Annealed Treatment Effect in Poly（3-hexylthiophene）：Methanofullerene Solar Cells

Polymer photovoltaic devices based on poly（3-hexylthiophene） （P3HT） ： [6,6]-phenyl-C61-butyricacid methyl ester （PCBM） 1：1 weight-ratio blend are reported. The effects of various annealing treatments on the device performance are investigated. Thermal annealing shows significant improvement of the device performances. For devices at 130℃ annealing, maximum power conversion efficiency （PCE） of 3.3% and All factor up to 60.3% is achieved under air mass 1.5, 100 m W/cm^2 illumination. We discuss the effect of thermal annealing by the results of ultraviolet-visible absorption spectroscopy （UV-vis）, dark current-voltage curve, atomic force microscopy （AFM）.     

Photovoltaic Cells with TiO2 Nanocrystals and Conjugated Polymer Composites

Various compositional photovoltaic cells based on the blend of poly(3-hexylthiophene) (P3HT) as donors and TiO₂ nanocrystals as acceptors are fabricated and investigated. It is demonstrated that the blend ratio of P3HT and TiO₂ nanocrystals could greatly influence the performance of the photovoltaic cells. The maximum of 0.411% in power conversion efficiency under AM 1.5, 100mW/cm², and 44.4% of fill factor are obtained in the solar cell with the blend weight ratio 1:1 of P3HT and TiO₂ nanocrystals. The function of nanocrystal composition is discussed in terms of the results of photoluminescence spectroscopy, atomic force microscopy, transmission electron microscopy, and charge transport I-V curve.     

Enhanced photovoltaic properties of polymer-fullerene bulk heterojunction solar cells by thermal annealing

The effect of a thermal annealing treatment on the performance of bulk heterojunction photovoltaic cells based on poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and fullerene (C₆₀) composites is investigated. Upon thermal annealing at 120 ^°C, short-circuit current and power conversion efficiency (η) are more than tripled, while a sharp rise by eight times in and η is found for the device annealed at 200 ^°C. It is concluded that the improved phase separation between MEH-PPV and C₆₀ leads to the enhancement of and η at 120 ^°C, while thermodynamic molecule arrangement at the higher temperature of ∼200 ^°C induces a significant increase in all photovoltaic parameters of composite devices except the open-circuit voltage .     

Enhanced Efficiency of Polymer： Fullerene Bulk Heterojunction Solar Cells with the Insertion of Thin TiO2 Layer near the LiF/A1 Electrode

The insertion layer of TiO2 between polymer-fullerene blend and LiF/AI electrode is used to enhance the shortcircuit current Isc and fill factor （FF）. The solar cell based on the blend of poly[2-methoxy-5-（2＇-ethylhexyloxy）- 1,4-phenylenevinylene] （MEH-PPV） and C60 with the modifying layer of TiO2 （about 20nm） shows the open- circuit Voc of about 0.62 V, short circuit current Isc of about 2.35 mA/cm^2, filling factor FF of about 0.284, and the power conversion efficiency （PCE） of about 2.4% under monochromatic light （50Onto） photoexcitation of about 17mW/cm^2. Compared to ceils without the TiO2 layer, the power conversion efficiency increases by about 17.5%. Similar effect is also obtained in cells with the undoped MEH-PPV structure of ITO/PEDOT：PASS/MEH- PPV/（TiO2）LiF/AI. The improved solar cell performance can be attributed to enhanced carrier extraction efficiency at the active layer/electrode interfaces when TiO2 is inserted.     

Role of TiO2 Nanotube on Improvement of Performance of Hybrid Photovoltaic Devices

The performance of TiO₂ nanotubes in bulk heterojunction of poly (2-methoxy-5-(2'-ethylhexyloxy)-1, 4-phenylenevinylene) (MEH-PPV)/TiO₂ nanotubes is investigated. The transport properties are studied by using the time-of-flight technique (TOF). The carrier mobilities of both holes and electrons are not improved for the MEH-PPV:TiO₂ composites compared with the pristine MEH-PPV. However, photoluminescence under the influence of the electric field indicates that the dissociation of excitons in the MEH-PPV:TiO₂ composites, which is facilitated by photoinduced charge transfer, only requires a smaller electric field.     

