Publications by authors named "Thuc-Quyen Nguyen"

128 Publications

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

Efficiency of Thermally Activated Delayed Fluorescence Sensitized Triplet Upconversion Doubled in Three-Component System.

Adv Mater 2021 Nov 18:e2103976. Epub 2021 Nov 18.

Center for Polymers and Organic Solids (CPOS) and Department of Chemistry & Biochemistry, University of California, Santa Barbara, CA-93106, USA.

As in many fields, the most exciting endeavors in photon upconversion research focus on increasing the efficiency (upconversion quantum yield) and performance (anti-Stokes shift) while diminishing the cost of production. In this vein, studies employing metal-free thermally activated delayed fluorescence (TADF) sensitizers have garnered increased interest. Here, for the first time, the strategy of ternary photon upconversion was utilized with the TADF sensitizer 4CzIPN, resulting in a doubling of the upconversion quantum yield in comparison to the binary system employing p-terphenyl as the emitter. In this ternary blend, the sensitizer 4CzIPN is paired with an intermediate acceptor, 1-methylnaphthalene, in addition to the emitter molecule, p-terphenyl yielding a normalized upconversion quantum yield of 7.6% while maintaining the 0.83 eV anti-Stokes shift. These results illustrate the potential benefits of utilizing this strategy of energy-funneling, previously used only with heavy-metal based sensitizers, to increase the performance of these photon upconversion systems. This article is protected by copyright. All rights reserved.
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http://dx.doi.org/10.1002/adma.202103976DOI Listing
November 2021

Current Progress of Interfacing Organic Semiconducting Materials with Bacteria.

Chem Rev 2021 Oct 29. Epub 2021 Oct 29.

Center for Polymers and Organic Solids & Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States.

Microbial bioelectronics require interfacing microorganisms with electrodes. The resulting abiotic/biotic platforms provide the basis of a range of technologies, including energy conversion and diagnostic assays. Organic semiconductors (OSCs) provide a unique strategy to modulate the interfaces between microbial systems and external electrodes, thereby improving the performance of these incipient technologies. In this review, we explore recent progress in the field on how OSCs, and related materials capable of charge transport, are being used within the context of microbial systems, and more specifically bacteria. We begin by examining the electrochemical communication modes in bacteria and the biological basis for charge transport. Different types of synthetic organic materials that have been designed and synthesized for interfacing and interrogating bacteria are discussed next, followed by the most commonly used characterization techniques for evaluating transport in microbial, synthetic, and hybrid systems. A range of applications is subsequently examined, including biological sensors and energy conversion systems. The review concludes by summarizing what has been accomplished so far and suggests future design approaches for OSC bioelectronics materials and technologies that hybridize characteristic properties of microbial and OSC systems.
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http://dx.doi.org/10.1021/acs.chemrev.1c00487DOI Listing
October 2021

The role of charge recombination to triplet excitons in organic solar cells.

Nature 2021 Sep 29;597(7878):666-671. Epub 2021 Sep 29.

Cavendish Laboratory, University of Cambridge, Cambridge, UK.

The use of non-fullerene acceptors (NFAs) in organic solar cells has led to power conversion efficiencies as high as 18%. However, organic solar cells are still less efficient than inorganic solar cells, which typically have power conversion efficiencies of more than 20%. A key reason for this difference is that organic solar cells have low open-circuit voltages relative to their optical bandgaps, owing to non-radiative recombination. For organic solar cells to compete with inorganic solar cells in terms of efficiency, non-radiative loss pathways must be identified and suppressed. Here we show that in most organic solar cells that use NFAs, the majority of charge recombination under open-circuit conditions proceeds via the formation of non-emissive NFA triplet excitons; in the benchmark PM6:Y6 blend, this fraction reaches 90%, reducing the open-circuit voltage by 60 mV. We prevent recombination via this non-radiative channel by engineering substantial hybridization between the NFA triplet excitons and the spin-triplet charge-transfer excitons. Modelling suggests that the rate of back charge transfer from spin-triplet charge-transfer excitons to molecular triplet excitons may be reduced by an order of magnitude, enabling re-dissociation of the spin-triplet charge-transfer exciton. We demonstrate NFA systems in which the formation of triplet excitons is suppressed. This work thus provides a design pathway for organic solar cells with power conversion efficiencies of 20% or more.
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http://dx.doi.org/10.1038/s41586-021-03840-5DOI Listing
September 2021

Understanding how Lewis acids dope organic semiconductors: a "complex" story.

Chem Sci 2021 Apr 19;12(20):7012-7022. Epub 2021 Apr 19.

Laboratory for Chemistry of Novel Materials, University of Mons Place du Parc, 20 7000 Mons Belgium

We report on computational studies of the potential of three borane Lewis acids (LAs) (B(CF) (BCF), BF, and BBr) to form stable adducts and/or to generate positive polarons with three different semiconducting π-conjugated polymers (PFPT, PCPDTPT and PCPDTBT). Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations based on range-separated hybrid (RSH) functionals provide insight into changes in the electronic structure and optical properties upon adduct formation between LAs and the two polymers containing pyridine moieties, PFPT and PCPDTPT, unravelling the complex interplay between partial hybridization, charge transfer and changes in the polymer backbone conformation. We then assess the potential of BCF to induce p-doping in PCPDTBT, which does not contain pyridine groups, by computing the energetics of various reaction mechanisms proposed in the literature. We find that reaction of BCF(OH) to form protonated PCPDTBT and [BCF(OH)], followed by electron transfer from a pristine to a protonated PCPDTBT chain is highly endergonic, and thus unlikely at low doping concentration. The theoretical and experimental data can, however, be reconciled if one considers the formation of [BCF(OH)BCF] or [BCF(OH)(OH)BCF] counterions rather than [BCF(OH)] and invokes subsequent reactions resulting in the elimination of H.
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http://dx.doi.org/10.1039/d1sc01268aDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8153436PMC
April 2021

Optical Expediency of Back Electrode Materials for Organic Near-Infrared Photodiodes.

