UV-photoassisted etching of GaN in KOH

The etch rate of GaN under ultraviolet-assisted photoelectrochemical conditions in KOH solutions is found to be a strong function of illumination intensity, solution molarity, sample bias, and material doping level. At low e-h pair generation rates, grain boundaries are selectively etched, while at higher illumination intensities etch rates for unintentionally doped (n～ 3×10¹⁶cm^?3) GaN are ≥1000Å·min^?1. The etching is diffusion-limited under our conditions with an activation energy of ～ 0.8kCal·mol^?1. The etched surfaces are rough, but retain their stoichiometry.     

Evidence of Interfacial Charge Transfer upon UV‐Light Irradiation in Novel Titanium Oxide Gel

We report on the combined magnetization and electron paramagnetic resonance characterization of a novel Ti‐O organic–inorganic gel hybrid and the related electron–hole generation process upon UV illumination. We find that electrons are injected into the conduction band of the Ti‐O framework, photoreducing Ti^IV to Ti^III. Ti^III sites are mainly located on the surface, owing to the nanometric dimensions of the inorganic component. Surprisingly, the symmetry of the TiO₆ octahedra depends on the level of illumination: in the lightly UV‐exposed samples Ti^III is sited in the weakly distorted TiO₆ octahedra to which methanoate groups are bonded, as suggested by electron spin echo envelope modulation (ESEEM) experiments. Extensive illumination causes structural rearrangements, leading to enhanced tetragonal TiO₆ distortion and shifting the Ti^III interaction towards the hydroxide groups or water. The results provide clear evidence for an interfacial charge transfer between the quantum‐size TiO lattice and coordinated species upon in situ and ex situ UV illumination at temperatures from room temperature to 5 K.     

A Time Error Correction Method Applied to High-Precision AER Asynchronous CMOS Image Sensor

A time error correction method for improving the accuracy of asynchronous CMOS (Complementary Metal Oxide Semiconductor) imager sensor (CIS) is proposed. By adding a time error measurement unit in each pixel, the time error can be recorded, and then is sent out together with the address-events. Besides, in the external processing circuit, the correction will be accomplished before making time-stamping. For the sake of clarifying the influences on the image accuracy, different scale of the pixel array, event collisions, accuracy of time-stamping and illumination are adopted in the simulation. The simulation results of the Matlab Simulink show that, in an 8?×?8 asynchronous CIS without time error correction, when the illumination is 10³?~?10⁵ lx, the error of the image accuracy is from 0.4 % to 28 %. With the decrease of the time-stamping accuracy, the expansion of pixel array, the increase of event collisions and the strengthening of illumination, the error would deteriorate further. As to the model of asynchronous CIS with time error correction, when the accuracy of the time-stamping is 10 ns, the time error can be controlled within 20 ns. For the same illumination, the maximal error of image is 0.31 %.     

A Self‐Powered Brain‐Linked Vision Electronic‐Skin Based on Triboelectric‐Photodetecing Pixel‐Addressable Matrix for Visual‐Image Recognition and Behavior Intervention

A new self‐powered brain‐linked vision electronic‐skin (e‐skin) for mimicking retina is realized from Polypyrrole/Polydimethysiloxane (Ppy/PDMS) triboelectric‐photodetecting pixel‐addressable matrix. The e‐skin can be driven by human motion, so no external electricity power is needed in both photodetecting and signal transmitting processes. The triboelectric output is significantly dependent on the photo illumination, which can act as visual bionic electric impulse. Taking blue illumination (405 nm) as an example, as the e‐skin is exposed to 100 µW cm^?2 illumination, the output current decreases from 7.5 to 4.9 nA, and the photosensitivity is 34.7. And the photosensitivity of the e‐skin keeps stable with different bending angles and force. The e‐skin is flexible enough to combine with human body and can be driven by blinking eyes to detect UV illumination. In addition, the 4 × 4 photodetecting unit matrix in the e‐skin can map single‐point and multipoint illumination‐stimuli (visual‐image recognition) via the multichannel data acquisition method. Furthermore, the e‐skin can directly transmit photodetecting signals into mouse brain for participating in the perception and behavior intervention. This new self‐powered perception device can lower down the production cost of traditional complex sensory‐substitution system, and can be easily extended to various brain–machine interaction applications.     

