Publications by Donal Bradley

Ultrastable Supramolecular Self-Encapsulated Wide-Bandgap Conjugated Polymers for Large-Area and Flexible Electroluminescent Devices.

Advanced materials (Deerfield Beach, Fla.) (2018) e1804811-

J Lin, B Liu, M Yu, X Wang, Z Lin, X Zhang, C Sun, J Cabanillas-Gonzalez, L Xie, F Liu, C Ou, L Bai, Y Han, M Xu, W Zhu, TA Smith, PN Stavrinou, DDC Bradley, W Huang

Controlling chain behavior through smart molecular design provides the potential to develop ultrastable and efficient deep-blue light-emitting conjugated polymers (LCPs). Herein, a novel supramolecular self-encapsulation strategy is proposed to construct a robust ultrastable conjugated polydiarylfluorene (PHDPF-Cz) via precisely preventing excitons from interchain cross-transfer/coupling and contamination from external trace H2 O/O2 . PHDPF-Cz consists of a mainchain backbone where the diphenyl groups localize at the 9-position as steric bulk moieties, and carbazole (Cz) units localize at the 4-position as supramolecular π-stacked synthon with the dual functionalities of self-assembly capability and hole-transport facility. The synergistic effect of the steric bulk groups and π-stacked carbazoles affords PHDPF-Cz as an ultrastable property, including spectral, morphological stability, and storage stability. In addition, PHDPF-Cz spin-coated gelation films also show thickness-insensitive deep-blue emission with respect to the reference polymers, which are suitable to construct solution-processed large-scale optoelectronic devices with higher reproducibility. High-quality and uniform deep-blue emission is observed in large-area solution-processed films. The electroluminescence shows high-quality deep-blue intrachain emission with a CIE (0.16, 0.12) and a very narrow full width at half-maximum of 32 nm. Finally, large-area and flexible polymer light-emitting devices with a single-molecular excitonic behavior are also fabricated. The supramolecular self-encapsulation design provides an effective strategy to construct ultrastable LCPs for optoelectronic applications.

Systematic investigation of self-organization behavior in supramolecular pi-conjugated polymer for multi-color electroluminescence


J-Y Lin, B Liu, M-N Yu, X-H Wang, L-B Bai, Y-M Han, C-J Ou, L-H Xie, F Liu, W-S Zhu, X-W Zhang, H-F Ling, PN Stavrinou, J-P Wang, DDC Bradley, W Huang

Large-area plastic nanogap electronics enabled by adhesion lithography

npj Flexible Electronics 2 (2018)

J Semple, DG Georgiadou, G Wyatt-Moon, M Yoon, A Seitkhan, E Yengel, S Rossbauer, F Bottacchi, MA McLachlan, DDC Bradley, TD Anthopoulos

Low-Voltage Solution-Processed Hybrid Light-Emitting Transistors.

ACS applied materials & interfaces 10 (2018) 18445-18449

MU Chaudhry, K Tetzner, Y-H Lin, S Nam, C Pearson, C Groves, MC Petty, TD Anthopoulos, DDC Bradley

We report the development of low operating voltages in inorganic-organic hybrid light-emitting transistors (HLETs) based on a solution-processed ZrO x gate dielectric and a hybrid multilayer channel consisting of the heterojunction In2O3/ZnO and the organic polymer "Super Yellow" acting as n- and p-channel/emissive layers, respectively. Resulting HLETs operate at the lowest voltages reported to-date (<10 V) and combine high electron mobility (22 cm2/(V s)) with appreciable current on/off ratios (≈103) and an external quantum efficiency of 2 × 10-2% at 700 cd/m2. The charge injection, transport, and recombination mechanisms within this HLET architecture are discussed, and prospects for further performance enhancement are considered.

Photophysical and Fluorescence Anisotropic Behavior of Polyfluorene β-Conformation Films.