ACS Appl Mater Interfaces 2021 Jun 3;13(23):27217-27226. Epub 2021 Jun 3.

Department of Physics, School of Sciences and Humanities, Nazarbayev University, Nur-Sultan City 010000, Republic of Kazakhstan.

Organic semiconductor devices, including organic photodetectors (OPDs) and organic photovoltaics (OPVs), have undergone vast improvements, thanks to the development of non-fullerene acceptors. The absorption range of such NFA-based systems is typically shifted toward the near-infrared (near-IR) region compared to early-generation fullerene-based systems, rendering organic semiconductor devices suitable for near-IR sensing applications. While most efforts are concentrated on the photoactive materials, less attention is paid to the impact of the back electrodes on the device performance. Therefore, this work focuses on the optical expediency of gold (Au), silver (Ag), aluminum (Al), and graphite as back electrode materials in organic optoelectronics. This work shows that the "one size fits all" methodology is not a valid approach for choosing the back electrode material. Instead, considering the active layer absorption, the active layer thickness, and the intended application is necessary. A traditional polymer/fullerene-based system, poly(3-hexylthiophene) with [6,6]-phenyl C butyric acid methyl ester (P3HT:PCBM), and a state-of-the-art narrow-band gap non-fullerene-based system, poly[4,8bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-; 4,5-']dithiophene-2,6-diyl-alt-(4-(2-ethy-lhexyl)3-fluorothieno[3,4-]thiophene-)-2-carboxylate-(2-6-diyl)] and 2,2'-((2Z,2'Z)-((5,5'-(4,4-bis(2-ethylhexyl)4-cyclopenta[1,2-:5,4-']dithiophene-2,6-diyl)bis(4-((2ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene)) bis(5,6-difluoro3-oxo-2,3-dihydro-1-indene-2,1-diylidene))dimalononitrile (PCE10:COTIC-4F), are investigated by combining optical transfer matrix modeling simulations with experimentally determined recombination and extraction losses. We find that the narrow-band gap system shows performance gains when employing Au as the back electrode. Furthermore, we show that these performance gains are dependent on active layer thickness, yielding the most significance for thin active layers (<100 nm). Such thin, ultra-narrow-band gap devices are the focus of near-IR sensing applications, highlighting the importance of methodically choosing the back electrode. Lastly, the impact of the back electrode on the OPV device performance is outlined.
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http://dx.doi.org/10.1021/acsami.1c04036DOI Listing
June 2021

Understanding and Countering Illumination-Sensitive Dark Current: Toward Organic Photodetectors with Reliable High Detectivity.

ACS Nano 2021 Jan 13;15(1):1753-1763. Epub 2021 Jan 13.

Center for Polymers and Organic Solids, University of California, Santa Barbara, California 93106, United States.

Continuously enhanced photoresponsivity and suppressed dark/noise current combinatorially lead to the recent development of high-detectivity organic photodetectors with broadband sensing competence. Despite the achievements, reliable photosensing enabled by organic photodetectors (OPDs) still faces challenges. Herein, we call for heed over a universal phenomenon of detrimental sensitivity of dark current to illumination history in high-performance inverted OPDs. The phenomenon, unfavorable to the attainment of high sensitivity and consistent figures-of-merit, is shown to arise from exposure of the commonly used electron transport layer in OPDs to high-energy photons and its consequent loss of charge selectivity systematic studies. To solve this universal problem, "double" layer tin oxide as an alternative electron transport layer is demonstrated, which not only eliminates the inconsistency between the initial and after-illumination dark current characteristics but also preserves the low magnitude of dark current, good external quantum efficiency, and rapid transient response.
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http://dx.doi.org/10.1021/acsnano.0c09426DOI Listing
January 2021

The Importance of Quantifying the Composition of the Amorphous Intermixed Phase in Organic Solar Cells.

Adv Mater 2020 Nov 21;32(47):e2005241. Epub 2020 Oct 21.

POLYMAT, University of the Basque Country UPV/EHU, Av. de Tolosa 72, San Sebastián, 20018, Spain.

