29 results on '"Shyamal K. K. Prasad"'
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2. Large, Tunable, and Reversible pH Changes by Merocyanine Photoacids
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Shyamal K. K. Prasad, Martin D. Peeks, Joakim Andréasson, Timothy W. Schmidt, Jonathon E. Beves, and Laura Wimberger
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Thermal equilibrium ,Spiropyran ,Photoswitch ,010405 organic chemistry ,General Chemistry ,010402 general chemistry ,Photochemistry ,01 natural sciences ,Biochemistry ,Catalysis ,0104 chemical sciences ,chemistry.chemical_compound ,Colloid and Surface Chemistry ,chemistry ,Ultrafast laser spectroscopy ,Merocyanine ,Irradiation ,Solubility ,Spectroscopy - Abstract
Molecular photoswitches capable of generating precise pH changes will allow pH-dependent processes to be controlled remotely and noninvasively with light. We introduce a series of new merocyanine photoswitches, which deliver reversible bulk pH changes up to 3.2 pH units (pH 6.5 to pH 3.3) upon irradiation with 450 nm light, displaying tunable and predictable timescales for thermal recovery. We present models to show that the key parameters for optimizing the bulk pH changes are measurable: the solubility of the photoswitch, the acidity of the merocyanine form, the thermal equilibrium position between the spiropyran and the merocyanine isomers, and the increased acidity under visible light irradiation. Using ultrafast transient absorption spectroscopy, we determined the quantum yields for the ring-closing reaction and found that the lifetimes of the transient cis-merocyanine isomers ranged from 30 to 550 ns. Quantum yields did not appear to be a limitation for bulk pH switching. The models we present use experimentally determined parameters and are, in principle, able to predict the change in pH obtained for any related merocyanine photoacid.
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- 2021
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3. Singlet and Triplet Exciton Dynamics of Violanthrone
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Timothy W. Schmidt, Lara V. Gillan, Shyamal K. K. Prasad, Michael P. Nielsen, Ned Ekins-Daukes, Murad J. Y. Tayebjee, Dane R. McCamey, and Elham M. Gholizadeh
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Violanthrone ,Materials science ,Exciton ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Molecular physics ,01 natural sciences ,0104 chemical sciences ,3. Good health ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,chemistry.chemical_compound ,General Energy ,chemistry ,Singlet fission ,Ultrafast laser spectroscopy ,Density functional theory ,Singlet state ,Triplet state ,Physical and Theoretical Chemistry ,Ground state ,0210 nano-technology - Abstract
The exciton dynamics of violanthrone-79 are investigated in solution and in the solidstate. In solution, the photo-prepared singlet is found to exhibit a strong ground-state bleachand stimulated emission feature, but when sensitized in its triplet state, exhibits only a narrowand weak ground-state bleach. As supported by density functional theory calculations,this is explained by the triplet state having absorptions in the same region, with a similaroscillator strength, as the ground state molecule. In solid films, the excited singlet isfound to survive only 100 ps, giving way to a long-lived transient absorption spectrum withcharacteristics reminiscent of the triplet in solution. This is interpreted in terms of singletfission in the solid film.
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- 2021
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4. Photochemical upconversion of near-infrared light from below the silicon bandgap
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Thilini Ishwara, Jared H. Cole, Shyamal K. K. Prasad, Richard D. Tilley, Soshan Cheong, Shujuan Huang, Elham M. Gholizadeh, John E. Anthony, Timothy W. Schmidt, Anthony J. Petty, Sarah Norman, and Zhi Li Teh
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Violanthrone ,Materials science ,Silicon ,Singlet oxygen ,Infrared ,Physics::Optics ,chemistry.chemical_element ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Photochemistry ,7. Clean energy ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,Photon upconversion ,Electronic, Optical and Magnetic Materials ,010309 optics ,Condensed Matter::Materials Science ,chemistry.chemical_compound ,chemistry ,Quantum dot ,0103 physical sciences ,Singlet fission ,0210 nano-technology ,Visible spectrum - Abstract
Photochemical upconversion is a strategy for converting infrared light into more energetic, visible light, with potential applications ranging from biological imaging and drug delivery to photovoltaics and photocatalysis. Although systems have been developed for upconverting light from photon energies in the near-infrared, upconversion from below the silicon bandgap has been out of reach. Here, we demonstrate an upconversion composition using PbS semiconductor nanocrystal sensitizers that absorb photons below the bandgap of silicon and populate violanthrone triplet states below the singlet oxygen energy. The triplet-state violanthrone chromophores luminesce in the visible spectrum following energy delivery from two singlet oxygen molecules. By incorporating organic chromophores as ligands onto the PbS nanocrystals to improve energy transfer, we demonstrate that violanthrone upconverts in the absence of oxygen by the triplet–triplet annihilation mechanism. The change in mechanism is shown by exploiting the magnetic field effect on triplet–triplet interactions. Photochemical upconversion of light with photon energy below the silicon bandgap has remained elusive, but the feat has now been demonstrated using PbS semiconductor nanocrystals and violanthrone.
