Photoconductive Polymer Research Articles

(Invited) Electrochemical Modulation of Energy Transfer in Plasmonic-Polymer Hybrid Nanomaterials

The absorption cross-section of plasmonic nanoparticles makes them attractive candidates for light-activated chemistry, but much of the absorbed energy is transformed into heat in the surrounding medium. We propose that one way to overcome this problem is through nonradiative energy transfer to a photo-conductive polymer shell. At the same time, understanding the mechanisms that drive energy-transfer is obscured by the innate heterogeneity of the nanoparticle/polymer interface. I will discuss our recent efforts to apply in-situ single-particle spectroelectrochemistry to quantify the energy transfer efficiency in hybrid nanomaterials comprised of plasmonic cores surrounded by polymer shells.

Photorefractivity and photocurrent dynamics of triphenylamine-based polymer composites

The photorefractive properties of triphenylamine polymer-based composites with various composition ratios were investigated via optical diffraction, response time, asymmetric energy transfer, and transient photocurrent. The composite consisted of a photoconductive polymer of poly((4-diphenylamino)benzyl acrylate), a photoconductive plasticizer of (4-diphenylamino)phenyl)methanol, a sensitizer of [6,6]-phenyl-C61-butyric acid methyl ester, and a nonlinear optical dye of (4-(azepan-1-yl)-benzylidene)malononitrile. The photorefractive properties and related quantities were dependent on the composition, which was related to the glass transition temperature of the photorefractive polymers. The quantum efficiency (QE) of photocarrier generation was evaluated from the initial slope of the transient photocurrent. Transient photocurrents were measured and showed two unique peaks: one in the range of 10−4 to 10−3 s and the other in the range of 10−1 to 1 s. The transient photocurrents was well simulated (or reproduced) by the expanded two-trapping site model with two kinds of photocarrier generation and recombination processes and two different trapping sites. The obtained photorefractive quantity of trap density was significantly related to the photoconductive parameters of QE.

Stereoselective Polymerization of 3,6-Disubstituted N-Vinylcarbazoles.

Poly(N-vinylcarbazole) (PNVC-H) is a valuable nonconjugated photoconductive polymer, but the free radical polymerization conditions typically used for its synthesis do not control polymer stereochemistry and are not tolerant to many substituted N-vinylcarbazoles. Here, we report the stereoselective cationic polymerization of a series of 3,6-disubtituted N-vinylcarbazole derivatives using a chiral scandium-bis(oxazoline) Lewis acid catalyst. The combination of asymmetric ion-pairing catalysis and inherent monomer stereoelectronics facilitated stereoselective polymerization at room temperature, which enabled the polymerization of less soluble 3,6-disubstituted-N-vinylcarbazole derivatives. Isotactic halogen-substituted PNVCs demonstrated self-assembly in solution through halogen-halogen bonding, which was not observed in their atactic counterparts. Initial spectral characterization displayed a wide range of excitation-emission profiles for substituted PNVCs, which demonstrate the promise of these materials as a new class of nonconjugated photoconductive polymers for optoelectronic applications. Overall, these results showcase a diverse class of isotactic poly(N-vinylcarbazoles), highlight the benefits of identifying alternative stereocontrol mechanisms for polymerization, and expand the suite of accessible nonconjugated hole-transport materials.

Preparation of Carbazoles Involving 6π-Electrocyclization, Photoredox-, Electrochemical-, and Thermal Cyclization Reactions: Mechanistic Insights.

Carbazole is a heterocyclic motif that can be found in a diverse array of natural and unnatural products displaying a wide range of biological and physiological properties. Furthermore, this heterocycle is part of electronic materials like photoconducting polymers and organic optoelectronic materials owing to its excellent photophysical characteristics. Consequently, the development of synthetic strategies for carbazole scaffolds holds potential significance in biological and material fields. In this regard, a variety of preparation methods has been developed to exploit their efficient and distinct formation of new C-C and C-heteroatom bonds under mild conditions and enabling broad substrate diversity and functional group tolerance. Therefore, this review focuses on the synthesis of a set of carbazole derivatives describing a variety of methodologies that involve direct irradiation, photosensitization, photoredox, electrochemical and thermal cyclization reactions.

