High Photoconductivity Research Articles

Investigation on ultraviolet photoconductivity in p-type ZnO thin films

The authors report on comparative study on ultraviolet (UV) photoconductivity of p-type ZnO:N films and n-type ZnO epilayer. As compared with the ZnO epilayer, the ZnO:N films show much higher photoconductivity but with faster decay. Surface adsorption of C and O, as identified by X-ray photoelectron spectroscopy, is believed to be a major contribution to the UV photoconductivity. The surface adsorption and photodesorption process, combined with a competition between holes and electrons, in p-type ZnO:N, is proposed tentatively, providing qualitative agreement with the observed behaviors.

Synthesis and properties of π conjugated organic molecular one-dimensional nanomaterials

Much attention is turning to nanoscale organic materials, due to many unique properties such as flexibility, high photoconductivity, and nonlinear optical effects that may offer novel applications in nano-optoelectronic devices. We have synthesised new π conjugated organic molecular aggregates using various methods, including template, solid phase organic reaction, thermal evaporation method and organic vapour-solid phase reaction. The dimension-dependent emission properties were observed in the nanowires and nanorods. The approaches are expected to form new general routes for the controlled morphosynthesis of organic molecular materials in restricted dimensions, with controlled size and shape, the solid-state physical properties of which are of great interest.

Hybridization of thiol-functionalized poly(phenylacetylene) with cadmium sulfide nanorods: improved miscibility and enhanced photoconductivity

Molecules of a thiol-functionalized phenylacetylene derivative were assembled on the CdS nanorod surface and copolymerized with phenylacetylene, affording an inorganic semiconductor-conjugated polymer hybrid with excellent solubility and high photoconductivity.

Photoluminescence and photoconductivity in CdTe crystals doped with Bi

Defect levels in CdTe doped with Bi are studied by low temperature photoluminescence, photoinduced current transient spectroscopy, photoconductivity measurements, and optical absorption. Two centers associated with the doping with Bi are reported. The first one, a deep level located at Ev+0.71eV, only present at low dopant concentrations, has donor character and hole-trap properties, and is mainly responsible for the high resistivity and very high photoconductivity of the samples. The second one, an acceptor center located at Ev+0.30eV, assigned to BiTe species, is only present at high dopant concentrations and is mainly responsible for the low resistivity and poor photoconductivity of these samples.

Intermediate type excitons in Schottky barriers of A 3 B 6 layer semiconductors and UV photodetectors

Photoelectric and photovoltaic spectra of Schottky barrier (SB) structures of InSe, GaSe and GaS layered semiconductors (LS) are investigated at quantum energies from the band edge excitons of corresponding materials up to 6.5eV. Spectral dependences of photoconductivity (PC) of photo resistors and barrier structures are strongly different at the quantum energies corresponding to the intermediate type excitons (ITE) observed in these semiconductors. It was suggested that high UV photoconductivity of A3B6 LS is due to existence of high mobility light carriers in the depth of the band structure. It is shown that SB of semitransparent Au-InSe is high sensitive photo detector in UV region of spectra. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Silicon thin films prepared in the transition region and their use in solar cells

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (sigma(ph)), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51 % and a stabilized efficiency of 8.01% (AM 1.5, 100 mw/cm(2)) at room temperature. (c) 2006 Published by Elsevier B.V.

Microcrystalline silicon–germanium alloys for solar cell application: Growth and material properties

Microcrystalline silicon–germanium (μc-Si1−xGex:H) alloy films have been grown by 100-MHz glow-discharge of a SiH4/GeH4/H2 gas mixture. Alloys over a full range of compositions were prepared to gain a comprehensive understanding of their growth and material properties. With increasing GeH4 concentration in the gas-phase, we observed a preferential Ge incorporation behavior in the solid. Growth rate studies revealed that the Ge incorporation efficiency from source gas to solid is about five times greater than for Si at growth temperature of 200°C, which accounts for the variation of alloy composition. With increasing Ge incorporation in the solid, on the other hand, we find a monotonic decrease in photoconductivity, followed by an electrical transition from weak n-type to strong p-type conduction at x>0.7. At x≈0.4, however, we obtained relatively high photoconductivity gains by a factor of 20 and strong infrared response in the solar cell structure. The Ge incorporation behavior and its effect on charge carrier transport are discussed.

