Electron Transport In Films Research Articles

Robust carbon–carbon singly bonded electron acceptors for efficient organic photovoltaics

To obtain photostable organic semiconductors is essential for advancing organic solar cells (OSCs), yet to be explored. Herein, a new type of robust carbon–carbon singly bonded acceptor–donor–acceptor (A–D–A) non-fullerene acceptor (NFA) is developed, namely IDT-SZ, via the Stille coupling between the designed electron-deficient indeno ([2,1-b]thiazole-8-ylidene) malononitrile (A unit, SZ) and indacenodithiophene (D unit, IDT). Different from the classic A–D–A NFAs containing vulnerable exocyclic vinyl groups, the planar IDT-SZ has robust carbon–carbon bond linkages mediated by the nitrogen-sulfur (N…S) noncovalent interaction, exhibiting not only excellent photo- and thermal stabilities in ambient, but also high electron transport in films. As a result, efficient OSCs have been developed from the bulk-heterojunction (BHJ) blend of PM6 polymer donor and IDT-SZ acceptor, representing the best-performed single carbon–carbon bonded A–D–A acceptors so far.

(Invited) Integrated Metal Oxide Hybrid Perovskite Photoelectrode for Efficient Photoelectrochemical Water Splitting

In these present days where developing unconventional sources of energy is a major concern, sunlight-driven water splitting has attracted much attention due to sustainable hydrogen fuel production as a substitute of conventional energy. Although sunlight-driven water splitting is an eco-friendly and interminable energy source, extensive implementation is hindered by the expense of the essential photo anode.1-2 In this contribution, we demonstrate an efficient and low-cost water-splitting cell by combining a charge carrier metal oxide semiconductor - processed multidimensional perovskite thin films photoanodes. Effort has been made to prepare nanostructured morphologies of metal oxide as well as perovskite photoanodes that possesses optimized surface catalytic reactions; as well as high photocurrent by controlling the morphology of the structure.3-4This nanostructure material capable to overcome the shortcoming of the short hole diffusion length and enhancing photocurrent. The spectral absorbance of the materials have been also fine-tuned by controlling dimensionality of the structures as well as through doping. We optimized the necessary parameters for high performance photo electrochemical water splitting, by balancing photon absorption and charge carrier transport. A significant improvement in the state-of-the-art perovskite photo anodes through an enhancement in electron transport in films attained and these porous photoanodes films could be easily modified by metal doping for further enhanced photo electrochemical activity for solar water splitting. This hybrid “modify perovskite nanostructures” composites is different from existing composites in that they (i) are produced using benign solvents and processing techniques, and (ii) possible to modify to introduce virtually any desired functionality tailored to the desired application. Our materials are much simpler to prepare and can easily be modified with virtually any desired functionality to optimize performance in different applications. Specific properties can also be dialled in by choosing the appropriate linking molecule (rigidity/flexibility, stimulus responsiveness, etc.). The flexibility of this approach leads to practically unlimited possibilities, while the technology is based on the most abundantly available in nature. References Y. Kuang, Q. Jia, H. Nishiyama,T. Yamada, A. Kudo, K. Domen, Advanced Energy Materials, 2016, 6, 1501645-51. A. Kudo, K. Omori, H. Kato, Journal of the American Chemical Society , 1999, 121 11459-67. K. Sayama , A. Nomura , Z. Zou , R. Abe , Y. Abe , H. Arakawa , H. Arakawa , Chemical Communications, 2003 , 39 , 2908-2909. K. Kim, P. Thiyagarajan, H.-J. Ahn, S.-I. Kim and J.-H. Jang, Nanoscale, 2013, 5, 6254–6260

Geobacter sulfurreducens pili support ohmic electronic conduction in aqueous solution.

The bacterium Geobacter sulfurreducens is a model biological catalyst in microbial electrochemical devices. G. sulfurreducens forms electrically conductive, electrode-associated biofilms, but the biological structures mediating electrical conduction from cells to the electrodes are a matter of debate. Bacteria in these communities produce a network of fiber-like Type IV pili, which have been proposed to act either as inherent, protein-based electronic conductors, or as electronically inert scaffolds for cytochromes mediating long-range charge transport. Previous studies have examined pilus conduction mechanisms under vacuum and in dry conditions, but their conduction mechanism under physiologically relevant conditions has yet to be characterized. In this work, we isolate G. sulfurreducens pili, and compare the electronic conduction mechanism of both live biofilms and purified pili networks under dry and aqueous conditions. Solid-state I-V characteristics indicate that electronic transport in films of purified pili is representative of conduction in a fiber percolation network. Electrochemical gating measurements in a bipotentiostat device configuration confirm previous results suggesting redox currents dominate live biofilm conduction. Purified pili films, however, exhibit non-redox electronic conduction under aqueous, buffered conditions, and their conductivity increases with decreasing temperature. These findings show that isolated pili possess inherent, non-redox-mediated conductivity consistent with a metallic-like model of charge carrier transport. The results demonstrate an experimental platform for studying electronic transport in biomaterials and suggest that pili serve as an exemplary model for designing bioelectronic interfaces.

