Photoinduced Charge Transport Research Articles

Inversion of the Photogalvanic Effect of Conductive Polymers by Porphyrin Dopants

Conductive polymers are widely used as active and auxiliary materials for organic photovoltaic cells due to their easily tunable properties, high electronic conductivity, and light absorption. Several conductive polymers show the cathodic photogalvanic effect in pristine state. Recently, photoelectrochemical oxygen reduction has been demonstrated for nickel complexes of Salen-type ligands. Herein, we report an unexpected inversion of the photogalvanic effect caused by doping of the NiSalen polymers with anionic porphyrins. The observed effect was studied by means of UV-Vis spectroscopy, cyclic voltammetry and chopped light chronoamperometry. While pristine NiSalens exhibit cathodic photopolarization, doping with porphyrins inverts the polarization. As a result, photoelectrochemical oxidation of the ascorbate proceeds smoothly on the NiSalen electrode doped with zinc porphyrins. The highest photocurrents were observed on NiSalen polymer with o-phenylene imine bridge, doped with anionic zinc porphyrin. Assuming this, porphyrin serves both as a catalytic center for the oxidation of ascorbate and an internal electron donor, facilitating the photoinduced charge transport and anodic depolarization.

Polaron Trapping and Migration in Iron-Doped Lithium Niobate

Photoinduced charge transport in lithium niobate for standard illumination, composition and temperature conditions occurs by means of small polaron hopping either on regular or defective lattice sites. Starting from Marcus-Holstein’s theory for polaron hopping frequency we draw a quantitative picture illustrating two underlying microscopic mechanisms besides experimental observations, namely direct trapping and migration-accelerated polaron trapping transport. Our observations will be referred to the typical outcomes of transient light induced absorption measurements, where the kinetics of a polaron population generated by a laser pulse then decaying towards deep trap sites is measured. Our results help to rationalize the observations beyond simple phenomenological models and may serve as a guide to design the material according to the desired specifications.

In Situ Spectroelectrochemical Investigation of Perovskite Quantum Dots for Tracking Their Transformation

All-inorganic lead-halide perovskite quantum dots (PQDs) (CsPbX3, where X is Cl, Br, or I) have been used successfully in optoelectronic applications, such as solar cells, light-emitting diodes, photocatalysts, and lasers. These PQDs work under electrochemical bias and/or illumination with charge separation/collection by interacting with the charge-transport medium. In this study, we discuss the spectroelectrochemical characteristics of PQDs to understand the oxidation and reduction processes that occur during photoinduced charge transport or charge injection under electrochemical conditions. We also found that the PQDs underwent irreversible transformation to the precursor state of plumbate complexes under electrochemical conditions. Furthermore,in situspectroelectrochemical analysis demonstrated that hole-mediated electrochemical oxidation of PQDs resulted in their irreversible transformation. Finally, the results presented herein contribute to our understanding of the charge-transfer-mediated process in PQDs and enhance their application potential in optoelectronic devices.

Direct Evidence of Photoinduced Charge Transport Mechanism in 2D Conductive Metal Organic Frameworks.

Conductive metal organic frameworks (MOFs) represent a promising class of porous crystalline materials that have demonstrated potential in photo-electronics and photocatalytic applications. However, the lack of fundamental understanding on charge transport (CT) mechanism as well as the correlation of CT mechanism with their structure hampered their further development. Herein, we report the direct evidence of CT mechanism in 2D Cu-THQ MOFs and the correlation of temporal and spatial behaviors of charge carriers with their photoconductivity by combining three advanced spectroscopic methods, including time resolved optical and X-ray absorption spectroscopy and terahertz spectroscopy. In addition to Cu-THQ, the CT in Cu/Zn-THQ after incorporating Zn2+ guest metal was also examined to uncover the contribution of through space pathway, as the presence of the redox inactive 3d10 Zn2+ is expected to perturb the long range in-plane CT. We show that the hot carriers in Cu-THQ generated after photoexcitation are highly mobile and undergo fast localization to a lower energy state (cool carriers) with electrons occupying Cu center and holes in ligands. The cool carriers, which have super long lifetime (>17 ns), are responsible for the long-term photoconductivity in Cu-THQ and transport through the O-Cu-O motif with negligible contribution from interlayer ligand π-π stacking, as incorporation of Zn2+ in Cu-THQ significantly reduced photoconductivity. These unprecedented results not only demonstrate the capability to experimentally probe CT mechanism but also provide important insight in the rational design of 2D MOFs for photoelectronic and photocatalytic applications.