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- 2020
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5. Large, tunable and reversible pH changes by spiropyran photoacids
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Timothy W. Schmidt, Shyamal K. K. Prasad, Laura Wimberger, Joakim Andréasson, and Jonathon E. Beves
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Molecular switch ,Spiropyran ,chemistry.chemical_compound ,Photochromism ,Materials science ,chemistry ,Photoswitch ,Ultrafast laser spectroscopy ,Merocyanine ,Solubility ,Photochemistry ,Spectroscopy - Abstract
Molecular photoswitches capable of generating pH changes with precision will allow pH-dependent processes to be controlled remotely and non-invasively with light. We introduce a series of new spiropyran photoswitches, delivering reversible bulk pH changes up to 3.2 pH units (pH 6.5 to pH 3.3) upon irradiation with 450 nm light, displaying tunable and predictable timescales for thermal recovery. We present models to show that the key parameters for optimizing the bulk pH changes are measurable: the solubility of the photoswitch, the acidity of the merocyanine form influenced by the thermal equilibrium position between the spiropyran and the merocyanine isomers, and the increased acidity under visible light irradiation. Using ultrafast transient absorption spectroscopy, we determine quantum yields for the ring closing reaction and observe the lifetime of the transient cis-merocyanine isomer ranging from 30 to 550 ns. Quantum yields did not appear to be a limitation of bulk pH switching. The models we present use experimentally determined parameters and are, in principle, able to predict the change in pH obtained for any related spiropyran photoacid.
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- 2021
- Full Text
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6. Singlet Fission in Concentrated TIPS-Pentacene Solutions: The Role of Excimers and Aggregates
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Chelsea R. Forest, Roslyn Forecast, Timothy W. Schmidt, Asaph Widmer-Cooper, Cameron B. Dover, Rowan W. MacQueen, Shyamal K. K. Prasad, Miroslav Dvořák, Akasha Kaleem, Jonathon E. Beves, Pall Thordarson, Murad J. Y. Tayebjee, John E. Anthony, and Anthony J. Petty
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Photoluminescence ,Chemistry ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Excimer ,Radial distribution function ,01 natural sciences ,Biochemistry ,Molecular physics ,Catalysis ,0104 chemical sciences ,Pentacene ,chemistry.chemical_compound ,Colloid and Surface Chemistry ,Reaction rate constant ,Singlet fission ,Ultrafast laser spectroscopy ,0210 nano-technology ,Excitation - Abstract
The excited-state dynamics of 6,13-bis(triisopropylsilylethynyl)pentacene is investigated to determine the role of excimer and aggregate formation in singlet fission in high-concentration solutions. Photoluminescence spectra were measured by excitation with the evanescent wave in total internal reflection, in order to avoid reabsorption effects. The spectra over nearly two magnitudes of concentration were nearly identical, with no evidence for excimer emission. Time-correlated single-photon counting measurements confirm that the fluorescence lifetime shortens with concentration. The observed rate constant grows at high concentrations, and this effect is modeled in terms of the hard-sphere radial distribution function. NMR measurements confirm that aggregation takes place with a binding constant of between 0.14 and 0.43 M-1. Transient absorption measurements are consistent with a diffusive encounter mechanism for singlet fission, with hints of more rapid singlet fission in aggregates at the highest concentration measured. These data show that excimers do not play the role of an emissive intermediate in exothermic singlet fission in solution and that, while aggregation occurs at higher concentrations, the mechanism of singlet fission remains dominated by diffusive encounters.
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- 2021
7. Intramolecular Versus Intermolecular Triplet Fusion in Multichromophoric Photochemical Upconversion
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Dane R. McCamey, Can Gao, Bolong Zhang, Timothy W. Schmidt, Wallace W. H. Wong, Trevor A. Smith, Shyamal K. K. Prasad, Miroslav Dvořák, and Murad J. Y. Tayebjee
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Photon ,Intermolecular force ,Physics::Optics ,02 engineering and technology ,Chromophore ,Photon energy ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,7. Clean energy ,01 natural sciences ,Photon upconversion ,0104 chemical sciences ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,General Energy ,Molecular geometry ,Intramolecular force ,Physical and Theoretical Chemistry ,0210 nano-technology ,Excitation - Abstract
Photon upconversion is a process that creates high-energy photons under low photon energy excitation. The effect of molecular geometry on the triplet fusion upconversion process has been investigat...
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- 2019
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8. Ultrafast Spectrally Resolved Photoinduced Complex Refractive Index Changes in CsPbBr3 Perovskites
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Justinas Butkus, Michael Price, Kai Chen, Shyamal K. K. Prasad, Parth Vashishtha, Jonathan E. Halpert, Ronnie R. Tamming, and Justin M. Hodgkiss
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Materials science ,business.industry ,Halide ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,Electronic, Optical and Magnetic Materials ,Photonic metamaterial ,010309 optics ,0103 physical sciences ,Optoelectronics ,Electrical and Electronic Engineering ,0210 nano-technology ,business ,Refractive index ,Ultrashort pulse ,Biotechnology - Abstract
The exceptional optoelectronic properties of metal halide perovskites have been illuminated by extensive spectroscopic studies. Recent measurements suggest strong photoinduced refractive index chan...