Si thin film solar cells using SiH2Cl2 by rf plasma-enhanced chemical vapor deposition

The fabrication of p–i–n Si thin film solar cells using a SiH2Cl2–H2 mixture by rf plasma-enhanced chemical vapor deposition is presented. A novel p-type nanocrystalline Si:H:Cl film was synthesized from H2-diluted SiH2Cl2 and SiCl4 with high conductivity and low optical absorption in the visible and near-infrared regions. The Si:H:Cl films showed a high photoconductivity of order of >10−6S/cm under 100mW/cm2 white light exposure. The efficiency of 6.2% has been achieved in the p–i–n solar cells fabricated from a SiH2Cl2–H2 mixture at Ts of 340°C for the first time despite a use of a single chamber system. Not only SiH4 but also SiH2Cl2 is one of possible candidates for fabricating Si thin film solar cells.

Insights into the high-rate growth of highly crystallized silicon films from inductively coupled plasma of H 2-diluted SiH 4

We have studied microcrystalline silicon (μc-Si:H) formation from inductively coupled plasma (ICP) of monosilane (SiH 4) diluted with hydrogen at a temperature of 250 °C using RF (13.56 MHz) and VHF (60 MHz) power sources and demonstrated the advantage of VHF-ICP for high-rate growth of highly crystallized Si films in comparison to the films deposited from RF-ICP. A growth rate of ∼ 6 nm/s was obtained for films deposited from VHF-ICP with a crystallinity as high as ∼ 5 in the TO-phonon intensity ratio of the crystalline phase to the disordered one. For microcrystalline films with a crystallinity of ∼ 3, the growth rate was enhanced to ∼ 12 nm/s. Optical emission measurements show that the emission intensity from atomic hydrogen generated in the VHF-ICP remains nearly constant in the SiH 4 concentration range from 5% to 22% while the emission intensity in RF-ICP decreases linearly with increasing SiH 4 concentration. It is likely that a sufficient flux of atomic hydrogen to the growing film surface promotes the surface migration of the precursors in VHF-ICP even under high-rate deposition conditions. Highly crystallized films prepared by VHF-ICP show a fairly high photoconductivity in the range of 0.1–0.2 mS/cm with an improved photosensitivity of 50–130 under AM1 (100 mW/cm 2) illumination.

Doping Properties of Polydithienylmethine: A Study on the Correlation between Polymer Chain Length, Spectroscopy, and Transport

(1S)-(+)-10-Camphorsulfonic acid-doped polydithienylmethine was prepared through an acid-catalyzed condensation reaction of alpha,alpha'-di-2-thienyl-(2,2'-bithiophene)-5,5'-dimethanol and was characterized by 1H NMR spectroscopy and size exclusion chromatography (SEC). The electronic and vibrational properties of the resulting polymer thin films vary with the loadings of the (1S)-(+)-10-camphorsulfonic acid. The dark conductivity and drift mobility, which is significantly high, of the polymer thin films were enhanced with increasing doping levels and reached maximum values of 8.0x10(-5) S.cm-1 and 3.5x10(-2) cm2.V-1.s-1, respectively, at a 7 mol % dopant loading. Higher doping levels (>7 mol %) result in nonuniform polymer thin films with degraded optical quality due to the formation of nanocrystalite and thus a decrease in conductivity and drift mobility was observed. The doped polydithienylmethine thin film also exhibited a photoconductivity response with an excitation at 457 nm and the highest photoconductivity (2x10(-4) S.cm-1) was again observed at the 7 mol % doping level. Spectroscopic investigation suggests that the enhanced transport properties can be attributed to polaronic species present. The electronic and vibrational changes which relate to such doping were characterized by electronic absorption spectroscopy, Raman spectroscopy, and FTIR spectroscopy. The changes in transport values can be directly related to the changes we see in our spectroscopic investigations.

Diamond-like carbon thin films by microwave surface-wave plasma CVD aimed for the application of photovoltaic solar cells

