| 1. |
- Andersson, Mattias, et al.
(författare)
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Conformation sensitive charge transport in conjugated polymers
- 2013
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 103:21, s. 213303-
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Tidskriftsartikel (refereegranskat)abstract
- Temperature dependent charge carrier mobility measurements using field effect transistors and density functional theory calculations are combined to show how the conformation dependent frontier orbital delocalization influences the hole-and electron mobilities in a donor-acceptor based polymer. A conformationally sensitive lowest unoccupied molecular orbital results in an electron mobility that decreases with increasing temperature above room temperature, while a conformationally stable highest occupied molecular orbital is consistent with a conventional hole mobility behavior and also proposed to be one of the reasons for why the material works well as a hole transporter in amorphous bulk heterojunction solar cells.
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| 2. |
- Chaudhuri, Subhajyoti, et al.
(författare)
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Electron Transfer Assisted by Vibronic Coupling from Multiple Modes
- 2017
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Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 13:12, s. 6000-6009
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the effect of vibronic coupling on electron transfer (ET) rates is a challenge common to a wide range of applications, from electrochemical synthesis and catalysis to biochemical reactions and solar energy conversion. The Marcus-Jortner-Levich (MJL) theory offers a model of ET rates based on a simple analytic expression with a few adjustable parameters. However, the MJL equation in conjunction with density functional theory (DFT) has yet to be established as a predictive first-principles methodology. A framework is presented for calculating transfer rates modulated by molecular vibrations, that circumvents the steep computational cost which has previously necessitated approximations such as condensing the vibrational manifold into a single empirical frequency. Our DFT MJL approach provides robust and accurate predictions of ET rates spanning over 4 orders of magnitude in the 10(6)-10(10) s(-1) range. We evaluate the full MJL equation with a Monte Carlo sampling of the entire active space of thermally accessible vibrational modes, while using no empirical parameters. The contribution to the rate of individual modes is illustrated, providing insight into the interplay between vibrational degrees of freedom and changes in electronic state. The reported findings are valuable for understanding ET rates modulated by multiple vibrational modes, relevant to a broad range of systems within the chemical sciences.
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| 3. |
- Gedefaw, Desta, et al.
(författare)
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Design, Synthesis and Computational Study of Fluorinated Quinoxaline-Oligothiophene-based Conjugated Polymers with Broad Spectral Coverage
- 2018
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Ingår i: ChemPhysChem. - : Wiley. - 1439-4235 .- 1439-7641. ; 19:24, s. 3393-3400
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Tidskriftsartikel (refereegranskat)abstract
- Donor-acceptor (D-A) copolymers typically show two absorption peaks in the visible region, flanking a valley region of limited absorptivity. One strategy for more panchromatic light harvesting is to incorporate side-groups orthogonal to the polymer backbone, which enable 2D pi conjugation and can give rise to additional absorption peaks. Here we design and synthesize two D-A polymers which both carry a fluorinated quinoxaline acceptor unit, but while P1 includes a benzodithiophene donor moiety with thiophene side-groups (2D-BDT), the P2 polymer lacks 2D conjugation in its simpler pentathiophene donor segment. The P1 polymer consequently shows an atypical absorption profile with more panchromatic absorption with no apparent valley in the spectrum. In order to understand the structure-electronic relations, the optical and electrochemical properties were predicted using a previously developed computational approach. The predicted optical properties show very good agreement with the experimental results. Solar cells made from P1 show a short-circuit current more than twice as large as P2, attributed to its enhanced spectral coverage. However, poor fill factors limit the preliminary power conversion efficiencies to 3.3 % for P1 and 1.0 % for P2 as blended with PCBM[70] in a 1 : 1.5 (w/w) ratio.
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| 4. |
- Goswami, Sreetosh, et al.