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- 2019
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9. End-Functionalized Semiconducting Polymers as Reagents in the Synthesis of Hybrid II–VI Nanoparticles
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Shyamal K. K. Prasad, Justin M. Hodgkiss, Ken Okamoto, Katherine A. Mazzio, and Christine K. Luscombe
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chemistry.chemical_classification ,Materials science ,Chalcogenide ,Nanoparticle ,Nanotechnology ,02 engineering and technology ,Surfaces and Interfaces ,Polymer ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Acceptor ,0104 chemical sciences ,Nanomaterials ,chemistry.chemical_compound ,Monomer ,chemistry ,Electrochemistry ,Surface modification ,General Materials Science ,0210 nano-technology ,Hybrid material ,Spectroscopy - Abstract
The functionalization of II-VI nanocrystals with semiconducting polymers is of fundamental interest for lightweight, solution-processed optoelectronics. The direct surface functionalization of nanocrystals is useful for facilitating charge transfer across the donor/acceptor interface, in addition to promoting good mixing properties and thereby helping prevent nanoparticle aggregation. In this work, we develop a new method for the direct attachment of semiconducting polymers to II-VI inorganic nanocrystals, where the polymer plays a dual role, acting as both the desired capping agent and a chalcogenide monomer during synthesis. The success of this hybridization procedure relies on the establishment of a new polymer end-functionalization scheme, where a route toward a thio-phosphonate polymer end-group is developed; this end-group resembles many chalcogenide precursor materials used in the synthesis of II-VI nanomaterials. We show the applicability of this hybrid functionalization procedure by attaching poly(3-hexylthiophene-2,5-diyl) to CdSe and CdS. We followed the progress of the reaction by NMR and used transmission electron microscopy to determine the morphology of the resulting materials, which we found to have narrow size distributions after hybridization. Polymer attachment to the nanocrystals was confirmed by examining the steady-state and time-resolved optical properties of the hybrid materials, which also provided an insight into excited-state processes occurring across the hybrid interface.
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- 2018
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10. Improved optical confinement in ambipolar field-effect transistors toward electrical injection organic lasers
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Shyamal K. K. Prasad, Girish Lakhwani, Evan T. Hockings, Randy P. Sabatini, Yun Li, and Timothy W. Schmidt
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Amplified spontaneous emission ,Materials science ,genetic structures ,Physics and Astronomy (miscellaneous) ,business.industry ,Ambipolar diffusion ,Transistor ,Physics::Optics ,Laser ,Cladding (fiber optics) ,eye diseases ,law.invention ,Semiconductor laser theory ,law ,Optoelectronics ,Field-effect transistor ,sense organs ,business ,Refractive index - Abstract
Increasing optical confinement is critical to lowering laser thresholds and increasing modal gain in semiconductor lasers. Here, mode-solver calculations are used to demonstrate that improvements to optical confinement are possible in organic field-effect transistor geometries by using high refractive index cladding layers. Optical experiments show that the proposed structure increases the efficiency of amplified spontaneous emission (ASE) and lowers ASE thresholds without incurring additional losses. The results suggest that the structure can be used to improve optical confinement for both optically pumped and electrical injection organic lasers where thin, low refractive index active materials are required.
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- 2021
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11. Impact of Acceptor Fluorination on the Performance of All-Polymer Solar Cells
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Lars Thomsen, Shyamal K. K. Prasad, Kedar D. Deshmukh, Dinesh Kabra, Michael Sommer, Amelia C. Y. Liu, Adam Welford, Rukiya Matsidik, Luke A. Connal, Justin M. Hodgkiss, Christopher R. McNeill, Eliot Gann, and Naresh Chandrasekaran
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chemistry.chemical_classification ,Materials science ,Energy conversion efficiency ,chemistry.chemical_element ,02 engineering and technology ,Polymer ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,Acceptor ,Polymer solar cell ,0104 chemical sciences ,chemistry ,Fluorine ,Naphthalene diimide ,General Materials Science ,Fill factor ,0210 nano-technology - Abstract
Here, we systematically study the effect of fluorination on the performance of all-polymer solar cells by employing a naphthalene diimide (NDI)-based polymer acceptor with thiophene-flanked phenyl co-monomer. Fluorination of the phenyl co-monomer with either two or four fluorine units is used to create a series of acceptor polymers with either no fluorination (PNDITPhT), bifluorination (PNDITF2T), or tetrafluorination (PNDITF4T). In blends with the donor polymer PTB7-Th, fluorination results in an increase in power conversion efficiency from 3.1 to 4.6% despite a decrease in open-circuit voltage from 0.86 V (unfluorinated) to 0.78 V (tetrafluorinated). Countering this decrease in open-circuit voltage is an increase in short-circuit current from 7.7 to 11.7 mA/cm2 as well as an increase in fill factor from 0.45 to 0.53. The origin of the improvement in performance with fluorination is explored using a combination of morphological, photophysical, and charge-transport studies. Interestingly, fluorination is ...
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- 2017
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12. The Evolution of Quantum Confinement in CsPbBr3 Perovskite Nanocrystals
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Shyamal K. K. Prasad, Parth Vashishtha, Kai Chen, Dani Z. Metin, Joseph K. Gallaher, Geoffry Laufersky, Jonathan E. Halpert, Justin M. Hodgkiss, Nicola Gaston, and Justinas Butkus
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Materials science ,Condensed matter physics ,Band gap ,General Chemical Engineering ,02 engineering and technology ,General Chemistry ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Kinetic energy ,01 natural sciences ,Fluence ,0104 chemical sciences ,Condensed Matter::Materials Science ,Nanocrystal ,Quantum dot ,Ultrafast laser spectroscopy ,Materials Chemistry ,Energy level ,0210 nano-technology ,Perovskite (structure) - Abstract
Colloidal nanocrystals (NCs) of lead halide perovskites are considered highly promising materials that combine the exceptional optoelectronic properties of lead halide perovskites with tunability from quantum confinement. But can we assume that these materials are in the strong confinement regime? Here, we report an ultrafast transient absorption study of cubic CsPbBr3 NCs as a function of size, compared with the bulk material. For NCs above ∼7 nm edge length, spectral signatures are similar to the bulk material–characterized by state-filling with uncorrelated charges–but discrete new kinetic components emerge at high fluence due to bimolecular recombination occurring in a discrete volume. Only for the smallest NCs (∼4 nm edge length) are strong quantum confinement effects manifest in TA spectral dynamics; focusing toward discrete energy states, enhanced bandgap renormalization energy, and departure from a Boltzmann statistical carrier cooling. At high fluence, we find that a hot-phonon bottleneck effect ...
