High Photoconductivity Research Articles

Photoelectric properties based on photo-induced electron transfer processes in flavin–porphyrin hetero-type Langmuir–Blodgett films

The photo-induced electron transfer processes were investigated in flavin–porphyrin hetero-type Langmuir–Blodgett (LB) films to clarify the photoelectric properties of metal-insulator-metal (MIM) devices composed of LB films sandwiched within aluminium electrodes. A hetero-type MIM device with a flavin and porphyrin molecular heterojunction (MHJ) showed highly efficient photovoltaic effects and high photoconductivity. In contrast, a homo-type MIM device with flavin and porphyrin LB films exhibited low photovoltaic effects and a short-circuit photocurrent density less than a tenth of that of the MHJ device. Furthermore, the transient photocurrent of the MHJ device showed that the time constant for the charge separation (CS) process in the MHJ device was of a sub-nanosecond order. This was more than two orders of magnitude shorter than the time constant for the CS process in the flavin homo-type MIM device. We concluded that the highly efficient photoelectric properties of the MHJ device were mainly attributable to the fast CS process from the photo-excited flavin to the porphyrin at the MHJ. Considering the long distance between the flavin and porphyrin at the MHJ and the moderate free energy difference, the mechanism underlying the fast CS process might be based on the quantum electron tunneling.

Preparation and characterization of the stable nc-Si/a-Si:H films

High-quality nc-Si/a-Si:H diphasic films with improved stability were prepared b y using the plasma-enhanced chemical vapor deposition technology. In comparison with typical amorphous silicon, the diphasic silicon films possess higher photoc onductivity (two orders larger than that of the amorphous silicon film) and fair ly good photosensitivity(the ratio of the photo-to dark-conductivity is about 10 4) and higher stability (the degradation of the photoconductivity is less than 10% after 24h long light soaking with 50 mW/cm2 intensity at room te mperature ). In addition, the diphasic silicon film has a better light spectra response in the longer wavelength range. The improvement in photoelectronic properties may be attributed to: the existence of the disorder within the amorphous matrix, whi ch breaks the momentum selection rule in the optical transition and, consequentl y, results in the large light absorption coefficient and high photosensitivity; the improved medium range order and low gap states density. Excess carriers gene rated in the amorphous matrix tend to recombine in the embedded crystallites, wh ich suppresses nonradiative recombination within the amorphous matrix and reduce s the subsequent defect creation.

Synergism in photosensitivity for a single-layered photoreceptor comprising crystalline mixture of metal-free phthalocyanine with chloroindium phthalocyanine

The photosensitivity of an organic single-layered photoreceptor comprising crystalline mixture of metal-free phthalocyanine (H 2Pc) and chloroindium phthalocyanine (InClPc) was investigated. The photoreceptor exhibited enhanced photoconductivity in the visible region while still remaining high photoconductivity in the near-infrared region. UV–Vis spectra suggested that there was no detectable interaction between InClPc and H 2Pc molecules in the ground state. X-ray diffraction measurements showed that an aggregate state change of H 2Pc/InClPc took place. Based on XPS, an intermolecular partial charge transfer was observed and taken as the microcosmic mechanism of coordination-enhanced photosensitivity.

Phase-pure a-SiC:H films prepared by closed chamber CVD

a-SiC:H films were prepared by using a new film deposition process, referred to as closed chamber CVD (CC-CVD). The CC-CVD is a layer-by-layer technique, which repeats a cycle of two steps: (1) deposition of ultra-thin a-Si:C:H layer by common rf-plasma CVD (rf-CVD), and (2) reconstruction of the layer, performed at `etching/redeposition' equilibrium within a closed plasma chamber. In contrast to the common a-Si:C:H, the atomic network of the CC-CVD material is predominantly Si–C bonded. Films of a large optical band gap E g are achieved at low CH 4/SiH 4 gas ratios. The dark conductivity is several orders of magnitude higher than that of the common a-Si:C:H. The CC-CVD films also show a high photoconductivity. The film growth mechanism, resulting in a dramatic modification of the a-SiC:H material, is discussed.

