Photovoltaic and pyroelectric effects in self-polarized ferroelectric <Emphasis Type="Italic">PZT</Emphasis>(25/75) films

The stationary photovoltaic effect in self-polarized Pb(Zr_0.25,Ti_0.75)O₃ films with a semitransparent top NiCr electrode was measured. The correlation between the pyroelectric response and short-circuit photocurrent was established. The effect of pre-polarization on the short-circuit photocurrent value is considered. It is assumed that upon film ageing, the abnormal photovoltaic effect degenerates into the barrier effect.     

Superlinear photovoltaic currents in LiNbO<Subscript>3</Subscript>: analyses under the two-center model

Superlinear photovoltaic currents in LiNbO₃ are theoretically studied by using a two active center model, with Fe²⁺/Fe³⁺ as primary center and Nb_Li⁴⁺/Nb_Li⁵⁺ as secondary center. Analytical instead of numerical results are provided, including close-form expressions for most common experimental situations. Recent photovoltaic parameters obtained for -phase proton-exchanged LiNbO₃ waveguides (very similar to the substrate) are used for applying the model and comparing with published experimental results. Thoroughly studied aspects are: the redistribution of donor/acceptor states for each species as a function of the light intensity, their contribution to the photovoltaic current density, the effect of the temperature, and the role of the center concentrations and their reduction state. This provides a detailed understanding of the photovoltaic current function shapes versus light intensity, predicts new features of interest for experimentalists and suggests further experiments to better determine the material parameters. Photovoltaic measurements and modeling appear a simpler and safer way of understanding the role of the two-center photovoltaic effect in photorefractive phenomena as well as for determining important photorefractive parameters. PACS 42.70.Nq; 72.40.+w.     

Al2O3增强的Co2-C98/Al2O3/Si异质结的光伏效应

随着能源危机的加剧,太阳能电池作为开发和利用太阳能的一种普遍形式, 日益受到世界各国的重视.随着太阳能电池向着高效率、薄膜化、无毒性和原材料丰富的方向发展, 单纯的硅系太阳能电池已经无法达到这样的要求,因此新的材料和工艺的开发利用迫在眉睫. 本文研究了碳材料在硅异质节上实现光伏效应的改善及其可能在太阳能电池上的应用. 采用脉冲激光沉积方法制备的Co₂-C₉₈/Al₂O₃/Si异质结构在标准日光照射 (AM1.5, 100 mW/cm²)条件下,可获得0.447 V的开路电压和18.75 mA/cm²的电流密度, 转换效率可达3.27%.通过电容电压特性和暗条件下的电输运性能测量, 证明了氧化铝层的引入不但对单晶硅的表面起到了物理钝化作用,减小了反向漏电流, 使异质结界面缺陷、界面能级和复合中心减少,还起到了场效应钝化作用, 增加了异质结界面的势垒高度,增加了开路电压,使异质结的光伏效应显著增强.     

Ionic liquid type dye-sensitized solar cells: increases in photovoltaic performances by adding a small amount of water

Increases in photovoltaic performances for dye-sensitized solar cells having ionic liquid type electrolytes are reported. These results are explained by diffusion coefficient for I₃⁻, charge transfer resistances on counter electrodes, flat band potentials of TiO₂, redox potentials for I⁻/I₃⁻, electron diffusion constants, electron life time, and diffusion length in TiO₂ layers. Methylpropylimidazolium iodide is selected because of the lowest viscosity and the highest conductivity. Increases in the photovoltaic performance are observed when a small amount of water was added into the ionic liquid consisting of both LiI and t-butylpyridine as the additives. These improvements are brought about by enhancements of all of J_sc, ff and V_oc. The increases in J_sc and ff are associated with the decrease in charge transfer resistances on counter electrodes and increases in ionic conductivities. V_oc may be explained by an increase in the difference between redox potentials of I⁻/I₃⁻ and Fermi level.     