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- 2017
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13. High Exciton Diffusion Coefficients in Fused Ring Electron Acceptor Films
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Shyamal K. K. Prasad, Xiaowei Zhan, Sreelakshmi Chandrabose, Alex J. Barker, Kai Chen, Jingshuai Zhu, Joshua J. Sutton, Justin M. Hodgkiss, Jiadong Zhou, Zengqi Xie, and Keith C. Gordon
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chemistry.chemical_classification ,Fullerene ,Exciton ,Transition dipole moment ,Heterojunction ,General Chemistry ,Electron acceptor ,Chromophore ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,010402 general chemistry ,01 natural sciences ,Biochemistry ,Catalysis ,0104 chemical sciences ,Organic semiconductor ,Condensed Matter::Materials Science ,Colloid and Surface Chemistry ,chemistry ,Chemical physics ,Diffusion (business) - Abstract
Modest exciton diffusion lengths dictate the need for nanostructured bulk heterojunctions in organic photovoltaic (OPV) cells; however, this morphology compromises charge collection. Here, we reveal rapid exciton diffusion in films of a fused-ring electron acceptor that, when blended with a donor, already outperforms fullerene-based OPV cells. Temperature-dependent ultrafast exciton annihilation measurements are used to resolve a quasi-activationless exciton diffusion coefficient of at least 2 × 10–2 cm2/s, substantially exceeding typical organic semiconductors and consistent with the 20–50 nm domain sizes in optimized blends. Enhanced three-dimensional diffusion is shown to arise from molecular and packing factors; the rigid planar molecular structure is associated with low reorganization energy, good transition dipole moment alignment, high chromophore density, and low disorder, all enhancing long-range resonant energy transfer. Relieving exciton diffusion constraints has important implications for OPVs...
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- 2019
14. Oxygen-Enhanced Upconversion of near Infrared Light from Below the Silicon Band Gap
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Shyamal K. K. Prasad, Thilini Ishwara, Zhi Li Teh, John E. Anthony, Sarah Norman, Shujuan Huang, Anthony J. Petty, Timothy W. Schmidt, and Elham M. Gholizadeh
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Materials science ,Near infrared light ,Band gap ,business.industry ,Singlet oxygen ,chemistry.chemical_element ,Oxygen enhanced ,Triplet triplet annihilation ,Oxygen ,Photon upconversion ,chemistry.chemical_compound ,chemistry ,Optoelectronics ,Semiconductor nanocrystals ,business - Abstract
Here we demonstrate an upconversion composition using semiconductor nanocrystal sensitizers that employs molecular triplet states below the singlet oxygen energy. We show that, contrary to the usual expectation, the admission of oxygen enhances the intensity of upconverted light and significantly speeds up the photochemical processes involved. Further, we demonstrate photochemical upconversion from below the silicon band gap in the presence of oxygen.
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- 2019
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15. Facile Exciton Diffusion in Fused Ring Electron Acceptor Films
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Zenqi Xie, Joshua J. Sutton, Kai Chen, Alex J. Barker, Sreelakshmi Chandrabose, Jingshuai Zhu, Keith C. Gordon, Justin M. Hodgkiss, Shyamal K. K. Prasad, and Xiaowei Zhan
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chemistry.chemical_classification ,Materials science ,chemistry ,Exciton ,Electron acceptor ,Diffusion (business) ,Ring (chemistry) ,Photochemistry - Published
- 2019
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16. Isolating and quantifying the impact of domain purity on the performance of bulk heterojunction solar cells
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Lars Thomsen, Shyamal K. K. Prasad, Yi-Bing Cheng, Christopher R. McNeill, Naresh Chandrasekaran, Dinesh Kabra, Wenchao Huang, Justin M. Hodgkiss, and Eliot Gann
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Morphology ,Materials science ,Phase-Separation ,Layer ,Thin-Films ,Poly(3-Hexylthiophene) ,Analytical chemistry ,02 engineering and technology ,X-Ray-Scattering ,010402 general chemistry ,01 natural sciences ,Polymer solar cell ,Domain (software engineering) ,law.invention ,Crystallinity ,law ,Phase (matter) ,Solar cell ,Environmental Chemistry ,Quantum Efficiency ,Renewable Energy, Sustainability and the Environment ,business.industry ,Scattering ,Energy conversion efficiency ,Miscibility ,021001 nanoscience & nanotechnology ,Pollution ,0104 chemical sciences ,Blends ,Nuclear Energy and Engineering ,Optoelectronics ,Organic Photovoltaics ,0210 nano-technology ,business ,Intensity (heat transfer) - Abstract
In solution-processed organic bulk heterojunction (BHJ) solar cells, the purity of the phase-separated domains is known to play an important role in determining device function. While the effects of domain purity have been investigated by tuning of the BHJ morphology, such tuning typically results in several parameters (for example domain size and crystallinity) being varied at once. Here we show that by varying the time between spin-coating and the application of an anti-solvent treatment, the domain purity of the polymer-rich phase in PBDTTT-EFT: PC71BM blends can be tuned while keeping other morphological parameters constant. This unique approach enables the effect of domain purity on device function to be isolated and quantified. Over the purity range explored, solar cell power conversion efficiency is observed to monotonically increase from 7.2% to 9.6% with increasing domain purity, with the cell fill factor most affected by changes in domain purity. Employing transient photovoltage measurements we find that purer phases result in a reduction in the rate constant of bimolecular recombination. A more thorough treatment is also presented on the relationship between the total scattering intensity (derived from resonant soft X-ray scattering measurements) and domain purity. In particular it is shown that domain purity does not scale linearly with total scattering intensity requiring an initial estimate of absolute domain composition.