Observation of inter-layer charge transmission resonance at optically excited graphene–TMDC interfaces

The transfer of charge carriers across the optically excited hetero-interface of graphene and semiconducting transition metal dichalcogenides (TMDCs) is the key to convert light to electricity, although the intermediate steps from the creation of excitons in TMDC to the collection of free carriers in the graphene layer are not fully understood. Here, we investigate photo-induced charge transport across graphene–MoS2 and graphene–WSe2 hetero-interfaces using time-dependent photoresistance relaxation with varying temperature, wavelength, and gate voltage. In both types of heterostructures, we observe an unprecedented resonance in the inter-layer charge transfer rate as the Fermi energy (EF) of the graphene layer is tuned externally with a global back gate. We attribute this to a resonant quantum tunneling from the excitonic state of the TMDC to EF of the graphene layer and outline a new method to estimate the excitonic binding energies (Eb) in the TMDCs, which are found to be 400 meV and 460 meV in MoS2 and WSe2 layers, respectively. The gate tunability of the inter-layer charge transfer timescales may allow precise engineering and readout of the optically excited electronic states at graphene–TMDC interfaces.

Photo-induced carbocation-enhanced charge transport in single-molecule junctions.

We report the first example of photo-induced carbocation-enhanced charge transport in triphenylmethane junctions using the scanning tunneling microscopy break junction (STM-BJ) technique. The electrical conductance of the carbocation state is enhanced by up to 1.5 orders of magnitude compared to the initial state, with stability lasting for at least 7 days. Moreover, we can achieve light-induced reversible conductance switching with a high ON–OFF ratio in carbocation-based single-molecule junctions. Theoretical calculations reveal that the conductance increase is due to a significant decrease of the HOMO–LUMO gap and also the enhanced transmission close to the Fermi levels when the carbocation forms. Our findings encourage continued research toward developing optoelectronics and carbocation-based devices at the single-molecule level.

Еxcitation Energy Relaxation Processes Involving Chlorophyll Molecules In Vitro: Solutions and Self-Organized Nanoassemblies

The review presents the main results obtained by the Belorussian scientific school of Acad. G.P. Gurinovich and his followers in the in the research on the spectral kinetic properties and photonics of pigment-pigment interactions of chlorophyll and its close analogs in vitro (from highly concentrated solutions to structurally organized heterogeneous nanoassemblies of various morphology), as well as on the interactions of multiporphyrin nanoassemblies with molecular oxygen. The adequacy of various theoretical models that describe the primary processes of photosynthesis (electronic excitation energy transfer and photoinduced charge transport) at short interchromophore distances, considering the properties of the medium and the conformational dynamics of interacting subunits, is substantiated. Application-oriented aspects concerning the use of multicomponent nanostructures on the basis of photostable tetrapyrrole compounds with controllable electron-transport properties and conformational mobility for the development of element base in molecular electronics, photonics, medicine, and nanobiotechnology are considered in brief.

Oxygen vacancy self-doped black TiO2 nanotube arrays by aluminothermic reduction for photocatalytic CO2 reduction under visible light illumination

In this work, black titania nanotube arrays (B-TiO2NTAs) were prepared by aluminothermic reduction of anodized TiO2-NTAs. It was found that the oxygen partial pressure atmicro-region of TiO2-NTAs surfacewas critical for the creation of black TiO2 NTAs. The oxygen vacancies in the prepared B-TiO2 NTAs induced new defect energy levels in the band gap of TiO2, which reduced the band gap and broadened their visible light absorption. Furthermore, the oxygen vacancies could also act as the catalytic sites and accelerate surface reactions for the photocatalytic reduction of CO2 to CO, which also was proved by isotope experiment. Especially, B-TiO2 NTAs annealed at 600℃ showed an excellent photocatalytic CO2 reduction to CO performance with the yield of 185.39 μmol g−1 h-1 under visible light illumination because the oxygen vacancy self-doping largely enhanced three key factors in this process, including photoinduced charge generation, charge separation and transportation and interfacial reaction. This facile and versatile method could be potentially used for large scale production of colored TiO2 with a high photocatalytic CO2 reduction capability in the visible light illumination.

