Terahertz radiation driven chiral edge currents in graphene

We observe photocurrents induced in single-layer graphene samples by illumination of the graphene edges with circularly polarized terahertz radiation at normal incidence. The photocurrent flows along the sample edges and forms a vortex. Its winding direction reverses by switching the light helicity from left to right handed. We demonstrate that the photocurrent stems from the sample edges, which reduce the spatial symmetry and result in an asymmetric scattering of carriers driven by the radiation electric field. The developed theory based on Boltzmann's kinetic equation is in a good agreement with the experiment. We show that the edge photocurrents can be applied for determination of the conductivity type and the momentum scattering time of the charge carriers in the graphene edge vicinity.     

Space-charge limited photocurrent

In 1971 Goodman and Rose predicted the occurrence of a fundamental electrostatic limit for the photocurrent in semiconductors at high light intensities. Blends of conjugated polymers and fullerenes are an ideal model system to observe this space-charge limit experimentally, since they combine an unbalanced charge transport, long lifetimes, high charge carrier generation efficiencies, and low mobility of the slowest charge carrier. The experimental photocurrents reveal all the characteristics of a space-charge limited photocurrent: a one-half power dependence on voltage, a three-quarter power dependence on light intensity, and a one-half power scaling of the voltage at which the photocurrent switches into full saturation with light intensity.     

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.     

调制光/电作用下染料敏化太阳电池中电荷传输和界面转移研究

详细讨论了染料敏化太阳电池(DSC)在稳态光照射或外加偏压下电荷的传输和转移过程,以及在调制光/电作用下电池的频率响应特点.通过电化学阻抗谱、光电化学阻抗谱、强度调制光电流谱和强度调制光电压谱等四种频谱光电测试手段,对DSC中TiO₂薄膜电子传输和界面转移的相关时间常数进行测量.详细分析和比较了电荷的传输及转移过程对时间常数的影响.结果表明,在低光强或低偏压下电荷传输和转移过程对时间常数影响较小,但在高光强或高偏压下对电子寿命影响明显.     

Electrical characteristics of UV photodetectors based on ZnO/diamond film structure

Ultraviolet photodetectors based on ZnO/diamond film structure were fabricated. The properties of Au/ZnO contacts and effects of grain sizes on the electrical characteristics of photodetectors were discussed. Due to the bombardment with Au atoms and the annealing process, fine ohmic contacts were formed between Au electrodes and ZnO films. Dark currents and photocurrents of the photodetectors were related to sputtering time and the grain size of ZnO films. For the photodetector with a bigger grain size, a lower dark current and a higher photocurrent were obtained under 10 V bias voltage. The time-dependent photocurrent confirmed the carrier trapping effect.     

Significant modifications in the electrical properties of poly(methyl methacrylate) thin films upon dispersion of silver nanoparticles

Electrical conductivity of thin solid films of PMMA with dispersed silver nanoparticles, synthesized by a novel method, was studied in dark conditions by changing the applied voltage and temperature and also under photoexcitation (by a mercury lamp, 125 W) at room temperature. Anomalous hysteresis in current-temperature characteristics during heating and cooling cycles was observed. The hysteresis-like behaviour was explained on the basis of the movements of molecules associated with different parts of a PMMA matrix and diffusion of silver nanoparticles in the PMMA matrix. Dark current in the PMMA films with dispersed silver nanoparticles has been observed to be higher than the corresponding current in the PMMA films without silver nanoparticles due to the creation of conduction paths by the silver nanoparticles/nanoclusters. The photoresponse in the thin solid films of PMMA with dispersed silver nanoparticles was the reverse of that observed in thin solid films of PMMA without silver nanoparticles. A decrease in photocurrent under illumination of light was observed due to the destruction of conduction paths by the illumination of light.     