Charge transport properties of undoped congruent lithium niobate crystals

Time-resolved light-induced birefringence measurements based on a phase compensation technique are used to determine the charge transport properties of congruent, nominally-undoped lithium niobate crystals (LiNbO₃). Some of the crystals are conventionally oxidized. The steady-state space-charge field E _pv, the bulk photovoltaic coefficient β, and the photoconductivity σ _ph are determined in the intensity range (30–30 000) W/cm². The photovoltaic coefficient β increases by one order of magnitude over this intensity range, the space-charge field E _pv even by two orders of magnitude. We discuss the results in the context of the known one- and two-center charge transport models for LiNbO₃. The experimental findings presented here are of relevance for the long-standing problem of optical damage in such crystals, which inhibits their use in high-intensity applications like nonlinear optics.     

The use of photoinduced light scattering for the evaluation of photoelectric fields in Lithium Niobate Crystals

The photovoltaic and diffusion fields in nominally pure single crystals of stoichiometric composition (R = Li/Nb = 1) grown from the melt with 58.6 mol % of Li₂O (LiNbO₃ stoich), in the nominally pure single crystals of congruent composition (LiNbO₃), and in congruent single crystals doped with Cu²⁺, Zn²⁺, and Gd³⁺ are found from the parameters of the photoinduced light scattering indicatrix obtained with the use of a 60-mW He-Ne laser.     

Polarization-enhanced bulk photovoltaic effect of BiFeO3 epitaxial film under standard solar illumination

In this Letter, we report the polarization-enhanced bulk photovoltaic effect (BPV) in pristine BiFeO₃ (BFO) epitaxial film under standard 1 sun AM 1.5 G illumination. High-quality epitaxial BFO films are grown on (001)-oriented niobium doped-SrTiO₃ substrates by pulsed laser deposition. The best BFO film based photovoltaic device achieves a power conversion efficiency of 0.0062% under standard illumination. Besides, it is found that the number of bipolar pulses plays a key role in improving the short-circuit current density and open-circuit voltage. These results are beneficial for further understanding of physical origin of the photovoltaic properties in ferroelectric oxides.     

Sm doped BiFeO3 nanofibers for improved photovoltaic devices

Interdigital electrodes, BiFeO₃ and Sm doped BiFeO₃ nanofibers and corresponding photovoltaic devices were obtained by semiconductor technology and electrostatic spinning method. The interdigital electrodes have prefect structure and the nanofibers have uniform morphology both before and after annealing process examined by SEM and AFM, respectively. The ferroelectric and piezoelectric properties of the BiFeO₃ and Sm doped BiFeO₃ nanofibers were demonstrated by PFM. After Sm doping, the BiFeO₃ nanofibers have improved ferroelectricity. Compared with BiFeO₃ nanofiber photovoltaic device, Sm doped BiFeO₃ nanofiber photovoltaic device has improved photovoltaic performance with increased photocurrent and energy conversion efficiency is improved to 1.65 times. The BiFeO₃-based nanofibers are very promising ferroelectric photovoltaic materials for high performance photovoltaic devices.     

Towards an optical coupler using fine-wire: a study of the photovoltaic effect of a heterojunction formed in a single fine-wire of tungsten oxides

Nanodevices using the photovoltaic effect of a single nanowire have attracted growing interest. In this paper, we consider potential applications of the photovoltaic effect to optical signal coupling and optical power transmission, and report on the realization of a heterojunction formed between WO₂ and WO₃ in a fine-wire having a diameter on the micrometer scale. Using a laser beam of 514.5~nm as a signal source, the WO₂--WO₃ heterojunction yields a maximum output power of up to 37.4 pico watt per heterojunction. Fast responses (less than a second) of both photovoltaic voltage and current are also observed. In addition, we demonstrate that it is a simple and effective way to adapt a commercial Raman spectrometer for the combined functions of fabrication, material characterization and photovoltaic measurement of an optical signal coupler and optical power transmitter based on a fine-wire. Our results show an attractive perspective of developing nanowire or fine-wire elements for coupling optical signals into and for powering a nanoelectronic or nano-optoelectronic integrated circuit that works under the condition of preventing it from directly electrically connecting with the optical coupler.     

