Publications by authors named "Veaceslav Coropceanu"

76 Publications

Lower limits for non-radiative recombination loss in organic donor/acceptor complexes.

Mater Horiz 2021 May 26. Epub 2021 May 26.

Department of Chemistry, University of Washington, Seattle, WA, 98195-2120, USA.

Understanding the factors controlling radiative and non-radiative transition rates for charge transfer states in organic systems is important for applications ranging from organic photovoltaics (OPV) to lasers and LEDs. We explore the role of charge-transfer (CT) energetics, lifetimes, and photovoltaic properties in the limit of very slow non-radiative rates by using a model donor/acceptor system with photoluminescence dominated by thermally activated delayed fluorescence (TADF). This blend exhibits an extremely high photoluminescence quantum efficiency (PLQY = ∼22%) and comparatively long PL lifetime, while simultaneously yielding appreciable amounts of free charge generation (photocurrent external quantum efficiency EQE of 24%). In solar cells, this blend exhibits non-radiative voltage losses of only ∼0.1 V, among the lowest reported for an organic system. Notably, we find that the non-radiative decay rate, , is on the order of 10 s, approximately 4-5 orders of magnitude slower than typical OPV blends, thereby confirming that high radiative efficiency and low non-radiative voltage losses are achievable by reducing . Furthermore, despite the high radiative efficiency and already comparatively slow , we find that is nevertheless much faster than predicted by Marcus-Levich-Jortner two-state theory and we conclude that CT-local exciton (LE) hybridization is present. Our findings highlight that it is crucial to evaluate how radiative and non-radiative rates of the LE states individually influence the PLQY of charge-transfer states, rather than solely focusing on the PLQY of the LE. This conclusion will guide material selection in achieving low non-radiative voltage loss in organic solar cells and high luminescence efficiency in organic LEDs.
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http://dx.doi.org/10.1039/d1mh00529dDOI Listing
May 2021

Resolving Atomic-Scale Interactions in Non-Fullerene Acceptor Organic Solar Cells with Solid-State NMR Spectroscopy, Crystallographic Modelling, and Molecular Dynamics Simulations.

Adv Mater 2021 Nov 24:e2105943. Epub 2021 Nov 24.

University of Lille, CNRS, Centrale Lille Institut, Univ. Artois, UMR 8181, Unité de Catalyse et Chimie du Solide, Lille, F-59000, France.

Fused-ring core non-fullerene acceptors (NFAs), designated "Y-series", have enabled high-performance organic solar cells (OSCs) achieving over 18% power conversion efficiency (PCE). Since the introduction of these NFAs, much effort has been expended to understand the reasons for their exceptional performance. While several studies have identified key optoelectronic properties that govern high PCEs, little is known about the molecular level origins of large variations in performance, spanning from 5 to 18% PCE, e.g., in the case of PM6:Y6 OSCs. Here, we introduce a combined solid-state NMR, crystallography, and molecular modelling approach to elucidate the atomic-scale interactions in Y6 crystals, thin films, and PM6:Y6 bulk heterojunction (BHJ) blends. We show the Y6 morphologies in BHJ blends are not governed by the morphology in neat films or single crystals. Notably, PM6:Y6 blends processed from different solvents self-assemble into different structures and morphologies, whereby the relative orientations of the sidechains and end groups of the Y6 molecules to their fused-ring cores play a crucial role in determining the resulting morphology and overall performance of the solar cells. The molecular-level understanding of BHJs enabled by this approach will guide the engineering of next-generation NFAs for stable and efficient OSCs. This article is protected by copyright. All rights reserved.
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http://dx.doi.org/10.1002/adma.202105943DOI Listing
November 2021

Strong Suppression of Thermal Conductivity in the Presence of Long Terminal Alkyl Chains in Low-Disorder Molecular Semiconductors.

Adv Mater 2021 Sep 3;33(37):e2008708. Epub 2021 Aug 3.

Optoelectronics Group, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, UK.

While the charge transport properties of organic semiconductors have been extensively studied over the recent years, the field of organics-based thermoelectrics is still limited by a lack of experimental data on thermal transport and of understanding of the associated structure-property relationships. To fill this gap, a comprehensive experimental and theoretical investigation of the lattice thermal conductivity in polycrystalline thin films of dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (Cn-DNTT-Cn with n = 0, 8) semiconductors is reported. Strikingly, thermal conductivity appears to be much more isotropic than charge transport, which is confined to the 2D molecular layers. A direct comparison between experimental measurements (3ω-Völklein method) and theoretical estimations (approach-to-equilibrium molecular dynamics (AEMD) method) indicates that the in-plane thermal conductivity is strongly reduced in the presence of the long terminal alkyl chains. This evolution can be rationalized by the strong localization of the intermolecular vibrational modes in C8-DNTT-C8 in comparison to unsubstituted DNTT cores, as evidenced by a vibrational mode analysis. Combined with the enhanced charge transport properties of alkylated DNTT systems, this opens the possibility to decouple electron and phonon transport in these materials, which provides great potential for enhancing the thermoelectric figure of merit ZT.
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http://dx.doi.org/10.1002/adma.202008708DOI Listing
September 2021

Impact of secondary donor units on the excited-state properties and thermally activated delayed fluorescence (TADF) efficiency of pentacarbazole-benzonitrile emitters.