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- 2017
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17. Efficient and tunable spectral compression using frequency-domain nonlinear optics
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Kai Chen, Shyamal K. K. Prasad, N R Monahan, Justin M. Hodgkiss, and Karen E. Thorn
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Materials science ,business.industry ,Bandwidth (signal processing) ,Physics::Optics ,Nonlinear optics ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Laser ,01 natural sciences ,Atomic and Molecular Physics, and Optics ,law.invention ,010309 optics ,Optics ,Narrowband ,law ,Picosecond ,Frequency domain ,0103 physical sciences ,Femtosecond ,0210 nano-technology ,business ,Ultrashort pulse - Abstract
A key requirement in the field of ultrafast vibrational spectroscopy is to efficiently generate intense tunable narrowband picosecond laser pulses synchronized to a broadband femtosecond laser source. Current nonlinear methods for picosecond pulse generation suffer from complexities in both experimental implementation and pulse frequency tunability. We present here a straightforward method for spectral bandwidth compression that produces frequency tunable picosecond pulses with efficient power conversion. Broadband femtosecond laser pulses are compressed to narrowband picosecond pulses using frequency domain sum-frequency generation of spatially chirped pulses, achieving spectral bandwidths of
- Published
- 2018
18. Two for one photon deal: Prospects for singlet fission photovoltaics (Conference Presentation)
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Timothy W. Schmidt and Shyamal K. K. Prasad
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Nuclear physics ,Physics ,Presentation ,Photon ,Photovoltaics ,business.industry ,media_common.quotation_subject ,Singlet fission ,business ,media_common - Published
- 2018
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19. Quantifying highly efficient incoherent energy transfer in perylene-based multichromophore arrays
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Justin M. Hodgkiss, Shyamal K. K. Prasad, James E. A. Webb, Kai Chen, Jonathan P. Wojciechowski, Pall Thordarson, and Alexander Falber
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Materials science ,Photoluminescence ,business.industry ,General Physics and Astronomy ,02 engineering and technology ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,7. Clean energy ,Resonance (particle physics) ,0104 chemical sciences ,chemistry.chemical_compound ,chemistry ,Picosecond ,Ultrafast laser spectroscopy ,Optoelectronics ,Physical and Theoretical Chemistry ,Photonics ,0210 nano-technology ,business ,Spectroscopy ,Ultrashort pulse ,Perylene - Abstract
Multichromophore perylene arrays were designed and synthesized to have extremely efficient resonance energy transfer. Using broadband ultrafast photoluminescence and transient absorption spectroscopies, transfer timescales of approximately 1 picosecond were resolved, corresponding to efficiencies of up to 99.98%. The broadband measurements also revealed spectra corresponding to incoherent transfer between localized states. Polarization resolved spectroscopy was used to measure the dipolar angles between donor and acceptor chromophores, thereby enabling geometric factors to be fixed when assessing the validity of Förster theory in this regime. Förster theory was found to predict the correct magnitude of transfer rates, with measured ∼2-fold deviations consistent with the breakdown of the point-dipole approximation at close approach. The materials presented, along with the novel methods for quantifying ultrahigh energy transfer efficiencies, will be valuable for applications demanding extremely efficient energy transfer, including fluorescent solar concentrators, optical gain, and photonic logic devices.
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- 2016
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20. An N-heterocyclic carbene phenanthroline ligand: synthesis, multi-metal coordination and spectroscopic studies
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Alyssa A. Webster, Justin M. Hodgkiss, John O. Hoberg, and Shyamal K. K. Prasad
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010405 organic chemistry ,Ligand ,Chemistry ,Phenanthroline ,chemistry.chemical_element ,Rhenium ,010402 general chemistry ,Photochemistry ,01 natural sciences ,3. Good health ,0104 chemical sciences ,Ruthenium ,Inorganic Chemistry ,Metal ,chemistry.chemical_compound ,visual_art ,Polymer chemistry ,visual_art.visual_art_medium ,Moiety ,Platinum ,Carbene - Abstract
Dimetal complexes of a new N-heterocyclic carbene/phenanthroline ligand have been synthesized. Coordination of both ruthenium and rhenium to the phenanthroline moiety in combination with platinum at the carbene moiety are reported. Steady-state and time-resolved optical absorption and photoluminescence spectra were obtained for the complexes. These results illustrate significant changes occur with the incorporation of the second metal, and that the specific metal bound to the phenanthroline moiety is important to the photophysical characteristics of this system.