Photocurrent analysis of organic photovoltaic cells based on CuPc/C60 with Alq3 as a buffer layer

The performance of an organic photovoltaic(OPV) cell based on copper phthalocyanine CuPc/C60 with a tris(8-hydroxyquinolinato) aluminum(Alq3) buffer layer has been investigated.It was found that the power conversion efficiency of the device was 1.51% under illumination with an intensity of 100 mW/cm2,which was limited by a squareroot dependence of the photocurrent on voltage.The photocurrent-optical power density characteristics showed that the OPV cell had a significant space-charge limited photocurrent with a varied saturation voltage and a three quarters power dependence on optical power density.Also,the absorption spectrum was measured by a spectrophotometer,and the results showed that the additional Alq3 layer has a minor effect on photocurrent generation.     

亮暗屏蔽光伏孤子在LiNbO3晶体中的大自偏转

用数值方法证实了亮、暗屏蔽光伏孤子在有外加电场的LiNbO₃晶体中都可以发生大自偏转,并验证了这种自偏转现象不但与晶体中受主浓度N_A有关而且还与外加电场E₀有关. 在E₀相同的条件下,N_A越小这种自偏转现象越明显,在N_A相同的条件下,E₀越大自偏转现象越明显. 还发现亮、暗屏蔽光伏孤子的自偏转现象不同：亮屏蔽光伏孤子整体都发生明显偏转,暗屏蔽光伏孤子的偏转只是发生在一侧,其极值位置和另一侧几乎不发生偏转. 关键词： 自偏转 空间孤子 光折变效应     

Influence of post-growth treatment on the holographic storage properties of In:Fe:LiNbO3

The congruent In (3 mol%):Fe (0.03 wt%): LiNbO₃ crystal has been grown by Czochralski method in air. Some crystal samples were reduced in Li₂CO₃ powder, and others were oxidized in Nb₂O₅ powder. The defects and ions location in crystal were investigated by infrared (IR) transmission spectrum. The photorefractive properties were measured by two-wave coupling and light-induced scattering resistance experiments. In the oxidized sample, the photovoltaic effect was the dominant process during recording. However, for the as-grown sample as well as the reduced, the photorefractive effect was governed by the diffuse field and the photovoltaic field, together. In addition, the reduction treatment made the photoconductivity increase, which resulted in shorter erasure time and lower diffraction efficiency, but higher light-induced scattering resistance ability. The oxidation treatment caused the inverse effect.     

Multilevel resistance switching of Ag/Nb-doped SrTiO<Subscript>3</Subscript>/Ti structure

Rectifying and photovoltaic properties of a La_0.7Sr_0.3CoO_3−σ /Si heterostructure have been experimentally studied. The heterostructure exhibits a rectifying behavior, similar to that of the traditional diode. A photovoltaic effect is observed when the heterostructure is exposed to the laser with a wavelength of 532 nm and a power of 6 mW/mm². It is interesting to note that the peak photovoltages initially increase with decreasing temperature, followed by a decrease at T<220 K. This reveals that the photovoltaic effect can be modulated by the intrinsic properties of the La_0.7Sr_0.3CoO_3−σ layer.     

Polar properties of hydrothermally synthesized BiFeO3 thin films

Multiferroic bismuth ferrite (BiFeO₃) has attracted considerable attention due to applications related to the bulk photovoltaic effect in which the direction of polarization determines the direction of the photocurrent. Epitaxial thin films are produced by means of techniques that usually require high temperature processes. The hydrothermal method can be seen as an alternative route to obtain highly textured thin films in quantities compatible with batch processing; nevertheless, the structural, dielectric and electric properties are generally affected by the presence of hydrogen and other reaction by-products. In this work, functional and highly textured BiFeO₃ films were successfully produced on metallic SrTiO₃:Nb (0.5 wt.%) (100) substrates via hydrothermal synthesis. X-ray diffraction (XRD) and Atomic Force Microscopy (AFM) were used to analyze the structural properties of the films. Piezoresponse Force Microscopy (PFM) and Photoconductive Atomic Force Microscopy (Pc-AFM) were used to determine their functional properties. We show the polarization switching and confirm the presence of the bulk photovoltaic effect for the first time in hydrothermally synthesized BiFeO₃.     