Development of high-performance organic photodetectors by understanding origin of dark current density with synthesis of photoconductive polymers

Photoconductive polymers (PCPs) are key materials that determine the static and dynamic performance of p-n bulk heterojunction organic photodetectors (OPDs). Herein, we developed benzo[1,2-d:4,5-d']bis(oxazole)-based novel PCPs with π-spacer molecular engineering to promote charge-transport while maintaining low dark-current densities (Jd) in OPDs. The origin of Jd was elucidated by measuring hole charge-transfer (CT) state energy. We found that the degree of hole accumulation at the CT state rather than the CT state energy was crucial to maintaining a low Jd and promoting a fast signal response in the devices. The optimized OPDs showed highly promising responsivity (R) of 0.43 A/W and specific detectivity (D*) of 6.85 × 1012 Jones at −1 V under 532 nm LED (5.0 mW cm−2) illumination, which are superior to those of the control device (R = 0.29 A/W and D* = 5.46 × 1011 Jones). In addition, the dynamic properties were significantly improved by the introduction of π-spacers on the PCPs. −3 dB cut-off frequency was 200.4 kHz, and the rising and falling response times was ultrafast as 2.3 and 2.8 μs, respectively. Understanding the origin of Jd through the molecular engineering of PCPs is expected to contribute greatly toward expanding the knowledge required to develop high-performance OPDs.

Light-activated SAW sensor structures with photoconductive polymer films for DMMP detection

We report the light-activated surface acoustic wave (SAW) investigations of DMMP (dimethyl methylphosphonate) sensing based on photoconductive polymer films such as: (rr)P3HT (regio-regular Poly-3-hexylthiophene) and SilPEG1.4. These SAW sensor structures were composed of ∼205 MHz quartz dual delay lines with a novel switchable digital generator set-up and a thin (∼50–100 nm) polymer films created by means of a simple spray coating method. The white light, low power laser LED (blue, green, red) and seven LED’s sources were applied for light activation of the SAW structures. The relative sensitivity increase (frequency shifts in light vs. the non activated state) at 2 ppm of DMMP is ∼275 for SilPEG1.4 with a blue LED (∼460 nm), to even ∼340 for (rr)P3HT with a yellow LED (∼585 nm) activation. This last structure has also acceptable stability and repeatability with the limit of detection ∼133 ppb, the response/recovery times ∼5–100 s / ∼240 s, and sensitivity coefficient ∼1.013 kHz/ppm in ppb ranges. The observed substantially enhanced light-activated SAW sensitivity should be interpreted in category of the acoustoelectric interactions, because the acoustoelectric parameter, estimated by resistive transducer results, has the greatest value for a yellow and blue LED light illumination.

Microscopic Structures, Dynamics, and Spin Configuration of the Charge Carriers in Organic Photovoltaic Solar Cells Studied by Advanced Time-Resolved Spectroscopic Methods.

Organic photovoltaics (OPVs) are promising solutions for renewable energy and sustainable technologies and have attracted much attention in recent years. Two types of organic semiconductors are used as donor materials to fabricate OPV cells. One type is a photoconductive polymer, and the other type is a small-molecule-based compound. The discovery of a bulk-heterojunction (BHJ) structure using a mixture of p- and n-type organic semiconductors has dramatically increased the power conversion efficiency (PCE) of OPV cells. In this feature article, we review our recent studies on organic BHJ thin films and OPVs by using advanced time-resolved spectroscopic techniques. Two topics regarding the microscopic behaviors of the charge carriers are discussed. The first topic is focused on how to quantify the local mobility of the charge carriers. Here, we discuss charge carrier dynamics in diketopyrrolopyrrole-linked tetrabenzoporphyrin (DPP-BP) BHJ thin films studied by time-resolved terahertz spectroscopy on a subpicosecond to several tens of picoseconds time scale and by transient photocurrent measurements on a microsecond time scale. The second topic concerns the spin configuration and interaction of the electron and hole of the polaron pairs in polymer-based BHJ thin films and OPV cells studied by the time-resolved electron paramagnetic resonance method, time-resolved simultaneous optical and electrical detection, and measurement of the magnetoconductance effect.