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- 2015
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21. Spectroscopically tracking charge separation in polymer : fullerene blends with a three-phase morphology
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Justin M. Hodgkiss, Jin Young Kim, Mohammad Afsar Uddin, Joseph K. Gallaher, Han Young Woo, Taehyo Kim, and Shyamal K. K. Prasad
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chemistry.chemical_classification ,Materials science ,Fullerene ,Renewable Energy, Sustainability and the Environment ,Analytical chemistry ,Energy landscape ,Charge (physics) ,Polymer ,Pollution ,Effective nuclear charge ,Amorphous solid ,Nuclear Energy and Engineering ,chemistry ,Chemical physics ,Ultrafast laser spectroscopy ,Environmental Chemistry ,Spectroscopy - Abstract
The coexistence of intermixed amorphous polymer : fullerene phases alongside pure semicrystalline polymer and fullerene phases provides a plausible explanation for effective charge separation in organic photovoltaic blends by providing a cascaded energy landscape. We sought to test this proposal by spectroscopically tracking charge dynamics in 3-phase blends compared with binary counterparts and linking these dynamics to free charge yields. Our study applies broadband transient absorption spectroscopy to a series of closely related alternating thiophene–benzothiadiazole copolymers in which the tuned curvature of the polymer backbone controls the nature and degree of polymer–fullerene intermixing. Free charge generation is most efficient in the 3-phase morphology that features intimately mixed polymer : PCBM regions amongst neat polymer and PCBM phases. TA spectral dynamics and polarization anisotropy measurements reveal the sub-nanosecond migration of holes from intermixed to pure polymer regions of such blends. In contrast, 2-phase blends lack the spectral dynamics of this charge migration process and suffer from severe geminate recombination losses. These results provide valuable spectroscopic evidence for an efficient charge separation pathway that relies on the 3-phase morphology.
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- 2015
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22. Influence of Fullerene Acceptor on the Performance, Microstructure, and Photophysics of Low Bandgap Polymer Solar Cells
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Dinesh Kabra, Lars Thomsen, Shyamal K. K. Prasad, Yang Yang, Sheng-Yung Chang, Christopher R. McNeill, Justin M. Hodgkiss, Eliot Gann, Wenchao Huang, Naresh Chandrasekaran, and Yi-Bing Cheng
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Morphology ,Fullerene ,Materials science ,Band gap ,Photovoltaic Cells ,02 engineering and technology ,Thermal treatment ,Blend ,010402 general chemistry ,Photochemistry ,01 natural sciences ,Polymer solar cell ,Crystallinity ,General Materials Science ,Indene-C-60 Bisadduct ,High-Efficiency ,Dependence ,chemistry.chemical_classification ,Renewable Energy, Sustainability and the Environment ,Energy conversion efficiency ,Miscibility ,Polymer ,021001 nanoscience & nanotechnology ,Acceptor ,0104 chemical sciences ,chemistry ,11-Percent ,0210 nano-technology ,Donor - Abstract
The morphology, photophysics, and device performance of solar cells based on the low bandgap polymer poly[[2,6'-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl (PBDTTT-EFT) (also known as PTB7-Th) blended with different fullerene acceptors: Phenyl-C-61-butyric acid methyl ester (PC61BM), phenyl-C-71-butyric acid methyl ester (PC71BM), or indene-C-60 bisadduct (ICBA) are correlated. Compared to PC71BM-based cells-which achieve a power conversion efficiency (PCE) of 9.4%-cells using ICBA achieve a higher open-circuit voltage (V-OC) of 1.0 V albeit with a lower PCE of 7.1%. To understand the origin of this lower PCE, the morphology and photophysics have been thoroughly characterized. Hard and soft X-ray scattering measurements reveal that the PBDTTT-EFT: ICBA blend has a lower crystallinity, lower domain purity, and smaller domain size compared to the PBDTTT-EFT: PC71BM blend. Incomplete photoluminescence quenching is also found in the ICBA blend with transient absorption measurements showing faster recombination dynamics at short timescales. Transient photovoltage measurements highlight further differences in recombination at longer timeframes due to the more intermixed morphology of the ICBA blend. Interestingly, a mild thermal treatment improves the performance of PBDTTT-EFT: ICBA cells which is exploited in the fabrication of a homo PBDTTT-EFT: ICBA tandem solar cell with PCE of 9.0% and V-OC of 1.93 V.
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- 2017
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23. Impact of Fullerene Mixing Behavior on the Microstructure, Photophysics, and Device Performance of Polymer/Fullerene Solar Cells
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Shyamal K. K. Prasad, Dinesh Kabra, Naresh Chandrasekaran, Justin M. Hodgkiss, Eliot Gann, Wenchao Huang, Christopher R. McNeill, Lars Thomsen, and Yi-Bing Cheng
- Subjects
Morphology ,Photoluminescence ,Fullerene ,Materials science ,Organic solar cell ,Mobility Conjugated Polymer ,Nanotechnology ,02 engineering and technology ,010402 general chemistry ,7. Clean energy ,01 natural sciences ,Polymer solar cell ,Ultrafast laser spectroscopy ,Physics::Atomic and Molecular Clusters ,Organic Photovoltaic Cells ,General Materials Science ,High-Efficiency ,Spectroscopy ,Charge Separation ,chemistry.chemical_classification ,Molecular-Orientation ,Organic Solar Cells ,Miscibility ,Polymer ,Semiconductor ,021001 nanoscience & nanotechnology ,Acceptor ,0104 chemical sciences ,Photophysics ,chemistry ,Chemical engineering ,Blends ,13. Climate action ,Mixing Behavior ,Device Physics ,Fullerenes ,0210 nano-technology ,Domain Purity - Abstract
Here, a comprehensive study of the influence of polymer:fullerene mixing behavior on the performance, thin-film microstructure, photophysics, and device physics of polymer solar cells is presented. In particular, blends of the donor polymer PBDTTT-EFT with the acceptor PC71BM that exhibit power conversion efficiencies over 9% are investigated. Through tuning of the fullerene concentration in PBDTTT-EFT:PC71BM blends, the impact of fullerene mixing behavior is systematically investigated via a combination of synchrotron-based X-ray scattering and spectroscopy techniques. The impact of fullerene loading on photophysics and device physics is further explored with steady-state photoluminescence measurements, ultrafast transient absorption spectroscopy, and transient photovoltage measurements. In the low fullerene concentration regime (70 wt %), large fullerene domains result in incomplete PC71BM exciton harvesting with the presence of fullerene molecules also disrupting the molecular packing of polymer crystallites. The optimum fullerene concentration of similar to 60-67 wt % balances the requirements of charge generation and charge collection. These findings demonstrate that controlling the fullerene concentration in the mixed phase and optimizing the balance between pure and mixed phases are critical for maximizing the efficiency of highly mixed polymer/fullerene solar cells.