The relation of phase morphology and solid-state microstructure with organic photovoltaic (OPV) device performance has intensely been investigated over the last twenty years. While it has been established that a combination of donor:acceptor intermixing and presence of relatively phase-pure donor and acceptor domains is needed to get an optimum compromise between charge generation and charge transport/charge extraction, a quantitative picture of how much intermixing is needed is still lacking. This is mainly due to the difficulty in quantitatively analyzing the intermixed phase, which generally is amorphous. Here, fast scanning calorimetry, which allows measurement of device-relevant thin films (<200 nm thickness), is exploited to deduce the precise composition of the intermixed phase in bulk-heterojunction structures. The power of fast scanning calorimetry is illustrated by considering two polymer:fullerene model systems. Somewhat surprisingly, it is found that a relatively small fraction (<15 wt%) of an acceptor in the intermixed amorphous phase leads to efficient charge generation. In contrast, charge transport can only be sustained in blends with a significant amount of the acceptor in the intermixed phase (in this case: ≈58 wt%). This example shows that fast scanning calorimetry is an important tool for establishing a complete compositional characterization of organic bulk heterojunctions. Hence, it will be critical in advancing quantitative morphology-function models that allow for the rational design of these devices, and in delivering insights in, for example, solar cell degradation mechanisms via phase separation, especially for more complex high-performing systems such as nonfullerene acceptor:polymer bulk heterojunctions.
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http://dx.doi.org/10.1002/adma.202005241DOI Listing
November 2020

Visualization of Charge Transfer from Bacteria to a Self-Doped Conjugated Polymer Electrode Surface Using Conductive Atomic Force Microscopy.

ACS Appl Mater Interfaces 2020 Sep 28;12(36):40778-40785. Epub 2020 Aug 28.

Center for Polymers and Organic Solids, Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States.

In this work, we aim to provide a better understanding of the reasons behind electron transfer inefficiencies between electrogenic bacteria and the electrode in microbial fuel cells. We do so using a self-doped conjugated polyelectrolyte (CPE) as the electrode surface, onto which is placed, then using conductive atomic force microscopy (C-AFM) to directly visualize and quantify the electrons that are transferring from each bacterium to the electrode, thereby helping us gain a better understanding for the overpotential losses in MFCs. In doing so, we obtain images that show can directly transfer electrons to an electrode surface without the use of pili, and that overpotential losses are likely due to cell death and poor distribution or performance of individual bacterium's OmcB cytochromes. This unique combination of CPEs with C-AFM can also be used for other studies where electron transfer loss mechanisms need to be understood on the nanoscale, allowing for direct visualization of potential issues in these systems.
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http://dx.doi.org/10.1021/acsami.0c10795DOI Listing
September 2020

Organic Electrochemical Transistors Based on the Conjugated Polyelectrolyte PCPDTBT-SO K (CPE-K).

Adv Mater 2020 Aug 12;32(33):e1908120. Epub 2020 Jul 12.

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

PCPDTBT-SO K (CPE-K), a conjugated polyelectrolyte, is presented as a mixed conductor material that can be used to fabricate high transconductance accumulation mode organic electrochemical transistors (OECTs). OECTs are utilized in a wide range of applications such as analyte detection, neural interfacing, impedance sensing, and neuromorphic computing. The use of interdigitated contacts to enable high transconductance in a relatively small device area in comparison to standard contacts is demonstrated. Such characteristics are highly desired in applications such as neural-activity sensing, where the device area must be minimized to reduce invasiveness. The physical and electrical properties of CPE-K are fully characterized to allow a direct comparison to other top performing OECT materials. CPE-K demonstrates an electrical performance that is among the best reported in the literature for OECT materials. In addition, CPE-K OECTs operate in the accumulation mode, which allows for much lower energy consumption in comparison to commonly used depletion mode devices.
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http://dx.doi.org/10.1002/adma.201908120DOI Listing
August 2020

Enzyme-assisted in vivo polymerisation of conjugated oligomer based conductors.

J Mater Chem B 2020 05 13;8(19):4221-4227. Epub 2020 Mar 13.

Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, SE-60174, Norrköping, Sweden.

Conjugated polymers conduct both electronic and ionic carriers and thus can stimulate and translate biological signals when used as active materials in bioelectronic devices. Self- and on-demand organization of the active material directly in the in vivo environment can result in the seamless integration of the bioelectronic interface. Along that line, we recently demonstrated spontaneous in vivo polymerization of the conjugated oligomer ETE-S in the vascular tissue of plants and the formation of conducting wires. In this work, we elucidate the mechanism of the in vivo polymerization of the ETE-S trimer and demonstrate that ETE-S polymerizes due to an enzymatic reaction where the enzyme peroxidase is the catalyst and hydrogen peroxide is the oxidant. ETE-S, therefore, represents the first example of a conducting polymer that is enzymatically polymerized in vivo. By reproducing the reaction in vitro, we gain further insight on the polymerization mechanism and show that hydrogen peroxide is the limiting factor. In plants the ETE-S triggers the catalytic cycle responsible for the lignification process, hacks this biochemical pathway and integrates within the plant cell wall, forming conductors along the plant structure.
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http://dx.doi.org/10.1039/d0tb00212gDOI Listing
May 2020

Highly efficient organic photovoltaics with enhanced stability through the formation of doping-induced stable interfaces.

Proc Natl Acad Sci U S A 2020 Mar 9;117(12):6391-6397. Epub 2020 Mar 9.