ZnSe quantum dots sensitized electrospun ZnO nanofibers as an efficient photoanode for improved performance of QDSSC

In the present study, zinc selenide (ZnSe) quantum dots (QDs) with an average size of ~2.6 nm were prepared by hot injection method and used as a sensitizer onto the electrospun ZnO nanofibers using 3-mercaptapropionic acid as a linker agent. The optical absorption, photoluminescence and time-resolved photoluminescence (TRPL) studies for ZnSe sensitized ZnO NFs were performed to give insight about the improvement in optical properties. The performances of fabricated QDSSCs was examined in detail using cobalt sulfide (CoS) as a counter electrode and polysulfide redox couple (S²⁻/S_x²⁻) as an electrolyte. The ZnSe QDs sensitized ZnO nanofibers showed an appreciable improvement in short circuit current density (6.60 mA/cm²) with a maximum power conversion efficiency of 1.24% under 1 sun illumination of 100 mW/cm². This enhancement is mainly due to better light harvesting ability of ZnSe QDs and ZnO NFs, and lower recombination of photoinjected electrons with the polysulfide electrolyte. The improvement in power conversion efficiency (PCE) and reduction in back electrons recombination are supported by photovoltaic and electrochemical impedance studies. Finally, stability test was carried out over a span of 30 days (720 h) under one sun illumination to know about the practical applicability of the resultant QDSSC.     

Optical generation of free charge carriers in thin films of tin oxide

The methods of infrared absorption spectroscopy and Raman spectroscopy are used to study nanocrystalline SnO _x films (1 ≤ x ≤ 2) prepared by thermal oxidation of metallic tin layers. A monotonic decrease in the transmittance of films in the infrared region has been observed as a result of exposure of the films to light with the wavelength of 380 nm at room temperature. The effect is at a maximum for the samples with x ≈ 2 and is observed for ∼10 min after switching off of illumination. The mentioned variations in optical properties, similarly to those observed in the case of heating of the samples in the dark, are accounted for by an increase in the concentration of free charge carriers (electrons) in nanocrystals of tin dioxide. The data of infrared spectroscopy and the Drude model are used to calculate the concentrations of photogenerated charge carriers (∼10¹⁹ cm⁻³); variations in these concentrations in the course of illumination and after switching off of illumination are determined. Mechanisms of observed photogeneration of charge carriers in SnO _x films and possible applications of this effect to gas sensors are discussed.     

Low-frequency noise in n-GaN

Low-frequency noise has been investigated in the hexagonal polytype of n-type gallium nitride (GaN) with equilibrium electron concentration at 300 K n ₀⋍7×10¹⁷ cm⁻³. The frequency and temperature dependence of the noise spectral density S _I/I² was studied in the range of analysis frequencies f from 20 Hz to 20 kHz in the temperature range from 80 to 400 K. Over the entire temperature range the frequency dependence of the dark noise is close to S _I/I²∼1/f (flicker noise). The rather weak temperature dependence of the noise level is characterized by very high values of the Hooge constant α⋍5–7. These large α values indicate a rather low level of structural quality of the material. The effects of infrared and band-to-band illumination on low-frequency noise in GaN are studied here for the first time. The noise level is unaffected by illumination with photon energy E _ph<E _g (E _g is the band gap) even for a relatively high value of the photoconductivity Δσ⋍50%. Band-to-band illumination (E _ph⩾E _g) influences the low-frequency noise level over the entire investigated temperature range. At relatively high temperatures the influence of illumination is qualitatively similar to that of band-to-band illumination on low-frequency noise in Si and GaAs. At relatively low temperatures the influence of illumination on the noise in GaN is qualitatively different from the results obtained earlier for Si and GaAs. Fiz. Tekh. Poluprovodn. 32, 285–289 (March 1998)     