A probabilistic model of the electron transport in films of nanocrystals arranged in a cubic lattice

The fabrication of nanocrystal (NC) films, starting from colloidal dispersion, is a very attractive topic in condensed matter physics community. NC films can be employed for transistors, light emitting diodes, lasers, and solar cells. For this reason the understanding of the film conductivity is of major importance. In this paper we describe a probabilistic model that allows the prediction of the conductivity of NC films, in this case of a cubic lattice of Lead Selenide or Cadmium Selenide NCs. The model is based on the hopping probability between NCs. The results are compared to experimental data reported in literature.

Effects of Metal Oxide Modifications on Photoelectrochemical Properties of Mesoporous TiO2 Nanoparticles Electrodes for Dye-Sensitized Solar Cells

Mesoporous TiO2 (m-TiO2) nanoparticles were used to prepare the porous film electrodes for dye-sensitized solar cells, and a second metal oxide (MgO, ZnO, Al2O3, or NiO) modification was carried out by dipping the m-TiO2 electrode into their respective nitrate solution followed by annealing at 500 °C. Experimental results indicated that the above second metal oxide modifications on m-TiO2 electrode are shown in all cases to act as barrier layer for the interfacial charge transfer processes, but film electron transport and interfacial charge recombination characteristics under applied bias voltage were dependent significantly on the existing states and kinds of these second metal oxides. Those changes based on second metal oxide modifications showed good correlation with the current-voltage analyses of dye-sensitized solar cell, and all modifications were found to increase the open-circuit photo-voltage in various degrees, while the MgO, ZnO, and NiO modifications result in 23%, 13%, and 6% improvement in cell conversion efficiency, respectively. The above observations indicate that controlling the charge transport and recombination is very important to improve the photovoltaic performance of TiO2-based solar cell.

Tuning structural, electrical, and optical properties of oxide alloys: ZnO1−xSex

Previously we showed that it is possible to narrow the band gap of zinc oxide from 3.3 to ∼2 eV through the addition of Se. Here, we use thin film samples of ZnO1−xSex grown by pulsed laser deposition to describe in detail the effect of growth parameters (temperature, pressure, and fluence) on the chemistry, structure, and optoelectronic properties of oxide alloys. We analyze the influences of temperature, laser fluence, and pressure during growth on the structure and composition of the films and define the parameter space in which homogeneous ZnO1−xSex alloy films can in fact be synthesized. Electronic transport in films grown under different conditions was characterized by resistivity, thermopower, and Hall effect measurements. We discuss how the electron affinity and native defects in polycrystalline oxide alloys enable reasonable mobilities (∼15 cm2/Vs) relative to their single crystalline counterparts. Finally, we elaborate on the model of optical structure in ZnO1−xSex and discuss the dependence of optical properties on growth temperature and fluence.

Conductive-atomic force microscopy study of local electron transport in nanostructured titanium nitride thin films

Simultaneous local current and topography measurements were made on the surface of titanium nitride thin films by conductive-atomic force microscopy (C-AFM). Two compositions, stoichiometric TiN and sub-stoichiometric TiN 0.76 were investigated. Local variation of current at grain and grain boundaries was examined. The current flow is filamentary in nature, with the number of percolation paths being smaller for sub-stoichiometric titanium nitride. Current-voltage characteristics of stoichiometric TiN reveal that the grain interiors are electrically conductive, while in sub-stoichiometric TiN 0.76 thin film, grains are electrically resistive, i.e., a potential barrier to electron transport exists at the junction between the grain and the grain boundary in sub-stoichiometric TiN 0.76. Therefore, electron transport in this film is due to tunneling through the junction, which leads to increased resistivity. The total resistance of the samples measured using the four probe technique is 1 and 400 kΩ for TiN and TiN 0.76 respectively. In both type of compounds the grain and grain boundary resistances are of the order of MΩ. The grain and grain boundaries are connected in a manner that causes the total resistivity to be lower than the local resistivity.