The journal of physical chemistry letters 9 (2018) 364-372

M-N Yu, H Soleimaninejad, J-Y Lin, Z-Y Zuo, B Liu, Y-F Bo, L-B Bai, Y-M Han, TA Smith, M Xu, X-P Wu, DE Dunstan, R-D Xia, L-H Xie, DDC Bradley, W Huang

We demonstrate a systematic visualization of the unique photophysical and fluorescence anisotropic properties of polyfluorene coplanar conformation (β-conformation) using time-resolved scanning confocal fluorescence imaging (FLIM) and fluorescence anisotropy imaging microscopy (FAIM) measurements. We observe inhomogeneous morphologies and fluorescence decay profiles at various micrometer-sized regions within all types of polyfluorene β-conformational spin-coated films. Poly(9,9-dioctylfluorene-2,7-diyl) (PFO) and poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl] (PODPF) β-domains both have shorter lifetime than those of the glassy conformation for the longer effective conjugated length and rigid chain structures. Besides, β-conformational regions have larger fluorescence anisotropy for the low molecular rotational motion and high chain orientation, while the low anisotropy in glassy conformational regions shows more rotational freedom of the chain and efficient energy migration from amorphous regions to β-conformation as a whole. Finally, ultrastable ASE threshold in the PODPF β-conformational films also confirms its potential application in organic lasers. In this regard, FLIM and FAIM measurements provide an effective platform to explore the fundamental photophysical process of conformational transitions in conjugated polymer.

Pronounced Side Chain Effects in Triple Bond-Conjugated Polymers Containing Naphthalene Diimides for n-Channel Organic Field-Effect Transistors.

ACS applied materials & interfaces 10 (2018) 12921-12929

S Nam, SG Hahm, D Khim, H Kim, T Sajoto, M Ree, SR Marder, TD Anthopoulos, DDC Bradley, Y Kim

Three triple bond-conjugated naphthalene diimide (NDI) copolymers, poly{[ N, N'-bis(2-R1)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-[(2,5-bis(2-R2)-1,4-phenylene)bis(ethyn-2,1-diyl)]} (PNDIR1-R2), were synthesized via Sonogashira coupling polymerization with varying alkyl side chains at the nitrogen atoms of the imide ring and 2,5-positions of the 1,4-diethynylbenzene moiety. Considering their identical polymer backbone structures, the side chains were found to have a strong influence on the surface morphology/nanostructure, thus playing a critical role in charge-transporting properties of the three NDI-based copolymers. Among the polymers, the one with an octyldodecyl (OD) chain at the nitrogen atoms of imide ring and a hexadecyloxy (HO) chain at the 2,5-positions of 1,4-diethynylbenzene, P(NDIOD-HO), exhibited the highest electron mobility of 0.016 cm2 V-1 s-1, as compared to NDI-based copolymers with an ethylhexyl chain at the 2,5-positions of 1,4-diethynylbenzene. The enhanced charge mobility in the P(NDIOD-HO) layers is attributed to the well-aligned nano-fiber-like surface morphology and highly ordered packing structure with a dominant edge-on orientation, thus enabling efficient in-plane charge transport. Our results on the molecular structure-charge transport property relationship in these materials may provide an insight into novel design of n-type conjugated polymers for applications in the organic electronics of the future.

Host Exciton Confinement for Enhanced Forster-Transfer-Blend Gain Media Yielding Highly Efficient Yellow-Green Lasers


Q Zhang, J Liu, Q Wei, X Guo, Y Xu, R Xia, L Xie, Y Qian, C Sun, L Lueer, J Cabanillas-Gonzalez, DDC Bradley, W Huang

Thermally Stable Zinc Disalphen Macrocycles Showing Solid-State and Aggregation-Induced Enhanced Emission.

Inorganic chemistry 56 (2017) 5688-5695

JA Marafie, DDC Bradley, CK Williams

In order to investigate the solid-state light emission of zinc salphen macrocycle complexes, 7 dinuclear zinc salphen macrocycle complexes (1-7), with acetate or hexanoate coligands, are synthesized. The complexes are stable in air up to 300 °C, as shown via thermogravimetric analysis (TGA), and exhibit green to orange-red emission in solution (λem = 550-600 nm, PLQE ≤ 1%) and slightly enhanced yellow to orange-red emission in the solid state (λem = 570-625 nm, PLQE = 1-5%). Complexes 1, 2, 4, 5, and 7 also display aggregation-induced enhanced emission (AIEE) when hexane (a nonsolvent) is added to a chloroform solution of the complexes, with complex 4 displaying a 75-fold increase in peak emission intensity upon aggregation (in 0.25:0.75 chloroform:hexane mixture).

Efficient Deep Red Light-Sensing All-Polymer Phototransistors with p-type/n-type Conjugated Polymer Bulk Heterojunction Layers.

