Investigation of helicity-dependent photocurrent of surface states in(Bi_(0.7)Sb_(0.3))_(2)Te_(3)nanoplate

Helicity-dependent photocurrent(HDPC)of the surface states in a high-quality topological insulator(Bi_(0.7)Sb_(0.3))_(2)Te_(3)nanoplate grown by chemical vapor deposition(CVD)is investigated.By investigating the angle-dependent HDPC,it is found that the HDPC is mainly contributed by the circular photogalvanic effect(CPGE)current when the incident plane is perpendicular to the connection of the two contacts,whereas the circular photon drag effect(CPDE)dominates the HDPC when the incident plane is parallel to the connection of the two contacts.In addition,the CPGE of the(Bi_(0.7)Sb_(0.3))_(2)Te_(3)nanoplate is regulated by temperature,light power,excitation wavelength,the source–drain and ionic liquid top-gate voltages,and the regulation mechanisms are discussed.It is demonstrated that(Bi_(0.7)Sb_(0.3))_(2)Te_(3)nanoplates may provide a good platform for novel opto-spintronics devices.

Oneε-Ga_(2)O_(3)-based solar-blind Schottky photodetector emphasizing high photocurrent gain and photocurrent-intensity linearity

Theε-Ga_(2)O_(3) thin film was grown on sapphire substrate by using metalorganic chemical vapor deposition(MOCVD)method,and then was used to fabricate a deep-ultraviolet(DUV)photodetector(PD).Theε-Ga_(2)O_(3) thin film shown good crystal quality and decent surface morphology.Irradiated by a 254-nm DUV light,the photodetector displayed good optoelectronic performance and high wavelength selectivity,such as photoresponsivity(R)of 175.69 A/W,detectivity(D*)of 2.46×10^(15) Jones,external quantum efficiency(EQE)of 8.6×10^(4)%and good photocurrent-intensity linearity,suggesting decent DUV photosensing performance.At 5 V and under illumination with light intensity of 800μW/cm2,the photocurrent gain is as high as 859 owing to the recycling gain mechanism and delayed carrier recombination;and the photocurrent gain decreases as the incident light intensity increases because of the recombination of photogenerated carriers by the large photon flux.

PHOTOCONDUCTIVE PROPERTIES OF PVK:Alq_3 BLEND FILMS STUDIED BY STEADY-STATE AND TIME-RESOLVED TRANSIENT PHOTOCURRENT SPECTRA

The carrier transport properties of the blends of the hole transport material poly(N-vinylcarbazole) (PVK) and the electron transport material tris (8-hydroxyquinolinolato) aluminumⅢ(Alq_3) are investigated at room temperature using steady-state and time-resolved transient photocurrent measurements as a function of doping concentration of Alq_3.Due to lower LUMO and higher HOMO energy level of Alq_3 than those of PVK,Alq_3 molecules may act as carrier trap states in PVK films at low concentration.However,at...

Helicity‐dependent THz emission induced by ultrafast spin photocurrent in nodal‐line semimetal candidate Mg3Bi2

Helicity-dependent ultrafast spin current generated by circularly polarized photons in topological materials holds the crux to many technological improvements,such as quantum communications,on-chip communication processing and storage.Here,we present the manipulation of helicity-dependent terahertz emission generated in a nodal line semimetal candidate Mg3Bi2 by using photon polarization states.The terahertz emission is mainly ascribed to the helicity-dependent photocurrent that is originated from circular photogalvanic effects,and the helicity-independent photocurrent that is attributed to linear photogalvanic effect.Our work will inspire more explorations into novel nodal line semimetals and open up new opportunities for developing ultrafast optoelectronics in the topological system.

Photocurrent improvement of an ultra-thin silicon solar cell using the localized surface plasmonic effect of clustering nanoparticles

The cluster-shaped plasmonic nanostructures are used to manage the incident light inside an ultra-thin silicon solar cell.Here we simulate spherical,conical,pyramidal,and cylindrical nanoparticles in a form of a cluster at the rear side of a thin silicon cell,using the finite difference time domain(FDTD)method.By calculating the optical absorption and hence the photocurrent,it is shown that the clustering of nanoparticles significantly improves them.The photocurrent enhancement is the result of the plasmonic effects of clustering the nanoparticles.For comparison,first a cell with a single nanoparticle at the rear side is evaluated.Then four smaller nanoparticles are put around it to make a cluster.The photocurrents of 20.478 mA/cm2,23.186 mA/cm2,21.427 mA/cm2,and 21.243 mA/cm2 are obtained for the cells using clustering conical,spherical,pyramidal,cylindrical NPs at the backside,respectively.These values are 13.987 mA/cm2,16.901 mA/cm2,16.507 mA/cm2,17.926 mA/cm2 for the cell with one conical,spherical,pyramidal,cylindrical NPs at the backside,respectively.Therefore,clustering can significantly improve the photocurrents.Finally,the distribution of the electric field and the generation rate for the proposed structures are calculated.