ZnO/P-Si异质结的光电特性研究

利用脉冲激光沉积方法在P-Si(100)衬底上生长ZnO薄膜,制备ZnO/P-Si异质结,研究衬底温度对异质结光电特性的影响.结果表明,在400℃,500℃,550℃和600℃下生长ZnO制备的异质结都有一定的整流特性,反向暗电流随着衬底温度的升高略有增加,在550℃下制备的样品具有最明显的光电效应.ZnO/P-Si异质结对可见光和紫外光呈现出不同的响应性.在可见光照射下,光电流随反向偏压急剧增大,偏压增大到某一值时,光电流增速变小,而在紫外光下,光电流有逐渐增大的趋势.根据ZnO的透射谱认为,可见光和紫外光是异质结不同的耗尽区诱导电子-空穴对产生光电流的.     

Optoelectronic characteristics of close-packed HgTe nanoparticles in the infrared range

Absorption, photoluminescence (PL), photoresponse, and I-V measurements were made for a close-packed HgTe nanoparticle film without organic capping materials to investigate its optoelectronic characteristics in the infrared (IR) range. In the absorption and PL spectra taken for the close-packed nanoparticle film, the wavelength of exciton peak was red-shifted, compared with 1-thioglycerol capped HgTe nanoparticles dispersed in solution. For the HgTe nanoparticle film, dark current was below several pA level, current was increased by about three orders of magnitude at a biased voltage of 3 V under the illumination, and photoresponse was very rapid compared with 1-thioglycerol capped HgTe nanoparticles. These optoelectronic characteristics illustrate that HgTe nanoparticles are one of promising materials for the photodetector in the IR range. Finally, the origin for the increase of photocurrent with increasing temperature observed in this study will be discussed.     

InGaN/GaN多量子阱LED载流子泄漏与温度关系研究

通过测量光电流,直接观察了InGaN/GaN量子阱中载流子的泄漏程度随温度升高的变化关系。当LED温度从300K升高到360K时,在相同的光照强度下,LED的光电流增大,说明在温度上升之后,载流子从量子阱中逃逸的数目更多,即载流子泄漏比例增大。同时,光电流的增大在激发密度较低的时候更为明显,而且光电流随温度的增加幅度与激发光子的能量有关。用量子阱-量子点复合模型能很好地解释所观察到的实验现象。实验结果直接证明,随着温度的升高,InGaN/GaN量子阱中的载流子泄漏将显著增加,而且在低激发密度下这一效应更为明显。温度升高导致的载流子泄漏增多是InGaN多量子阱LED发光效率随温度升高而降低的重要原因。     

磁场对聚合物光电池中电流的影响

制备了结构为 ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al的聚合物光电池器件,并在不同偏压下,分别测量了器件的光电流和暗电流随外加磁场的变化. 发现随外加磁场增加,光电流增强,暗电流减弱. 从聚合物光电池中光电流和暗电流的产生机制出发,对该现象进行了解释,认为外加磁场可以有效改变单重态极化子对和三重态极化子对之间的相对比例,进而使自由载流子浓度增加. 光生自由载流子浓度增加是光生电流增强的原因,而自由载流子与三重态激子的相互作用导致了暗电流减弱. 开路电压附近,光电流随磁场增加而增强可以近似 关键词： 聚合物光电池 磁场效应 光生电流 极化子对     

Influence of white light illumination on the performance of a-IGZO thin film transistor under positive gate-bias stress

The influence of white light illumination on the stability of an amorphous In GaZnO thin film transistor is investigated in this work. Under prolonged positive gate bias stress, the device illuminated by white light exhibits smaller positive threshold voltage shift than the device stressed under dark. There are simultaneous degradations of field-effect mobility for both stressed devices, which follows a similar trend to that of the threshold voltage shift. The reduced threshold voltage shift under illumination is explained by a competition between bias-induced interface carrier trapping effect and photon-induced carrier detrapping effect. It is further found that white light illumination could even excite and release trapped carriers originally exiting at the device interface before positive gate bias stress, so that the threshold voltage could recover to an even lower value than that in an equilibrium state. The effect of photo-excitation of oxygen vacancies within the a-IGZO film is also discussed.     