Light-controlled molecular resistive switching ferroelectric heterojunction

Abstract Molecular ferroelectrics have attained significant advancement as a promising approach towards the development of next-generation non-volatile memory devices. Herein, the semiconducting-ferroelectric heterojunctions which is composed of molecular ferroelectrics (R)-(−)-3-hydroxlyquinuclidinium chloride together with organic charge transfer complex is reported. The molecular ferroelectric domain provides polarization and bistability while organic charge transfer phase allows photo-induced charge generation and transport for photovoltaic effect. By switching the direction of the polarization in the ferroelectric phase, the heterojunction-based devices show non-volatile resistive switching under external electric field and photocurrent/voltage induced by light excitation, stable fatigue properties and long retention time. Overall, the photovoltaic controlled resistive switching provides a new route for all-organic multiphase non-volatile memories.

Ultrasonically sprayed-on perovskite solar cells-effects of organic cation on defect formation of CH3NH3PbI3 films

Methylammonium lead iodide (CH3NH3PbI3) based perovskite having low degrees of the disorder is of great interest for optoelectronic and photovoltaic applications. In this work, a layer of CH3NH3PbI3 was successfully prepared using an ultrasonically sprayed-nebulous method. Changes in structural and optical properties alongside with photo-induced charge separation and transportation behavior were systematically studied. The surface photovoltage spectra reveal a significant reduction of the density of deep defect states as the organic content was increased. It was observed that the measured values of Urbach energies decrease from 33.36 to 28.24 meV as the amount of organic content was increased to an optimum value. The best perovskite solar cells obtained using the sprayed-on approach exhibited a Jsc of 16.54 mA/cm2, a Voc of 0.99 V, and a FF of 62.4, resulting in an overall PCE of 10.09%.

Light‐Stimulated Charge Transport in Bilayer Molecular Junctions for Photodetection

AbstractEleven bilayer molecular junctions (MJs) consisting of two different 5–7 nm thick molecular layers between conducting contacts are investigated to determine how orbital energies and optical absorbance spectra of the oligomers affect the photocurrent (PC) response, the direction of photoinduced charge transport, and maximum response wavelength. Photometric sensitivity of 2 mA W−1 and a detection limit of 11 pW are demonstrated for MJs, yielding an internal quantum efficiency of 0.14 electrons per absorbed photon. For unbiased MJs, the PC tracks the absorption spectrum of the molecular layer, and is stable for >5 h of illumination. The organic/organic (O/O) interface between the molecular layers within bilayer MJs is the primary determinant of PC polarity, and the bilayer MJ mechanism is conceptually similar to that of a single O/O heterojunction studied in bilayers of much greater thickness. The charge transport direction of the 11 MJs is completely consistent with hole‐dominated transport of photogenerated carriers. For MJs illuminated while an external bias is applied, the PC greatly exceeds the dark current by factors of 102 to 105, depending on bias, bilayer structure, and wavelength. The bilayer MJs are amenable to flexible substrates, and may have applications as sensitive, wavelength‐specific photodetectors.

Synthesis of an Ag2WO4/Ti3C2 Schottky composite by electrostatic traction and its photocatalytic activity

The reduction and low current-carrying charge of Ag2WO4 limit its application in photocatalysis; thus, a cocatalyst that can improve the stability of Ag2WO4 is urgently needed. Here, we successfully synthesized an Ag2WO4/Ti3C2 composite material and found that the presence of conductive Ti3C2 greatly enhanced the catalytic activity and corrosion resistance of Ag2WO4. Notably, the properties exhibited by the cocatalyst resulted in excellent application prospects that solve the above problems as follows: (1) The large surface area and exposed groups of Ti3C2 not only make the heterogeneous Ag2WO4 catalyst more practical to prepare but also serve as possible growth sites for additional nanostructures, and (2) the metallic conductivity of Ti3C2 ensures efficient photoinduced charge transport and increases the charge-carrier lifetime of Ag2WO4/Ti3C2 (34.5 μs). Notably, the Ag2WO4/Ti3C2 composite has good photocatalytic activity. The photocatalytic removal rates for tetracycline hydrochloride (TC) and sulfadimidine (SFE) were 62.9% and 88.6%, respectively. The experimental results showed that this Schottky heterojunction can be used as a photocatalytic material.