Simple Method to Fabricate Au Nanoparticle-Decorated TiO2 Nanotube Arrays for Enhanced Visible Light Photocurrent

Au nanoparticle-decorated TiO2 nanotube arrays are prepared by a simple method, which is a thermal annealing thin gold film deposited on anodie oxidized TiO2 nanotube arrays. These electron microscope images present that Au nanoparticles are well dispersed within the wall and on the surface of the XiO2 nanotubes. Meanwhile, the morphologies of Au nanoparticles can be controlled by changing the thickness of the deposited gold film. Associ- ated with the excitation of localized surface plasmon resonances, the prepared Au nanoparticle-decorated TiO2 nanotube arrays could work as visible light responsive photocatalysts to produce a greatly enhanced photocurrent density. By varying the initial gold film thickness, such Au nanoparticle-decorated TiO2 nanotube arrays could be optimized to obtain the highest photocurrent generation efficiency in the visible and UV light regions.

Sub-bandgap photocurrent response and carrier transport properties of undoped semi-insulating LEG GaAs as a composite

Undoped (ND) semi-insulating (SI) liquid encapsulated Czochralski (LEC) GaAscrystals were investigated by photocurrent and temperature-dependent Hall measurements. It isindicated that strong nonuniformities in the distributions of impurities and defects can occur forthe NDSILEC GaAs crystal grown under a condition with strong constitutional supercooling. In suchcase, the deep level that dominates Fermi level is spacial location dependent, and the GaAs crystalbecomes a composite consisting of a large number of elementary domains with differentconductivities. The sub-bandgap photocurrent response and the carrier transport properties for thiskind of composite are quite different from those for homogeneous NDSILEC GaAs.

Studying the charge carrier properties in CuInS_2 films via femtosecond transient absorption and nanosecond transient photocurrents

The carrier behavior in CuInS_2 thin films at femtosecond and microsecond time scales is discussed in detail. Transient absorption data suggests that the photo-generated carriers relax rapidly accompanied by a change in energy. The photogenerated charge carriers are extracted by a bias electric field E in the nanosecond transient photocurrent system. An applied E improves the efficiency of photon conversion to charge carriers and enhances the velocity of the extracted charge carriers. In addition, there exists a threshold of illumination intensity in the extraction process of charge carriers in the CuInS_2 thin film, above which carrier recombination occurs. The corresponding loss further increases with illumination intensity and the recombination rate is almost independent of E. Our results provide useful insights into the characteristics of carriers in the CuInS_2 thin film and are important for the operation of optoelectronic devices realized with these films.

TRANSIENT PHOTOCURRENT INVESTIGATION OF PHOTO PROCESSES OF LATTICE-MATCHED MQW GaAs/As_xGa_l-xAs ELECTRODE

Abstract: Transient photocurrents induced by shoft light pulses at lanice-mathed GaAs/Alx.Ga1-x multiple quantum well (MQW) electrodes were studied as a function of electrode potential. Dual exponential photocurrent decay indents were observed at various potentials. By analysis of the dual exponented decay transients, information on steady state photocurrents (Is). surface collection of photoexcoted mmority cCarriers(GO) and lifetimes of surfaCe states (Ts) was obtained. The kinetic behaviors of photoprocesses at illuminated MQW/electrolyte interface were discussed.

Solvent and electric field dependence of the photocurrent generation in donor：acceptor blend system

This paper reports that the blend films of poly （2-methoxy-5-（2＇-ethyl-hexyloxy）-p-phenylene vinylene） （MEH-PPV） and N,N＇-bis（1-ethylpropyl）-3,4： 9,10-perylene bis （tetracarboxyl diimide） （EP-PDI） with the weight ratio of 1：2.5 have been prepared by spin-coating from chloroform （CF） and chlorobenzene （CB） solutions respectively. The absorption spectra and the morphology of the blend films show that large crystal-like EP-PDI aggregates are formed in film prepared from CB solution, which corresponds to a new absorption shoulder near 590 nm, while there is no shoulder around 590 nm in the UV-Vis absorption spectra of the blend film from CF solution. The electric-field dependence spectra of the photocurrent generation quantum yield of the film from CB solution shows that at weak electric field the EP-PDI aggregates act as more efficient sensitizers, but at strong electric field the quantum yields become almost invariable over the entire spectral range no matter what the state of EP-PDI, monomer or aggregate. At strong electric field, the photocurrent generation yields of both films from CF and CB solution saturate and their yield spectra become spectrally similar, mentioning that at strong electric field the photoexcitons dissociate efficiently and the free charges are collected by the electrodes almost completely.