An Electroluminescence Delay Time Model of Bilayer Organic Light-Emitting Diodes

Based on the mechanism of injection, transport and recombination of the charge carriers, we develop a model to calculate the delay time of electroluminescence （EL） from bilayer organic light emitting diodes. The effect of injection, transport and recombination processes on the EL delay time is discussed, and the relationship between the internal interface barrier and the recombination time is revealed. ＂]~he results show that the EL delay time is dominated by the recombination process at lower applied voltage and by the transport process at higher applied voltage. When the internal interface barrier varies from 0.15 eV to 0.3 eV, the recombination delay time increases rapidly~ while the internal interface barrier exceeds about 0.3eV~ the dependence of the recombination delay time on applied voltage is almost undiversified, which may serve as a guideline for designing of a high-speed EL response device.     

Photocurrents in p-n Si diodes under high intensity (pulsed-laser) illumination: Quantum yields and kinetic evaluation

The photoelectric response of p-n Si photodiodes under pulsed laser illumination (half width 10 ns) at 532 nm was studied as a function of dose which was varied over 6 orders of magnitude. The photocurrent transients are dominated by a plateau-like feature due to the build up of space charge at the intensities used. Increasing bias voltage increases the height of the plateau and decreases its length. In the low-dose range the length of the transient increases linearly with dose and the collected charge (integrated current) reaches a constant value. At high doses (above 10^–5 J/pulse · cm² or 2.7×10¹³ quanta/pulse · cm²) considerable charge loss (decrease in quantum yields) is accompanied by a less than proportional increase of the transient lifetime. From model calculations the dose and voltage dependence of the quantum yield of charge collection is shown to be the result of competition between current flow and first and higher order recombination. The model calculations are consistent with experimental results. Rate constants have been obtained by fitting.     

硅衬底Ba1-xSrxNbyTi1-yO3薄膜光敏特性的研究

利用硅平面工艺和氩离子束镀膜技术在SiO2/Si衬底上淀积一层厚45nm的钛铌酸锶钡(Ba1-xSrxNbyTi1-yO3)薄膜，制成Al/Ba1-xSrxNbyTi1-yO3/SiO2/Si结构平面型薄膜电阻器。测试了该薄膜电阻器的吸收光谱，不同照度、电压下的光电流，调制光下的频率特性。实验结果表明：在钛酸钡中掺入锶(Sr)和铌(Nb)后，禁带内引入了杂质能级，钛铌酸锶钡禁带宽度变为2.7eV，且可见光区域存在连续的吸收峰。该薄膜在近紫外及可见光范围内具有良好的光敏特性，其灵敏度和光电导增益较高，并且具有较好的线性光照特性。光照强度较低时，电阻器的光照特性属于单分子复合过程；光照强度较高时，电阻器的光照特性属于双分子复合过程。通过对该薄膜电阻器频率特性的测量得出：在光照度为200lx时，薄膜中光生载流子的寿命为27ms。     

Phenomena caused by the capture of carriers injected into illuminated cadmium selenide single crystals

The current-voltage characteristics of the dark current and the photocurrent excited by light from the edge region of the fundamental absorption have been measured for CdSe single crystals fitted with sandwichtype indium-gallium electrodes and guard rings to eliminate surface currents. At a certain voltage, the photocurrent characteristics show a tendency toward saturation, due to the filling of traps by charge carriers injected from the cathode, which causes a reduction of the conduction-electron mobility.     