Thin-Film Device Laboratory, RIKEN, Wako, 351-0198 Saitama, Japan;

Flexible organic photovoltaics (OPVs) are promising power sources for wearable electronics. However, it is challenging to simultaneously achieve high efficiency as well as good stability under various stresses. Herein, we demonstrate the fabrication of highly efficient (efficiency, 13.2%) and stable OPVs based on nonfullerene blends by a single-step postannealing treatment. The device performance decreases dramatically after annealing at 90 °C and is fully recovered after annealing at 150 °C. Glass-encapsulated annealed OPVs show good environmental stability with 4.8% loss in efficiency after 4,736 h and an estimated lifetime (80% of the initial power conversion efficiency) of over 20,750 h in the dark under ambient condition and lifetime of 1,050 h at 85 °C and 30% relative humidity. This environmental stability is enabled by the synergetic effect of the stable morphology of donor/acceptor blends and thermally stabilized interfaces due to doping. Furthermore, the high efficiency and good stability are almost 100% retained in ultraflexible OPVs and minimodules which are mechanically robust and have long-term operation capability and thus are promising for future self-powered and wearable electronics.
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http://dx.doi.org/10.1073/pnas.1919769117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7104015PMC
March 2020

A High-Performance Solution-Processed Organic Photodetector for Near-Infrared Sensing.

Adv Mater 2020 Jan 12;32(1):e1906027. Epub 2019 Nov 12.

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

Sensitive detection of near-infrared (NIR) light enables many important applications in both research and industry. Current organic photodetectors suffer from low NIR sensitivity typically due to early absorption cutoff, low responsivity, and/or large dark/noise current under bias. Herein, organic photodetectors based on a novel ultranarrow-bandgap nonfullerene acceptor, CO1-4Cl, are presented, showcasing a remarkable responsivity over 0.5 A W in the NIR spectral region (920-960 nm), which is the highest among organic photodiodes. By effectively delaying the onset of the space charge limited current and suppressing the shunt leakage current, the optimized devices show a large specific detectivity around 10 Jones for NIR spectral region up to 1010 nm, close to that of a commercial Si photodiode. The presented photodetectors can also be integrated in photoplethysmography for real-time heart-rate monitoring, suggesting its potential for practical applications.
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http://dx.doi.org/10.1002/adma.201906027DOI Listing
January 2020

Orbital-Energy Modulation of Tetrabenzoporphyrin-Derived Non-Fullerene Acceptors for Improved Open-Circuit Voltage in Organic Solar Cells.

J Org Chem 2020 Jan 25;85(1):168-178. Epub 2019 Nov 25.

Division of Materials Science, Graduate School of Science and Technology , Nara Institute of Science and Technology (NAIST) , 8916-5 Takayama-cho , Ikoma , Nara 630-0192 , Japan.

Tetrabenzoporphyrin (BP) holds attractive characteristics for optoelectronic applications, such as the large π-conjugated framework and high photoabsorption capability. However, its use in organic solar cells (OSCs) has been limited because of the extremely low solubility that hampers direct solution processing and also the high frontier-orbital energies that lead to low open-circuit voltage (). Herein, we examine BP derivatives equipped with multiple strongly electron-withdrawing groups for photovoltaic applications. The derivatives are generated in thin films through a thermal precursor approach, wherein the corresponding bicyclo[2.2.2]octadiene-fused porphyrin derivatives are solution-cast, and then annealed to carry out the in situ retro-Diels-Alder reaction. The frontier-orbital energies of the resulting derivatives are effectively stabilized as compared to pristine BP to such a degree that they afford high of up to 0.94 V when used as a donor or can even work as a new class of nonfullerene acceptor in OSCs. Single-crystal X-ray diffraction analyses demonstrate that the conformation of the BP framework largely varies from being near planar to highly curved depending on its substituents. The morphology of polymer:BP-derivative bulk-heterojunction films prepared by the thermal precursor approach also varies between the BP derivatives. These results can greatly extend the scope of both molecular design and morphology control for utilization of the BP chromophore toward achieving viable organic optoelectronic devices.
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http://dx.doi.org/10.1021/acs.joc.9b02386DOI Listing
January 2020

Understanding the High Performance of over 15% Efficiency in Single-Junction Bulk Heterojunction Organic Solar Cells.

Adv Mater 2019 Nov 9;31(48):e1903868. Epub 2019 Oct 9.

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

The highly efficient single-junction bulk-heterojunction (BHJ) PM6:Y6 system can achieve high open-circuit voltages (V ) while maintaining exceptional fill-factor (FF) and short-circuit current (J ) values. With a low energetic offset, the blend system is found to exhibit radiative and non-radiative recombination losses that are among the lower reported values in the literature. Recombination and extraction dynamic studies reveal that the device shows moderate non-geminate recombination coupled with exceptional extraction throughout the relevant operating conditions. Several surface and bulk characterization techniques are employed to understand the phase separation, long-range ordering, as well as donor:acceptor (D:A) inter- and intramolecular interactions at an atomic-level resolution. This is achieved using photo-conductive atomic force microscopy, grazing-incidence wide-angle X-ray scattering, and solid-state F magic-angle-spinning NMR spectroscopy. The synergy of multifaceted characterization and device physics is used to uncover key insights, for the first time, on the structure-property relationships of this high-performing BHJ blend. Detailed information about atomically resolved D:A interactions and packing reveals that the high performance of over 15% efficiency in this blend can be correlated to a beneficial morphology that allows high J and FF to be retained despite the low energetic offset.
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http://dx.doi.org/10.1002/adma.201903868DOI Listing
November 2019

Towards understanding the doping mechanism of organic semiconductors by Lewis acids.

Nat Mater 2019 12 16;18(12):1327-1334. Epub 2019 Sep 16.

Center for Polymers and Organic Solids, Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, USA.