The nitrogen doped diamond-like carbon (DLC) thin films were deposited on quartz and silicon substrates by a newly developed microwave surface-wave plasma chemical vapor deposition, aiming the application of the films for photovoltaic solar cells. For film deposition, we used argon as carrier gas, nitrogen as dopant and hydrocarbon source gases, such as camphor (C 10H 16O) dissolved with ethyl alcohol (C 2H 5OH), methane (CH 4), ethylene (C 2H 4) and acetylene (C 2H 2). The optical and electrical properties of the films were studied using X-ray photoelectron spectroscopy, Nanopics 2100/NPX200 surface profiler, UV/VIS/NIR spectroscopy, atomic force microscope, electrical conductivity and solar simulator measurements. The optical band gap of the films has been lowered from 3.1 to 2.4 eV by nitrogen doping, and from 2.65 to 1.9 eV by experimenting with different hydrocarbon source gases. The nitrogen doped (flow rate: 5 sccm; atomic fraction: 5.16%) film shows semiconducting properties in dark (i.e. 8.1 × 10 − 4 Ω − 1 cm − 1 ) and under the light illumination (i.e. 9.9 × 10 − 4 Ω − 1 cm − 1 ). The surface morphology of the both undoped and nitrogen doped films are found to be very smooth (RMS roughness ≤ 0.5 nm). The preliminary investigation on photovoltaic properties of DLC (nitrogen doped)/p-Si structure show that open-circuit voltage of 223 mV and short-circuit current density of 8.3 × 10 − 3 mA/cm 2. The power conversion efficiency and fill factor of this structure were found to be 3.6 × 10 − 4 % and 17.9%, respectively. The use of DLC in photovoltaic solar cells is still in its infancy due to the complicated microstructure of carbon bondings, high defect density, low photoconductivity and difficulties in controlling conduction type. Our research work is in progress to realize cheap, reasonably high efficiency and environmental friendly DLC-based photovoltaic solar cells in the future.

Efficient photoconductive devices at infrared wavelengths using quantum dot-polymer nanocomposites

We report high photoconductivity and narrow luminescence in nanocomposites of PbSe quantum dots in a polymeric matrix. The nanocomposites are photoactive at infrared wavelengths with narrow emission bands, tunable with the quantum dot sizes. The quantum dots sensitize the polymer at distinct wavelengths across the infrared range between 800 nm and 2μm, by virtue of quantum-size effects. Photoconductivity measured in the nanocomposites results in a quantum efficiency of ∼3%, which is among the highest reported so far in this spectral range.

Electronic processes in end-functionalized oligomeric poly(diphenylvinylene)s

The photoluminescence and photoelectrical properties of soluble poly(diphenylvinylene)s (PDV) of the mean number-average degree of polymerization ranging from 7 to 9 with main chains end-capped with: (i) diphenylmethylene (PDV-P), (ii) phenylcarbonyl (PDV-C), and (iii) fluorene-9,9-diyl (PDV-F) groups are reported. The introduced groups were found to affect the decay path of photogenerated excitons significantly: PDV-P shows the medium-intensity photoluminescence in the blue spectral region and poor photoconductivity decreasing about one order of magnitude when going from the excitation wavelength λ exc of 254–366 nm; PDV-C exhibits fluorescence quenching but the photoconductivity 10 times as high as that of PDV-P, which, however, decreases about almost two orders of magnitude when changing λ exc from 254 to 366 nm; PDV-F shows strong photoluminescence and high photoconductivity which does not decrease significantly if λ exc is changed from 254 to 366 nm. The observed behavior of PDV-F shows that not always a strong photoluminescence should be accompanied with a weak photoconductivity, which is explained by a formation of aggregates in PDV-F, into which excitons are effectively transferred. This trapping results in a prolonged lifetime of excitons and, consequently, in an increase in the charge carrier formation at high electric field. On the other hand, the aggregates also act as traps hindering the charge carrier transport in PDV-F. Since PDV-C also shows a reasonable photoelectrical sensitivity at low electric fields, it was tested as an active material for photovoltaic cells. The obtained parameters of the ITO/PDV-C/Al sandwich cell (illumination with white light of the intensity ca. 10 mW cm −2): short-circuit current I sc = 2 × 10 −10 A, open circuit voltage U oc = 0.5 V and fill factor FF = 0.5 provided a rather discouraging value of the power efficiency of about 10 −7.

High Pressure Conductivity and Photoconductivity of Polyveratrole

Electropolymerization of o-(CH3O)2C6H4 (Veratrole) in dry acetonitrile/tetrabutylammonium tetrafluoroborate at platinum electrode, yields a green polymeric conducting solid. As the pressure is increased, the material becomes more conductive, but sharp changes towards lower conductivity occur at 4.2 and 6.2 metric tons pressure, probably due to phase transitions. The photoconductivity shows an exponential increase as temperature is increased, but it shows reversible changes at 200 K, 225 K and 250 K during slow warming, also attributable to phase transitions. Photoconductivity of Polyveratrole increases with the radiation frequency, being higher under blue light than under red of infrared radiation and there is a linear increase of photoconductivity with radiation density. Conductivity transients show that the conductivity increases exponentially with time to the saturation point within few minutes, and it also decreases exponentially when the light is turned off and the circuit is open. This behaviour can be explained by formation and encounter of electron and hole as current carriers. A preliminary study indicates a piezoelectric effect for the material. absorption spectroscopy allowed the measurements of the material band gap.

