High Photoconductivity Research Articles

Directed assembly of optoelectronically active alkyl-π-conjugated molecules by adding n-alkanes or π-conjugated species.

Supramolecular assembly can yield ordered structures by taking advantage of the cumulative effect of multiple non-covalent interactions between adjacent molecules. The thermodynamic origin of many self-assembled structures in water is the balance between the hydrophilic and hydrophobic segments of the molecule. Here, we show that this approach can be generalized to use solvophobic and solvophilic segments of fully hydrophobic alkylated fullerene molecules. Addition of n-alkanes results in their assembly--due to the antipathy of C60 towards n-alkanes--into micelles and hexagonally packed gel-fibres containing insulated C60 nanowires. The addition of pristine C60 instead directs the assembly into lamellar mesophases by increasing the proportion of π-conjugated material in the mixture. The assembled structures contain a large fraction of optoelectronically active material and exhibit comparably high photoconductivities. This method is shown to be applicable to several alkyl-π-conjugated molecules, and can be used to construct organized functional materials with π-conjugated sections.

Effects of H2O2 treatment on the optoelectronic property of poly(3-hexylthiophene) doped with the reduced graphene oxide sheets/Si-nanowire arrays/n-type Si diodes

The effect of H2O2 treatment on the optoelectronic property of the poly(3-hexylthiophene) doped with reduced graphene oxide sheets (P3HT:RGO)/Si-nanowire (SiNW) arrays/n-type Si diode was examined. SiNW surface passivation influences device performance. Compared to P3HT:RGO/SiNWs/n-type Si diodes, P3HT:RGO/H2O2-treated SiNWs/n-type Si diodes exhibit much higher photoconductivity. The results revealed that SiNW surface passivation influences the photoconductivity by reducing the number of electron traps that serve to decrease the photocurrent time constant. High responsivity thus originates from efficient light absorption and carrier collection.

Enhanced vacuum-photoconductivity of chemically synthesized ZnO nanostructures

We report on the enhanced ultraviolet (UV) photoconductivity of zinc oxide (ZnO) nanostructures in vacuum. Nanoparticles and nanorods of ZnO were fabricated using a simple cost-effective solid state grinding method. Morphology of the nanostructures was studied using transmission electron microscopy, while the optical properties were investigated using UV–visible absorption and photoluminescence spectroscopy. The emission spectra of the nanostructures revealed the existence of various native defect states of ZnO and also indicated the presence of surface adsorbed water molecules. In the photoconductivity measurements, although the ZnO nanoparticles exhibited lower photoconductivity in comparison to the nanorods, a similar trend of photoresponse was observed for both the cases. An initial decrease in the photoconductivity followed by a large enhancement was observed in vacuum compared to that in ambient condition. Such unusually increased photoconductivity has been correlated to the desorption of physisorbed water molecules from nanostructure surfaces under vacuum. This desorption is responsible for the rise in dark current and an initial decrease in photoconductivity. Continual UV irradiation in vacuum leads to the desorption of chemisorbed water molecules from the defect sites of the nanostructures, resulting in the occurrence of high photoconductivity.

Effect of cooling oxygen pressure on the photoconductivity in Bi0.9La0.1FeO3 thin films

A direct correlation is observed between the magnitude of the photoconductivity and cooling oxygen pressure in Bi0.9La0.1FeO3 films. With lower oxygen vacancies (VOs) films cooled in higher oxygen pressure show obviously decrease in leakage current. Higher level of VOs led to higher free carrier concentration and hence is believed to be responsible for the increase in higher photoconductivity. It is evident that BLFO films with higher VOs cooled in lower oxygen pressure and/or light illumination induce a higher carrier injection, ni and thus the conductive mechanism is dominated by space-charge-limited conduction.

Photoconductivity of Polymer Composite Films Containing an Mn(III)/Cu(II) Complex

We have studied the optical, photoelectric and dielectric properties of polymer composite films based on polyvinyl butyral with additives of a mixed-metal Mn(III)/Cu(II) complex. We observed high photoconductivity of the films obtained in the region of absorption by the complex. The slow photocurrent rise and relaxation kinetics are connected with the low mobility of the photogenerated charge carriers.