- Published
- 2016
24. Capturing ultrafast spectral evolution with transient grating photoluminescence spectroscopy
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Kai Chen, Justin M. Hodgkiss, Joseph K. Gallaher, James E. A. Webb, Pall Thordarson, and Shyamal K. K. Prasad
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Diffraction ,Photoluminescence ,Kerr effect ,Materials science ,business.industry ,Relaxation (NMR) ,Physics::Optics ,02 engineering and technology ,Grating ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Laser ,01 natural sciences ,0104 chemical sciences ,law.invention ,Optics ,law ,Physics::Atomic and Molecular Clusters ,Optoelectronics ,0210 nano-technology ,business ,Spectroscopy ,Ultrashort pulse - Abstract
We have developed a new method, transient grating photoluminescence spectroscopy (TGPLS), allowing the collection of broadband ultrafast photoluminescence spectroscopy with low photoluminescence background. In TGPLS, two ultrafast laser pulses generate a multiplexed transient grating (TG) by the optical Kerr effect. The gated signal is diffracted by the TG and spatially separated from background fluorescence. This high performance nonlinear optical gate delivers time resolution less than 200 fs, spectral bandwidth covering the entire visible region with extremely low fluorescence background. Here we present two applications of TGPLS that provide deeper insight into ultrafast energy transfer in multi-chromophore perylene arrays and ultrafast structural relaxation in oligothiophenes.
- Published
- 2016
- Full Text
- View/download PDF
25. Incoherent charge separation dynamics in organic photovoltaics
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Joseph K. Gallaher, Shyamal K. K. Prasad, Justin M. Hodgkiss, Mamatimin Abbas, Han Young Woo, Lionel Hirsch, Alex J. Barker, MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, Laboratoire de l'intégration, du matériau au système (IMS), and Université Sciences et Technologies - Bordeaux 1-Institut Polytechnique de Bordeaux-Centre National de la Recherche Scientifique (CNRS)
- Subjects
Materials science ,Organic solar cell ,transient absorption ,02 engineering and technology ,010402 general chemistry ,01 natural sciences ,7. Clean energy ,intermixed phase ,organic photovoltaic ,Optics ,Phase (matter) ,Ultrafast laser spectroscopy ,three phase ,Diffusion (business) ,Spectroscopy ,business.industry ,Charge (physics) ,021001 nanoscience & nanotechnology ,Charge separation ,0104 chemical sciences ,mobility relaxation ,Chemical physics ,Picosecond ,[PHYS.COND.CM-MS]Physics [physics]/Condensed Matter [cond-mat]/Materials Science [cond-mat.mtrl-sci] ,0210 nano-technology ,business ,Excitation - Abstract
International audience; There is mounting evidence that long-range charge separation determines the efficiency of organic photovoltaic cells, yet different mechanisms remain under debate. One class of proposed mechanism is ultrafast coherent long-range charge separation, and another is a slower process whereby charges incoherently hop apart with a transiently enhanced mobility due to morphology and disorder. Here, we use transient absorption spectroscopy to probe incoherent charge separation dynamics in two different ways. First, we use a family of polymers whose backbone structures allows us to compare 2-phase donor-acceptor morphologies with 3-phase morphologies that feature an intermixed region. In the 3-phase system, we see pronounced spectral signatures associated with charges (holes) occupying the disordered intermixed region, and we track separation via biased charge diffusion to more ordered neat regions on the timescale of hundreds of picoseconds. Secondly, by resolving bimolecular charge recombination at high excitation density, we show that charge mobilities must be substantially enhanced on early timescales, which may be sufficient for separation to occur. Together, these measurements provide support for models of incoherent and relatively slow charge separation.
- Published
- 2016
- Full Text
- View/download PDF
26. Mapping Polymer Donors toward High-Efficiency Fullerene Free Organic Solar Cells
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Shyamal K. K. Prasad, Wei Ma, Jingshuai Zhu, Zhi-Guo Zhang, Lijun Huo, Yuxin Xia, Yuze Lin, Justin M. Hodgkiss, Zhishan Bo, Olle Inganäs, Zhixiang Wei, Feng Gao, Chunru Wang, Fuwen Zhao, Kai Chen, Haijun Bin, Yang Wu, Yongfang Li, Maojie Zhang, Yanming Sun, Guangwu Li, Xia Guo, and Xiaowei Zhan
- Subjects
chemistry.chemical_classification ,Solid-state chemistry ,Materials science ,Fullerene ,Organic solar cell ,Mechanical Engineering ,Inorganic chemistry ,02 engineering and technology ,Polymer ,Hybrid solar cell ,Electron acceptor ,010402 general chemistry ,021001 nanoscience & nanotechnology ,01 natural sciences ,Polymer solar cell ,0104 chemical sciences ,Planar ,chemistry ,Chemical engineering ,Mechanics of Materials ,General Materials Science ,0210 nano-technology - Abstract
Five polymer donors with distinct chemical structures and different electronic properties are surveyed in a planar and narrow-bandgap fused-ring electron acceptor (IDIC)-based organic solar cells, which exhibit power conversion efficiencies of up to 11%.