J Chem Phys 2020 Oct;153(14):144708

Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721-0088, USA.

The performance of organic light-emitting diodes based on thermally activated delayed fluorescence emitters depends on the efficiency of reverse intersystem crossing (RISC) processes, which are promoted by a small energy gap between the lowest singlet (S) and triplet (T) excited states and large spin-orbit couplings. Recently, it was proposed that the introduction of secondary donor units into 2,3,4,5,6-penta(9H-carbazol-9-yl)benzonitrile (5CzBN) can significantly increase the mixing between triplet states with charge-transfer (CT) and local-excitation characteristics and consequently increase the spin-orbit couplings. Here, the results of long-range corrected density functional theory calculations show that the main impact on the RISC rates of substituting 5CzBN with secondary donors is due to a decrease in adiabatic singlet-triplet energy gaps and intramolecular reorganization energies rather than to a change in spin-orbit couplings. Our calculations underline that at least two singlet and three triplet excited states contribute to the ISC/RISC processes in 5CzBN and its derivatives. In addition, we find that in all emitters, the lowest singlet excited-state potential energy surface has a double-minimum shape.
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http://dx.doi.org/10.1063/5.0028227DOI Listing
October 2020

Organic Neutral Radical Emitters: Impact of Chemical Substitution and Electronic-State Hybridization on the Luminescence Properties.

J Am Chem Soc 2020 Oct 30;142(41):17782-17786. Epub 2020 Sep 30.

Department of Chemistry and Biochemistry The University of Arizona, Tucson, Arizona 85721-0088, United States.

Neutral donor-acceptor (D-A) organic radicals have recently attracted a great deal of attention as promising luminescent materials due to their strong doublet emission. Here, we consider a series of emitters based on substituted triarylamine (TAA) donors and a radical-carrying perchlorotriphenylmethyl (PTM) acceptor. We evaluate, by means of quantum-chemical calculations and theoretical modeling, how chemical substitution affects the electronic structures and radiative and nonradiative decay rates. Our calculations show that the radiative decay rates are dominated in all instances by the electronic coupling between the lowest excited state, which has charge-transfer (CT) character, and the ground state. On the other hand, the nonradiative decay rates in the case of TAA-PTM radicals that have high CT energies are defined by the electronic hybridization of the CT state with local excitations (LE) on the PTM moiety; also, these nonradiative rates deviate significantly from the gap law dependence that is observed in the TAA-PTM radicals that have low CT energies. These findings underscore that hybridization of the emissive state with high-energy states can, in analogy with the intensity borrowing effect commonly invoked for radiative transitions, enhance as well the nonradiative decay rates. Our results highlight that in order to understand the emissive properties of D-A radicals, it is required that the electronic hybridization of the CT states with both the ground and the LE states be properly considered.
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http://dx.doi.org/10.1021/jacs.0c08997DOI Listing
October 2020

Delocalization of exciton and electron wavefunction in non-fullerene acceptor molecules enables efficient organic solar cells.

Nat Commun 2020 Aug 7;11(1):3943. Epub 2020 Aug 7.

State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, School of Materials Science and Engineering, South China University of Technology, 381 Wushan Road, 510640, Guangzhou, P. R. China.

A major challenge for organic solar cell (OSC) research is how to minimize the tradeoff between voltage loss and charge generation. In early 2019, we reported a non-fullerene acceptor (named Y6) that can simultaneously achieve high external quantum efficiency and low voltage loss for OSC. Here, we use a combination of experimental and theoretical modeling to reveal the structure-property-performance relationships of this state-of-the-art OSC system. We find that the distinctive π-π molecular packing of Y6 not only exists in molecular single crystals but also in thin films. Importantly, such molecular packing leads to (i) the formation of delocalized and emissive excitons that enable small non-radiative voltage loss, and (ii) delocalization of electron wavefunctions at donor/acceptor interfaces that significantly reduces the Coulomb attraction between interfacial electron-hole pairs. These properties are critical in enabling highly efficient charge generation in OSC systems with negligible donor-acceptor energy offset.
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http://dx.doi.org/10.1038/s41467-020-17867-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7414148PMC
August 2020

Electronic Structure of Multicomponent Organic Molecular Materials: Evaluation of Range-Separated Hybrid Functionals.