Morphology Control of Nanofibril Donor–Acceptor Heterojunction To Achieve High Photoconductivity: Exploration of New Molecular Design Rule

Donor-acceptor nanofibril composites have been fabricated, and the dependence of their photocurrent response on the structure and morphology of the donor part has been systematically investigated. The nanofibril composites were composed of template nanofibers, assembled from an electron acceptor molecule, perylene tetracarboxylic diimide (PTCDI), onto which (through drop-casting) various electron donor molecules (D1-D4) were coated. The donor molecules have the same π-conjugated core, but different side groups. Due to the different side groups, the four donor molecules showed distinctly different propensity for intermolecular aggregation, with D1-D3 forming segregated phases, while D4 prefers homogeneous molecular distribution within the film. It was found that the nanofibril composites with D4 exhibit the highest photocurrent, whereas those with aggregation-prone D1-D3 exhibited much lower photocurrent under the same illumination condition. Solvent annealing is found to further enhance the aggregation of D1-D3 but facilitate more uniform molecular distribution of D4 molecules. As a result, the photocurrent response of PTCDI fibers coated with D1-D3 decreased after vapor annealing, whereas those coated with D4 further increased. The detrimental effect of the aggregation of donor molecules on the PTCDI fiber is likely due to the enhanced local electrical field built up by the high charge density around the aggregate-nanofiber interface, which hinders the charge separation of the photogenerated electron-hole pair. The results reported in this study give further insight into the molecular structural effect on photoconductivity of hybrid materials, particularly those based on donor-acceptor composites or interfaces, and provide new molecular design rules and material processing guidelines to achieve high photoconductivity.

Composition-dependent ultra-high photoconductivity in ternary CdS x Se1−x nanobelts as measured by optical pump-terahertz probe spectroscopy

We employ optical pump-terahertz probe spectroscopy to investigate the composition-dependent photoconductivity in ternary CdS x Se1−x nanobelts. The observed carrier dynamics of CdS nanobelts display much shorter lifetime than those of ternary CdS x Se1−x nanobelts. This indicates the implementation of CdS nanobelts as ultrafast switching devices with a switching speed potentially up to 46.7 GHz. Surprisingly, ternary CdS x Se1−x nanobelts are found to exhibit much higher photoconductivity than binary CdS and CdSe. This is attributed to the higher photocarrier densities in ternary compounds. In addition, the presence of Se in samples resulted in prominent CdSe-like transverse optical (TO) phonon modes due to electron-phonon interactions. The strength of this mode shows a large drop upon photoexcitation but recovers gradually with time. These results demonstrated that growth of ternary nanostructures can be utilized to alleviate the high surface defect density in nanostructures and improve their photoconductivity. Open image in new window

Insight into the structural, electrical and photoresponse properties of individual Fe:SrTiO3 nanotubes

Titanates are suitable for many applications such as oxygen sensing and tunable HTS (high temperature superconducting) microwave filters. The potential advantages of the nanostructured forms have been however scarcely explored compared to other oxides. In this work, the structural and electrical properties of individual iron-doped strontium titanate nanotubes (Fe:SrTiO3) grown by electrophoretic deposition (EPD) were assessed for the first time, showing high stability and reproducibility. This result paves the way to further development of more complex titanate-based devices, as for instance nanostructured oxygen STFO sensors. From experimental data, it was concluded that the polycrystalline form of Fe:SrTiO3 nanotubes is the major limitation to attain high photoconductivity gains when exposed to UV-light.