ACS applied materials & interfaces 9 (2017) 14983-14989

S Nam, J Seo, H Han, H Kim, DDC Bradley, Y Kim

Here we demonstrate deep red light-sensing all-polymer phototransistors with bulk heterojunction layers of poly[4,8-bis[(2-ethylhexyl)-oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) and poly[[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)] (P(NDI2OD-T2)). The device performances were investigated by varying the incident light intensity of the deep red light (675 nm), while the signal amplification capability was examined by changing the gate and drain voltages. The result showed that the present all-polymer phototransistors exhibited higher photoresponsivity (∼14 A/W) and better on/off photoswitching characteristics than the devices with the pristine polymers under illumination with the deep red light. The enhanced phototransistor performances were attributed to the well-aligned nanofiber-like morphology and nanocrystalline P(NDI2OD-T2) domains in the blend films, which are beneficial for charge separation and charge transport in the in-plane direction.

Understanding the molecular gelation processes of heteroatomic conjugated polymers for stable blue polymer light-emitting diodes


J-Y Lin, B Liu, M-N Yu, C-J Ou, Z-F Lei, F Liu, X-H Wang, L-H Xie, W-S Zhu, H-F Ling, X-W Zhang, PN Stavrinou, J-P Wang, DDC Bradley, W Huang

Polyacetylene-based polyelectrolyte as a universal interfacial layer for efficient inverted polymer solar cells


S Nam, J Seo, M Song, H Kim, M Ree, Y-S Gal, DDC Bradley, Y Kim

Electron Hopping Across Hemin-Doped Serum Albumin Mats on Centimeter-Length Scales.

Advanced materials (Deerfield Beach, Fla.) 29 (2017)

N Amdursky, X Wang, P Meredith, DJ Riley, DJ Payne, DDC Bradley, MM Stevens

Exploring long-range electron transport across protein assemblies is a central interest in both the fundamental research of biological processes and the emerging field of bioelectronics. This work examines the use of serum-albumin-based freestanding mats as macroscopic electron mediators in bioelectronic devices. In particular, this study focuses on how doping the protein mat with hemin improves charge-transport. It is demonstrated that doping can increase conductivity 40-fold via electron hopping between adjacent hemin molecules, resulting in the highest measured conductance for a protein-based material yet reported, and transport over centimeter length scales. The use of distance-dependent AC impedance and DC current-voltage measurements allows the contribution from electron hopping between adjacent hemin molecules to be isolated. Because the hemin-doped serum albumin mats have both biocompatibility and fabrication simplicity, they should be applicable to a range of bioelectronic devices of varying sizes, configurations, and applications.

Photovoltaic limitations of BODIPY:fullerene based bulk heterojunction solar cells

SYNTHETIC METALS 226 (2017) 25-30

D Baran, S Tuladhar, SP Economopoulos, M Neophytou, A Savva, G Itskos, A Othonos, DDC Bradley, CJ Brabec, J Nelson, SA Choulis

Thickness Effect of Bulk Heterojunction Layers on the Performance and Stability of Polymer:Fullerene Solar Cells with Alkylthiothiophene-Containing Polymer


S Nam, M Song, H Kim, DDC Bradley, Y Kim

Influence of the Hole Transporting Layer on the Thermal Stability of Inverted Organic Photovoltaics Using Accelerated-Heat Lifetime Protocols.

ACS applied materials & interfaces 9 (2017) 14136-14144

F Hermerschmidt, A Savva, E Georgiou, SM Tuladhar, JR Durrant, I McCulloch, DDC Bradley, CJ Brabec, J Nelson, SA Choulis

High power conversion efficiency (PCE) inverted organic photovoltaics (OPVs) usually use thermally evaporated MoO3 as a hole transporting layer (HTL). Despite the high PCE values reported, stability investigations are still limited and the exact degradation mechanisms of inverted OPVs using thermally evaporated MoO3 HTL remain unclear under different environmental stress factors. In this study, we monitor the accelerated lifetime performance under the ISOS-D-2 protocol (heat conditions 65 °C) of nonencapsulated inverted OPVs based on the thiophene-based active layer materials poly(3-hexylthiophene) (P3HT), poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), and thieno[3,2-b]thiophene-diketopyrrolopyrrole (DPPTTT) blended with [6,6]-phenyl C71-butyric acid methyl ester (PC[70]BM). The presented investigation of degradation mechanisms focus on optimized P3HT:PC[70]BM-based inverted OPVs. Specifically, we present a systematic study on the thermal stability of inverted P3HT:PC[70]BM OPVs using solution-processed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and evaporated MoO3 HTL. Using a series of measurements and reverse engineering methods, we report that the P3HT:PC[70]BM/MoO3 interface is the main origin of failure of the P3HT:PC[70]BM-based inverted OPVs under intense heat conditions, a trend that is also observed for the other two thiophene-based polymers used in this study.