Photoconductive Core–Shell Liquid‐Crystals of a Perylene Bisimide J‐Aggregate Donor–Acceptor Dyad

A novel core-shell structured columnar liquid crystal composed of a donor-acceptor dyad of tetraphenoxy perylene bisimide (PBI), decorated with four bithiophene units on the periphery, was synthesized. This molecule self-assembles in solution into helical J-aggregates guided by π-π interactions and hydrogen bonds which organize into a liquid-crystalline (LC) columnar hexagonal domain in the solid state. Donor and acceptor moieties exhibit contrasting exciton coupling behavior with the PBIs' (J-type) transition dipole moment parallel and the bithiophene side arms' (H-type) perpendicular to the columnar axis. The dyad shows efficient energy and electron transfer in solution as well as in the solid state. The synergy of photoinduced electron transfer (PET) and charge transport along the narcissistically self-assembled core-shell structure enables the implementation of the dye in two-contact photoconductivity devices giving rise to a 20-fold increased photoresponse compared to a reference dye without bithiophene donor moieties.

Photoconductive Core–Shell Liquid‐Crystals of a Perylene Bisimide J‐Aggregate Donor–Acceptor Dyad

AbstractA novel core–shell structured columnar liquid crystal composed of a donor‐acceptor dyad of tetraphenoxy perylene bisimide (PBI), decorated with four bithiophene units on the periphery, was synthesized. This molecule self‐assembles in solution into helical J‐aggregates guided by π–π interactions and hydrogen bonds which organize into a liquid‐crystalline (LC) columnar hexagonal domain in the solid state. Donor and acceptor moieties exhibit contrasting exciton coupling behavior with the PBIs’ (J‐type) transition dipole moment parallel and the bithiophene side arms’ (H‐type) perpendicular to the columnar axis. The dyad shows efficient energy and electron transfer in solution as well as in the solid state. The synergy of photoinduced electron transfer (PET) and charge transport along the narcissistically self‐assembled core–shell structure enables the implementation of the dye in two‐contact photoconductivity devices giving rise to a 20‐fold increased photoresponse compared to a reference dye without bithiophene donor moieties.

Computationally guided synthesis of (2D/3D/2D) rGO/Fe2O3/g-C3N4 nanostructure with improved charge separation and transportation efficiency for degradation of pharmaceutical molecules

In this study, we designed and successfully prepared all solid state 2D/3D/2D rGO/Fe2O3/g-C3N4 nanocomposite by embedding 3D Fe2O3 nanoparticles on 2D g-C3N4 nanosheets to for 3D/2D Fe2O3/g-C3N4 followed by the addition of 2D rGO nanosheets via a simple hydrothermal technique with the support of response surface methodology for the first time. The formation of this unique 2D/3D/2D heterojunction leads to generate several nanochannels in their interfacial contact for high-speed photoinduced charge transfer. The considerable enhancement in photoinduced charge transportation and migration efficiency resulted in significant visible-light-driven degradation of emerging pharmaceutical condemnations. The 3D/2D Fe2O3/g-C3N4 nanocomposite was optimized by various concentrations of Fe2O3 in g-C3N4, followed by the optimization of rGO concentration in 2D/3D/2D rGO/Fe2O3/g-C3N4 nanocomposite to obtain maximum degradation efficiency. We observed that the 3% of rGO in 4% Fe2O3/g-C3N4 nanocomposite exhibited superior photocatalytic ability, nearly 22 times and 16 times higher than pristine g-C3N4 nanosheets towards tetracycline and ciprofloxacin degradation, respectively. The synergistic effect between 2D/3D/2D g- rGO/Fe2O3/g-C3N4 nanocomposites and the photocatalytic mechanism was well studied through various characterization techniques like XRD, FTIR, SEM-EDX-mapping, HR-TEM, UV–vis DRS, PL, XPS and EPR. In addition, the 2D/3D/2D rGO/Fe2O3/g-C3N4 nanocomposite exhibits excellent recyclability and stability, establishing a promising application in environmental remediation. This research would provide a noteworthy platform for the extensive photocatalytic properties of 2D/3D/2D heterojunction nanocomposite system with enhanced charge migration and separation.