J Chem Theory Comput 2020 Jun 8;16(6):3712-3719. Epub 2020 May 8.

Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721-0088, United States.

Range-separated hybrid (RSH) functionals have become a tool of choice to study the and -molecular electronic states in organic materials. These functionals provide the most accurate descriptions of the electronic structure when the range-separation parameter is optimally tuned (OT). However, since the range-separation parameter is molecule dependent, this approach faces consistency issues when applied to the multicomponent systems typically found in the active layers of organic solar cells or organic light-emitting diodes (OLEDs). Here, we investigate the performance of four common RSH functionals in the description of the excited states of three molecular compounds used as components of the active layer in a hyperfluorescence OLED device. Our results indicate that the excited-state energies of the investigated molecules show a very weak dependence on the range-separation parameter value when they are evaluated by means of a screened version of RSH functionals. In this instance, the excited states of all three molecular compounds can be derived accurately and consistently with the exact same functional.
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http://dx.doi.org/10.1021/acs.jctc.0c00138DOI Listing
June 2020

High stability and luminescence efficiency in donor-acceptor neutral radicals not following the Aufbau principle.

Nat Mater 2019 Sep 22;18(9):977-984. Epub 2019 Jul 22.

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, China.

With their unusual electronic structures, organic radical molecules display luminescence properties potentially relevant to lighting applications; yet, their luminescence quantum yield and stability lag behind those of other organic emitters. Here, we designed donor-acceptor neutral radicals based on an electron-poor perchlorotriphenylmethyl or tris(2,4,6-trichlorophenyl)methyl radical moiety combined with different electron-rich groups. Experimental and quantum-chemical studies demonstrate that the molecules do not follow the Aufbau principle: the singly occupied molecular orbital is found to lie below the highest (doubly) occupied molecular orbital. These donor-acceptor radicals have a strong emission yield (up to 54%) and high photostability, with estimated half-lives reaching up to several months under pulsed ultraviolet laser irradiation. Organic light-emitting diodes based on such a radical emitter show deep-red/near-infrared emission with a maximal external quantum efficiency of 5.3%. Our results provide a simple molecular-design strategy for stable, highly luminescent radicals with non-Aufbau electronic structures.
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http://dx.doi.org/10.1038/s41563-019-0433-1DOI Listing
September 2019

Assessing the nature of the charge-transfer electronic states in organic solar cells.

Nat Commun 2018 12 13;9(1):5295. Epub 2018 Dec 13.

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia, 30332-0400, USA.

The charge-transfer electronic states appearing at the donor-acceptor interfaces in organic solar cells mediate exciton dissociation, charge generation, and charge recombination. To date, the characterization of their nature has been carried out on the basis of models that only involve the charge-transfer state and the ground state. Here, we demonstrate that it is essential to go beyond such a two-state model and to consider explicitly as well the electronic and vibrational couplings with the local absorbing state on the donor and/or acceptor. We have thus developed a three-state vibronic model that allows us: to provide a reliable description of the optical absorption features related to the charge-transfer states; to underline the erroneous interpretations stemming from the application of the semi-classical two-state model; and to rationalize how the hybridization between the local-excitation state and charge-transfer state can lead to lower non-radiative voltage losses and higher power conversion efficiencies.
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http://dx.doi.org/10.1038/s41467-018-07707-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6294259PMC
December 2018

Design rules for minimizing voltage losses in high-efficiency organic solar cells.

Nat Mater 2018 08 16;17(8):703-709. Epub 2018 Jul 16.

Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping, Sweden.

The open-circuit voltage of organic solar cells is usually lower than the values achieved in inorganic or perovskite photovoltaic devices with comparable bandgaps. Energy losses during charge separation at the donor-acceptor interface and non-radiative recombination are among the main causes of such voltage losses. Here we combine spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between donor and acceptor molecular states and (2) high photoluminescence yield of the low-gap material in the blend. Following these rules, we present a range of existing and new donor-acceptor systems that combine efficient photocurrent generation with electroluminescence yield up to 0.03%, leading to non-radiative voltage losses as small as 0.21 V. This study provides a rationale to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.
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http://dx.doi.org/10.1038/s41563-018-0128-zDOI Listing
August 2018

Langmuir-Blodgett Thin Films of Diketopyrrolopyrrole-Based Amphiphiles.