Precise doping of organic semiconductors allows control over the conductivity of these materials, an essential parameter in electronic applications. Although Lewis acids have recently shown promise as dopants for solution-processed polymers, their doping mechanism is not yet fully understood. In this study, we found that B(CF) is a superior dopant to the other Lewis acids investigated (BF, BBr and AlCl). Experiments indicate that Lewis acid-base adduct formation with polymers inhibits the doping process. Electron-nuclear double-resonance and nuclear magnetic resonance experiments, together with density functional theory, show that p-type doping occurs by generation of a water-Lewis acid complex with substantial Brønsted acidity, followed by protonation of the polymer backbone and electron transfer from a neutral chain segment to a positively charged, protonated one. This study provides insight into a potential path for protonic acid doping and shows how trace levels of water can transform Lewis acids into powerful Brønsted acids.
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http://dx.doi.org/10.1038/s41563-019-0479-0DOI Listing
December 2019

Tuning Geobacter sulfurreducens biofilm with conjugated polyelectrolyte for increased performance in bioelectrochemical system.

Biosens Bioelectron 2019 Nov 28;144:111630. Epub 2019 Aug 28.

Center for Polymers and Organic Solids, Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, CA, 93106, USA. Electronic address:

Bioelectrochemical systems (BESs) are emerging as a platform technology with great application potentials such as wastewater remediation and power generation. Materials for electrode/microorganism modification are being examined in order to improve the current production in BESs. Herein, we report that the current production increased almost one fold in single-chamber BES reactors, by adding a conjugated polyelectrolyte (CPE-K) in the growth medium to co-form the anodic biofilm with Geobacter sulfurreducens cells. The CPE-K treated BESs had a maximum current density as high as 12.3 ± 0.5 A/m, with that of the controls being 6.2 ± 0.7 A/m. Improved current production was sustained even after CPE-K was no longer added to the medium. It was demonstrated that increased current resulted from improvement of certain biofilm properties. Analysis using electrochemical impedance spectroscopy (EIS) showed that CPE-K addition decreased the charge transfer resistance at the cell/electrode interface and the diffusion resistance through the biofilm. Protein quantification showed increased biomass growth on the electrode surface, and confocal scanning microscopy images revealed enhanced biofilm permeability. These results demonstrated for the first time that conjugated polyelectrolytes could be used for G. sulfurreducens biofilm augmentation to achieve high electricity production through tuning the anodic biofilm in BESs.
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http://dx.doi.org/10.1016/j.bios.2019.111630DOI Listing
November 2019

Tuning Optical Properties of Conjugated Molecules by Lewis Acids: Insights from Electronic Structure Modeling.

J Phys Chem Lett 2019 Aug 1;10(16):4632-4638. Epub 2019 Aug 1.

Center for Polymers and Organic Solids (CPOS) , University of California Santa Barbara , Santa Barbara , California 93106 , United States.

Understanding and controlling the optoelectronic properties of organic semiconductors at the molecular level remains a challenge due to the complexity of chemical structures and intermolecular interactions. A common strategy to address this challenge is to utilize both experimental and computational approaches. In this contribution, we show that density functional theory (DFT) calculation is a useful tool to provide insights into the bonding, electron population distribution, and optical transitions of adducts between conjugated molecules and Lewis acids (CM-LA). Adduct formation leads to relevant modifications of key properties, including a red shift in optical transitions and an increase in charge carrier density and charge mobility, compared to the parent conjugated molecules. We show that electron density transfer from the CM to the LA, which was hypothesized to cause the experimental red shift in absorption spectra upon LA binding, can be quantified and interpreted by population analysis. Experimental red shifts in optical transitions for all molecular families can also be predicted by time-dependent DFT calculations with different density functionals. These detailed insights help to optimize a priori design guidelines for future applications.
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http://dx.doi.org/10.1021/acs.jpclett.9b01572DOI Listing
August 2019

Solution-Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance.

Adv Mater 2019 Jul 30;31(30):e1900904. Epub 2019 May 30.

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

Recent research efforts on solution-processed semitransparent organic solar cells (OSCs) are presented. Essential properties of organic donor:acceptor bulk heterojunction blends and electrode materials, required for the combination of simultaneous high power conversion efficiency (PCE) and average visible transmittance of photovoltaic devices, are presented from the materials science and device engineering points of view. Aspects of optical perception, charge generation-recombination, and extraction processes relevant for semitransparent OSCs are also discussed in detail. Furthermore, the theoretical limits of PCE for fully transparent OSCs, compared to the performance of the best reported semitransparent OSCs, and options for further optimization are discussed.
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http://dx.doi.org/10.1002/adma.201900904DOI Listing
July 2019

High-k Fluoropolymer Gate Dielectric in Electrically Stable Organic Field-Effect Transistors.

ACS Appl Mater Interfaces 2019 May 22;11(17):15821-15828. Epub 2019 Apr 22.

Center for Polymers and Organic Solids, Department of Chemistry and Biochemistry , University of California at Santa Barbara , Santa Barbara , California 93106 , United States.