Determination of bulk and surface transport properties by photocurrent spectral measurements

Reliable minority carrier diffusion length and surface recombination velocity values have been obtained from stationary photocurrent measurements. A modified surface photovoltage method has been used to determine diffusion lengths longer than the wafer thickness in high-purity Si, whereas the spectral variation of the photocurrent has been employed to measure the surface recombination velocity. The novelty presented in this paper is that a Schottky diode has been employed in both the methods to collect generated charged carriers. Moreover the same Schottky diode has been employed in both the methods in order to avoid any a priori assumptions on the material transport parameters. This combined application of the two methods at the same device enables the determination of highly reliable results. Received: 17 February 2000 / Accepted: 28 March 2000 / Published online: 30 June 2000     

Two-beam coupling of a series of reduced BaTiO3:Rh at wavelengths of 514 nm and 633 nm

Dedicated to Prof. Dr. Eckard Kr?tzig on the occasion of his 60th birthday.Five annealed BaTiO₃:Rh crystals, including as-grown and those reduced at 10^-5 atm, 10^-10 atm, 10^-12 atm, and 10^-14 atm oxygen partial pressures, were produced and investigated by two-beam coupling at wavelengths of 514 nm and 633 nm. The carrier type is a function of reduction degree and incident wavelength. The compensation points are around 10^-3 atm and 10^-11 atm oxygen partial pressures for 514 nm and 633 nm, respectively. The electrooptic gain is controlled by the photoconductivity–dark conductivity ratio, effects from the deep and shallow traps, and hole–electron competition, which are dependent on the reduction degree and incident wavelength. Received: 23 November 1998 / Revised version: 10 February 1999 / Published online: 12 April 1999     

Effect of TiO2 Nanotubes on Polymer-Fullerene Bulk Heterojunction Solar Cells

We investigate the photovoltaic properties of hybrid organ/c solar cell based on the blend of poly[2-methoxy-5-（2- ethylhexoxy-l,4-phenylenevinylene） （MEH-PPV）, C60 and titanium dioxide （TiO2） nanotubes. In comparison of the composite devices with different TiO2：[MEH-PPV ＋C60] weight ratios of lw$.% （D1-1）, 2wt.% （D1-2）, 3wt.% （D1-3）, 5wt.% （D1-4）, 10wt.% （D1-5） and 20wt.% （D1-6）, it is found that the device Dl-a exhibits the best performance. The conversion efficiency is improved by a factor of 3 compared with the MEH-PPV：C60 device.     

Time-dependent photocurrent of a CdTe nanoparticle film under the above-gap illumination

Time-dependent photocurrent of a solid film of 1-thioglycerol-capped CdTe nanoparticles under the illumination of the 325-nm wavelength light is characterized to investigate the transport mechanism of photo-generated charge carriers in this nanoparticle film. Under the illumination of the above-gap light, photocurrent rises rapidly, and subsequently it decays or rises slowly, depending on the magnitude of bias voltage. A careful investigation into the variation in the magnitude of the current measured as a function of time while the light was switched on and off periodically reveals that rapidly and slowly respondent photocurrents overlap in the time-dependent photocurrent. Charge carriers contributing to the rapidly and slowly respondent photocurrents are electrons and holes separated from a fraction of excitons excited by the above-gap light. The transport behaviors of these charge carriers may explain the monotonously decay and slow rising of the photocurrent after its rapidly rising under the illumination for the solid film at unbiased and biased voltage, respectively.     