ACS Appl Mater Interfaces 2018 Apr 30;10(14):11995-12004. Epub 2018 Mar 30.

Stanford Synchrotron Radiation Lightsource , SLAC National Accelerator Laboratory , Menlo Park , California 94025 , United States.

We report on two π-conjugated donor-acceptor-donor (D-A-D) molecules of amphiphilic nature, aiming to promote intermolecular ordering and carrier mobility in organic electronic devices. Diketopyrrolopyrrole was selected as the acceptor moiety that was disubstituted with nonpolar and polar functional groups, thereby providing the amphiphilic structures. This structural design resulted in materials with a strong intermolecular order in the solid state, which was confirmed by differential scanning calorimetry and polarized optical microscopy. Langmuir-Blodgett (LB) films of ordered mono- and multilayers were transferred onto glass and silicon substrates, with layer quality, coverage, and intermolecular order controlled by layer compression pressure on the LB trough. Organic field-effect transistors and organic photovoltaics devices with active layers consisting of the amphiphilic conjugated D-A-D-type molecules were constructed to demonstrate that the LB technique is an effective layer-by-layer deposition approach to fabricate self-assembled, ordered thin films.
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http://dx.doi.org/10.1021/acsami.7b18239DOI Listing
April 2018

Understanding the effects of electronic polarization and delocalization on charge-transport levels in oligoacene systems.

J Chem Phys 2017 Jun;146(22):224705

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.

Electronic polarization and charge delocalization are important aspects that affect the charge-transport levels in organic materials. Here, using a quantum mechanical/embedded-charge (QM/EC) approach based on a combination of the long-range corrected ωB97X-D exchange-correlation functional (QM) and charge model 5 (CM5) point-charge model (EC), we evaluate the vertical detachment energies and polarization energies of various sizes of crystalline and amorphous anionic oligoacene clusters. Our results indicate that QM/EC calculations yield vertical detachment energies and polarization energies that compare well with the experimental values obtained from ultraviolet photoemission spectroscopy measurements. In order to understand the effect of charge delocalization on the transport levels, we considered crystalline naphthalene systems with QM regions including one or five-molecules. The results for these systems show that the delocalization and polarization effects are additive; therefore, allowing for electron delocalization by increasing the size of the QM region leads to the additional stabilization of the transport levels.
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http://dx.doi.org/10.1063/1.4984783DOI Listing
June 2017

Energetic fluctuations in amorphous semiconducting polymers: Impact on charge-carrier mobility.

J Chem Phys 2017 Oct;147(13):134904

Institut des Sciences Moléculaires, UMR 5255, University of Bordeaux, Talence, France.

We present a computational approach to model hole transport in an amorphous semiconducting fluorene-triphenylamine copolymer (TFB), which is based on the combination of molecular dynamics to predict the morphology of the oligomeric system and Kinetic Monte Carlo (KMC), parameterized with quantum chemistry calculations, to simulate hole transport. Carrying out a systematic comparison with available experimental results, we discuss the role that different transport parameters play in the KMC simulation and in particular the dynamic nature of positional and energetic disorder on the temperature and electric field dependence of charge mobility. It emerges that a semi-quantitative agreement with experiments is found only when the dynamic nature of the disorder is taken into account. This study establishes a clear link between microscopic quantities and macroscopic hole mobility for TFB and provides substantial evidence of the importance of incorporating fluctuations, at the molecular level, to obtain results that are in good agreement with temperature and electric field-dependent experimental mobilities. Our work makes a step forward towards the application of nanoscale theoretical schemes as a tool for predictive material screening.
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http://dx.doi.org/10.1063/1.4996969DOI Listing
October 2017

Impact of interfacial molecular orientation on radiative recombination and charge generation efficiency.

Nat Commun 2017 07 19;8(1):79. Epub 2017 Jul 19.

Center for Polymers and Organic Solids, University of California, Santa Barbara, CA, 93106, USA.

A long standing question in organic electronics concerns the effects of molecular orientation at donor/acceptor heterojunctions. Given a well-controlled donor/acceptor bilayer system, we uncover the genuine effects of molecular orientation on charge generation and recombination. These effects are studied through the point of view of photovoltaics-however, the results have important implications on the operation of all optoelectronic devices with donor/acceptor interfaces, such as light emitting diodes and photodetectors. Our findings can be summarized by two points. First, devices with donor molecules face-on to the acceptor interface have a higher charge transfer state energy and less non-radiative recombination, resulting in larger open-circuit voltages and higher radiative efficiencies. Second, devices with donor molecules edge-on to the acceptor interface are more efficient at charge generation, attributed to smaller electronic coupling between the charge transfer states and the ground state, and lower activation energy for charge generation.Molecular orientation profoundly affects the performance of donor-acceptor heterojunctions, whilst it has remained challenging to investigate the detail. Using a controllable interface, Ran et al. show that the edge-on geometries improve charge generation at the cost of non-radiative recombination loss.
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http://dx.doi.org/10.1038/s41467-017-00107-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5517510PMC
July 2017

Effect of Solid-State Polarization on Charge-Transfer Excitations and Transport Levels at Organic Interfaces from a Screened Range-Separated Hybrid Functional.