Photoconductivity of Regular Low Dimensional Arrays of GaAs Wires

A reactive ion etching (RIE) technique of the surface of GaAs/AlGaAs heterostructures through nitrocellulose mesoscopic regular network precursor was used to produce a novel network-confined 2D system for the study of charge transport. The characteristic dimensions of the fabricated honeycomb GaAs structures (the size of hexagonal cell and the thickness of bonds) were approximately 500 and 50 nm, respectively. The manufactured structures were found to be completely depleted due to RIE damage to the sidewalls. The LED excitation was used in order to increase the conductance of the structures. The dependencies of the photoconductivity on the temperature and on the light intensity in the temperature range 20-120 K were measured. High photoconductivity is believed to be related to inhomogeneity of the investigated structures due to high density of the surface defects.

Donor formation in plasma-deposited amorphous silicon (a-Si:H) by erbium incorporation

Erbium-doped amorphous silicon films have been prepared by plasma decomposition of silane doped with the volatile erbium compound tris(2,4-pentanedionato)-Er(III), which has a more favourable erbium–carbon ratio than previously used material. The films exhibit a rather high n-type dark conductivity ( σ d ≈3×10 −4 S/ cm at T=300 K), a high photoconductivity and a relatively low defect density (N D ≈3×10 17 cm −3) . These properties are similar to those of films doped with approximately 10 ppm phosphine. By co-doping with diborane we obtain compensated (near-intrinsic) a-Si:H(Er) films and p-type samples. These films thus may be suitable for the fabrication of light emitting p–i–n diodes.

Characteristics of Photoconductive MoS2 Films Grown on NaCl Substrates by a Sequential Process

MoS 2 thin film deposition has been achieved on NaCl substrates. The films were obtained by solid state reaction between the constituents in thin film form. Multilayered sequences Mo/S/Mo…Mo/S were annealed under argon flow at 1023 K for half an hour. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron microprobe analysis (EPMA), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and photo-electrical measurements. The films are nearly stoichiometric and crystallized in the expected 2H-MoS 2 structure. Moreover, the films are textured with the c-axis of the crystallites perpendicular to the plane of the substrate. XPS analysis reveals a small oxygen contamination of the films, while Na, but no Cl, was detected at the surface of the samples. The high conductivity and photoconductivity of the films agrees with a high crystalline quality. It is shown that Na is the key to achieve photoconductive films. The crystallization process is discussed in the frame of a van der Waals texturation model (pseudo epitaxy) onto dangling bond sulfur terminated surfaces, assisted and strongly enhanced by Na diffusion through the MoS 2 films thickness.

Temperature Dependence of Two Beam Coupling, Light-induced Erasure Decay and Dark Decay in As-Grown and Reduced BaTiO3:Rh

Two beam coupling, light-induced erasure decay and dark decay are investigated in as-grown and reduced BaTiO3:Rh as a function of temperature. The photorefractive sensitivity increases with temperature, mainly due to the increase of the speed. As deduced from the dark decay measurement, the shallow trap effect decreases while the dark conductivity increases with increasing temperature. These two factors determine how the two beam coupling gain depends on intensity and the intensity dependence power coefficient with response time varies as temperature increases. For the as-grown sample, the photorefractive properties are controlled by the competition between deep and shallow levels when the temperature is below 60°C. Above 60°C, large dark conductivity plays a dominant role. For the sample reduced at an atmosphere of 10-10 atm oxygen partial pressure, the photorefractive properties are controlled by the competition between deep and shallow traps from room temperature to 100°C. For the sample reduced at an atmosphere of 10-14 atm oxygen partial pressure, the photorefractive properties are mainly dependent on the dark conductivity though it has a high photoconductivity.

High Speed Diamond Photoconductive Devices for UV Detection

Deep UV visible blind photoconductive devices can be fabricated on polycrystalline CVD diamond, a material that is intrinsically radiation hard and visible blind. However, the performance of detectors fabricated on as-grown material is insufficient to meet the requirements of many laserbased applications. In this paper, it is shown that sequentially applied post-growth treatments can progressively change both the gain and speed of these devices. Charge sensitive deep level transient spectroscopy (Q-DLTS) and transient photoconductivity (TPC) has been used to study the effect of these treatments on the defect structure of our thin film diamond detector material. For the first time, we report the successful operation of a diamond photoconductive device with linear bias and fluence response characteristics at more than 1 kHz at 193 nm.