MoSx/CdS nano-heterostructures accurately constructed on the defects of CdS for efficient photocatalytic H2 evolution under visible light irradiation

Exploration of low-cost and earth-abundant photocatalysts for efficient photocatalytic H2 evolution from water is an attractive solution to the global energy and environmental problems. Although MoS2 has been proved as an efficient co-catalyst for the H2 evolution reaction (HER), controllably and accurately constructing MoS2-hybridized photocatalysts at where the electrons extracted still remains a challenge. Here, a facile two-step approach of photoetching-photodeposition has been developed to prepare MoSx/etched-CdS (Mo-Cd(e)-S) with heterostructure accurately constructed on the CdS surface defects. This structure can significantly accelerate photo-induced charge separation and transportation owing to the place of heterostructure construction matching well with that of electrons extracted, numerous active sites introduced, and shorter charge transfer path from bulk to surface. Therefore the charge recombination is significantly retarded. As a result, the Mo-Cd(e)-S exhibits outstanding photocatalytic performance with a state-of-the-art H2 generation rate of 22.5 mmol g−1 h−1 under visible light irradiation (λ ≥ 420 nm) which is 70 times higher than that of pristine CdS. This work opens a new avenue for the low-cost but high efficient photocatalysts development.

Significant Enhancement of Single-Walled Carbon Nanotube Based Infrared Photodetector Using PbS Quantum Dots

We show that the performance of single-walled carbon nanotubes (SWCNTs) based infrared photodetector can be greatly enhanced through the combination with colloidal PbS quantum dots (QDs). To improve the photo-induced charge transport efficiency and the carrier mobility, the colloidal PbS QDs are modified by short-chain inorganic. Under illumination, the light-induced electron-hole pairs can be effectively separated by the internal electric field formed at the interfaces between SWCNTS and PbS QDs, which will lead to the increase of both conductivities in them. Photocurrent is formed under the driving of source-drain voltage ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ds</sub> ) applied by the interdigital finger electrodes. Our hybrid phototransistor achieves a responsivity of 7.2 A/W, a specific detectivity (defined below) of 7.1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> Jones, and a response time of 1.58 ms at the same time under 1550-nm illumination with low intensity. Through gate voltage tuning, the responsivity can be increased to 353.4 A/W. In addition, our hybrid phototransistor is stable, low-cost, and compatible with complementary metal oxide semiconductor, which benefits a lot in real applications.

Dark and photo-induced charge transport across molecular spacers

Dithiol self-assembled molecular layers (SAMs) are exploited as controllable spacers between CdSe light absorbers and a gold collector. Using two sets of molecules, alkanes with different chain lengths and biphenyl derivatives with varied conjugation, the charge transport across corresponding SAMs is investigated by chemically resolved electrical measurements and photoluminescence (PL). Complementarity is found between the photovoltage and the PL intensity as a function of spacer thickness. However, it is not obeyed for varied conjugation, a feature shown to arise from reduced charge selectivity in the transport efficiencies. Interplay between dark and photo-induced mechanisms are revealed, where the emergence of charge traps by itself becomes spacer dependent via a remote effect of the substrate on surface chemical activity.

Photocurrent, Photovoltage, and Rectification in Large‐Area Bilayer Molecular Electronic Junctions

AbstractBilayer donor–acceptor molecular junctions are successfully fabricated with active layer thickness &lt;15 nm to study the mechanism of photo‐induced charge transport. The bilayer devices exhibit lifetimes greater than 1 h with laser irradiation, and a strong dependence of the sign and magnitude of the photovoltage and photocurrent on structure and order of the molecular layers. A distinct feature of the bilayer structure is covalent bonding between the donor and acceptor molecular layers, which permits simultaneous absorption and charge separation across a covalent heterojunction. In addition to providing probes of internal energy levels and charge transport in molecular junctions, photo‐induced currents and voltages may potentially be useful in light‐weight, durable, and flexible photodetectors.

Augmenting Photoinduced Charge Transport in a Single-Component Gel System: Controlled In Situ Gel-Crystal Transformation at Room Temperature.

Smart single-component materials with versatile functions require pre-programming of a higher order molecular assembly. An electroactive supergelator (c=0.07 wt %) triphenylamine core-appended poly(aryl ether) dendron (TPAPAE) is described, where substantial dendritic effects improve the order and crystallinity by switching the local minima from self-assembled molecular wires to thermodynamically favorable global minima of ordered crystals, ripened within the fibers. Controlled in situ phase change at room temperature ultimately stabilized the mixed valence states in the single-component supramolecular assembly with photoluminescence and photoinduced charge transport amplified by two orders of magnitude.