Photocatalytic characteristics of hydro-oxygenated amorphous titanium oxide films prepared using remote plasma enhanced chemical vapor deposition

The photocatalytic activity and photoconductive characteristics of hydro-oxygenated amorphous titanium oxide (a-TiO x :OH) films prepared using remote plasma enhanced chemical vapor deposition (RPE-CVD) were studied. The a-TiO x :OH films have OH groups, which when exposed to UV irradiation, show high photoconductivity. The photoconductivity is drastically affected by oxygen gas. The results suggest that the recombination states present in the film are non-activated by the OH endings in the dangling bond like the hydrogen endings in hydrogenated amorphous silicon. Oxygen-sensitive photoconductivity is useful for environmental sensor applications.

The contribution of the amorphous phase in nanocrystalline thin films of germanium and gallium arsenide

AbstractThe nanoscale semiconductor structures obtained by evaporation in high vacuum at different temperatures show anomalous high photoconductivity. This may be connected with the structure of clusters which can consist of both crystalline and amorphous phases. We propose a method of extracting amorphous and crystalline contributions to the clusters. This method is based on the extraction of corresponding contributions to the extended x‐ray absorption fine structure (EXAFS) spectra. The method of EXAFS spectroscopy allows information to be obtained about the local atomic structure of materials in different states, including gases and liquids. The macro‐structure of films was examined by different methods, i.e. x‐ray diffraction, transmission electron microscopy (TEM), and atomic force microscopy (AFM). We obtained the volume portion of the amorphous phase to the nanosized particles of Ge as about 70% and for GaAs about 50%. Copyright © 2004 John Wiley & Sons, Ltd.

High photoconductivity properties of perylene polyimide containing triarylamine unit

High photoconductivity properties of perylene polyimide containing triarylamine unit

Characterization of Polymorphous Silicon Thin Film and Solar Cells

Polymorphous silicon (pm-Si:H) films have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the pm-Si:H has higher photoconductivity (sigma(ph)), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. There are a blue shift for the stretching mode of IR spectra and a red shift for the wagging mode. The shifts are attributed to the variation of the microstructure. By using pm-Si:H film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51% and a stabilized efficiency of 8.01% (AM1.5, 100mw/cm(2)) at room temperature (T-R).

Synthesis and characterization of a new photoconducting poly(siloxane) having pendant diphenylhydrazone for photorefractive applications

A new photoconducting polymer, diphenyl hydrazone-substituted polysiloxane, was successfully synthesized by the hydrosilylation method and characterized by FT-IR,1H-NMR, and29Si-NMR spectroscopy. The glass transition temperature (Tg) of the polysiloxane having pendant diphenyl hydrazone was ca. 62 °C, which enabled a component of a low-Tg photorefractive material to be prepared without the addition of any plasticizers. This polysiloxane, with 1 wt% of C60 dopant, showed a high photoconductivity (2.8×10−12 S/cm at 70 V/μm) at 633 nm, which is necessary for fast build-up of the space-charge field. A photorefractive composite was prepared by adding a nonlinear optical chromophore, 2-{3-[2-(dibutylamino)-1-ethenyl]-5,5-dimethyl-2-cyclohexenylidene} malononitrile, into the photoconducting polysiloxane together with C60. This composite shows a large orientation birefringence (Δn=2.6×10−3 at 50 V/μm) and a high diffraction efficiency of 81% at an electric field of 40 V/μm.

Theoretical Calculations of the Electronic Absorption Spectra of Oxotitanium(IV) Phthalocyanine in the Solid State

Oxotitanium(IV) phthalocyanine (OTiPc) has been reported to be a near-IR-active photoconductor, where crystal modifications termed phases I, II, and Y exist. In this paper, solid-state electronic absorption spectra of OTiPc have been theoretically calculated by considering three origins (i.e., exciton, charge transfer (CT), and molecular distortion) to clarify the crystal form dependence on the large red shift of the Q absorption band and high photoconductivity of the photoactive phase II in particular. In these calculations, intermolecular interactions are included in the form of configuration interactions between exciton and CT configurations, and matrix elements were calculated using the ZINDO/S Hamiltonian in order to calculate 3D metallocomplexes. By considering 243 neighboring OTiPc molecules, our theoretical calculations have succeeded in reproducing the experimental trends of the solid-state optical properties, such as the large red shift of the Q absorption band, dependence on the incident light direction, phase dependence, and CT character, indicating the reliability of our calculations. In addition, it is demonstrated that the photoconductive properties correlate well with intermolecular resonance integrals. Calculations shown here are useful not only for understanding the solid-state electronic structures of OTiPc but also for designing new functional molecular crystals.