Co-reporter:Christopher W. Marvin, Haley M. Grimm, Nathaniel C. Miller, W. Seth Horne, and Geoffrey R. Hutchison
The Journal of Physical Chemistry B November 9, 2017 Volume 121(Issue 44) pp:10269-10269
Publication Date(Web):October 16, 2017
DOI:10.1021/acs.jpcb.7b10085
Many biomaterials are piezoelectric (i.e., mechanically deform under an applied electric field); however, the molecular origin of this phenomenon remains unclear. In the case of protein-based scaffolds, one possibility involves flexible response of local folding motifs to the applied field. Here, we test this hypothesis by examining the piezoresponse in a series of helical peptide-based oligomers. Control over folding propensity is exerted through systematic variation in both side-chain sequence and backbone composition. Piezoresponse is quantified by piezo-force microscopy on polar self-assembled monolayers. The results indicate backbone rigidity is an important determinant in peptide electromechanical responsiveness.
Co-reporter:Shaopeng Zhang, Nicole E. Bauer, Ilana Y. Kanal, Wei YouGeoffrey R. Hutchison, Tara Y. Meyer
Macromolecules January 10, 2017 Volume 50(Issue 1) pp:
Publication Date(Web):December 22, 2016
DOI:10.1021/acs.macromol.6b02215
To understand the influence of monomer sequence on the properties and performance of conjugated oligomers, a series of dimers, trimers, and tetramers were prepared from phenylene (P) and benzothiadiazole (B) monomers linked by vinylene groups. Optical and electrochemical studies established the influence of sequence on both the λmax and redox potentials of this series of structurally related oligomers. For tetramers with bromo end groups (PBBP, BPPB, PBPB, PPBB), the λmax ranged from 493 to 512 nm (Δ = 19 nm), the electrochemical oxidation potential from 0.65 to 0.82 (Δ = 0.17 V) and the reduction potential from −1.45 to −1.31 (Δ = 0.14 V), all of which are sequence-dependent. The effect of end groups (cyano, bromo, and alkyl) was also demonstrated to be important for the properties of these oligomers. DFT calculations of the tetramers were performed and the energy levels were correlated well with the experimentally determined spectroscopic data. Bulk heterojunction (BHJ) solar cells fabricated with selected tetramers as the donor and PC61BM as the acceptor exhibited power conversion efficiencies that varied by a factor of 3 as a function of sequence (0.47–1.85%). These results suggest that sequence control is important for tuning optoelectronic properties and photovoltaic performance of these structurally related conjugated oligomers.
Co-reporter:M. J. Moody, C. W. Marvin and G. R. Hutchison
Journal of Materials Chemistry A 2016 vol. 4(Issue 20) pp:4387-4392
Publication Date(Web):26 Apr 2016
DOI:10.1039/C6TC00613B
We report flexible piezoelectric polyurethane foams with d33 piezocoefficients up to 244 ± 30 pC N−1. Polymer foams have large volume changes under applied force, and dipole-doped polymers can have large polarizations even when poled at fields two orders of magnitude lower than space-charge electrets. Combining these features results in piezocoefficients an order of magnitude higher than conventional polymer piezomaterials, and independent selection of matrix and dopant should permit easy processing and tailorable properties. We further motivate the use of mesostructured materials by noting that the theoretical piezocoefficient limit is much higher for such materials than for conventional ceramics or polymers.
Co-reporter:Shaopeng Zhang;Tara Y. Meyer
Macromolecular Rapid Communications 2016 Volume 37( Issue 11) pp:882-887
Publication Date(Web):
DOI:10.1002/marc.201600086
Co-reporter:Adam G. Gagorik;Jacob W. Mohin;Tomasz Kowalewski
Advanced Functional Materials 2015 Volume 25( Issue 13) pp:1996-2003
Publication Date(Web):
DOI:10.1002/adfm.201402332
Monte Carlo simulations of charge transport in organic solar cells are performed for ideal and isotropic bulk heterojunction morphologies while altering the delocalization length of charge carriers. Previous device simulations have either treated carriers as point charges or with a highly delocalized mean-field treatment. This new model of charge delocalization leads to weakening of Coulomb interactions and more realistic predicted current and fill factors at moderate delocalization, relative to point charges. It is found that charge delocalization leads to significantly increased likelihood of escaping interface traps. In isotopic two-phase morphologies, increasing the domain sizes leads to slight decreases in predicted device efficiencies. It was previously shown that tortuous pathways in systems with small domain sizes can decrease device performance in thin film systems. However, the diminishing effects of Coulomb interactions with delocalization and efficient separations of excitons by small domains make morphological effects less pronounced. The importance of delocalization, which has largely been ignored in past simulations, as a parameter to consider and optimize when choosing materials for organic solar cells is emphasized.
