F8imBT-Br PLEDs exhibit much faster electroluminescence turn-on times (<10 μs) despite both EILs possessing the same tethered imidazolium and mobile bromide ions. The F8imBT-Br devices represent, to the best of our knowledge, the highest efficiency in thin (70 nm) single-layer F8BT PLEDs in conventional device architecture with the fastest EL response time using CPE EIL with mobile ions. Our results clearly indicate the importance of an additional factor of EIL materials, specifically the conjugated backbone structure, to determine the device efficiency and response times.

  • Journal article
    Abdulla M, Renero-Lecuna C, Kim JS, Friend RHet al., 2015,

    Morphological study of F8BT:PFB thin film blends

    , ORGANIC ELECTRONICS, Vol: 23, Pages: 87-98, ISSN: 1566-1199
  • Journal article
    Wade J, Wood S, Beatrup D, Hurhangee M, Bronstein H, McCulloch I, Durrant JR, Kim JSet al., 2015,

    Operational electrochemical stability of thiophene-thiazole copolymers probed by resonant Raman spectroscopy.

    , Journal of Chemical Physics, Vol: 142, Pages: 244904-244904, ISSN: 1089-7690

    We report on the electrochemical stability of hole polarons in three conjugated polymers probed by resonant Raman spectroscopy. The materials considered are all isostructural to poly(3-hexyl)thiophene, where thiazole units have been included to systematically deepen the energy level of the highest occupied molecular orbital (HOMO). We demonstrate that increasing the thiazole content planarizes the main conjugated backbone of the polymer and improves the electrochemical stability in the ground state. However, these more planar thiazole containing polymers are increasingly susceptible to electrochemical degradation in the polaronic excited state. We identify the degradation mechanism, which targets the C=N bond in the thiazole units and results in disruption of the main polymer backbone conjugation. The introduction of thiazole units to deepen the HOMO energy level and increase the conjugated backbone planarity can be beneficial for the performance of certain optoelectronic devices, but the reduced electrochemical stability of the hole polaron may compromise their operational stability.

  • Journal article
    Wood S, Kim J-H, Hwang D-H, Kim J-Set al., 2015,

    Effects of Fluorination and Side Chain Branching on Molecular Conformation and Photovoltaic Performance of Donor-Acceptor Copolymers

    , CHEMISTRY OF MATERIALS, Vol: 27, Pages: 4196-4204, ISSN: 0897-4756
  • Journal article
    Fei Z, Boufflet P, Wood S, Wade J, Moriarty J, Gann E, Ratcliff EL, McNeill CR, Sirringhaus H, Kim JS, Heeney Met al., 2015,

    Influence of backbone fluorination in regioregular poly(3-alkyl-4-fluoro)thiophenes.

    , Journal of the American Chemical Society, Vol: 137, Pages: 6866-6879, ISSN: 1520-5126

    We report two strategies toward the synthesis of 3-alkyl-4-fluorothiophenes containing straight (hexyl and octyl) and branched (2-ethylhexyl) alkyl groups. We demonstrate that treatment of the dibrominated monomer with 1 equiv of alkyl Grignard reagent leads to the formation of a single regioisomer as a result of the pronounced directing effect of the fluorine group. Polymerization of the resulting species affords highly regioregular poly(3-alkyl-4-fluoro)thiophenes. Comparison of their properties to those of the analogous non-fluorinated polymers shows that backbone fluorination leads to an increase in the polymer ionization potential without a significant change in optical band gap. Fluorination also results in an enhanced tendency to aggregate in solution, which is ascribed to a more co-planar backbone on the basis of Raman and DFT calculations. Average charge carrier mobilities in field-effect transistors are found to increase by up to a factor of 5 for the fluorinated polymers.

  • Journal article
    Higgins SG, Muir BVO, Wade J, Chen J, Striedinger B, Gold H, Stadlober B, Caironi M, Kim J-S, Steinke JHG, Campbell AJet al., 2015,

    Self-aligned megahertz organic transistors solution-processed on plastic

    , Advanced Electronic Materials, Vol: 1, ISSN: 2199-160X
  • 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
  • Journal article
    Perevedentsev A, Sonnefraud Y, Belton CR, Sharma S, Cass AEG, Maier SA, Kim J-S, Stavrinou PN, Bradley DDCet al., 2015,

    Dip-pen patterning of poly(9,9-dioctylfluorene) chain-conformation-based nano-photonic elements

    , Nature Communications, ISSN: 2041-1723

    Metamaterials are a promising new class of materials, in which sub-wavelength physical structures, rather than variations in chemical composition, can be used to modify the nature of their interaction with electromagnetic radiation. Here we show that a metamaterials approach, using a discrete physical geometry (conformation) of the segments of a polymer chain as the vector for a substantial refractive index change, can be used to enable visible wavelength, conjugated polymer photonic elements. In particular, we demonstrate that a novel form of dip-pen nanolithography provides an effective means to pattern the so-called β-phase conformation in poly(9,9-dioctylfluorene) thin films. This can be done on length scales ≤500 nm, as required to fabricate a variety of such elements, two of which are theoretically modelled using complex photonic dispersion calculations.

  • Journal article
    Kim JS, Wood S, Shoaee S, Spencer SJ, Castro FA, Tsoi WC, Murphy CE, Sim M, Cho K, Durrant JR, Kim J-Set al., 2015,

    Morphology-performance relationships in polymer/fullerene blends probed by complementary characterisation techniques - effects of nanowire formation and subsequent thermal annealing

    , JOURNAL OF MATERIALS CHEMISTRY C, Vol: 3, Pages: 9224-9232, ISSN: 2050-7526
  • Journal article
    Wood S, Wade J, Shahid M, Collado-Fregoso E, Bradley DDC, Durrant JR, Heeney M, Kim J-Set al., 2015,

    Natures of optical absorption transitions and excitation energy dependent photostability of diketopyrrolopyrrole (DPP)-based photovoltaic copolymers

    , ENERGY & ENVIRONMENTAL SCIENCE, Vol: 8, Pages: 3222-3232, ISSN: 1754-5692
  • Journal article
    Wade J, Steiner F, Niedzialek D, James DT, Jung Y, Yun D-J, Bradley DDC, Nelson J, Kim J-Set al., 2014,

    Charge mobility anisotropy of functionalized pentacenes in organic field effect transistors fabricated by solution processing

    , JOURNAL OF MATERIALS CHEMISTRY C, Vol: 2, Pages: 10110-10115, ISSN: 2050-7526

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