Enhancement Of Photoconductivity Research Articles

Electrical and Optical Properties of Conducting Polymer - Fullerene and Conducting Polymer - Carbon Nanotube Composites

Various novel photo-physical properties such as a drastic photoluminescence quenching and photoconductivity enhancement and a photo-induced enhancement of low-field microwave absorption due to a highly effective photo-induced electron transfer have been observed in various conducting polymers doped with fullerenes and also acceptor-type molecules and conducting polymers. New types of junction devices utilizing effective charge separation at the interface of conducting polymer/C60, C60-doped conducting polymer/C60 and acceptor type conducting polymer / donor type conducting polymer have been proposed and their novel characteristics have been demonstrated. Novel concepts such as an interpenetrating network, a condensed interface and the effect of introduction of photo-harvesting antenna molecule at the interface have been discussed. Nano-composite films of carbon nanotube (NT) and conducting polymer were prepared. With increasing the volume fraction of NT in conducting polymer the conductivity increases drastically at relatively low concentration of NT, which can be explained in terms of conduction by percolation process. Enhancement of photoconductivity of composite films has been found at near percolation threshold. Heterojunctions made of carbon NT and conducting polymer have also been found to be photosensitive.

Effects of Doping of Fullerene Derivative in a Self-Assembled Multilayer of Conducting Polymers

Thermally labile Diels-Alder adduct of C60 and 8-(9-anthryl)-7-oxaoctanoic acid (AOOA), C60AOOA2, has been introduced into a self-assembled multilayer of positively and negatively charged conducting polymers. C60AOOA2 of high concentration in a polymer deposition solution suppresses the multilayer formation and its low concentration solution yields the C60AOOA2-doped multilayered structure. Thermal treatment of the doped multilayer results in marked enhancement of photoconduction with C60AOOA2 doping. Thermal decomposition of C60AOOA2 into C60 and the difference in the electronic accepting natures of C60 and C60AOOA2 are discussed to be the origins of the enhanced doping effect in the photoconduction of the conducting polymer multilayer film.

Transport properties of Ni–WS2 photoconductive thin films

It is shown that the annealing under Ar of sputtered WSx amorphous films deposited on Ni coated substrates gives bidimensional polycrystalline 2H–WS2 films. Ni enhances the formation of large crystallites. The temperature dependence of the mobility and its dependence versus the Ni content clearly show that transport properties are governed by grain boundaries. A basic grain boundary model like the model of Seto is well suited to explain the electrical properties. The photoconductivity enhancement resulting from nickel is assigned to a decrease of the number of the electrical barriers. However, noise measurements are not consistent with Hall measurements; a model is thus proposed to explain the 1/f noise excess in relation to the traces of Ni–W revealed by x-ray diffraction.

Charge Transfer in Fullerene-Conducting Polymer Compositex: Electronic and Excitonic Properties

C60 doping into conducting polymer with highly extended π-electron system in the main chain induces remarkable quenching of photoluminescence in conducting polymer and drastic enhancement of photoconductivity. These results can be explained in terms of photo-induced charge transfer between conducting polymer and C60. That is, photoexcited excitons or exciton-polarons on conducting polymer are effectively dissociated at C60 molecules transferring electrons to C60. Photoexcitation of C60 results in the transfer of hole from C60 to conducting polymer. These novel C60 doping effects have been observed not only in conducting polymers with non-degenerated ground state structures but also those with degenerated ground state structure such as di-substituted acetylene polymers with solitonic electronic systems. Highly effective photo-induced charge transfer has been also observed in conducting polymer/C60 heterojunctions, which are interpreted as donor (D)-acceptor (A) photocell. Based on this finding we have demonstrated an organic photovoltaic cell with D-A double heterojunction, Al/C60/OEP/conducting polymer/TTO, in which OEP is octaethylporphine as an light absorbing antenna molecule. Novel characteristics have also been observed in various other junction devices utilizing C60 doped conducting polymer. Granular and multiphase superconductivity has been found in C60-conducting polymer-alkali metal composites. Effect of other type of fullerenes such as C70, modified C60 and C60 polymers, and also effect of C60 doping in polysilanes and their derivatives have also been studied.