Photoelectrochemical Characteristics Of α-Fe2O3 Nanocrystalline Semiconductor Thin Film

-Fe₂O₃ single crystal thin films have been prepared from 45nm diameter colloid. These thin nanocrystalline films exhibit a typical behavior of n-type semiconductor material because of the anodic photocurrent generation. The anodic photocurrent response upon illumination and the reversal spike of cathodic current upon the light switched off suggests that the electrons can flow in both directions and no space charge layer exists at the thin film/electrolyte interface. The decreased photocurrent responses of thicker films can be explained by the electric resistance effect and recombination effect. Moreover, the thicker film will lead to a poor photocurrent response for short wavelength light. Considering the use of sunlight, the thin film thickness should be controlled to an optimal value.     

Effect of the oligothiophene chain length on the performance of organic photovoltaic cells

We investigate photovoltaic cells based on α-oligothiophene molecules with different chain lengths nT (n=4,6,8) deposited on a sandwich structure between two electrodes: ITO and Al. From the analysis of the absorption and photocurrent spectrum, we have shown that the structure of the oligothiophene affects considerably the absorbing character of the evaporated film, which is increasingly significant from 4T towards 8T, and a significant photocurrent in agreement with strong absorption in the region Al/nT. Moreover, a theoretical model of Ghosh is used to display the photocurrent origin, proved to be sufficient after adjustment with a random term. Then we deduce that the crystalline order of the film improves the transport of charge and the photocurrent. The results are confirmed by electrical characterization under illumination that shows effectively an influence of the expansion of the molecular chains on the increase in the energy efficiencies and on the performances of organic photovoltaic cells.     

Specific features of photo-and electrical conduction in manganese germanium garnets

The specific features of photo-and electrical conduction in manganese germanium garnet crystals are investigated in the temperature range 4.2–370 K for the first time. Under exposure of samples with ohmic contacts to visible light, the photocurrent in these samples is observed only at high temperatures. The characteristic times of the photocurrent rise differ from those of photocurrent relaxation after the light is switched off. The inference is made that the photo-and electrical conduction is determined by the electrical recharging of manganese ions. The generation and transport of charge carriers are controlled by centers with electrical inhomogeneities and shallow attachment levels.     

Enhanced photogalvanic effect in the two-dimensional MgCl2/ZnBr2 vertical heterojunction by inhomogenous tensile stress

The photogalvanic effect (PGE) occurring in noncentrosymmetric materials enables the generation of a dc photocurrent at zero bias with a high polarization sensitivity, which makes it very attractive in photodetection. However, the magnitude of the PGE photocurrent is usually small, leading to a low photoresponsivity, and therefore hampers its practical application in photodetection. Here, we propose an approach to largely enhancing the PGE photocurrent by applying an inhomogenous mechanical stretch, based on quantum transport simulations. We model a two-dimensional photodetector consisting of the wide-bandgap MgCl₂/ZnBr₂ vertical van der Waals heterojunction with the noncentrosymmetric C_3v symmetry. Polarization-sensitive PGE photocurrent is generated under the vertical illumination of linearly polarized light. By applying inhomogenous mechanical stretch on the lattice, the photocurrent can be largely increased by up to 3 orders of magnitude due to the significantly increased device asymmetry. Our results propose an effective way to enhance the PGE by inhomogenous mechanical strain, showing the potential of the MgCl₂/ZnBr₂ vertical heterojunction in the low-power UV photodetection.     

Metal-semiconductor-metal ultraviolet photodetector based on GaN

A metal-semiconductor-metal (MSM) ultraviolet photodetector has been fabricated using unintentionally doped n-GaN films grown on sapphire substrates. Its dark current, photocurrent under the illumination with λ = 360 nm light, responsivity, and the dependence of responsivity on bias voltage were measured at room temperature. The dark current of the photodetector is 1.03 Na under 5 V bias, and is 15.3 Na under 10 V bias. A maximum responsivity of 0.166 A/W has been achieved under the illumination with λ= 366 nm light and 15 V bias. It exhibits a typical sharp band-edge cutoff at the wavelength of 366 nm, and a high responsivity at the wavelength from 320 nm to 366 nm. Its responsivity under the illumination with λ= 360 nm light increases when the bias voltage increases.