High-quality narrow gap (∼1.52 eV) a-Si:H with improved stability fabricated by excited inert gas treatment

Narrow band gap (∼1.5 eV) hydrogenated amorphous silicon (a-Si:H) were fabricated by a chemical annealing technique using noble gases (Ar, He, Ne). Although hydrogen content in the film was reduced to ∼1 atm% and band gap was decreased to 1.52 eV, high photoconductivity and large mobility–lifetime products were maintained and no marked changes in the short-range structure was found. Using these narrow band gap a-Si:H for photoactive layer in n-i-p solar cells, reasonable photovoltaic performances were obtained, i.e., open-circuit voltage of 0.71 V and fill factor of 57%. Also enhanced red response was observed with the 1.58 eV band gap i-layer solar cell prepared on textured substrate.

Synthesis and properties of a novel polyazomethine, the polymer with high photoconductivity and second-order optical nonlinearity

Condensation of N-(3-isopentyl)-3,6-diformylcarbazole and 4-[4-[bis(2-aminoethyl)amino]phenylazo]nitrobenzene gave a new polyazomethine, Cbz1-Azo, which has a good solubility in organic solvents. The polymer possessing carbazole moieties and azo type nonlinear optical chromophores in the polymer backbone showed high photoconductivity and nonlinear optical property.

Photoelectrical and electrical properties of polycrystalline CdxHg1−x Te layers on GaAs substrates

Temperature dependences of electrical conductivity, concentration, and mobility of electrons, as well as photoconductivity spectra and conductivity-illumination characteristics of Cd0.8Hg0.2Te polycrystalline layers grown on GaAs substrates are studied. The features of charge transport and photoconductivity of CdxHg1−xTe/CdTe/GaAs structures are discussed. It is established that a high photoconductivity at a temperature of 300 K and a jump in conductivity-illumination characteristics at high levels of excitation are caused by the influence of electrically active grain boundaries, which produce the potential barriers for the drift and recombination of charge carriers. It is shown within the framework of the semiconductor barrier model with a random potential relief pattern that, for high levels of excitation by the radiation pulses of ruby or neodymium lasers, the height of potential barriers at the grain boundaries lowers due to screening by nonequilibrium carriers.

Photoconductivity of 3,5-dinitrobenzoates of poly[1-(p-methoxyphenyl)penta-1,3-diyn-5-ol] and poly[1-(p-N,N-dimethylaminophenyl)penta-1,3-diyn-5-ol

The 3,5-dinitrobenzoates of poly[1-(p-methoxyphenyl)penta-1,3-diyn-5-ol] and poly[1-(p-N,N-dimethylaminophenyl)penta-1,3-diyn-5-ol] [DN-1,4-PMDO and DN-1,4-PDDO] showed high photoconductivity due to the conjugated main chain and the electron donating and accepting side chains.

Electrical properties before and after light-soaking of hydrogenated amorphous silicon carbide films prepared by the hydrogen radical CVD method

Hydrogenated amorphous silicon carbide (a-Si1-xCx:H) films were prepared by use of the preparation system possessing a hydrogen plasma chamber and a silane/methane plasma chamber (the hydrogen radical chemical vapour deposit (CVD) method) and their electrical properties before and after light-soaking were investigated. The photo- and dark conductivities both before and after light-soaking increased with increasing rf power on the hydrogen plasma side; however, the photosensitivity remained unchanged. The activation energy of the dark conductivity before light-soaking decreased with increasing rf power on the hydrogen plasma side, while that after light-soaking increased. The defect density decreased with increasing rf power on the hydrogen plasma side. The hydrogen radical CVD method was found to be useful for preparing a-Si1-xCx:H films with high photoconductivity and low defect density. The electrical properties obtained in this study were explained by the two-energy-level model. Although the hydrogen radical CVD method mainly reduced the defect density of the lower energy level, it also reduced that of the upper energy level.