A Versatile Light‐Switchable Nanorod Memory: Wurtzite ZnO on Perovskite SrTiO3

AbstractIntegrating materials with distinct lattice symmetries and dimensions is an effective design strategy toward realizing novel devices with unprecedented functionalities, but many challenges remain in synthesis and device design. Here, a heterojunction memory made of wurtzite ZnO nanorods grown on perovskite Nb‐doped SrTiO3 (NSTO) is reported, the electronic properties of which can be drastically reconfigured by applying a voltage and light. Despite of the distinct lattice structures of ZnO and NSTO, a consistent nature of single crystallinity is achieved in the heterojunctions via the low‐temperature solution‐based hydrothermal growth. In addition to a high and persistent photoconductivity, the ZnO/NSTO heterojunction diode can be turned into a versatile light‐switchable resistive switching memory with highly tunable ON and OFF states. The reversible modification of the effective interfacial energy barrier in the concurrent electronic and ionic processes most likely gives rise to the high susceptibility of the ZnO/NSTO heterojunction to external electric and optical stimuli. Furthermore, this facile synthesis route is promising to be generalized to other novel functional nanodevices integrating materials with diverse structures and properties.

SYNTHESIS AND PHOTOSENSITIVE PERFORMANCE OF NANOSTRUCTURED ZnO/DYE HYBRID FILM FOR ENERGY CONVERSION

The ZnO film prepared by doctor blade method are highly porous in presence of structure directing agent PEG and then can be easily sensitized by various molecules. In the present work, the nanostructured ZnO /dye hybrid film prepared by doctor blade method has been investigated in conductivity and photoconductivity measurements in view of applications in dye-sensitized solar cells (DSSC) and in optoelectronics, since electron transport in these devices plays a decisive role for the electron collection efficiency and therefore for the overall efficiency. The absorption of ZnO film sensitized by Rose Bengal dye (RB) has been studied by UV spectroscopy which indicates that after the dye is adsorbed on the ZnO electrode, its absorption spectra showed red-shift in the peak position compared to the absorbance spectra of dye in ethanol. Highly porous ZnO films are found to have high dark conductivity, probably because of a higher n-doping, which is due to higher concentration of Zn atoms in the film. On the other hand, ZnO /RB hybrid film is found to show a much higher sensitivity to illumination with visible light in photoconductivity measurements due to a higher absolute photoconductivity and lower conductivity in the dark.

Improvement of ultraviolet photoresponse of bent ZnO microwires by coupling piezoelectric and surface oxygen adsorption/desorption effects

Localized ultraviolet photoresponse properties of bent ZnO microwires bridging two perfect Ohmic contacts in both atmospheric and high vacuum (8 × 10(-6) torr) environments have been investigated for the first time to explore the bending strain effect on the photoelectrical properties of ZnO. It is found that the ZnO microwire has higher photoconductivity and faster rising speed when photo-excitation is localized at the bending region in an atmospheric environment, while the rising speeds are almost the same when photo-excitations are localized at the bending and straight regions under vacuum. The bending strain induced improvement of the UV photoresponse in air was well explained by considering the coupling of piezoelectric effects and the surface oxygen adsorption/desorption procedure on the bent ZnO microwire. Our results are valuable for designing and fabricating strain modulated photoelectrical micro/nano-devices.

Photoconducting nanocomposite films of PbS nanocrystals in insulating polystyrene

AbstractLead sulfide (PbS)/polystyrene (PS) nanocomposite (NCP) thin films have been synthesized from solution by in situ thermolysis. PbS nanocrystals (NCs) are of 3–5 nm size as determined from Transmission Electron Microscope. The films are p‐type with dark conductivity of ∼10−6 S cm−1 and high photoconductivity with linear current–voltage (I–V) characteristics. The electrical properties are explained on the basis of strong interaction between PbS and PS as revealed by infrared spectroscopy and thermogravimetric analysis (TGA). magnified image

Fast Photorefractive Response in Triphenylamine-based Molecular Glass

Abstract A new bifunctional photorefractive active succinate (TPD-2DCVA) consisting of a tetraphenyldiaminobiphenyl (TPD) unit and 4-dicyanovinylaniline (DCVA) moieties was prepared, which forms stable molecular glass. The glass-transition temperatures (Tg’s) of TPD-2DCVA-based composites were efficiently controlled by the doping of small amount of plasticizers. The compound possesses a high photoconductivity of 2.1 × 10−9 cm Ω−1 W−1 at 50 V µm−1. A gain coefficient of 56.7 cm−1 at electric field of 56 V µm−1 was achieved. Fast response times of 29–37 ms were also obtained at low electric fields for these composites.