Light Activated Surface Acoustic Wave Investigations of Photoconductive Polymer Films for DMMP Detection

Light Activated Surface Acoustic Wave Investigations of Photoconductive Polymer Films for DMMP Detection

(Invited) Energy Transfer in Hybrid Plasmonic Nanostructures

A surface plasmon in a metal nanoparticle is the coherent oscillation of the conduction band electrons leading to both strong absorption and scattering as well as large local electromagnetic fields. Plasmonic nanoparticles therefore have the potential to induce electronic transitions in nearby materials. However, but fast internal relaxation limits the efficiency of such plasmonic mediated processes. In this talk, I will discuss our recent results of a new type of nanomaterial: hybrid nanoantennas comprised of plasmonic nanorods with photoconductive polymer coatings that were grown by photo-electrochemistry. The formation of the conductive polymer shell is selective to the nanorods, while the polymerization is enhanced by photoexcitation. In situ single pareticle dark-field scattering spectroscopy and simulations support an energy transfer mechanism with up to 50% efficiency. These hybrid nanoantennas combine the unmatched light harvesting properties of a plasmonic nanoparticles with the similarly unmatched device processability of polymersl.

Study of the capacity of poly(N‐vinylcarbazole) derivatives to form honeycomb‐like patterns

AbstractPoly(N‐vinylcarbazole), two dihalogenated derivatives, and nanocellulose/poly(N‐vinylcarbazole) nanocomposite thin films have been fabricated with micrometric pore topography. This patterned structure has been developed by using breath figure method, which is an easy and cost‐effective method to obtain honeycomb‐like arrays. In addition, the effect of the dihalogenated substitutions and the presence of nanocellulose fillers in the topography and conformational entropy were evaluated. The conformational entropy of each sample was calculated by using Voronoi tessellation. It is important to notice that highly regular patterns have been obtained from this photoconductive polymer, since the conductivity of this kind of material is proportional to the absorbed light, this honeycomb‐like topography could improve the light capture and the efficiency of photoelectric conversion, increasing the potential applicability of these materials.

Photoconductive Coordination Polymer with a Lead-Sulfur Two-Dimensional Coordination Sheet Structure.

Coordination polymers with metal-sulfur (M-S) bonds in their nodes have interesting optical properties and can be used as photocatalysts for water splitting. A wide range of inorganic-organic hybrid materials with M-S bonds have been prepared in recent years. However, there is a dearth of structural information because of their low crystallinity, which has hampered the understanding of their underlying chemistry and physics. Thus, we conducted a structural study of a novel, highly crystalline coordination polymer with M-S bonds. Theoretical calculations were performed to elucidate its photoconductivity mechanism. The photoconductive, three-dimensional coordination polymer [Pb(tadt)]n (denoted as KGF-9; tadt = 1,3,4-thiadiazole-2,5-dithiolate) was synthesized and confirmed to have a three-dimensional structure containing a two-dimensional Pb-S framework by single-crystal X-ray diffraction. We also performed diffuse-reflectance ultraviolet-visible-near-infrared spectroscopy, time-resolved microwave conductivity, and photoelectron yield spectroscopy measurements on the bulk powder samples, as well as first-principles calculations. Additionally, direct-current photoconductivity measurements were conducted on a single-crystal sample.

A Hippocampus-Inspired Dual-Gated Organic Artificial Synapse for Simultaneous Sensing of a Neurotransmitter and Light.

Organic neuromorphic devices and sensors that mimic the functions of chemical synapses and sensory perception in humans have received much attention for next-generation computing and integrated logic circuits. Despite recent advances, organic artificial synapses capable of detecting both neurotransmitters in liquid environments and light are not reported. Herein, inspired by hippocampal synapses, a dual-gate organic synaptic transistor platform with a photoconductive polymer semiconductor, a ferroelectric insulator of P(VDF-TrFE), and an extended-gate electrode functionalized with boronic acid is developed to simultaneously detect the neurotransmitter dopamine and light. The developed synaptic transistor enables memory consolidation upon repetitive exposure to dopamine and polychromatic light, exhibiting effectively modulated postsynaptic currents. This proof-of-concept hippocampal-synapse-mimetic organic neuromorphic system combining a chemical sensor and a photosensor opens new possibilities for developing low-power organic artificial synaptic multisensors and light-induced memory consolidative artificial synapses, and can also contribute to the development of human-machine interfaces.