Co-reporter:Ma. Helen M. Cativo, Amanda C. Kamps, Jian Gao, John K. Grey, Geoffrey R. Hutchison, and So-Jung Park
The Journal of Physical Chemistry B 2013 Volume 117(Issue 16) pp:4528-4535
Publication Date(Web):November 28, 2012
DOI:10.1021/jp308638w
Here, we report an unusual oxidation-induced photoluminescence (PL) turn-on response of a poly(3-alkoxythiophene), poly(3-{2-[2-(2-ethoxyethoxy)ethoxy]ethoxy}thiophene) (PEEEET). PEEEET shows a significantly red-shifted absorption spectrum compared to polyalkylthiophenes and is almost nonfluorescent (quantum yield ≪ 1%) in its pristine state. The introduction of sulfonyl defects along the polymer backbone by the oxidation of PEEEET with meta-chloroperbenzoic acid (m-CPBA) increased the emission quantum yield with the intensity increasing with the degree of oxidation. Molecular modeling data indicated that the oxidation-induced PL increase cannot be explained by the nature of monomer units and radiative rate changes. We attributed the enhanced fluorescence to the reduced nonradiative rate caused by the increased band gap, according to the energy gap law, which is consistent with the observed blue shifts in absorption and PL spectra accompanied by the PL increase.
Co-reporter:Xinfeng Quan, Christopher W. Marvin, Leah Seebald, and Geoffrey R. Hutchison
The Journal of Physical Chemistry C 2013 Volume 117(Issue 33) pp:16783-16790
Publication Date(Web):July 17, 2013
DOI:10.1021/jp404252v
Conventional piezoelectric materials change shape in response to an applied external electric field, frequently deforming at grain boundaries in addition to intrinsic unit cell changes. We detail a computational investigation, using density functional theory (DFT) calculations of single-molecule piezoelectrics. Rather than deforming along covalent bond lengths or angles, these molecular springs, derivatives of [6]helicene and phenanthrene, change conformation in response to the applied field, up to 15% of the molecular length. A substituted [6]helicene has a predicted piezoelectric coefficient of 48.8 pm/V, and one of the phenanthrenes yields a piezoelectric coefficient of up to 54.3 pm/V, which is significantly higher than polymers such as polyvinylidine difluoride (PVDF) and comparable to conventional inorganic materials such as zinc oxide (ZnO). We discuss structural properties that are found to yield large piezoresponse and hypothetical target molecules with up to 64% length change and a predicted piezoelectric coefficient of 272 pm/V. Based on these findings, we believe a new class of highly responsive piezoelectric materials may be created from the “bottom up”, yielding immense electromechanical response.
Co-reporter:Ilana Y. Kanal, Steven G. Owens, Jonathon S. Bechtel, and Geoffrey R. Hutchison
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 10) pp:1613-1623
Publication Date(Web):April 25, 2013
DOI:10.1021/jz400215j
There has been increasing interest in rational, computationally driven design methods for materials, including organic photovoltaics (OPVs). Our approach focuses on a screening “pipeline”, using a genetic algorithm for first stage screening and multiple filtering stages for further refinement. An important step forward is to expand our diversity of candidate compounds, including both synthetic and property-based measures of diversity. For example, top monomer pairs from our screening are all donor–donor (D–D) combinations, in contrast with the typical donor–acceptor (D–A) motif used in organic photovoltaics. We also find a strong “sequence effect”, in which the average HOMO–LUMO gap of tetramers changes by ∼0.2 eV as a function of monomer sequence (e.g., ABBA versus BAAB); this has rarely been explored in conjugated polymers. Beyond such optoelectronic optimization, we discuss other properties needed for high-efficiency organic solar cells, and applications of screening methods to other areas, including non-fullerene n-type materials, tandem cells, and improving charge and exciton transport.