Sub-picosecond photo-induced melting of a charge-ordered state in a perovskite manganite

Ultrafast melting of a charge-ordered state has been observed in the photo-irradiated colossal magnetoresistive compound Pr_0.7Ca_0.3MnO₃. Pump-and-probe spectroscopy experiments reveal the formation of a conducting phase with typical features of an insulator–metal transition (IMT) after less than 1 ps. This phase is metastable and can be maintained for about 1 μs unless it is stabilized persistently into a pathlike metallic region by an electric field. Although laser-induced lattice heating may play a role in the initial excitation, electronic correlations are the dominant effect which leads to the formation of the metallic state upon the breakdown of the charge-ordered state. Received: 26 January 2000 / Published online: 16 June 2000     

Thermal and carrier transport originating from photon recycling and non-radiative recombination in laser micromachining of GaAs thin films

Coupled thermal and carrier transports (electron/hole generation, recombination, diffusion and drifting) in laser photoetching of GaAs thin film is investigated. A new volumetric heating mechanism originating from SRH (Shockley–Read–Hall) non-radiative recombination and photon recycling is proposed and modeled based on recent experimental findings. Both volumetric SRH heating and Joule heating are found to be important in the carrier transport, as well as the etching process. SRH heating and Joule heating are primarily confined within the space-charge region, which is about 20 nm from the GaAs surface. The surface temperature rises rapidly as the laser intensity exceeds 10⁵ W/m². Below a laser intensity of 10⁵ W/m², the thermal effect is negligible. The etch rate is found to be dependent on the competition between photovoltaic and photothermal effects on surface potential. At high laser intensity, the etch rate is increased by more than 100%, due to SRH and Joule heating. Received: 24 January 2002 / Accepted: 11 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-310/206-2302, E-mail: xiang@seas.ucla.edu     

Photoconductivity and the structural phase transition in SrTiO3

Detailed photoconductivity measurements have been performed in nominally pure SrTiO₃ in order to elucidate the effect of the antiferrodistorsive cubic-tetragonal phase transition. Small features in the photoconductivity’s temperature dependence in the phase transition region were found using low intensity interband UV or 514 nm light illumination. Such features are associated with a transformation of the defect system controlling the photoconductivity. At the same time, the temperature behavior of the photoconductivity spectral maximum reveals a rather unusual feature which is connected with changes in the absorption band edge structure in the phase transition region.     

Temperature dependence of absorption edge anisotropy in 2H- MoSe2 layered semiconductors

The absorption-edge anisotropy of 2H- MoSe₂ was studied by photoconductivity (PC) measurements as a function of temperature in the range of 12-300 K. A significant shift towards lower energies has been observed in the PC spectra on the edge plane with respect to those corresponding to the van der Waals (VdW) plane. The parameters that describe the temperature dependence of the absorption edges are evaluated by the Bose-Einstein empirical expression. Effective phonon energy was estimated from the temperature dependence of the Urbach energy. The estimated effective phonon energy for the VdW and edge planes, respectively, can be correlated to the observed Raman active and modes.     

On the photoconductivity of layered molecular complex of fullerene C60 with saturated amine TMPDA

It was revealed that the photoconductivity of layered molecular complex of fullerene C₆₀ with saturated amine TMPDA: TMPDA · C₆₀ is caused by intermolecular electronic processes in the fullerene layers. The intermediate magneto‐sensitive stage of photogenerating free charge carriers was found to be due to the effect of magnetic field on the rate constant of the triplet charge transfer exciton annihilation process. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reflection holograms in iron-doped lithium niobate

Received: 7 November 1996/Revised version: 16 December 1996     

Light–induced charge transport processes in photorefractive barium titanate crystals doped with iron

3 ) are annealed in a hydrogen atmosphere at various temperatures. After these reducing treatments, absorption, light–induced absorption changes, two–beam coupling direction, photo electron paramagnetic resonance (photo EPR), dark and photoconductivity as well as bulk photovoltaic current density are investigated. The samples are electron conductive and the charge transport is governed by only one level, which is identified by photo EPR as Fe²⁺/Fe³⁺. The photoconductivity exceeds the dark conductivity for intensities above 1 kWm^-2. A relation between the absorption constant and the Fe²⁺ concentration is derived. From the known charge transport parameters the advantageous photorefractive properties of optimized reduced BaTiO₃:Fe are deduced; possible response times in the millisecond range at an intensity of 10 kWm^-2 are estimated. Received: 22 January 1997/Accepted: 23 January 1997     