J Phys Chem Lett 2017 Jul 6;8(14):3277-3283. Epub 2017 Jul 6.

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology , Atlanta, Georgia 30332-0400, United States.

We develop a robust approach for the description of the energetics of charge-transfer (CT) excitations and transport levels at organic interfaces based on a screened range-separated hybrid (SRSH) functional. We find that SRSH functionals correctly capture the effect of solid-state electronic polarization on transport gap renormalization and on screening of the electrostatic electron-hole interaction. With respect to calculations based on nonscreened optimally tuned RSH (long-range corrected) functionals, the SRSH-based calculations can be performed for both isolated molecular complexes and systems embedded in a dielectric medium with the same range-separation parameter, which allows a clear physical interpretation of the results in terms of solid-state polarization without any perturbation of the molecular electronic structure. By considering weakly interacting donor/acceptor complexes of pentacene with C and poly-3-hexylthiophene (P3HT) with PCBM, we show that this new approach provides CT-state energies that compare very well with experimental data.
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http://dx.doi.org/10.1021/acs.jpclett.7b01276DOI Listing
July 2017

Electron-phonon coupling in anthracene-pyromellitic dianhydride.

J Chem Phys 2017 Jun;146(21):214705

Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3255, USA.

In this study, the electron-phonon coupling constants of the mixed-stack organic semiconductor anthracene-pyromellitic dianhydride (A-PMDA) are determined from experimental resonant Raman and absorption spectra of the charge transfer (CT) exciton using a time-dependent resonant Raman model. The reorganization energies of both intermolecular and intramolecular phonons are determined and compared with theoretical estimates derived from density functional theory calculations; they are found to agree well. We found that the dominant contribution to the total reorganization energy is due to intramolecular phonons, with intermolecular phonons only contributing a small percentage. This work goes beyond prior studies of the electron-phonon coupling in A-PMDA by including the coupling of all Raman-active phonons to the charge transfer exciton. The possibility of orientational disorder in A-PMDA at 80 K is inferred from the inhomogeneous broadening of the absorption line shape.
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http://dx.doi.org/10.1063/1.4984268DOI Listing
June 2017

Charge-Transfer States in Organic Solar Cells: Understanding the Impact of Polarization, Delocalization, and Disorder.

ACS Appl Mater Interfaces 2017 May 17;9(21):18095-18102. Epub 2017 May 17.

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology , Atlanta, Georgia 30332-0400, United States.

We investigate the impact of electronic polarization, charge delocalization, and energetic disorder on the charge-transfer (CT) states formed at a planar C/pentacene interface. The ability to examine large complexes containing up to seven pentacene molecules and three C molecules allows us to take explicitly into account the electronic polarization effects. These complexes are extracted from a bilayer architecture modeled by molecular dynamics simulations and evaluated by means of electronic-structure calculations based on long-range-separated functionals (ωB97XD and BNL) with optimized range-separation parameters. The energies of the lowest charge-transfer states derived for the large complexes are in very good agreement with the experimentally reported values. The average singlet-triplet energy splittings of the lowest CT states are calculated not to exceed 10 meV. The rates of geminate recombination as well as of dissociation of the triplet excitons are also evaluated. In line with experiment, our results indicate that the pentacene triplet excitons generated through singlet fission can dissociate into separated charges on a picosecond time scale, despite the fact that their energy in C/pentacene heterojunctions is slightly lower than the energies of the lowest CT triplet states.
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http://dx.doi.org/10.1021/acsami.7b02193DOI Listing
May 2017

Up-Conversion Intersystem Crossing Rates in Organic Emitters for Thermally Activated Delayed Fluorescence: Impact of the Nature of Singlet vs Triplet Excited States.

J Am Chem Soc 2017 03 13;139(11):4042-4051. Epub 2017 Mar 13.

Laboratory for Computational and Theoretical Chemistry of Advanced Materials, Physical Science and Engineering Division, King Abdullah University of Science and Technology , Thuwal 23955-6900, Kingdom of Saudi Arabia.