A detailed study of a high-k fluoropolymer gate dielectric material, poly(vinylidene fluoride- co-hexafluoropropylene) [P(VDF-HFP)], is presented as a guide to achieve low operational voltage and electrically stable device performance. The large dipole moment of C-F dipoles in P(VDF-HFP) is responsible for its high dielectric constant as well as its potentially ferroelectric behavior that must be minimized to avoid hysteretic current-voltage characteristics. A range of material grades and processing conditions are explored and are shown to have a significant effect on the degree of hysteresis observed in device-transfer characteristics. The percentage of HFP monomer in the P(VDF-HFP) dielectric has an effect on gate-dependent mobility induced by disorder at the semiconductor-dielectric interface. Most importantly, we present the considerations that must be made to achieve optimal performance in multiple device architectures of organic field-effect transistors when using P(VDF-HFP) as a dielectric layer.
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http://dx.doi.org/10.1021/acsami.8b20827DOI Listing
May 2019

Photoluminescence Quenching Probes Spin Conversion and Exciton Dynamics in Thermally Activated Delayed Fluorescence Materials.

Adv Mater 2019 May 8;31(21):e1804490. Epub 2019 Apr 8.

Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, 93106, USA.

Fluorescent materials that efficiently convert triplet excitons into singlets through reverse intersystem crossing (RISC) rival the efficiencies of phosphorescent state-of-the-art organic light-emitting diodes. This upconversion process, a phenomenon known as thermally activated delayed fluorescence (TADF), is dictated by the rate of RISC, a material-dependent property that is challenging to determine experimentally. In this work, a new analytical model is developed which unambiguously determines the magnitude of RISC, as well as several other important photophysical parameters such as exciton diffusion coefficients and lengths, all from straightforward time-resolved photoluminescence measurements. From a detailed investigation of five TADF materials, important structure-property relationships are derived and a brominated derivative of 2,4,5,6-tetrakis(carbazol-9-yl)isophthalonitrile that has an exciton diffusion length of over 40 nm and whose excitons interconvert between the singlet and triplet states ≈36 times during one lifetime is identified.
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http://dx.doi.org/10.1002/adma.201804490DOI Listing
May 2019

Tuning the Potential of Electron Extraction from Microbes with Ferrocene-Containing Conjugated Oligoelectrolytes.

Adv Biosyst 2019 02 10;3(2):e1800303. Epub 2018 Dec 10.

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

Synthetic systems that facilitate electron transport across cellular membranes are of interest in bio-electrochemical technologies such as bio-electrosynthesis, waste water remediation, and microbial fuel cells. The design of second generation redox-active conjugated oligoelectrolytes (COEs) bearing terminal cationic groups and a π-delocalized core capped by two ferrocene units is reported. The two COEs, DVFBO and F -DVFBO, have similar membrane affinity, but fluorination of the core results in a higher oxidation potential (422 ± 5 mV compared to 365 ± 4 mV vs Ag/AgCl for the neutral precursors in chloroform). Concentration-dependent aggregation is suggested by zeta potential measurements and confirmed by cryogenic transmission electron microscopy. When the working electrode potential (E ) is poised below the oxidation potential of the COEs (E = 200 mV) in three-electrode electrochemical cells containing Shewanella oneidensis MR-1, addition of DVFBO and F -DVFBO produces negligible biocurrent enhancement over controls. At E = 365 mV, DVFBO increases steady-state biocurrent by 67 ± 12% relative to controls, while the increase with F -DVFBO is 30 ± 5%. Cyclic voltammetry supports that DVFBO increases catalytic biocurrent and that F -DVFBO has less impact, consistent with their oxidation potentials. Overall, electron transfer from microbial species is modulated via tailoring of the COE redox properties.
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http://dx.doi.org/10.1002/adbi.201800303DOI Listing
February 2019

n-Type Ionic-Organic Electronic Ratchets for Energy Harvesting.

ACS Appl Mater Interfaces 2019 Jan 18;11(1):1081-1087. Epub 2018 Dec 18.

Center for Polymers and Organic Solids, Department of Chemistry and Biochemistry , University of California at Santa Barbara , Santa Barbara , California 93106 , United States.

Ionic-organic ratchets are three-terminal electronic devices with asymmetric conductivity of the active layer.  These devices  are capable of generating useful direct current electrical power by converting electromagnetic noise signals available in any environment. In this work, we demonstrate for the first time an n-type ionic-organic ratchet which can generate a current of up to 7.29 μA and power up to 12.5 μW that exceed the values reported for many of the presently state-of-the-art, p-type organic electronic ratchets. We show that n-type ratchets require elimination of electron traps at the SiO surface, which is not required in p-type devices. This can be achieved by using a trap-free passivation layer such as benzocyclobutene, where the traditional silane treatment is insufficient. Chemical doping is employed to further fill electron traps in the channel and increase carrier concentration and mobilities. Scanning Kelvin probe force microscopy studies provide evidence of a pn-like rectifying junction in the n-type ratchets fabricated in this work, which inherently differs from the rectification mechanism of previous ionic-organic p-type ratchets.
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http://dx.doi.org/10.1021/acsami.8b15042DOI Listing
January 2019

Solution-Processed Ion-Free Organic Ratchets with Asymmetric Contacts.

Adv Mater 2018 Nov 4;30(46):e1804794. Epub 2018 Oct 4.

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

Ion-free organic ratchets with asymmetric injecting contacts (AICs) are fabricated using solution-processable organic semiconductors. Scanning Kelvin probe microscopy analysis reveals that the rectifying function is achieved via the "charge pump" mechanism. Electrical characterizations show that the device can readily operate under industrial standard radio frequency and its high-frequency performance may be enhanced through further material/device engineering. The built-in asymmetric feature exempts the devices from the complicated material design, device processing, and performance decay associated with the use of ion/semiconductor blends in ionic-organic ratchets. Thus, the AIC ratchets can deliver a persisting ratchet effect and have excellent material compatibility toward organic semiconductors.
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http://dx.doi.org/10.1002/adma.201804794DOI Listing
November 2018

Mesomorphic Behavior in Silver(I) -(4-Pyridyl) Benzamide with Aromatic π⁻π Stacking Counterions.