Superlinear photovoltaic currents in proton-exchanged LiNbO<Subscript>3</Subscript> waveguides

Photovoltaic currents along the c axis have been measured in α-phase LiNbO₃ proton-exchanged waveguides at several visible wavelengths for a guided-beam configuration. The light-intensity dependence is superlinear and all experimental curves are very well fitted by computer simulations using a two-centre model, with Fe²⁺/Fe³⁺ as primary and Nb_Li ⁴⁺/Nb_Li ⁵⁺ as secondary photovoltaic centres. The superlinear behaviour arises from a much higher effective photovoltaic length of Nb_Li ⁴⁺ (small polaron) compared with that of Fe²⁺. In β₁-phase guides, the photocurrents are much smaller than in α-phase guides and apparently do not show superlinear behaviour. Received: 22 October 2002 / Revised version: 6 January 2003 / Published online: 12 May 2003 RID="*" ID="*"Corresponding author. Fax: +34-91/3978-579, E-mail: m.carrascosa@uam.es     

Atomistic processes and the strain distribution in the early stages of thin film growth

Structural relaxations in small Co islands on the Cu(001) surface are investigated performing atomistic calculations. We demonstrate that the strain relief at the metal interface in the early stages of heteroepitaxy is more complicated than suggested by simple considerations based on the small mismatch between the Co and Cu bulk metals. We found that the strain distribution in the surface region near the islands varies strongly on an atomic scale. The effect of strain on the shape of the Co islands is revealed. Diffusion on the top of strained islands and edge diffusion are considered. Received: 10 April 2000 / Accepted: 15 May 2000 / Published online: 7 March 2001     

Photoconduction and polarization effects in a heat-treated Au/Pb2CrO5/SnO2 film device

2 CrO₅/SnO₂ sandwich-structured film device (1.39 μm thick) was measured at room temperature as a function of frequency in the range 5 Hz-5 kHz, both in the dark and under illumination with visible light of different intensities. The Pb₂CrO₅ film was prepared at a substrate temperature of 200 °C and annealed at a temperature of 460 °C, and has been characterized to be polycrystalline-rich in sizable micro-grains. The experimental frequency-dependence of the total ac-impedance was found to be adequately described over the entire frequency range used by a proposed equivalent RC-circuit model that took into consideration the contributions of the bulk, grain-boundaries, and electrode–Pb₂CrO₅ interface to the device’s ac behavior. Bulk conduction within crystalline grains due to the semiconductivity of Pb₂CrO₅ and space-charge effects in the highly resistive grain-boundary regions have been found to dominate the device’s ac-behavior at frequencies higher than 100 Hz and were strongly dependent on light intensity. The light-intensity behavior of the circuit-parameters associated with grain-boundary effects can be understood by the use of grain-boundary trapping models. The effect of interfacial space-charge polarization in the region near the electrode–Pb₂CrO₅ junction has been noted to be most significant at the low-frequency side and was not highly affected by illumination of the device. Received: 13 February 1997/Accepted: 30 July 1997     

Photo-EMF-based adaptive detectors as optimal photodetectors for detection of light-pattern displacements

It is shown that non-steady-state photo-EMF-based adaptive photodetector operates like an optimal detecting medium when detection of a general spatial shift of a complicated light pattern is needed. This detecting medium can be considered as some spatial distribution of photo-induced p–n junctions matched with the original light pattern and its sensitivity is limited basically by the thermal (Johnson) noise of the detector volume itself. Received: 8 December 1998 / Revised version: 1 February 1999 / Published online: 12 April 1999