(författare)
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Charge disproportionate molecular redox for discrete memristive and memcapacitive switching
- 2020
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Ingår i: Nature Nanotechnology. - : Springer Science and Business Media LLC. - 1748-3387 .- 1748-3395. ; 15:5, s. 380-389
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Tidskriftsartikel (refereegranskat)abstract
- Electronic symmetry breaking by charge disproportionation results in multifaceted changes in the electronic, magnetic and optical properties of a material, triggering ferroelectricity, metal/insulator transition and colossal magnetoresistance. Yet, charge disproportionation lacks technological relevance because it occurs only under specific physical conditions of high or low temperature or high pressure. Here we demonstrate a voltage-triggered charge disproportionation in thin molecular films of a metal-organic complex occurring in ambient conditions. This provides a technologically relevant molecular route for simultaneous realization of a ternary memristor and a binary memcapacitor, scalable down to a device area of 60 nm(2). Supported by mathematical modelling, our results establish that multiple memristive states can be functionally non-volatile, yet discrete-a combination perceived as theoretically prohibited. Our device could be used as a binary or ternary memristor, a binary memcapacitor or both concomitantly, and unlike the existing 'continuous state' memristors, its discrete states are optimal for high-density, ultra-low-energy digital computing. Charge disproportionation in thin molecular films of a metal-organic complex enables the realization of a ternary memristor and binary memcapacitor.
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| 5. |
- Hedström, Svante
(författare)
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Computational Predictions of Conjugated Polymer Properties for Photovoltaic Applications
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Organic solar cells employing fullerenes blended with conjugated polymers as the main light-absorbing material have achieved power conversion efficiencies exceeding 10%. They hold promise as an alternative energy source with many advantages in terms of long-term sustainability and reduced greenhouse gas emissions. Detailed information on the electronic and geometric structure of the molecules involved is generally not accessible through experimental means, as the typically amorphous polymer films are not readily studied with e.g. X-ray crystallography. Computational chemistry, and in particular quantum chemistry as used for the research presented in this thesis, can however provide molecular level insight into the properties of these conjugated polymers. (Time-dependent) density functional theory calculations are here employed on various polymers, mainly of donor–acceptor (D–A) and D–A1–D–A2 types. Systematic studies demonstrate how the energy levels and optical properties relate to each other, as well as to the chemical composition of the polymers. In particular focus are the traits that are important for efficient solar cells: strong absorption, suitably narrow band gap, appropriate LUMO energy vs. the fullerene LUMO, and extended conjugation promoting high charge carrier mobilities. Several polymers with high-performance solar cells are studied, including TQ1 where a computationally revealed unique helical geometry is used to partially rationalize its 7.08% efficiency, and the D–A1–D–A2 polymer P3TQTIF whose two distinct acceptor units allow two strong low-energy electronic transitions, greatly enhancing its spectral coverage. Size-converged optical properties are obtained through a scheme based on extrapolations from oligomer calculations, and a detailed comparison to experiments has facilitated the development of an empirical correction for absorption energies and strengths. These corrections are subsequently used for a priori predictions of polymer absorption spectra with good agreement to experiments. Finally, a strategy is presented that includes the effect of temperature, in form of thermally populated conformations with reduced conjugation and weaker and blue-shifted absorption, yielding trends in excellent agreement with experimental optical properties. Calculations are in summary able to provide deeper insights into the fundamental properties of conjugated polymers, constituting a valuable tool for the ongoing development of materials for application in high-performance organic solar cells.
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| 6. |
- Hedström, Svante, et al.
(författare)
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Defining donor and acceptor strength in conjugated copolymers
- 2017
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Ingår i: Molecular Physics. - : Informa UK Limited. - 1362-3028 .- 0026-8976. ; 115:5, s. 485-496
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Tidskriftsartikel (refereegranskat)abstract
- The progress in efficiency of organic photovoltaic devices is largely driven by the development of new donor-acceptor (D-A) copolymers. The number of possible D-A combinations escalates rapidly with the ever-increasing number of donor and acceptor units, and the design process often involves a trial-and-error approach. We here present a computationally efficient methodology for the prediction of optical and electronic properties of D-A copolymers based on density functional theory calculations of donor- and acceptor-only homopolymers. Ten donors and eight acceptors are studied, as well as all of their 80 D-A copolymer combinations, showing absorption energies of 1.3-2.3eV, and absorption strengths varying by up to a factor of 2.5. Focus lies on exhibited trends in frontier orbital energies, optical band gaps, and absorption intensities, as well as their relation to the molecular structure. Based on the results, we define the concept of donor and acceptor strength, and calculate this quantity for all investigated units. The light-harvesting capabilities of the 80 D-A copolymers were also assessed. This gives a valuable theoretical guideline to the design of D-A copolymers with the potential to reduce the synthesis efforts in the development of new polymers.
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| 7. |
- Hedström, Svante, et al.