Photocurrent enhancement in light-soaked chalcogenide glasses

Photocurrent changes with light soaking have been studied in amorphous ${\mathrm{As}}_{2}{\mathrm{S}(\mathrm{S}\mathrm{e},\mathrm{T}\mathrm{e})}_{3}$ and Se and crystalline ${\mathrm{As}}_{2}{\mathrm{S}}_{3}$ and Se at 10--300 K. In the chalcogenide glasses the photoconductive degradation occurs at 300 K, while at low temperatures photocurrents in subband-gap spectral regions increase with light soaking. In the crystalline samples, no such changes are detected. The photoconductive enhancement appears to be related to the photoinduced midgap absorption, and the microscopic mechanism is discussed in a unified way. The observations are also compared with those in the Staebler-Wronski effect in amorphous Si:H films.

Indirect enhancement of PbS photoconductivity by ferroelectric field effect in a PbS/PbTiO3/Si heterostructure

A lead sulphide/lead titanate/silicon (PbS/PbTiO3/Si) heterostructure was manufactured by successive deposition of PbTiO3 and PbS thin film on a single crystalline, p-type, Si substrate. Chemical methods were used for deposition. A three electrode configuration was used to control the photoconductivity of PbS thin film by field effect, through the ferroelectric PbTiO3 thin film. The spectral distribution of the photoconductive signal shows two maxima, situated at 1.1 and 2.45 μm. It was found that the photoconductive signal at 1 μm varies of about eight times when the voltage applied on the Si substrate is varied between −1 and +1 V. At 2 μm the photoconductive signal is almost independent of the applied voltage on the Si substrate. The observed results are explained considering a ferroelectric field effect by which the photogenerated carriers in Si influence the photoconductive signal in the PbS thin film.

Computer simulation of photoinduced conductivity in oriented polymer/fullerene films

Monte Carlo computer simulations for photoconducting polymers with nondegenerated ground states/ fullerene films have been performed. The dependencies of photoconductivity and concentrations of photocarriers on fullerene concentrations have been studied. The enhancement of photoconductivity and the persistent conductivity, which have been observed in experiments, were confirmed by computer simulations.

Growth and properties of CdS thin films deposited from aqueous solutions, using different cadmium salts

Cadmium sulphide (CdS) thin films were deposited on glass substrates using the homogeneous precipitation technique and different cadmium salts. The structure of the films was analysed using scanning electron microscopy (SEM). Thermally stimulated current (TSC) measurements had revealed the existence of several trapping levels in as deposited and annealed films. The activation energy of these traps was determined from TSC measurements. The following values were found for CdS films deposited from cadmium acetate: E 1 = 0.3 eV; E 2 = 0.45 eV, E 3 = 0.51 eV. For CdS films deposited from cadmium chloride several trapping levels were found with the activation energies between 0.04 and 0.33 eV. A considerable enhancement of photoconduction after annealing in air was found. The experimental results were explained considering a sensitizing process due to the chemisorbtion of the oxygen.

A-Si:H and a-SiGe:H Alloys Fabricated Close to Powder Regime of RF PECVD

ABSTRACTWe have been observing that a-Si:H and a-SiGe:H films deposited under high chamber pressure or close to powder regime (500 to 2000 mT for a-Si:H and 200 to 1000 mT for a-SiGe:H) show many new features : the mobility lifetime product is 10 to 100 times higher and the density of states above Fermi level of a-Si:H and a-SiGe:H ([Ge] < 0.20) is lower than that of standard samples. This enhancement in photoconductivity can neither be attributed to autodoping nor to creation of sensitization centers in the samples. Hydrogen bonding of these films, is mostly monohydride and the density of microstructural (polyhydrides and microvoids) defects is low. Evidence for the presence of nanocrystals is noted. Though the films seems amorphous by all conventional techniques, they crystallize easily under low laser intensity. Kinetics of light induced degradation is very fast (less than 20 hrs), for a-Si:H and the value of photoconductivity in the light soaked state is comparable to that of the standard samples in the annealed state. The process gas utilization efficiency in this regime is also higher than in the low power and pressure regime.