Light-induced thermal gratings in LiNbO3: Fe

Holographic gratings are recorded in LiNbO³: Fe using frequency-doubled pulses of aQ-switched Nd : YAG laser (light wavelength = 532 nm, pulse durationt _p = 20ns). We monitor diffraction efficiencies of the holograms during and several milliseconds after exposure. Variations of Fe²⁺ concentration, light intensity, and fringe spacing show that different types of gratings are involved: photochromic, thermooptic, pyroelectric, and space-charge gratings. The influence of the internal pyroelectric field on the light-induced charge transport is discussed in terms of a two-center model. Pyroelectric and bulk photovoltaic effect are shown to partially compensate each other. In LiNbO₃: Fe, the photovoltaic effect is the dominant charge-driving force for our recording conditions.     

Photoreflectance study on the photovoltaic effect in InAs/GaAs quantum dot solar cell

To investigate the effect of quantum dot (QD) layers on the photovoltaic process of InAs/GaAs QD solar cell (QDSC), QD layers were embedded in conventional GaAs p-n junction SC (GaAs SC) structures. The photoreflectance (PR) was examined at different temperatures (T) and excitation light intensities (I_ex) to investigate the photovoltaic effects through observation of the Franz-Keldysh oscillations (FKOs) in the PR spectra. The evaluated the p-n junction electric fields (F_pn) of the InAs QDSC was different from that of the GaAs SC. Moreover, InAs QDSC show that the different photovoltaic behaviors compared with GaAs SC by varying I_ex and T. From these considerations, we suggest that the different photovoltaic behaviors are caused by the effect of the additional photo-carrier generation in InAs QD layers resulting in enhancement of the field screening effect in F_pn.     

Investigation of Cu2ZnSnS4 solar cell buffer layer fabricated via spray pyrolysis

Copper zinc tin sulfide solar cells were fabricated by using spray pyrolysis from a window layer to an absorber layer. ZnS and In₂S₃ buffer layers were deposited on the TiO₂ layer, and the photovoltaic characteristics were investigated. The ZnS buffer demonstrated a poor photovoltaic performance because of its poor surface coverage and micro-cracks at fluorine-doped tin oxide/TiO₂ layers. The In₂S₃ buffer layer sprayed at low temperature (<360 °C) showed a large difference between photo and dark currents beyond the open-circuit voltage (V_OC). When the spraying temperature exceeded 390 °C, the devices showed high dark leakage currents at reverse biases because of the high conductivity of the buffer layer, resulting in decreased V_OC and short-circuit current density (J_SC). The optimum temperature for spraying In₂S₃ is 360 °C, and the best performing device showed 410 mV, 30.4 mA/cm², 35.3%, and 4.4% of V_OC, J_SC, fill factor, and efficiency, respectively.     

Electronic processes at the interfaces between photoactive layers and TiO<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript> buffer layers in organic solar cells

The effect a layer of TiO_x located between a photoactive layer and a metallic Al electrode has on the photovoltaic properties of an organic solar cell based on P3HT:PC₇₀BM polymer is studied. The optimum thickness of the TiO_x layer at which the efficiency of the solar cells is highest and the TiO_x layer ensures the transfer of electrons from the photoactive polymer layer to the electrode while blocking vacancies is found to be 10 nm. The effect oxygen has on electronic processes during the operation of the photovoltaic cell is discussed.     

Space charge field limitations in photorefractive LiNbO3:Fe crystals

We use holographic techniques for the investigation of strongly oxidized LiNbO₃:Fe crystals with small Fe²⁺ concentrations and compare the results with theoretical predictions. Experimental evidence is presented for enhanced phase shifts between light intensity pattern and refractive index grating and for limitations of optically induced space charge fields in photovoltaic crystals due to the low concentration of filled traps. Our findings do not support the model of a nonlocal photovoltaic effect in LiNbO₃.     

One-dimensional steady-state bright photovoltaic solitons in LiNbO3:Fe crystal with background illumination

We investigate theoretically and experimentally one-dimensional bright photovoltaic solitons in LiNbO₃:Fe crystal by use of the background illumination. We find that, in LiNbO₃:Fe crystal, bright photovoltaic solitons can be obtained with background illumination for κ>1, where κ is the ratio of the background illumination photovoltaic constant to the soliton optical beam's photovoltaic constant. For κ<1, dark photovoltaic solitons are generated. On the other hand, our experiments show good agreement with theoretical prediction for the soliton existence curve in a special intensity ratio.