The rates for up-conversion intersystem crossing (UISC) from the T state to the S state are calculated for a series of organic emitters with an emphasis on thermally activated delayed fluorescence (TADF) materials. Both the spin-orbit coupling and the energy difference between the S and T states (ΔE) are evaluated, at the density functional theory (DFT) and time-dependent DFT levels. The calculated UISC rates and ΔE values are found to be in good agreement with available experimental data. Our results underline that small ΔE values and sizable spin-orbit coupling matrix elements have to be simultaneously realized in order to facilitate UISC and ultimately TADF. Importantly, the spatial separation of the highest occupied and lowest unoccupied molecular orbitals of the emitter, a widely accepted strategy for the design of TADF molecules, does not necessarily lead to a sufficient reduction in ΔE; in fact, either a significant charge-transfer (CT) contribution to the T state or a minimal energy difference between the local-excitation and charge-transfer triplet states is required to achieve a small ΔE. Also, having S and T states of a different nature is found to strongly enhance spin-orbit coupling, which is consistent with the El-Sayed rule for ISC rates. Overall, our results indicate that having either similar energies for the local-excitation and charge-transfer triplet states or the right balance between a substantial CT contribution to T and somewhat different natures of the S and T states, paves the way toward UISC enhancement and thus TADF efficiency improvement.
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http://dx.doi.org/10.1021/jacs.6b12124DOI Listing
March 2017

Polymorphism in the 1:1 Charge-Transfer Complex DBTTF-TCNQ and Its Effects on Optical and Electronic Properties.

Adv Electron Mater 2016 Oct 14;2(10). Epub 2016 Sep 14.

Department of Physics, Wake Forest University, Winston Salem, NC 27109, USA.

The organic charge-transfer (CT) complex dibenzotetrathiafulvalene - 7,7,8,8-tetracyanoquinodimethane (DBTTF-TCNQ) is found to crystallize in two polymorphs when grown by physical vapor transport: the known α-polymorph and a new structure, the β-polymorph. Structural and elemental analysis via selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), and polarized IR spectroscopy reveal that the complexes have the same stoichiometry with a 1:1 donor:acceptor ratio, but exhibit unique unit cells. The structural variations result in significant differences in the optoelectronic properties of the crystals, as observed in our experiments and electronic-structure calculations. Raman spectroscopy shows that the α-polymorph has a degree of charge transfer of about 0.5, while the β-polymorph is nearly neutral. Organic field-effect transistors fabricated on these crystals reveal that in the same device structure both polymorphs show ambipolar charge transport, but the α-polymorph exhibits electron-dominant transport while the β-polymorph is hole-dominant. Together, these measurements imply that the transport features result from differing donor-acceptor overlap and consequential varying in frontier molecular orbital mixing, as suggested theoretically for charge-transfer complexes.
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http://dx.doi.org/10.1002/aelm.201600203DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5788010PMC
October 2016

Description of the Charge Transfer States at the Pentacene/C60 Interface: Combining Range-Separated Hybrid Functionals with the Polarizable Continuum Model.

J Phys Chem Lett 2016 Jul 28;7(13):2616-21. Epub 2016 Jun 28.

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States.

Density functional theory (DFT) approaches based on range-separated hybrid functionals are currently methods of choice for the description of the charge-transfer (CT) states in organic donor/acceptor solar cells. However, these calculations are usually performed on small-size donor/acceptor complexes and as result do not account for electronic polarization effects. Here, using a pentacene/C60 complex as a model system, we discuss the ability of long-range corrected (LCR) hybrid functionals in combination with the polarizable continuum model (PCM) to determine the impact of the solid-state environment on the CT states. The CT energies are found to be insensitive to the interactions with the dielectric medium when a conventional time-dependent DFT/PCM (TDDFT/PCM) approach is used. However, a decrease in the energy of the CT state in the framework of LRC functionals can be obtained by using a smaller range-separated parameter when going from an isolated donor/acceptor complex to the solid-state case.
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http://dx.doi.org/10.1021/acs.jpclett.6b00911DOI Listing
July 2016

Static and Dynamic Energetic Disorders in the C60, PC61BM, C70, and PC71BM Fullerenes.

J Phys Chem Lett 2015 Sep 4;6(18):3657-62. Epub 2015 Sep 4.

Solar and Photovoltaics Engineering Research Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology , Thuwal 23955-6900, Kingdom of Saudi Arabia.