Enhancement of persistent photoconductivity by uv excitation in GdBa2Cu3O6.3.

Enhancement of persistent photoconductivity by uv excitation in GdBa2Cu3O6.3.

Electrical and optical properties of molecularly doped conducting polymers

Novel electrical and optical characteristics have been observed in conducting polymers doped with molecular dopants such as fullerenes (C 60, C 70 etc.), photochromic dyes and another (guest) conducting polymer. Highly effective photo-induced charge transfer results in various effects such as photoluminescence quenching, photoconductivity enhancement, electroluminescence quenching and persistent photoconductivity which have been observed in fullerene-doped conducting polymers. Unique photo-sensitive characteristics have also been found in conducting polymer/ fullerene systems. Photoluminescence and photoconductivity in conducting polymers doped with photochromic dyes have been changed dramatically by photo-induced isomerization of photochromic dyes and memory effects in both dark conductivity and photoconductivity have been observed. We also present here some results on conducting polymer-conducting polymer mixtures and discuss general features of such composites. These characteristics can be explained by the asymmetry of the relative electronic energy states of conducting polymer and molecular dopants on account of self-trapping effects. Superconductivity evolved upon alkali metal doping of C 60-conducting polymer composites, and has been confirmed by low-field microwave absorption (LFMA) and SQUID magnetometry measurements. The origin of superconducting phases in this doubly doped conducting polymer is discussed.

Effect of hydrogen dilution on the deposition rate of hydrogenerated amorphous silicon films in a modified pulsed plasma discharge

Hydrogen dilution effects on the growth of a-Si:H films in a modified pulsed plasma discharge are studied for two different silane flow conditions with hydrogen dilution ranging from 0% to 80%. The increase of deposition rate (rd) due to hydrogen dilution is attributed mainly to changes in electron density (ne) and electron decay time constant (τe). Concurrently, it appears that hydrogen dilution mitigates the deleterious effects of secondary plasma reactions. Increasing the dwell time of the high power period under hydrogen dilution also increases rd, accompanied by an enhancement of photoconductivity (σph) as compared to films grown under similar conditions without dilution. The decrease in rd at dilution ≳25% is seen as the onset of a process of etching, presumably by atomic hydrogen, and this limits rd irrespective of the availability of the number of silane molecules at such dilutions. Time-resolved optical emission spectroscopy results for H* emission supports the above views.

Photoconductivity of C 60-doped phthalocyanine composites

The influence of the dopant C 60 on the photoconductivity of zinc phthalocyanine (ZnPc) was examined. Measurement of the photoconductive properties shows that C 60 doping leads to a remarkable enhancement of the photosensitivity compared with the corresponding undoped materials. The X-ray powder diffraction pattern and IR spectrum suggest that the enhancement of photoconductivity may be due to a polymorphic transformation and change in molecular interaction in the phthalocyanine composites, as well as the formation of an intermolecular complex through charge transfer interaction between C 60 and ZnPc.

Doping effects of higher fullerene in conducting polymer

Doping effects of higher fullerenes such as C70 on electrical and optical properties of conducting polymers such as poly(3-alkylthiophene), poly(2,5-dialkoxy-p-phenylene vinylene) and poly(9,9-dialkylfluorene) have been studied and compared with those by C60 doping. Fluorescence intensity was quenched and photoconductivity was enhanced upon doping of higher fullerenes in the conducting polymers just as the case of C60 doping. However, there also exist differences in detailed aspects in quenching of photoluminescence and enhancement of photoconductivity between C60 doping and higher fullerenes dopings. These results are interpreted by taking into account the electronic energy states of C60 and higher fullerenes.