Plasmon Energy Transfer in Hybrid Nanoantennas.

Plasmonic metal nanoparticles exhibit large dipole moments upon photoexcitation and have the potential to induce electronic transitions in nearby materials, but fast internal relaxation has to date limited the spatial range and efficiency of plasmonic mediated processes. In this work, we use photo-electrochemistry to synthesize hybrid nanoantennas comprised of plasmonic nanoparticles with photoconductive polymer coatings. We demonstrate that the formation of the conductive polymer is selective to the nanoparticles and that polymerization is enhanced by photoexcitation. In situ spectroscopy and simulations support a mechanism in which up to 50% efficiency of nonradiative energy transfer is achieved. These hybrid nanoantennas combine the unmatched light-harvesting properties of a plasmonic antenna with the similarly unmatched device processability of a polymer shell.

Z-scheme-type conductive-polymer-P3HT/KTa(Zr)O3 heterojunction composites for enhancing the photocatalytic activity of water splitting

Photocatalytic water splitting on Pt/KTa(Zr)O3 modified with poly(3-hexylthiophene-2,5-diyl) (P3HT) as a photoconductive organic polymer is investigated. The photocatalyst powder of Pt/P3HT/KTa(Zr)O3 was synthesized by evaporation to dryness method. The H2/O2 production ratio of the unmodified catalyst was 2.90, whereas the H2/O2 production ratio become close to 2 (1.66–2.26) with loading P3HT (0.1 wt% to 0.8 wt%). The optimum amount of P3HT added was 0.4 wt% and the H2/O2 production ratio was 2.00, which increased the hydrogen production rate by 476 % compared to the unmodified photocatalyst. As a result of the fluorescence lifetime measurement, when the dye and platinum were supported on KTa(Zr)O3, charge separation into the dye and platinum was confirmed in both UV and visible light irradiation. Irradiation with visible light in addition to UV light increased the water splitting activity, while water decomposition was not performed only with visible light, suggests Z-scheme type water splitting reaction.

Theoretical Analysis of the Radiation-Induced Conductivity in Polymers Exposed to Pulsed and Continuous Electron Beams.

We have performed comparative numerical calculations using a multiple trapping (MT) formalism with an exponential and an aggregate two-exponential trap distributions for describing two mostly used experimental setups for studying the radiation-induced conductivity (RIC) and the time-of-flight (TOF) effects. Computations have been done for pulsed and long-time electron-beam irradiations in a small-signal regime. Predictions of these two approaches differ appreciably in both setups. The classical MT approach proved very popular in photoconductive polymers generally and in molecularly doped polymers in particular, while a newly proposed complex MT worked well in common polymers. It has been shown that the complex MT successfully accounts for the presence of inherent deep traps, which may or may not have an energy distribution.

Polarization Dependence of Charge Conduction in Conjugated Polymer Films Investigated with Time-Resolved Terahertz Spectroscopy.

Room temperature Time-Domain Terahertz (TDS) and Time-Resolved Terahertz (TRTS) spectroscopic methods are employed to measure carrier mobility and charge generation efficiency in thin-film semiconductor polymers. Interrogation of the dependence on excitation and probe polarizations yields insight into the underlying material properties that guide charge transport. We apply THz polarization anisotropy probes to analyze charge conduction in preparations of the copolymer PCDTPT, consisting of alternating cyclopenta-dithiophene (donor) and thiadiazolo-pyridine (acceptor) units. Comparisons are made among films of different ordering and morphology, including aligned films prepared by blade coating, a near isotropic dropcast film, and isotropic liquid dispersion. They are further contrasted with their population dynamics ascertained through transient absorption and the traditional photoconductive polymer poly-3-hexylthiophene (P3HT). Polarization anisotropy is observed as preferential charge conduction along the backbone propagation direction of PCDTPT, with various factors disproportionately influencing directional mobility and charge pair yield. PCDTPT exhibits unexpectedly strong conductivity when isolated in toluene dispersion. Quantitative comparisons yield a better understanding of polaron/free-charge relaxation and transfer mechanisms and illustrate dynamics among photoexcited charge carriers and their motion and diffusion through different material morphologies.