Deep Levels in Undoped Semi-insulating Liquid Encapsulated Czochralski GaAs Detected by Photocurrent Measurement

Deep levels in undoped semi insulating (SI) liquid encapsulated czochralski (LEC) GaAs were investigated through measuring extrinsic photocurrent spectra at 300 K. Two broad photoresponse bands M 1 and M 2 were observed in the range of 0.40 ～ 0.70 eV and the range from about 0.80 eV to the bandgap energy, respectively. It was shown that M 2 band is related to the photoionization of the deep defect EL2. M 1 band, which is reported for the first time, reveals several characteristic structures including five peaks at 0.46, 0.49, 0.56, 0.65 and 0.69 eV, respectively. The relative magnitudes of these peaks vary from sample to sample and are related to the thermal histories of the samples. These peaks are likely due to different deep levels.

Exploring photocurrent output from donor/acceptor bulk-heterojunctions by monitoring exciton quenching

In this work, a series of polymer bulk-heterojunctions is fabricated based on the combinations of different donors （Ds） （P3HT and PCPDTBT） and acceptors （As） （PCBM, ICBA, and F8BT）. Exciton quenching efficiencies of the D-A pairs are obtained in order to quantify charge-transfer between the donor and the acceptor via a modified approach developed in conjunction with experimental results of optical absorption and photoluminescence spectra. It is discovered that the exciton quenching efficiency in the combination of PCPDTBT：PCBM and P3HT：PCBM reaches 70% and over, but in PCPDTBT：ICBA it is about 12%. A relatively high ALUMOdonor_acceptor results in a relatively high exciton quenching efficiency, which is responsible for better charge separation. The results agreed well with the photocurrent effect of the heterojunction layers. The work offers a convenient way to predict a potentially promising photovoltaic material with a selected D-A pair.

A positive correlation between local photocurrent and grain size in a perovskite solar cell

Developing an interplay between the local morphological character and its local photovoltaic(PV)parameters in a perovskite thin film is essential for guiding the construction of highly-efficient perovskite solar cells(PSCs). To achieve a higher PSC performance, great efforts have been devoted to the growth of larger perovskite grains;however, how the gain size can influence the PSC performance in a working device remains unclear. Herein, using laser-scanned confocal microscopy coupled with a photocurrent detection module, we realize local photocurrent, photoluminescence(PL) intensity and PL lifetime mappings directly in a working PSC. For perovskite grains of various sizes(from ~500 nm to a few micrometers), their local photocurrent exhibit a statically positive correlation with the grain size, but anti-correlated with the grain’s local PL intensity. This result suggests that a larger perovskite grain likely has fewer defects and more importantly better interfacial contact with the charge collection layers and thus leads to higher charge collection efficiency, and the optimum grain size is found to be ≥2 μm.Our result provides important guidance to the growth and control of perovskite microstructures toward the further improvement of PSC performance.

Recombination-induced voltage-dependent photocurrent collection loss in CdTe thin film solar cell

Recently, the efficiency of CdTe thin film solar cell has been improved by using new type of window layer Mg_(x)Zn_(1-x)O(MZO). However, it is hard to achieve such a high efficiency as expected. In this report a comparative study is carried out between the MZO/CdTe and CdS/CdTe solar cells to investigate the factors affecting the device performance of MZO/CdTe solar cells. The efficiency loss quantified by voltage-dependent photocurrent collection efficiency(ηC(V′)) is 3.89% for MZO/CdTe and 1.53% for CdS/CdTe solar cells. The higher efficiency loss for the MZO/CdTe solar cell is induced by more severe carrier recombination at the MZO/CdTe p-n junction interface and in CdTe bulk region than that for the CdS/CdTe solar cell. Activation energy(Ea) of the reverse saturation current of the MZO/CdTe and CdS/CdTe solar cells are found to be 1.08 e V and 1.36 e V, respectively. These values indicate that for the CdS/CdTe solar cell the carrier recombination is dominated by bulk Shockley-Read-Hall(SRH) recombination and for the MZO/CdTe solar cell the carrier recombination is dominated by the p-n junction interface recombination. It is found that the tunneling-enhanced interface recombination is also involved in carrier recombination in the MZO/CdTe solar cell. This work demonstrates the poor device performance of the MZO/CdTe solar cell is induced by more severe interface and bulk recombination than that of the CdS/CdTe solar cell.