- Published
- 2016
27. Balanced Partnership between Donor and Acceptor Components in Nonfullerene Organic Solar Cells with >12% Efficiency
- Author
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Xiaowei Zhan, Wanzhu Cai, Qianqian Zhang, Yang Wu, Shyamal K. K. Prasad, Yuze Lin, Kai Chen, Wei Ma, Chunru Wang, Wei You, Fuwen Zhao, Jing-De Chen, Feng Gao, Jian-Xin Tang, and Justin M. Hodgkiss
- Subjects
chemistry.chemical_classification ,Photocurrent ,Materials science ,Fullerene ,Organic solar cell ,Mechanical Engineering ,Exciton ,02 engineering and technology ,Electron acceptor ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Photochemistry ,01 natural sciences ,Acceptor ,Polymer solar cell ,0104 chemical sciences ,chemistry ,Mechanics of Materials ,General Materials Science ,0210 nano-technology ,Absorption (electromagnetic radiation) - Abstract
Relative to electron donors for bulk heterojunction organic solar cells (OSCs), electron acceptors that absorb strongly in the visible and even near-infrared region are less well developed, which hinders the further development of OSCs. Fullerenes as traditional electron acceptors have relatively weak visible absorption and limited electronic tunability, which constrains the optical and electronic properties required of the donor. Here, high-performance fullerene-free OSCs based on a combination of a medium-bandgap polymer donor (FTAZ) and a narrow-bandgap nonfullerene acceptor (IDIC), which exhibit complementary absorption, matched energy levels, and blend with pure phases on the exciton diffusion length scale, are reported. The single-junction OSCs based on the FTAZ:IDIC blend exhibit power conversion efficiencies up to 12.5% with a certified value of 12.14%. Transient absorption spectroscopy reveals that exciting either the donor or the acceptor component efficiently generates mobile charges, which do not suffer from recombination to triplet states. Balancing photocurrent generation between the donor and nonfullerene acceptor removes undesirable constraints on the donor imposed by fullerene derivatives, opening a new avenue toward even higher efficiency for OSCs.
- Published
- 2018
- Full Text
- View/download PDF
28. Tuning the Molecular Weight of the Electron Accepting Polymer in All‐Polymer Solar Cells: Impact on Morphology and Charge Generation
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Amelia C. Y. Liu, Rukiya Matsidik, Michael Sommer, Luke A. Connal, Christopher R. McNeill, Lars Thomsen, Shyamal K. K. Prasad, Justin M. Hodgkiss, Eliot Gann, and Kedar D. Deshmukh
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chemistry.chemical_classification ,Materials science ,Band gap ,02 engineering and technology ,Polymer ,010402 general chemistry ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,7. Clean energy ,Acceptor ,Polymer solar cell ,0104 chemical sciences ,Electronic, Optical and Magnetic Materials ,law.invention ,Biomaterials ,chemistry ,Chemical engineering ,law ,Ultrafast laser spectroscopy ,Solar cell ,Electrochemistry ,0210 nano-technology ,Absorption (electromagnetic radiation) ,Spectroscopy - Abstract
Molecular weight is an important factor determining the morphology and performance of all-polymer solar cells. Through the application of direct arylation polycondention, a series of batches of a fluorinated naphthalene diimide-based acceptor polymer are prepared with molecular weight varying from M-n = 20 to 167 kDa. Used in conjunction with a common low bandgap donor polymer, the effect of acceptor molecular weight on solar cell performance, morphology, charge generation, and transport is explored. Increasing the molecular weight of the acceptor from M-n = 20 to 87 kDa is found to increase cell efficiency from 2.3% to 5.4% due to improved charge separation and transport. Further increasing the molecular weight to M-n = 167 kDa however is found to produce a drop in performance to 3% due to liquid-liquid phase separation which produces coarse domains, poor charge generation, and collection. In addition to device studies, a systematic investigation of the microstructure and photophysics of this system is presented using a combination of transmission electron microscopy, grazing-incidence wide-angle X-ray scattering, near-edge X-ray absorption fine-structure spectroscopy, photoluminescence quenching, and transient absorption spectroscopy to provide a comprehensive understanding of the interplay between morphology, photophysics, and photovoltaic performance.
- Published
- 2018
- Full Text
- View/download PDF
29. HIGH-SENSITIVITY ULTRAFAST TRANSIENT ABSORPTION SPECTROSCOPY OF ORGANIC PHOTOVOLTAIC DEVICES
- Author
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Shyamal K. K. Prasad, Kai Chen, Alex J. Barker, and Justin M. Hodgkiss
- Subjects
Photocurrent ,Microsecond ,Materials science ,Organic solar cell ,business.industry ,Femtosecond ,Ultrafast laser spectroscopy ,Optoelectronics ,Absorption (electromagnetic radiation) ,business ,Spectroscopy ,Ultrashort pulse - Abstract
The design of effective materials for efficient organic photovoltaic cells requires developing a detailed photophysical model of the processes that link photon absorption to photocurrent collection. Transient absorption spectroscopy offers the potential to do so, but its value depends on the ability to carry out measurements with sensitivity of better than 10 and from femtosecond to microsecond timescales. In this article, we describe the transient absorption spectroscopy tools that we have developed specifically for probing charge photogeneration and recombination in organic photovoltaic cells, and we illustrate their implementation with several examples.
- Published
- 2014
- Full Text
- View/download PDF
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