We use a combination of molecular dynamics simulations and density functional theory calculations to investigate the energetic disorder in fullerene systems. We show that the energetic disorder evaluated from an ensemble average contains contributions of both static origin (time-independent, due to loose packing) and dynamic origin (time-dependent, due to electron-vibration interactions). In order to differentiate between these two contributions, we compare the results obtained from an ensemble average approach with those derived from a time average approach. It is found that in both amorphous C60 and C70 bulk systems, the degrees of static and dynamic disorder are comparable, while in the amorphous PC61BM and PC71BM systems, static disorder is about twice as large as dynamic disorder.
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http://dx.doi.org/10.1021/acs.jpclett.5b01709DOI Listing
September 2015

Correlating Non-Geminate Recombination with Film Structure: A Comparison of Polythiophene: Fullerene Bilayer and Blend Films.

J Phys Chem Lett 2014 Nov 14;5(21):3669-76. Epub 2014 Oct 14.

†Centre for Plastic Electronics, Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom.

The morphology of the active layer in polymer:fullerene solar cells is a key parameter in determining their performance. In this study we monitor the charge carrier dynamics in bilayers of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (fabricated by sequential spin coating and vacuum deposition) before and after thermal annealing, and compare these against conventional solution processed bulk heterojunction (BHJ) blend films. Transmission electron microscopy images, supported by field effect mobility data show that while not-annealed P3HT/PC61BM bilayers possess a sharp interface, intermixing proceeds upon annealing. Transient absorption studies indicate that the not-annealed bilayer yields fewer, but longer lived, charge carriers compared to the BHJ. Monte Carlo (MC) simulations further suggest that the difference in non-geminate recombination dynamics observed for the BHJ and bilayer films could be related to the confinement of charge carriers to the interface, with the lower dimensionality for the flat interface bilayer films relative to the intercalated donor-acceptor network BHJ films leading to lower recombination.
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http://dx.doi.org/10.1021/jz5018575DOI Listing
November 2014

Mode-selective vibrational modulation of charge transport in organic electronic devices.

Nat Commun 2015 Aug 6;6:7880. Epub 2015 Aug 6.

Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel.

The soft character of organic materials leads to strong coupling between molecular, nuclear and electronic dynamics. This coupling opens the way to influence charge transport in organic electronic devices by exciting molecular vibrational motions. However, despite encouraging theoretical predictions, experimental realization of such approach has remained elusive. Here we demonstrate experimentally that photoconductivity in a model organic optoelectronic device can be modulated by the selective excitation of molecular vibrations. Using an ultrafast infrared laser source to create a coherent superposition of vibrational motions in a pentacene/C60 photoresistor, we observe that excitation of certain modes in the 1,500-1,700 cm(-1) region leads to photocurrent enhancement. Excited vibrations affect predominantly trapped carriers. The effect depends on the nature of the vibration and its mode-specific character can be well described by the vibrational modulation of intermolecular electronic couplings. This presents a new tool for studying electron-phonon coupling and charge dynamics in (bio)molecular materials.
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http://dx.doi.org/10.1038/ncomms8880DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4538862PMC
August 2015

Theoretical Study of the Local and Charge-Transfer Excitations in Model Complexes of Pentacene-C60 Using Tuned Range-Separated Hybrid Functionals.

J Chem Theory Comput 2014 Jun;10(6):2379-88

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology , Atlanta, Georgia 30332-0400, United States.

The characteristics of the electronic excited states and the charge-transfer processes at organic-organic interfaces play an important role in organic electronic devices. However, charge-transfer excitations have proven challenging to describe with conventional density functional theory (DFT) methodologies due to the local nature of the exchange-correlation potentials often employed. Here, we examine the excited states of model pentacene-C60 complexes using time-dependent DFT with, on one hand, one of the most popular standard hybrid functionals (B3LYP) and, on the other hand, several long-range corrected hybrid functionals for which we consider both default and nonempirically tuned range-separation parameters. The DFT results based on the tuned functionals are found to agree well with the available experimental data. The results also underline that the interface geometry of the complex has a strong effect on the energies and ordering of the singlet and triplet charge-transfer states.
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http://dx.doi.org/10.1021/ct500259mDOI Listing
June 2014

Defect-driven interfacial electronic structures at an organic/metal-oxide semiconductor heterojunction.

Adv Mater 2014 Jul 15;26(27):4711-6. Epub 2014 May 15.

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia, 30332-0400.

The electronic structure of the hybrid interface between ZnO and the prototypical organic semiconductor PTCDI is investigated via a combination of ultraviolet and X-ray photoelectron spectroscopy (UPS/XPS) and density functional theory (DFT) calculations. The interfacial electronic interactions lead to a large interface dipole due to substantial charge transfer from ZnO to 3,4,9,10-perylenetetracarboxylicdiimide (PTCDI), which can be properly described only when accounting for surface defects that confer ZnO its n-type properties.
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http://dx.doi.org/10.1002/adma.201305351DOI Listing
July 2014

Bistetracene: an air-stable, high-mobility organic semiconductor with extended conjugation.