Co-reporter:Adam G. Gagorik, Jacob W. Mohin, Tomasz Kowalewski, and Geoffrey R. Hutchison
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 1) pp:36-42
Publication Date(Web):December 9, 2012
DOI:10.1021/jz3016292
The effect of morphology on charge transport in organic photovoltaics is assessed using Monte Carlo. In isotopic two-phase morphologies, increasing the domain size from 6.3 to 18.3 nm improves the fill factor by 11.6%, a result of decreased tortuosity and relaxation of Coulombic barriers. Additionally, when small aggregates of electron acceptors are interdispersed into the electron donor phase, charged defects form in the system, reducing fill factors by 23.3% on average, compared with systems without aggregates. In contrast, systems with idealized connectivity show a 3.31% decrease in fill factor when domain size was increased from 4 to 64 nm. We attribute this to a decreased rate of exciton separation at donor–acceptor interfaces. Finally, we notice that the presence of Coulomb interactions increases device performance as devices become smaller. The results suggest that for commonly found isotropic morphologies the Coulomb interactions between charge carriers dominates exciton separation effects.Keywords: charge transfer; disordered transport; dynamic Monte Carlo; organic electronics; organic semiconductors; photovoltaics;
Co-reporter:Paula B. Hoffmann, Adam G. Gagorik, Xialing Chen, and Geoffrey R. Hutchison
The Journal of Physical Chemistry C 2013 Volume 117(Issue 36) pp:18367-18374
Publication Date(Web):August 8, 2013
DOI:10.1021/jp406781x
Organic electronic devices promise cheaper solution processability than their inorganic counterparts and allow for the vast tailorability of synthetic chemistry to tune properties and efficiency. A critical fundamental challenge is to understand the dynamics and mechanisms of charge transport, particularly the role of defects and traps. We use Kelvin probe force microscopy to compare potential energy distributions of organic, semiconducting thin films to comparable histograms calculated via our dynamic Monte Carlo simulation. A combination of theoretical and experimental investigations indicates that the common assumption of a Gaussian disorder model (GDM) is not always a good approximation for these materials. Instead we find asymmetric distributions due to nanoscale heterogeneity of charge and a resulting mix of Lorentzian and Gaussian disorder fit through a Voigt profile. Determining a more accurate model for disorder in these commonly used materials has the potential to influence the future of organic electronics design.
Co-reporter:Benjamin N. Norris, Shaopeng Zhang, Casey M. Campbell, Jeffrey T. Auletta, Percy Calvo-Marzal, Geoffrey R. Hutchison, and Tara Y. Meyer
Macromolecules 2013 Volume 46(Issue 4) pp:1384-1392
Publication Date(Web):February 11, 2013
DOI:10.1021/ma400123r
Although sequence must necessarily affect the photophysical properties of oligomers and copolymers prepared from donor and acceptor monomers, little is known about this effect, as nearly all the donor/acceptor materials have an alternating structure. A series of sequenced p-phenylene–vinylene (PV) oligomers was synthesized and investigated both experimentally and computationally. Using Horner–Wadsworth–Emmons (HWE) chemistry, a series of dimers, trimers, tetramers, pentamers, and hexamers were prepared from two building block monomers, a relatively electron-poor unsubstituted p-phenylene–vinylene (A) and an electron-rich dialkoxy-substituted p-phenylene–vinylene (B). UV–vis absorption/emission spectra and cyclic voltammetry demonstrated that the optoelectronic properties of these oligomers depended significantly on sequence. Calculations predicting the HOMO–LUMO gap of the sequenced oligomers correlated well with the experimental properties for the 2- to 4-mers, and the consensus model developed was used to design hexameric sequences with targeted characteristics. Despite the weak acceptor qualities of the “A” monomer employed in the study, HOMO–LUMO gap differences of ∼0.25 eV were found for isomeric, sequenced oligomers. In no case did the alternating structure give the largest or smallest gap. The use of sequence as a strategy represents a new dimension in tailoring properties of π-conjugated polymers.