Materials (Basel) 2018 Sep 9;11(9). Epub 2018 Sep 9.

Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968, USA.

Organic semiconductor materials composed of π⁻π stacking aromatic compounds have been under intense investigation for their potential uses in flexible electronics and other advanced technologies. Herein we report a new family of seven π⁻π stacking compounds of silver(I) bis--(4-pyridyl) benzamide with varying counterions, namely [Ag(NPBA)2]X, where NPBA is -(4-pyridyl) benzamine, X = NO₃ (), ClO₄ (), CF₃SO₃ (), PF₆ (), BF₄ (), CH₃PhSO₃ (), and PhSO₃ (), which form extended π-π stacking networks in one-dimensional (1D), 2D and 3D directions in the crystalline solid-state via the phenyl moiety, with average inter-ring distances of 3.823 Å. Interestingly, the counterions that contain π⁻π stacking-capable groups, such as in and , can induce the formation of mesomorphic phases at 130 °C in dimethylformamide (DMF), and can generate highly branched networks at the mesoscale. Atomic force microscopy studies showed that 2D interconnected fibers form right after nucleation, and they extend from ~30 nm in diameter grow to reach the micron scale, which suggests that it may be possible to stop the process in order to obtain nanofibers. Differential scanning calorimetry studies showed no remarkable thermal behavior in the complexes in the solid state, which suggests that the mesomorphic phases originate from the mechanisms that occur in the DMF solution at high temperatures. An all-electron level simulation of the band gaps using NRLMOL (Naval Research Laboratory Molecular Research Library) on the crystals gave 3.25 eV for (), 3.68 eV for (), 1.48 eV for (), 5.08 eV for (), 1.53 eV for (), and 3.55 eV for (). Mesomorphic behavior in materials containing π⁻π stacking aromatic interactions that also exhibit low-band gap properties may pave the way to a new generation of highly branched organic semiconductors.
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http://dx.doi.org/10.3390/ma11091666DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6163786PMC
September 2018

Elucidating Aggregation Pathways in the Donor-Acceptor Type Molecules p-DTS(FBTTh) and p-SIDT(FBTTh).

J Phys Chem B 2018 10 25;122(39):9191-9201. Epub 2018 Sep 25.

Biofluid Simulation and Modeling, Theoretische Physik VI , Universität Bayreuth , 95440 Bayreuth , Germany.

We investigated the aggregation behavior of the donor-acceptor molecules p-DTS(FBTTh) (T1) and p-SIDT(FBTTh) (H1) in MTHF solutions. Using optical spectroscopy, we found that T1 forms aggregates in solution while H1 aggregates only when processed as a thin film, but not in solution. Free energy molecular dynamics (MD) simulations based on force fields derived from quantum-mechanical density functional theory fully reproduce this difference. Our simulations reveal that this difference is not due to the lengthy carbon side chains. Rather, the molecular symmetry of T1 allows for an aggregated state in which the central donor units are spatially well-separated while a similar configuration is sterically impossible for H1. As a consequence, any aggregation of H1 necessarily involves aggregation of the central donors which requires, as a first step, stripping the central donor of its protective MTHF solvation shell. This unfavorable process leads to a significant kinetic hindrance for aggregation and explains the strongly differing aggregation behavior of T1/H1 in MTHF despite their otherwise similar structures.
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http://dx.doi.org/10.1021/acs.jpcb.8b06283DOI Listing
October 2018

Donor-Acceptor-Collector Ternary Crystalline Films for Efficient Solid-State Photon Upconversion.

J Am Chem Soc 2018 07 9;140(28):8788-8796. Epub 2018 Jul 9.

Department of Chemistry and Biochemistry, Faculty of Engineering , Center for Molecular Systems (CMS) , Kyushu University , Moto-oka 744 , Nishi-ku , Fukuoka 819-0395 , Japan.

It is pivotal to achieve efficient triplet-triplet annihilation based photon upconversion (TTA-UC) in the solid-state for enhancing potentials of renewable energy production devices. However, the UC efficiency of solid materials is largely limited by low fluorescence quantum yields that originate from the aggregation of TTA-UC chromophores and also by severe back energy transfer from the acceptor singlet state to the singlet state of the triplet donor in the condensed state. In this work, to overcome these issues, we introduce a highly fluorescent singlet energy collector as the third component of donor-doped acceptor crystalline films, in which dual energy migration, i.e., triplet energy migration for TTA-UC and succeeding singlet energy migration for transferring energy to a collector, takes place. To demonstrate this scheme, a highly fluorescent singlet energy collector was added as the third component of donor-doped acceptor crystalline films. An anthracene-based acceptor containing alkyl chains and a carboxylic moiety is mixed with the triplet donor Pt(II) octaethylporphyrin (PtOEP) and the energy collector 2,5,8,11-tetra- tert-butylperylene (TTBP) in solution, and simple spin-coating of the mixed solution gives acceptor films of nanofibrous crystals homogeneously doped with PtOEP and TTBP. Interestingly, delocalized singlet excitons in acceptor crystals are found to diffuse effectively over the distance of ∼37 nm. Thanks to this high diffusivity, only 0.5 mol % of doped TTBP can harvest most of the singlet excitons, which successfully doubles the solid-state fluorescent quantum yield of acceptor/TTBP blend films to 76%. Furthermore, since the donor PtOEP and the collector TTBP are separately isolated in the nanofibrous acceptor crystals, the singlet back energy transfer from the collector to the donor is effectively avoided. Such efficient singlet energy collection and inhibited back energy transfer processes result in a large increase of UC efficiency up to 9.0%, offering rational design principles toward ultimately efficient solid-state upconverters.
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http://dx.doi.org/10.1021/jacs.8b04542DOI Listing
July 2018