Synthesis of photoconducting nonlinear optical side-chain polymers containing carbazole derivatives

Two kinds of multifunctional monomers, 2-methyl-acrylic acid 6-[3-[2-cyano-2-(4-nitrophenyl)-vinyl]-carbazol-9-yl]hexyl ester (M1) and 2-methyl-acrylic acid 6-[3-[2-(4-nitrophenyl)-vinyl]-carbazol-9-yl]hexyl ester (M2) were synthesized to possess both photoconductivity and nonlinear optical (NLO) properties. Photoconducting NLO polymers (P1, P2) were obtained by the copolymerization of these multifunctional monomers with methylmethacrylate. Charge-transporting monomer, 2-methyl-acrylic acid 6-[3-(diphenyl-hydrazonomethyl)-carbazol-9-yl]hexyl ester (M3) was also synthesized and copolymerized with M1 or M2 to give the photoconducting NLO polymers (P3, P4) with enhanced photoconductivity. The successful polymerization was identified by spectroscopic means and gel permeation chromatography analysis. These polymers were very soluble in common solvents and high optical quality films could be easily fabricated by spin coating. Second harmonic generation (SHG) measurement confirmed P1 and P2 possessed high NLO activity. From the preliminary measurement of photoconductivity, it was found that P1 possessed distinct photoconductivity. It was also found that P3 and P4 showed much higher photoconductivity than P1, presumably due to the enhanced charge transporting property of M3 structure.

Photoconductivity of 3,5-dinitrobenzoate of poly[1-phenyl-1-penten-3-yn-5-ol] (DN-PPPYO) blended with poly[4-(p-N,N-diphenylaminophenyl)-1-phenyl-1-buten-3-yne] (PPAPBEY) as a hole transporting polymer

The 3,5-dinitrobenzoate of poly[1-phenyl-1-penten-3-yn-5-ol] (DN-PPPYO) blended with a charge transporting polymer, poly[4-(p-N,N-diphenylaminophenyl)-1-phenyl-1-buten-3-yne] (PPAPBEY), and polystyrene shows high photoconductivity due to the high concentration of the triphenylamine (TPA) moiety and the fully conjugated backbone as verified by xerographic measurements.

Photoeffects in cobalt doped pyrite (FeS2) films

By indiffusion of a thin metallic cobalt layer into a pyrite film deposited by metal organic chemical vapour deposition (MOCVD), cobalt doped pyrite (FeS2) films have been prepared. The cobalt in these films acts as a donor and transforms the originally p-type into n-type conductivity. To our knowledge this is the first time that n-type pyrite films have been prepared. Compared to the undoped p-type pyrite films, the cobalt-diffused films exhibit a much higher photoconductivity, as revealed by time resolved microwave conductivity analysis.From Hall and conductivity measurements a charge carrier concentration of about 1020cm−3 and a Hall mobility of about 1.5cm2/(Vs) was calculated. This has to be compared with p-type pyrite films which do not show a Hall mobility above 0.1cm2/(Vs), the detection limit of our Hall system. By analytical techniques (Rutherford backscattering and photoelectron spectroscopy) it was confirmed that the increase of the photoactivity is a bulk property of the pyrite films and not merely due to a surface passivation (for instance, due to metallic CoS2). The presented results stimulate further experiments on in-situ-doping of pyrite by MOCVD and open the opportunity for the preparation of pn-junctions and pn-solar cells with pyrite.

高分子チャンピオンデータ 光導電性が最も大きい高分子は

高分子チャンピオンデータ 光導電性が最も大きい高分子は

Defects in CdTe-Based Photodetectors

AbstractThe photoluminescence spectra, defect composition and optoelectronic properties of chlorine doped CdTe monocrystals, thick layers and thin films are investigated. It is supposed that the complex defect (VCd-2ClTe) is a neutral defect that causes high resistivity of Cl doped CdTe. This complex can dissociate into two charged defects (VCd-ClTe) and ClTe at 300 K. Acentre (VCd-ClTe) is a stable defect that is responsible for p-type conductivity of Cl doped CdTe. Depending on the C1 concentration high resistivity, high photoconductivity or high p-type conductivity can be formed in CdTe that is only chlorine doped.

The Optimization of the Deposition Variables for High Photoconductivity a-Si:H Films Prepared by Electron Cyclotron Resonance Plasma Chemical Vapor Deposition

The physical parameters of the amorphous silicon films with high photoconductivity of 10-5–10-4 (Ωcm)-1 were optimized in the range of the various experimental conditions using electron cyclotron resonance plasma chemical vapor deposition. Films with high photoconductivity shared the following properties with each other: the optical band gap was 1.7–1.8 eV, FWHM was below 75, the ratio of dihydride to the monohydride was 1–1.5 and hydrogen content was 18–24 at.%.