School of Chemistry - Research Publications
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ItemNo Preview AvailableFluorescence anisotropy imaging of a polydiacetylene photopolymer film(CANADIAN SCIENCE PUBLISHING, NRC RESEARCH PRESS, 2019-06)UV-illumination of phase-separated surfactant films prepared from mixtures of photopolymerizable 10,12-pentacosadiynoic acid and perfluorotetradecanoic acid results in the formation of fluorescent polydiacetylene fibers and aggregates. In this work, the orientation of polymer strands that comprise the resulting photopolymer structures has been probed using fluorescence anisotropy imaging in combination with defocused single-molecule fluorescence imaging. Imaging experiments indicate the presence of significant fiber-to-fiber heterogeneity, as well as anisotropy within each fiber (or aggregate), with both of these properties changing as a function of film preparation conditions. This anisotropy can be attributed to various alignments of the constituent polymer strands that comprise the larger fibers and aggregates. Intriguingly, when using defocused imaging, fiber images consisted of a series of discrete “doughnut” fluorescence emission patterns, which exhibited intermittent on–off blinking behavior; both of these properties are characteristic of individual emission transition dipoles (single molecules). Further, all of the individual emission transition dipoles had a uniform orientation with respect to the axis of the fiber, indicating a common orientation of discrete emitters in the larger polymer fiber. The implications of these results for future studies of the electronic properties of conjugated polymers in larger macroscopic systems are noted.
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ItemImproved compatibility of DDAB-functionalized graphene oxide with a conjugated polymer by isocyanate treatment(Royal Society of Chemistry, 2017-01-01)2-Chlorophenyl isocyanate (CI) reacts with didodecyl dimethyl ammonium bromide (DDAB) functionalized graphene oxide (DDAB-GO) dispersed in ortho-dichlorobenzene under mild conditions. The CI treatment complements DDAB functionalization to further decrease the hydrophilicity of pristine GO sheets. The resulting CI–DDAB-GO exhibits improved compatibility with the conjugated polymer poly(3-hexylthiophene) (P3HT), compared to DDAB-GO. CI–DDAB-GO sheets can be homogeneously dispersed when blended with P3HT, resulting in an improved morphology compared to P3HT:DDAB-GO composites. The incorporation of CI–DDAB-GO can effectively reduce the dark current of photo-diodes based on P3HT composites, showing potential to enhance the performance of electronic devices based on conjugated polymer composites.
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ItemLiquid Crystallinity as a Self-Assembly Motif for High-Efficiency, Solution-Processed, Solid-State Singlet Fission Materials(WILEY-V C H VERLAG GMBH, 2019-08)Abstract Solution and solution‐deposited thin films of the discotic liquid crystalline electron acceptor–donor–acceptor (A‐D‐A) p‐type organic semiconductor FHBC(TDPP)2, synthesized by coupling thienyl substituted diketopyrrolopyrrole (TDPP) onto a fluorenyl substituted hexa‐peri‐hexabenzocoronene (FHBC) core, are examined by ultrafast and nanosecond transient absorption spectroscopy, and time‐resolved photoluminescence studies to examine their ability to support singlet fission (SF). Grazing incidence wide‐angle X‐ray (GIWAX) studies indicate that as‐cast thin films of FHBC(TDPP)2 are “amorphous,” while hexagonal packed discotic liquid crystalline films evolve during thermal annealing. SF in as‐cast thin films is observed with an ≈150% triplet generation yield. Thermally annealing the thin films improves SF yields up to 170%. The as‐cast thin films show no long‐range order, indicating a new class of SF material where the requirement for local order and strong near neighbor coupling has been removed. Generation of long‐lived triplets (µs) suggests that these materials may also be suitable for inclusion in organic solar cells to enhance performance.
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ItemTriggering the Passivation Effect of Potassium Doping in Mixed-Cation Mixed-Halide Perovskite by Light Illumination(Wiley, 2019-06-01)Potassium (K+) doping has been recently discovered as an effective route to suppress hysteresis and improve the performance stability of perovskite solar cells. However, the mechanism of these K+ doping effects is still under debate, and rationalization of the improved performance in these perovskites is needed. Herein, the photoluminescence (PL) properties and device performance of mixed‐cation mixed‐halide perovskite are dynamically monitored with and without K+ doping under bias light illumination via a confocal fluorescence microscope, together with ultrafast transient absorption as well as time‐dependent and time‐resolved PL measurements. It is demonstrated that illumination is essential to trigger the passivation effect of K+ by forming KBr‐like compounds, leading to the elimination of interface trapping defects and suppression of mobile ion migration, thus resulting in improved power conversion efficiency and negligible current–voltage hysteresis of solar cells. This work provides novel insight into the hysteresis suppression upon K+ doping and highlights the significance of light illumination when using this protocol.