Acceptor Percolation Determines How Electron-Accepting Additives Modify Transport of Ambipolar Polymer Organic Field-Effect Transistors.

ACS Nano 2018 Jul 18;12(7):7134-7140. Epub 2018 Jun 18.

Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices , Soochow University , Suzhou , Jiangsu 215123 , P. R. China.

Organic field-effect transistors (OFETs) that utilize ambipolar polymer semiconductors can benefit from the ability of both electron and hole conduction, which is necessary for complementary circuits. However, simultaneous hole and electron transport in organic field-effect transistors result in poor ON/OFF ratios, limiting potential applications. Solution processing methods have been developed to control charge transport properties and transform ambipolar conduction to hole-only conduction. The electron-acceptor phenyl-C61-butyric acid methyl ester (PCBM), when mixed in solution with an ambipolar semiconducting polymer, can reduce electron conduction. Unipolar p-type OFETs with high, well-defined ON/OFF ratios and without detrimental effects on hole conduction are achieved for a wide range of blend compositions, from 95:5 to 5:95 wt % semiconductor polymer:PCBM. When introducing the alternative acceptor N, N'-bis(1-ethylpropyl)-3,4:9,10-perylenediimide (PDI), high ON/OFF ratios are achieved for 95:5 wt % semiconductor polymer:PDI; however, electron conduction increases for 50:50 and 5:95 wt % semiconductor polymer:PDI. As described within, we show that electron conduction is practically eliminated when additive domains do not percolate across the OFET channel, that is, electrons are "morphologically trapped". Morphologies were characterized by optical, electron, and atomic force microscopy as well as X-ray scattering techniques. PCBM was substituted with an endohedral LuN fullerene, which enhanced contrast in electron microscopy and allowed for more detailed insight into the blend morphologies. Blends with alternative, nonfullerene acceptors further emphasize the importance of morphology and acceptor percolation, providing insights for such blends that control ambipolar transport and ON/OFF ratios.
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http://dx.doi.org/10.1021/acsnano.8b03006DOI Listing
July 2018

Thermally stable, highly efficient, ultraflexible organic photovoltaics.

Proc Natl Acad Sci U S A 2018 05 16;115(18):4589-4594. Epub 2018 Apr 16.

Center for Emergent Matter Science, RIKEN, Wako, 351-0198 Saitama, Japan;

Flexible photovoltaics with extreme mechanical compliance present appealing possibilities to power Internet of Things (IoT) sensors and wearable electronic devices. Although improvement in thermal stability is essential, simultaneous achievement of high power conversion efficiency (PCE) and thermal stability in flexible organic photovoltaics (OPVs) remains challenging due to the difficulties in maintaining an optimal microstructure of the active layer under thermal stress. The insufficient thermal capability of a plastic substrate and the environmental influences cannot be fully expelled by ultrathin barrier coatings. Here, we have successfully fabricated ultraflexible OPVs with initial efficiencies of up to 10% that can endure temperatures of over 100 °C, maintaining 80% of the initial efficiency under accelerated testing conditions for over 500 hours in air. Particularly, we introduce a low-bandgap poly(benzodithiophene-cothieno[3,4-b]thiophene) (PBDTTT) donor polymer that forms a sturdy microstructure when blended with a fullerene acceptor. We demonstrate a feasible way to adhere ultraflexible OPVs onto textiles through a hot-melt process without causing severe performance degradation.
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http://dx.doi.org/10.1073/pnas.1801187115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5939109PMC
May 2018

Doping Polymer Semiconductors by Organic Salts: Toward High-Performance Solution-Processed Organic Field-Effect Transistors.

ACS Nano 2018 04 16;12(4):3938-3946. Epub 2018 Apr 16.

Center for Polymers and Organic Solids, Department of Chemistry and Biochemistry , University of California at Santa Barbara , Santa Barbara , California 93106 , United States.

Solution-processed organic field-effect transistors (OFETs) were fabricated with the addition of an organic salt, trityl tetrakis(pentafluorophenyl)borate (TrTPFB), into thin films of donor-acceptor copolymer semiconductors. The performance of OFETs is significantly enhanced after the organic salt is incorporated. TrTPFB is confirmed to p-dope the organic semiconductors used in this study, and the doping efficiency as well as doping physics was investigated. In addition, systematic electrical and structural characterizations reveal how the doping enhances the performance of OFETs. Furthermore, it is shown that this organic salt doping method is feasible for both p- and n-doping by using different organic salts and, thus, can be utilized to achieve high-performance OFETs and organic complementary circuits.
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http://dx.doi.org/10.1021/acsnano.8b01460DOI Listing
April 2018
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