(Bi-4I16)4- Iodobismuthate Tetramer Directed by Viologen-like Template:Enhanced Absorption Behavior, Optical Gap and Photocurrent Response Property Induced by I···I Interactions

Based on the template of protonated BPDO(B PDO = 4,4’-bipyridine N,N’-dioxide), a new iodobismuthate hybrid with formula of(H2 DPDO)2(Bi4 I16)(1) was synthesized under solvothermal conditions. The Bi4 I164- tetramer is built from four edge-sharing BiI6 octahedra. I···I interactions stabilize the structure and contribute to the structural extending from 0-D cluster to 1-D chain. The broader absorption band and lower energy gap of 1.76 eV are led by strong I···I interactions, so the small and rigid template can result in strong halogen···halogen interaction, which will be beneficial for its charge transfer. The photocurrent response property of this complex was discussed, and its mechanism has also been proposed.

Enhancement of Photocurrent Generation of a Novel Stilbazolium Dye Dimer in LB Monolayer Films

A novel dye dimer, bis-{[1-(N-hexadecyl-4-pyridinium)-2-(4-N, N-dimethylamino- phenyl)] ethenyl}methane diiodide (C16BP) was synthesized, and the photoelectrochemistry of the dye Langmuir-Blodgett monolayer modified ITO electrode was investigated. For comparison, the photoelectrochemistry of the monomer (E)-N-hexadecyl-4-[2-(4-N, N-dimethylaminophenyl) ethenyl] pyridinium iodide (C16P) was also measured. The results show that the photocurrent generation property of the dimer is enhanced. The photocurrent generation quantum yield is 0.38% for C16BP, while that for C16P is 0.23%.

Ethoxycarbonylmethyl Viologen Induced Metal Halide-based Hybrids:Structures,Photoluminescence and Photocurrent Response Behavior

Two new ethoxycarbonylmethyl viologen induced metal halide-based hybrids,(Ae V)(BiI)(DMF)(1)and(Ae V)(CoCl)(2)(AeV=N,N?-bis(ethoxycarbonylmethyl)-4,4?-bipyridinium)have been synthesized and structurally determined by X-ray diffraction method.Under the direction of a new template AeV,the(BiI)tetramer constructed from four edge-sharing BiIoctahedra(for 1)and(CoCl)mono-nuclear(for 2)were obtained.Furthermore,C–H···O and C–H···X(X=I,Cl)hydrogen bonds contribute to the extension of structures from 0-D to 1-D chains.Their energy band gaps of 2.18 and 2.41 eV indicate their semiconductor properties,and the photoluminescence was detected on 1.Interestingly,2 exhibits good photocurrent response behavior.Electronic structure analysis was executed to correlate the structure/property.

Iodoplumbate(Ⅱ)-based Hybrid Templated by 1,4-Diazabicyclo[2.2.2]octane Derivative：Structure,Photocurrent Response Behavior and Photocatalytic Activity for the Degradation of Organic Dye

A new iodoplumbate/organic hybrid,[（Et_2DABCO）_2（Pb_3I_（11））（H3 O）]n（1,Et_2DABCO = N,N？-diethyl-1,4-diazabicyclo[2.2.2] octane） has been synthesized using solution method.According to X-ray diffraction structural analysis,the unique（Pb_3I_（11））_n^（5n-） chain in 1 is constructed from face-and edge-sharing PbI_6 octahedra,which is templated by（Et_2DABCO）^（2＋） dication possessing both rigidity and flexibility.C-H...I hydrogen bonds contribute to the structure extending from 1D chains to a 3D network.Its energy band gap of 2.64 eV indicates its broad-gap semiconductor nature.It exhibits both photocurrent response property and photocatalytic activity for the degradation of rhodamine B.

TiO2-Loaded WO3 Composite Films for Enhancement of Photocurrent Density

Titanium dioxide （TiO2） loaded tungsten trioxide （WO3） composite films are prepared by an E-beam vapor system. Associated with the existence of a heterojunction at the interface of TiO2 and WO3, the prepared TiO2-WO3 composite film shows enhanced photocurrent density, four times than the pure WO3 film illuminated under xenon lamp, and higher incident-photon-to-current conversion e^ciency. By varying the initial TiO2 film thickness, such composite structures could be optimized to obtain the highest photocurrent density. We believe that thin TiO2 films improve the light response and increase the surface roughness of WO3 films. Furthermore, the existence of the heterojunction results in the e^cient charge carriers＇ separation, transfer process, and a lower recombination of electron-hole pairs, which is beneficial for the enhancement of photocurrent density.