Steric-Hindrance-Functionalized Polydiarylfluorenes: Conformational Behavior, Stabilized Blue Electroluminescence, and Efficient Amplified Spontaneous Emission.

ACS applied materials & interfaces 9 (2017) 37856-37863

L Bai, B Liu, Y Han, M Yu, J Wang, X Zhang, C Ou, J Lin, W Zhu, L Xie, C Yin, J Zhao, J Wang, DDC Bradley, W Huang

Control of the hierarchical molecular organization of polydiarylfluorenes by synthetic strategies is significant for optimizing photophysical properties as well as the performance of light-emitting devices. Herein, for the suppression of molecular aggregation and enhancement of luminescence efficiency, a series of steric units were introduced into polydiarylfluorenes by copolymerization, with the aim of integrating the advantages of the steric-hindrance effect and of the β-phase. Optical and Raman spectroscopies revealed a β-phase conformation for a polymer copolymerized with spiro[fluorene-9,9'-xanthene] (SFX), with photoluminescence (PL) peaks at 454, 482, and 517 nm. Moreover, the morphological stability and electroluminescence (EL) stability were also improved without compromising the performance of the polymer light-emitting diodes (PLEDs). Furthermore, three steric-hindrance-functionalized copolymers showed significantly decreased thresholds for amplified spontaneous emission (EthASE) and enhanced stability following thermal annealing treatment. These results indicate that steric-hindrance functionalization is a superior approach to improve the overall stability and optoelectronic properties for blue-light-emitting π-conjugated polymers.

Room temperature dielectric bistability in solution-processed spin crossover polymer thin films


G Bovo, I Braunlich, WR Caseri, N Stingelin, TD Anthopoulos, KG Sandeman, DDC Bradley, PN Stavrinou

Charge-Carrier Density Independent Mobility in Amorphous Fluorene-Triarylamine Copolymers


A Campbell, R Rawcliffe, A Guite, JCD Faria, A Mukherjee, MA McLachlan, M Shkunov, DDC Bradley

Nanoscale current spreading analysis in solution-processed graphene oxide/silver nanowire transparent electrodes via conductive atomic force microscopy

Journal of Applied Physics 119 (2016)

JE Shaw, A Perumal, DDC Bradley, PN Stavrinou, TD Anthopoulos

© 2016 Author(s). We use conductive atomic force microscopy (CAFM) to study the origin of long-range conductivity in model transparent conductive electrodes composed of networks of reduced graphene oxide (rGOX) and silver nanowires (AgNWs), with nanoscale spatial resolution. Pristine networks of rGOX(1-3 monolayers-thick) and AgNWs exhibit sheet resistances of ∼100-1000 kΩ/□ and 100-900 Ω/□, respectively. When the materials are deposited sequentially to form bilayer rGOX/AgNW electrodes and thermally annealed at 200 °C, the sheet resistance reduces by up to 36% as compared to pristine AgNW networks. CAFM was used to analyze the current spreading in both systems in order to identify the nanoscale phenomena responsible for this effect. For rGOXnetworks, the low intra-flake conductivity and the inter-flake contact resistance is found to dominate the macroscopic sheet resistance, while for AgNW networks the latter is determined by the density of the inter-AgNW junctions and their associated resistance. In the case of the bilayer rGOX/AgNWs' networks, rGOXflakes are found to form conductive "bridges" between AgNWs. We show that these additional nanoscopic electrical connections are responsible for the enhanced macroscopic conductivity of the bilayer rGOX/AgNW electrodes. Finally, the critical role of thermal annealing on the formation of these nanoscopic connections is discussed.

Supramolecular Polymer-Molecule Complexes as Gain Media for Ultraviolet Lasers

ACS MACRO LETTERS 5 (2016) 967-971

J-Y Lin, G-Y Zhu, B Liu, M-N Yu, X-H Wang, L Wang, W-S Zhu, L-H Xie, C-X Xu, J-P Wang, PN Stavrinou, DDC Bradley, W Huang