(författare)
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Light-harvesting capabilities of low band gap donor-acceptor polymers
- 2014
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 16:45, s. 24853-24865
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Tidskriftsartikel (refereegranskat)abstract
- A series of nine donor-acceptor polymers, including three new and six polymers from previous work, have been investigated experimentally and theoretically. The investigation focuses on narrow band gaps and strong absorptions of the polymers, where experimentally determined first peak absorption energies range from 1.8 to 2.3 eV, and peak absorption coefficients vary between 19-67 L g(-1) cm(-1). An overall assessment of each polymer's light-harvesting capability is made, and related to the chemical structure. Oligomer calculations using density functional theory are extrapolated to obtain size-converged polymer properties, and found to reproduce the experimental absorption trends well. Accurate theoretical predictions of absorption energies to within 0.06 eV of experiments, and absorption strength to within 12%, are obtained through the introduction of an empirical correction scheme. The computational and experimental results provide insight for the design of polymers with efficient absorption, concerning the intrinsic properties of the constituent units and the use of bulky side-groups.
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| 8. |
- Hedström, Svante, et al.
(författare)
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Photodriven CO dimerization on Cu2O from an electronic-structure perspective
- 2020
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Ingår i: Sustainable Energy & Fuels. - : Royal Society of Chemistry (RSC). - 2398-4902. ; 4:2, s. 670-677
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Tidskriftsartikel (refereegranskat)abstract
- Electrochemically driven CO2 reduction into alcohols and hydrocarbons is a topic of intense study. Photocatalytic approaches, which instead are powered by light, are also reported, but these generally rely on two-component catalysts and yield only moderately reduced products with a single carbon atom. In this report, we use density functional theory, including its linear-response time-dependent implementation, to investigate the feasibility of photocatalytically driving the dimerization of CO chemisorbed on Cu2O, a crucial step in the chemical conversion of CO2 into C-2 products, such as ethanol and ethylene. We find that CO dimerization into OCCO is greatly aided by the photoinduced population of a low-lying LUMO that is bonding with respect to the C-C bond of two adjacently chemisorbed CO molecules.
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| 9. |
- Hedström, Svante, et al.
(författare)
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Quantum Chemical Calculations of Side-Group Stacking and Electronic Properties in Thiophene-Quinoxaline Polymers
- 2012
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:51, s. 26700-26706
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Tidskriftsartikel (refereegranskat)abstract
- Organic bulk heterojunction (BHJ) solar cells offer a viable source of solar energy. Structural organization is crucial in BHJ cells but hard to achieve and assess due to limitations in experimental methodology. Quantum chemical methods have here been used to gain further insight into the geometric and optical properties of a promising light-harvesting polymer, poly[2,3-bis(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1). Calculations show that favorable positions of the two alkoxyphenyl side groups on each TQ1 monomer allow nonbonded side-group stacking interactions with the neighboring units in both directions. This yields a unique, helical geometry with enhanced intramolecular ordering that promotes extensive electronic conjugation. Adequate description of this effect requires computational methods that include dispersion corrections. A strategy based on such side-group interactions is proposed for designing new polymers.
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| 10. |
- Hedström, Svante, et al.
(författare)
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Rational design of D-A1-D-A2 conjugated polymers with superior spectral coverage
- 2015
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 17:40, s. 26677-26689
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Tidskriftsartikel (refereegranskat)abstract
- The spectral coverage of a light-harvesting polymer largely determines the maximum achievable photocurrent in organic photovoltaics, and therefore constitutes a crucial parameter for improving their performance. The D-A1-D-A2 copolymer motif is a new and promising design strategy for extending the absorption range by incorporating two acceptor units with complementary photoresponses. The fundamental factors that promote an extended absorption are here determined for three prototype D-A1-D-A2 systems through a combination of experimental and computational methods. Systematic quantum chemical calculations are then used to reveal the intrinsic optical properties of ten further D-A1-D-A2 polymer candidates. These investigated polymers are all predicted to exhibit intense primary absorption peaks at 615-954 nm, corresponding to charge-transfer (CT) transitions to the stronger acceptor, as well as secondary absorption features at 444-647 nm that originate from CT transitions to the weaker acceptors. Realization of D-A1-D-A2 polymers with superior spectral coverage is thereby found to depend critically on the spatial and energetic separation between the two distinct acceptor LUMOs. Two promising D-A1-D-A2 copolymer candidates were finally selected for further theoretical and experimental study, and demonstrate superior light-harvesting properties in terms of significantly extended spectral coverage. This demonstrates great potential for enhanced light-harvesting in D-A1-D-A2 polymers via multiple absorption features compared to traditional D-A polymers.
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