Photoferroelectric Effects of Undoped and Extrinsic Ion-Doped PZT Thin Films

ABSTRACTDeposition by aqueous acetate solution (DAAS) technique was used to synthesize undoped and 5 wt% Cr-, Mn-, Eu-, or Pr-doped Pb(Zr0.53Ti0.47O3 [PZT] thin films. The dopant was incorporated into PZT either in the precursor coating solution or via thermal diffusion into undoped PZT. X-ray diffraction shows that ion-doped PZT thin films on Pt<l 1 l>/Ti/SiO2/Si<100> (in particular, the Mn- and Eu- doped samples) display better crystallinity and smaller lattice parameters than those on sapphire substrates. The enhancement of photoconductivity at visible wavelengths measured by excitation photocurrent spectroscopy (EPS) goes as Cr-doped∼Mn-doped > Eu-doped ∼ Pr-doped. Only Mn-doped PZT perovskites have the band gap energy red-shifted to 360 nm from 330 nm. The space charge (or photovoltaic) field was estimated to be < 8 v when about 100 v was applied to ion-doped PZT thin films with 260 or 320 nm light. By using laser irradiation at selected wavelengths and switching the polarity of applied bias voltage, the expected changes in resistance (Ry) and remanent polarization (±Pr) states were observed in the P-E hysteresis loops. The possible applications of extrinsic ion-doped PZT thin films in optical memory devices are discussed.

Enhancement of photoconductivity in r.f. glow-discharge-deposited a-SiC: H films doped with nitrogen

Abstract Nitrogen-doped amorphous hydrogenated silicon-carbon alloy (a-SiC: N: H) films have been prepared by introducing nitrogen during the radio frequency (r.f.) glow-discharge decomposition of methane-silane-helium mixtures. The effects of nitrogen on the structural, optical, electrical and optoelectronic properties of the films deposited at various methane-silane flow rates have been investigated. It is found that nitrogen doping enhances the photoconductivity of the films, while the optical band-gap remains unchanged. The incorporation of nitrogen atoms leads to a lower hopping-assisted contribution to the dark d.c. conductivity at low temperatures and increases the conductivity in the thermally activated conduction regimes. Activation energies of the dark conductivity and the photoconductivity are lower in the nitrogen-doped films. These results indicate that nitrogen in the films relaxes the structural disorder and/or decreases the density of defects.

Role of photoinduced defects in amorphous Ge xSe 1− x photoconductivity

The spectral dependence of photoconductivity of amorphous pure Se and GeSe alloys (4 and 8 at.% Ge) evaporated thin films was measured for a spectral range between 250 and 800 nm. A comparison of the Se and GeSe spectra shows that the magnitude of photocurrent decreases as the Ge content increases. This result indicates a correlation between the magnitude of the photocurrent and the density of photoinduced defects. The contribution of these defects to the enhancement of photoconductivity in a specific region of the spectrum as well as the influence of temperature on their density are also discussed.

Quenching and enhancement of photoconductivity in semi-insulating GaAs

The time evolution of low temperature photoconductivity (PC) in Liquid Encapsulated Czochralski Semi-insulating GaAs for below-the- gap energy photons is studied. For a certain range of photon flux densities both the quenching and enhancement part of the PC curve can be observed. It is shown that the transformation of EL2 defects into their metastable states cannot be responsible for the quenching part of PC curve. The good correlation of the PC with deep trap occupancy is established during both quenching and enhancement part of PC curve, leading to the conclusion that filling/emptying of deep traps plays the key role in peculiar time evolution of PC.

Trap-induced photoconductivity in semi-insulating GaAs

Photoconductivity of semi-insulating gallium arsenide illuminated with monochromatic photons in the 0.7–1.8 eV range has been studied. It has been found that photoconductivity strongly depends on the occupancy of deep traps present in the material, so that the photoconductivity measured for the full trap regime is several orders of magnitude larger than one taken when the traps are empty. By selective emptying or filling of traps it was possible to identify the contribution of each particular trap to the photoconductivity enhancement in different temperature intervals.

Photoconductive Enhancement of Degenerate Four-wave Mixing Reflectivity in BSO

Abstract A method of enhancing the degenerate four-wave mixing (DFWM) conjugate reflectivity in photorefractive BSO is described, which uses an auxiliary beam of different wavelength to modify the local conductivity inside the crystal. With an applied external electric field of 6 kV cm−1, and small diameter DFWM beams, the auxiliary enhancing beam is used to increase local conductivity and hence drop essentially the entire voltage across the small DFWM region producing local fields of ⋍30 kV cm−1. A simple model is derived for three separate experimental configurations, and the predictions are compared with experimental results using yellow and blue beams for DFWM and enhancing beams respectively.