Co-reporter:Hai-Jing Nie;Xialing Chen;Chang-Jiang Yao;Dr. Yu-Wu Zhong;Dr. Geoffrey R. Hutchison;Dr. Jiannian Yao
Chemistry - A European Journal 2012 Volume 18( Issue 45) pp:14497-14509
Publication Date(Web):
DOI:10.1002/chem.201201813
Abstract
Electron delocalization of new mixed-valent (MV) systems with the aid of lateral metal chelation is reported. 2,2′-Bipyridine (bpy) derivatives with one or two appended di-p-anisylamino groups on the 5,5′-positions and a coordinated [Ru(bpy)2] (bpy=2,2′-bipyridine), [Re(CO)3Cl], or [Ir(ppy)2] (ppy=2-phenylpyridine) component were prepared. The single-crystal molecular structure of the bis-amine ligand without metal chelation is presented. The electronic properties of these complexes were studied and compared by electrochemical and spectroscopic techniques and DFT/TDDFT calculations. Compounds with two di-p-anisylamino groups were oxidized by a chemical or electrochemical method and monitored by near-infrared (NIR) absorption spectral changes. Marcus–Hush analysis of the resulting intervalence charge-transfer transitions indicated that electron coupling of these mixed-valent systems is enhanced by metal chelation and that the iridium complex has the largest coupling. TDDFT calculations were employed to interpret the NIR transitions of these MV systems.
Co-reporter:Tamika A. Madison, Adam G. Gagorik, and Geoffrey R. Hutchison
The Journal of Physical Chemistry C 2012 Volume 116(Issue 22) pp:11852-11858
Publication Date(Web):May 15, 2012
DOI:10.1021/jp207421n
Monte Carlo simulations were used to study the effects of explicit charge traps on charge transport in small-molecule organic field effect transistors. The results show that the source-drain current decreases as the trap/barrier concentration increases, reaches a minimum around 30/70%, and increases as the concentration reaches 100%, regardless of the trap/barrier distribution. Greater current is predicted for heterogeneous trap distributions than for homogeneous trap distributions, due to wider conduction pathways that allow for more charge carriers to reach the drain electrode. Also, the distributions of distances and potential energy between charge carriers and trap sites were shown to depend on the heterogeneity of the traps and device geometry and, in most cases, are non-Gaussian in shape, due to electrostatic effects between charged traps, unlike previous assumptions. For some ranges of heterogeneity, these densities of states exhibit exponential tails. These results suggest that more experimental work is needed to gain insight into the energetic density of states under operating conditions in electronic devices made from mixed films of organic semiconductors, such as solar cells.
Co-reporter:Adam G. Gagorik and Geoffrey R. Hutchison
The Journal of Physical Chemistry C 2012 Volume 116(Issue 40) pp:21232-21239
Publication Date(Web):September 5, 2012
DOI:10.1021/jp306597n
Monte Carlo simulations were used to investigate the carrier dynamics in realistic, finite-sized, small-molecule, organic field-effect transistors (OFETs) within the first few nanoseconds of device turn-on as well as when the system equilibrates. The results show that the device current exhibits large magnitude oscillations (64 ± 27 nA) during device turn-on if the initial configuration assumed no carriers in the device (i.e., carriers only arrive through injection from the source electrode). After equilibration (125 ns), the current continues to oscillate, however, at lower magnitude (64 ± 2 nA), even if the initial configuration assumed randomly placed charges. Fourier transforms of device current as a function of simulation time show that these oscillations occur at well-defined device geometry-dependent frequencies, independent of initial configuration of the system. Examination of the carrier lifetimes and path lengths, which were found to vary nonlinearly with device length, are used to argue that the oscillations are the result of the charge injection procedure, which assumed a constant probability event. The results suggest that carriers travel in waves in realistically finite-sized devices and that carrier lifetime and path length vary nonlinearly by device geometry. Alternating current studies of OFETs may be useful in confirming these findings.