J Am Chem Soc 2014 Jul 12;136(26):9248-51. Epub 2014 May 12.

Department of Polymer Science & Engineering, Conte Polymer Research Center, University of Massachusetts , Amherst, Massachusetts 01003, United States.

We report the synthesis and characterization of "bistetracene", an unconventional, linearly extended conjugated core with eight fused rings. The annellation mode of the system allows for increased stability of the conjugated system relative to linear analogues due to the increased number of Clar aromatic sextets. By attaching the appropriate solubilizing substituents, efficient molecular packing with large transfer integrals can be obtained. The electronic structure calculations suggest these large polycyclic aromatic hydrocarbons (PAHs) exhibit excellent intrinsic charge transport properties. Charge carrier mobilities as large as 6.1 cm(2) V(-1) s(-1) and current on/off ratios of 10(7) were determined experimentally for one of our compounds. Our study provides valuable insight into the design of unconventional semiconductor compounds based on higher PAHs for use in high-performance devices.
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http://dx.doi.org/10.1021/ja503643sDOI Listing
July 2014

Triisopropylsilylethynyl-functionalized graphene-like fragment semiconductors: synthesis, crystal packing, and density functional theory calculations.

Chemistry 2013 Dec;19(52):17907-16

Tri-isopropylsilylethynyl (TIPS)-functionalized polycyclic aromatic hydrocarbon (PAH) molecules incorporate structural components of graphene nanoribbons and represent a family of model molecules that form organic crystal semiconductors for electronic devices. Here, we report a series of TIPS-functionalized PAHs and discuss their electronic properties and crystal packing features. We observe that these soluble compounds easily form one-dimensional (1 D) packing arrangements and allow a direct evolution of the π stacking by varying the geometric shape. We find that the aspect ratio between length and width plays an important role on crystal packing. Our result indicates that when the PAH molecules have zigzag edges, these can provide enough volume for the molecules to rotate and reorient, alleviating the unfavorable electrostatic interactions found in perfectly cofacial π-π stacking. Density functional theory calculations were carried out to provide insights into how the molecular geometric shape influences the electronic structure and transport properties. The calculations indicate that, among the compounds studied here, "brick-layer" stacks provide the highest hole mobility.
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http://dx.doi.org/10.1002/chem.201303308DOI Listing
December 2013

Vibration-assisted crystallization improves organic/dielectric interface in organic thin-film transistors.

Adv Mater 2013 Dec 23;25(48):6956-62. Epub 2013 Sep 23.

Department of Physics, Wake Forest University, Winston-Salem, NC, 27109, USA.

Solution processability of organic semiconductors allows high-throughput fabrication on arbitrary substrates at low-cost, but the films often exhibit low performance. Here, we report on a new method for device fabrication, vibration assisted crystallization (VAC) that produces superior films, which approach the fundamental performance limits shown in corresponding single-crystal measurements.
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http://dx.doi.org/10.1002/adma.201302838DOI Listing
December 2013

Nonlocal electron-phonon coupling in organic semiconductor crystals: the role of acoustic lattice vibrations.

J Chem Phys 2013 May;138(20):204713

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, 901 Atlantic Drive NW, Atlanta, Georgia 30332-0400, USA.

We discuss, in the context of a tight-binding description, how the electronic and charge-transport properties in single crystals of molecular organic semiconductors are affected by the nonlocal electron-phonon coupling to both acoustic and optical lattice vibrations. While the nonlocal electron-phonon interactions can in general be divided into contributions from symmetric modes and antisymmetric modes, we show that only the antisymmetric coupling mechanism is operational in the case of acoustic vibrations. Interestingly, when the quantum nature of the phonons can be neglected, the effect of electron-phonon interactions with acoustic phonons is found to be equivalent to that of the electron-phonon interactions with optical phonons, in the case where contributions from symmetric and antisymmetric modes are equal.
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http://dx.doi.org/10.1063/1.4807886DOI Listing
May 2013

Intramolecular reorganization energy in zinc phthalocyanine and its fluorinated derivatives: a joint experimental and theoretical study.

Chem Commun (Camb) 2013 Jul;49(54):6069-71

Institute of Physics, University of Brasília, C.P. 04455, DF CEP 70919-970, Brazil.

We report a high-resolution gas-phase UPS spectrum of zinc phthalocyanine (ZnPc) together with a detailed analysis of the vibronic structure of the first ionization band, showing that presents the lowest value of the intramolecular reorganization energy experimentally reported for a molecular organic semiconductor.
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http://dx.doi.org/10.1039/c3cc42003eDOI Listing
July 2013
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