Vapor treatment of nanocrystalline WO3 photoanodes for enhanced photoelectrochemical performance in the decomposition of water

WO3 mesoporous films were prepared using a sol–gel method to serve as the photoanode for water cleavage in a 1 M HClO4 solution with Pt as the counter electrode. Post-treatment of the as-synthesized WO3 film with methanol and ethanol vapors, especially with methanol, improves the photocurrent during photoelectrolysis of water. Water and hexane vapors have a deleterious influence on the WO3 film. Scanning electron microscopy and X-ray diffraction analyses showed that treatment with methanol vapors did not alter the configuration of the nanocrystalline framework or the WO3 mesoscopic structure. Optical absorption and W L3-edge X-ray absorption near-edge structural analyses also revealed that the W-ion electronic and oxidation states remained unchanged after methanol treatment. However, extended X-ray absorption fine structure analysis of the W L3-edge showed that the coordination number of W6+ sites in the WO3 film significantly increased with the methanol treatment, indicating a corresponding decrease in the defect-state density of the film. The observed increase in the coordination number resulted in a 25% increase in the electron transit rate of the WO3 film and enhanced solar energy conversion by 32% for the photoelectrolysis of water. We conclude that post-treatment with methanol vapor remedies the defect region in nanocrystalline WO3 films, and improves the film's electron transport performance in a photoelectrochemical cell.

Biological synthesis of triangular gold nanoprisms

The optoelectronic and physicochemical properties of nanoscale matter are a strong function of particle size. Nanoparticle shape also contributes significantly to modulating their electronic properties. Several shapes ranging from rods to wires to plates to teardrop structures may be obtained by chemical methods; triangular nanoparticles have been synthesized by using a seeded growth process. Here, we report the discovery that the extract from the lemongrass plant, when reacted with aqueous chloroaurate ions, yields a high percentage of thin, flat, single-crystalline gold nanotriangles. The nanotriangles seem to grow by a process involving rapid reduction, assembly and room-temperature sintering of 'liquid-like' spherical gold nanoparticles. The anisotropy in nanoparticle shape results in large near-infrared absorption by the particles, and highly anisotropic electron transport in films of the nanotriangles.

Electronic transport in films of colloidal CdSe nanocrystals

We present results for electronic transport measurements on large three-dimensional arrays of CdSe nanocrystals. In response to a step in the applied voltage, we observe a power-law decay of the current over five orders of magnitude in time. Furthermore, we observe no steady-state dark current for fields up to 10^6 V/cm and times as long as 2x10^4 seconds. Although the power-law form of the decay is quite general, there are quantitative variations with temperature, applied field, sample history, and the material parameters of the array. Despite evidence that the charge injected into the film during the measurement causes the decay of current, we find field-scaling of the current at all times. The observation of extremely long-lived current transients suggests the importance of long-range Coulomb interactions between charges on different nanocrystals.

Dispersive electron transport in polycrystalline films of CdTe.

Electron transient transport has been studied in CdTe polycrystallilne films by means of the time-of-flight technique. The samples studied were intrinsic CdTe polycrystalline films with a grain size from about 100 nm to 20 \ensuremath{\mu}m. The transit time was measured in the largest-grain-size films. The transient transport has the characteristics of dispersive transient transport as observed in disordered materials such as amorphous semiconductors, glasses, etc. For the smaller-grain-size films it is not possible to determine a transit time in the photocurrent traces, due to the effects of recombination and deep trapping of charge. However, the transport characterization allows the determination of the dispersive nature of the electron transport in these films. In addition, in one kind of these smaller-grain-size films, it was possible to identify the presence of a localized state as being responsible for the photocurrent peak exhibited in the photocurrent traces. This peak is explained by a multiple-trapping model with a single-trapping level along with a recombination center.

Electron transport in a molecularly doped polymer

Electron transport in films of an electron acceptor, n-butyl 9-dicyanomethylenefluorene-4-carboxylate (BDFC) stabilized with an electron donor, N,N'-diphenyl-N-N'-bis(3-methylphenyl)-(1-1'-biphenyl)-4-4'diamine (TPD) and dispersed in bisphenol A polycarbonate polymer matrix was investigated. Electron mobility was determined using the time-of-flight technique and was found to be strongly dependent on the electric field. The trapped space charge, which results in the residual potential, is distributed uniformly in the bulk of the charge-transport layer.

Hole transport in solid solutions of substituted triarylmethanes in bisphenol-A-polycarbonate

The time-of-flight and xerographic discharge techniques have been used to study electronic transport in films of dialkylamine-substituted triarylmethanes in bisphenol-A-polycarbonate binder. The hole mobility has been measured as a function of applied electric field, concentration of the molecules, and temperature. The results indicate that charge transport occurs by hopping among localized states associated with the molecule. In chemical terms, the hole transport involves a series of one-electron oxidation-reduction steps involving the organic molecules. The influence of some substituent groups on the electronic transport in these aromatic diamines has also been studied.