Co-reporter:Noel M. O’Boyle, Casey M. Campbell, and Geoffrey R. Hutchison
The Journal of Physical Chemistry C 2011 Volume 115(Issue 32) pp:16200-16210
Publication Date(Web):July 6, 2011
DOI:10.1021/jp202765c
Conjugated organic polymers are key building blocks of low-cost photovoltaic materials. We have examined over 90 000 copolymers using computational predictions to solve the “inverse design” of molecular structures with optimum properties for highly efficient solar cells (specifically matching optical excitation energies and excited-state energies). Our approach, which uses a genetic algorithm to search the space of synthetically accessible copolymers of six or eight monomer units, yields hundreds of candidate copolymers with predicted efficiencies over 8% (the current experimental record), including many predicted to be over 10% efficient. We discuss trends in polymer sequences and motifs found in the most frequent monomers and dimers in these highly efficient targets and derive design rules for the selection of appropriate donor and acceptor molecules. We show how additional computationally intensive filtering steps can be used, for example, to eliminate targets likely to have poor hole mobilities. Our method effectively targets optimum electronic structure and optical properties far more efficiently than time-consuming serial experiments or computational studies and can be applied to similar problems in other areas of materials science.
Co-reporter:Tamika A. Madison and Geoffrey R. Hutchison
The Journal of Physical Chemistry C 2011 Volume 115(Issue 35) pp:17558-17563
Publication Date(Web):August 3, 2011
DOI:10.1021/jp2047085
Standard and constrained density functional theory calculations were used to study the degree of charge localization in positively charged bithiophene clusters. Although polarization effects due to the crystalline environment are known, many charge-transport models in π-conjugated organic materials assume a highly localized picture of carriers due to the strong electron–phonon interaction. These first-principles calculations show that the positive charge delocalizes over at least eight molecules in one-dimensional herringbone stacks. For such one-dimensional clusters, positive charge is more likely to localize on “tilted” molecules than on “parallel” molecules because of polarization effects. For two-dimensional clusters, whereas polarization effects cancel due to symmetry, positive charge is shown to affect molecular sites anisotropically. Differences in computed HOMO energies between the localized and delocalized charges are ∼1–2 eV, about the same as the difference in energy computed between a singly charged and doubly charged stack. These results suggest that models for charge transport in organic semiconductors should be revised to account for significant charge delocalization and intermolecular interactions such as polarization.
Co-reporter:Marcus D. Hanwell ; Tamika A. Madison
The Journal of Physical Chemistry C 2010 Volume 114(Issue 48) pp:20417-20423
Publication Date(Web):August 18, 2010
DOI:10.1021/jp104416a
The effects of defects and electrostatics on charge transport in realistic organic field effect transistors were studied using a combination of first principles quantum chemistry calculations and Monte Carlo simulations with explicit introduction of defect sites. The results show that electrostatic interactions dramatically affect the field and carrier concentration dependence of charge transport in devices that include a significant number of traps, as well as its “switch-on” characteristics. Our results also show that charge transport decreases linearly as a function of neutral defect concentration as conduction pathways are turned off. For charged defects, mobility of imperfect devices is lower relative to defect-free devices but is surprisingly unaffected by the concentration of charged defects. The exact statistics of electrostatic disorder introduced by charged defects is found to obey a Poisson distribution rather than Gaussian or exponential as previously assumed. We also demonstrate that without including electrostatic interactions, simulations of transistors exhibit an unphysical negative differential resistance at higher defect levels.
Co-reporter:M. J. Moody, C. W. Marvin and G. R. Hutchison
Journal of Materials Chemistry A 2016 - vol. 4(Issue 20) pp:NaN4392-4392
Publication Date(Web):2016/04/26
DOI:10.1039/C6TC00613B
We report flexible piezoelectric polyurethane foams with d33 piezocoefficients up to 244 ± 30 pC N−1. Polymer foams have large volume changes under applied force, and dipole-doped polymers can have large polarizations even when poled at fields two orders of magnitude lower than space-charge electrets. Combining these features results in piezocoefficients an order of magnitude higher than conventional polymer piezomaterials, and independent selection of matrix and dopant should permit easy processing and tailorable properties. We further motivate the use of mesostructured materials by noting that the theoretical piezocoefficient limit is much higher for such materials than for conventional ceramics or polymers.
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![Acetamide, N-[2-(dimethylamino)-5-nitrophenyl]-](http://img.cochemist.com/ccimg/5400/5367-36-2.png)
![Acetamide, N-[2-(dimethylamino)-5-nitrophenyl]-](http://img.cochemist.com/ccimg/5400/5367-36-2_b.png)
![Phenanthro[3,4-c]phenanthrene](http://img.cochemist.com/ccimg/200/187-83-7.png)
![Phenanthro[3,4-c]phenanthrene](http://img.cochemist.com/ccimg/200/187-83-7_b.png)