Information Properties of Halogen-Containing Media for Photothermoplastic Recording of Holograms

The effect of halogens in carbazolyl-containing oligomers on the information properties of recording media in the photothermoplastic method of recording holograms is studied. An increase of photosensitivity was observed for such media with halogens present in their composition. It was shown by photoactivated electron paramagnetic resonance that the lifetime of the photogenerated charge pairs is increased in the presence of halogens. This is due to the increased probability of photogeneration of charge pairs in the triplet state for the modified oligomers compared with their analogs. It is presumed that modification of the photoconductive polymers with halogens is one way of increasing their photosensitivity and for other practical applications (photovoltaics, molecular electronics).

Tổng hợp polymer quang dẫn poly(4-diphenylamino styrene) cho vật liệu quang phi tuyến

The typical synthesis procedure for the monomer of N,N-diphenyl-4-vinylaniline was conducted and has been proved to be ineffective in the university laboratory conditions. The modified procedure was applied successfully using freshly formed Wittig reagent. The monomer was characterized by proton nuclear magnetic resonance (1H NMR) spectroscopy and confirmed the chemical structure with the correct peaks. The photoconductive polymer of poly((4-diphenylamino styrene)) could be obtained only by using the white crystal monomer.

Optimal composition of the poly(triarylamine)-based polymer composite to maximize photorefractive performance

A holographic display system requires the external diffraction efficiency to be greater than 10% and four orders of magnitude of sensitivity for practical usage. To achieve such requirements, the photorefractive (PR) performance of PR composite based on poly[bis(2,4,6-trimethylpheneyl)amine] (PTAA) has been investigated. In the present report, the change of the content of PTAA as a photoconductive polymer, (2,4,6-trimethylphenyl)diphenylamine (TAA) as a photoconductive plasticizer, and second trap agent bathophenanthroline (BPhen) reasonably optimized the PR response time and external diffraction efficiency. High sensitivity of 1851 cm2 J−1 with response time of 494 μs and external diffraction efficiency of 23.9% were achieved at 532 nm and 60 V μm−1 by reducing the content of PTAA and increasing the contents of TAA and BPhen. Decreasing the amount of PTAA and increasing the contents of TAA and BPhen lowered the absorption coefficient, resulting in the high external diffraction efficiency. The narrower distribution of the electronic density of states (DOS) for PTAA/TAA (43.5/20 and 33.5/30) also contributed to the shorter PR response time of hundreds of microseconds.

Flexible All-Organic Photorefractive Devices

The objective of the present study is to demonstrate and evaluate the photorefractive (PR) performance of an all-organic PR device with a self-assembled monolayer (SAM)-modified organic conductive electrode of PEDOT:PSS coated on polyethylene terephthalate (PET). The PR composite consisted of a triphenylamine-based photoconductive polymer: poly(4-(diphenylamino)benzyl acrylate) (PDAA), a triphenylamine photoconductive plasticizer: (4-(diphenylamino)phenyl)methanol (TPAOH), a nonlinear optical dye based on aminocyanostyrene: (4-asacycloheptylbenzylidenemalononitrile) (7-DCST), and soluble fullerene: [6,6]-phenyl C61 butyric acid-methyl ester (PCBM). For comparison with the all-organic PR device, the PR performances using PET/ITO, glass/ITO, and glass/PEDOT:PSS substrates were also evaluated. At an applied electric field of 40 V μm–1, the diffraction efficiency and the response time of the PR device using the PET/PEDOT:PSS-SAM substrate were 21.9%, and 390 ms, respectively. As a result of repeating bending ...