Rectification effect in poly-p-xylylene-cadmium sulfide graded nanocomposites

The hybrid poly-p-xylylene-cadmium sulfide nanocomposites characterized by gradients of concentrations and sizes of CdS nanoparticles along the lines of an applied electric field have been synthesized using the vapor deposition polymerization in an inhomogeneous electric field. The maximum concentration of cadmium sulfide can exceed 10 vol %, while the average effective sizes of the nanoparticles depend on the concentration and do not exceed 5 nm. The synthesized thin-film nanocomposites exhibit a quantum confinement effect. According to the estimates obtained from the shift of the absorption band edge, the radius of nanoparticles is 2.7 nm near the negative electrode and 3.5 nm near the positive electrode. It has been found that the sample formed in an electric field of 10 kV/cm manifests a rectification effect, which can be associated with the gradient of nanoparticle sizes. The measurements of current-voltage characteristics and photoconductivity have demonstrated that the synthesized samples possess high photoconductivity. The photocurrent in the sample prepared in an electric field of 10 kV/cm can exceed the dark current by two orders of magnitude, and the rectification effect in this case disappears.

Molecular Photoconductor with Simultaneously Photocontrollable Localized Spins

UV irradiation reversibly switches a new insulating and nonmagnetic molecular crystal, BPY[Ni(dmit)(2)](2) (BPY = N,N'-ethylene-2,2'-bipyridinium; Ni(dmit)(2) = bis(1,3-dithiole-2-thione-4,5-dithiolato)nickelate(III)), into a magnetic conductor. This is possible because the bipyridyl derivative cations (BPY(2+)) trigger a photochemical redox reaction in the crystal to produce a change of ∼10% in the filling of the Ni(dmit)(2) valence band, leaving localized spins on the BPY themselves. In the dark, almost all of the BPY molecules are closed-shell cations, and most of the Ni(dmit)(2) radical anions form spin-singlet pairs; thus, this material is a diamagnetic semiconductor. Under UV irradiation, a photocurrent is observed, which enhances the conductivity by 1 order of magnitude. Electron spin resonance measurements indicate that the UV irradiation reversibly generates carriers and localized spins on the Ni(dmit)(2) and the BPY, respectively. This high photoconductivity can be explained by charge transfer (CT) transitions between Ni(dmit)(2) and BPY in the UV region. In other words, the photoconduction and "photomagnetism" can be described as reversible optical control of the electronic states between an ionic salt (BPY(2+)/[Ni(dmit)(2)](-), nonmagnetic insulator) and a CT complex (BPY(2(1-δ)+)/[Ni(dmit)(2)]((1-δ)-) (δ ≈ 0.1), magnetic conductor) in the solid state.

High Photoconductive Response of Gas-Sensitized Porous Nanocrystalline TiO2 Film in Formaldehyde Ambience and Carrier Transport Kinetics

We propose a gas-sensitized porous nanocrystalline TiO2 film with a potential application in photovoltaic devices and report about the systematic photoconductivity study of it. The quantitative results show that the gas-sensitized TiO2 film in formaldehyde atmosphere exhibits much higher photoconductivity (3–4 orders of magnitude) and longer carrier lifetime than usual. The intriguing performance of the gas-sensitized TiO2 film indicates the distinct charge carrier transport kinetic courses, whose contributions to the photoconductivity are shown in a designed flowchart. From the flowchart, it is clearly found that two electron loss processes, recombination and electron scavenging, are suppressed for the gas-sensitized TiO2 film in formaldehyde gas, leading to large improvements of photoconductivity and carrier lifetime. The results provide the potential of improving efficiency of photovoltaic devices, and measuring photoconductivity under target gas appears to be a useful tool for research on photocatalytic and photoelectrical processes.