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ItemThe Dominant Energy Transport Pathway in Halide Perovskites: Photon Recycling or Carrier Diffusion?(WILEY-V C H VERLAG GMBH, 2019-05-23)Abstract Photon recycling and carrier diffusion are the two plausible processes that primarily affect the carrier dynamics in halide perovskites, and therefore the evaluation of the performance of their photovoltaic and photonic devices. However, it is still challenging to isolate their individual contributions because both processes result in a similar emission redshift. Herein, it is confirmed that photon recycling is the dominant effect responsible for the observed redshifted emission. By applying one‐ and two‐photon confocal emission microscopy on Ruddlesden–Popper type 2D perovskites, of which interplane carrier diffusion is strictly suppressed, the substantial PL redshift (72 meV) is well reproduced by the photon transport model. A comparison of 3D bulk CH3NH3PbBr3 single crystal to 2D perovskite by depth‐resolved two‐photon PL spectra reveals the contribution of carrier diffusion on energy transport at a distance beyond diffusion length is constantly negligible, though the carrier diffusion indeed exists in the 3D crystal. The investigation resolves the fundamental confusion and debate surrounding the issue and provides significant insights into carrier kinetics in perovskites, which is important for future developments in solar cells and other optoelectronic devices.
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ItemSolution-Processable, Solid State Donor-Acceptor Materials for Singlet Fission(WILEY-V C H VERLAG GMBH, 2018-10-25)Abstract The exploitation of singlet fission (SF) materials in optoelectronic devices is restricted by the limited number of SF materials available and developing new organic materials that undergo singlet fission is a significant challenge. Using a new strategy based on conjugating strong donor and acceptor building blocks, the small molecule (BDT(DPP)2) and polymer (p‐BDT‐DPP) systems are designed and synthesized knowing that bisthiophene‐2,5‐dihydropyrrolo[3,4‐c]pyrrole‐1,4‐dione (DPP) has a low lying triplet energy level, which is further confirmed by time‐dependent density functional theory (TD‐DFT) calculations. TD‐DFT and natural transition orbital (NTO) analysis are conducted to gain insight into the photophysical properties and features of excited states in BDT(DPP)2, respectively. Femtosecond and nanosecond transient absorption spectroscopies are used to investigate the excited state kinetics in the synthesized compounds. Fast formation of triplet pairs in thin film of p‐BDT‐DPP and BDT(DPP)2 and the equilibrium formation of correlated triplet pairs and S1 from triplet–triplet annihilation in solution of BDT(DPP)2 are further evidence of SF in these compounds. The short triplet lifetime, as a result of fast biexcitonic recombination, provides additional support for triplet pair formation through singlet fission.
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ItemConcentrating Aggregation-Induced Fluorescence in Planar Waveguides: A Proof-of-Principle(NATURE PORTFOLIO, 2014-04-10)The photophysical properties of fluorescent dyes are key determinants in the performance of luminescent solar concentrators (LSCs). First-generation dyes--coumarin, perylenes, and rhodamines--used in LSCs suffer from both concentration quenching in the solid-state and small Stokes shifts which limit the current LSC efficiencies to below theoretical limits. Here we show that fluorophores that exhibit aggregation-induced emission (AIE) are promising materials for LSC applications. Experiments and Monte Carlo simulations show that the optical quantum efficiencies of LSCs with AIE fluorophores are at least comparable to those of LSCs with first-generation dyes as the active materials even without the use of any optical accessories to enhance the trapping efficiency of the LSCs. Our results demonstrate that AIE fluorophores can potentially solve some key limiting properties of first-generation LSC dyes.
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ItemEfficient light harvesting of a luminescent solar concentrator using excitation energy transfer from an aggregation-induced emitter(ROYAL SOC CHEMISTRY, 2014)The compromise between light absorption and reabsorption losses limits the potential light conversion efficiency of luminescent solar concentrators (LSCs). Current approaches do not fully address both issues. By using the excitation energy transfer (EET) strategy with a donor chromophore that exhibits aggregation-induced emission (AIE) behaviour, it is shown that both transmission and reabsorption losses can be minimized in a LSC device achieving high light collection and concentration efficiencies. The light harvesting performance of the LSC developed has been characterized using fluorescence quantum yield measurements and Monte Carlo ray tracing simulations. Comparative incident photon conversion efficiency and short-circuit current data based on the LSC coupled to a silicon solar cell provide additional evidence for improved performance.
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ItemA Transparent Planar Concentrator Using Aggregates of gem-Pyrene Ethenes(WILEY-V C H VERLAG GMBH, 2015-10-07)The luminescence properties of pyrene ethenes, both as monomer and aggregate species, are found to depend on the regioisomer structure. Systematic shifts in absorption, emission, and excitation spectra of the gem‐pyrene ethenes, both in solution and in rigid polymer hosts, are consistent with weakly interacting H‐aggregate formation. This aggregation leads to excimer‐like emission with Stokes shifts greater than 1 eV. Planar concentrators fabricated from gem‐pyrene diphenylethenes show comparable performance to previously reported inorganic phosphors. The UV absorption and emission properties of the planar concentrator devices exhibit potential for transparent solar concentrators or visible–blind photodetector applications. This is the first demonstration of exploiting the unusual photophysics of molecular aggregates in planar concentrators.
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ItemEnergy Migration in Organic Solar Concentrators with a Molecularly Insulated Perylene Diimide(AMER CHEMICAL SOC, 2016-06-23)