Defect induced high photocurrent in solution grown vertically aligned ZnO nanowire array films

Highly oriented and homogeneously distributed single crystalline zinc oxide nanowires (NWs) are fabricated on amorphous glass substrates using soft solution growth approach. The nanowire films and sol-gel grown ZnO films are devised and tested for UV light detection applying four-probe conductivity measurements. As-grown ZnO NWs film device demonstrates three orders enhancement (sensitivity = 440) in conductivity at room temperature under an illumination of 365 nm UV light, while the sol-gel based thick film reveals two orders of enhancement in device conductance. A clear correlation of conductivity and photoluminescence measurements suggest that surface oxygen vacancies (singly charged/Vo+) which render higher green defect luminescence intensity (IG/IUV = 1.8) in ZnO NWs leads to poor dark conductance and higher photo-conductance. Post growth annealing of nanowire arrays either in air (IG/IUV = 0.85) or oxygen ambience (IG/IUV = 0.38) results in reduction of green defects and corresponding suppression of photocurrent. Higher concentration of surface traps also leads to persistent photocurrent due to ionization of oxygen vacancies and creation of perturb host states under UV light excitation.

Solution Synthesis of Ultrathin Single-Crystalline SnS Nanoribbons for Photodetectors via Phase Transition and Surface Processing

We report the solution-phase synthesis and surface processing of ~2-5 μm long single-crystalline IV-VI tin(II) sulfide (SnS) ultrathin nanoribbons, with thicknesses down to 10 nm, and their use in single nanoribbon based photodetectors. The SnS nanoribbons grow via a metastable-to-stable phase transition from zinc blende (ZB) nanospheres to orthorhombic nanoribbons; dual-phase intermediate heterostructures with zinc blende nanosphere heads and orthorhombic nanoribbon tails were observed. Exchange of long, insulating organic oleylamine ligands by short, inorganic HS(-) ligands converts the organic SnS nanoribbons into completely inorganic, hydrophilic structures. Field-effect transistor (FET) devices were made from single SnS nanoribbons, both before and after ligand exchange, which exhibit p-type semiconductor behavior. The SnS single nanoribbon based photodetector devices showed highly sensitive and rapid photocurrent responses to illumination by blue, green, and red light. The switching behavior of photocurrent generation and annihilation is complete within approximately 1 ms and exhibits high photoconductivity gains (up to 2.3 × 10(4)) and good stability. The ON/OFF ratio of the photodetector can be engineered to 80 (4 nA/50 pA) using a small drain current (10 mV) for the all inorganic SnS nanoribbons. This work paves the way for the colloidal growth of low-cost, environmentally benign, single-crystalline narrow band gap semiconductor nanostructures from abundant elements for applications in photodetectors and other nanoscale devices.

Photoconducting Properties of a Unit Nanostructure of ZnO Assembled Between Microelectrodes

The photoconducting properties of a unit microflower of zinc oxide are investigated as a function of wavelength from UV to IR region at constant illumination intensity. Synthesized flowers were trapped in 2 microm gap, between pre-prepared gold microelectrodes, using AC dielectrophoresis. Photocurrent drastically increases upon illumination in the UV region, whereas it gradually reduces when irradiated in visible and IR region. Higher photoconductivity in UV region is correlated to band to band transition upon illumination. In visible region, deep level transitions are expected which intern exhibits comparatively low photocurrent. Photoconduction in IR region is only due to the adsorbed surface oxygen species. This investigation suggests the potential application of ZnO nanostructures for various optoelectronic device applications.

Photoconductivity of doped hexa-alkoxy triphenylene nano composite structures

The photoconductivity of undoped as well as nanostructures of gold chloride doped hexa-alkoxytriphenylene series (HAT4, HAT5 and HAT6) has been studied. After doping with gold chloride, the photoconductivity increased by more than ten times for HAT4 and HAT5 and not for HAT6. The high photoconductivity can be explained in terms of the transition temperatures, the core-core distances and their length of side chains. Photoconductivity of the gold chloride doped hexa-alkoxytriphenylene derivatives are in the region 10−4–10−5Sm−1, which are five orders of magnitude higher than that of undoped compounds. The high NIR absorbance combined with the photoconductivity makes this nanocomposite a potential candidate for organic solar cell applications.