Characterization of polymer bulk heterojunction photocell with unmodified C70 prepared with halogen-free solvent for indoor light harvesting

Although photocells are commonly characterized under AM1.5G 100 mW cm⁻² (1 sun) illumination, their performance under low light illumination is also important, because photocells are frequently used for indoor applications. In this study, polymer photocells based on a bulk heterojunction composite consisting of a low energy gap polymer PTB7 and unmodified C₇₀ prepared with a halogen-free solvent 1,2,4-trimethylbenzene have been characterized under the illumination of 1 sun or below. A typical photocell with the power conversion efficiency (PCE) of 4% at 1 sun shows the PCE of approximately 7% at 10⁻³ sun, which seems to fit for some indoor applications such as a permanent power source for a wireless sensor node. The sublinear dependence of short-circuit photocurrent on light intensity as well as the increase of fill-factor under low light illumination yields the increased efficiency under low light illumination. An analysis employing a one-diode equivalent circuit model suggests that the increased parallel resistance as well as the decreased saturation current of the diode under low light illumination accounts for the latter feature. It is also pointed out that the parallel resistance and/or the saturation current under dark strongly influence the PCE of a photocell under low light illumination. In addition, the dependence of the device performance on the light intensity is found to be useful for analyzing the effects of the thermal treatment and the PFN interlayer at cathode.     

Novel Ω‐Shaped Core–Shell Photodetector with High Ultraviolet Selectivity and Enhanced Responsivity

The design of nanostructure plays an important role in performance enhancement of low‐dimensional optoelectronic devices. Herein, a novel photodetector (PD) based on electrospun SnO₂ nanofibers with Ω‐shaped ZnO shell (SnO₂@ZnO) is fabricated. With 87.4% transmittance at 550 nm, SnO₂@ZnO PD exhibits a high photo‐to‐dark current ratio up to 10⁴ at around 280 nm. Owing to the additional Ω‐shaped ZnO shell, SnO₂@ZnO PD possesses a responsivity of nearly 100 A W^?1 under 5 V bias and the illumination of 250 nm light, which is 30‐time enhancement of pristine SnO₂ PD. The enhancement is mainly attributed to type‐II energy band structure. Furthermore, by changing the direction of incident light, SnO₂@ZnO PD has a high UV selectivity with an UV–vis rejection ratio (R _{250 nm}/R _{400 nm}) as much as 2.0 × 10³ at 5 V bias under back illumination, which is fourfold higher than that under face illumination. The UV selectivity improvement may be attributed to light confinement in the Ω‐shaped structure. With both theoretical simulations and experimental comparisons, it is demonstrated that the unique compact Ω‐shaped nanostructure does contribute to photon trapping and gaining process, especially in back‐illumination configuration. The approach can be easily extended to other materials, preparing novel building blocks for optoelectronic devices.     

Conversion of commercial si solar cells to keep their efficient performance at 15 suns

The screen‐printing method is an economical metallization technique used by most manufacturers of conventional silicon solar cells. This method limits the cells' use under concentrated light owing to high series resistance losses caused, among other reasons, by low metal density in the fingers. This paper describes increasing the finger metal density by electrolytic deposition. The electrolytic deposition of silver is an economical, controllable and readily commercializable deposition method to reduce the front and back metallization series resistance contributions. With an optimized grid design, compatible with 1 sun silicon cell technology, and later electrolytic silver deposition we have obtained cells that maintain their efficiency up to 15 suns. In addition, an analysis of the performance of these cells under uniform and non‐uniform illumination were carried out on n⁺p and n⁺pn⁺ structures. Copyright © 2004 John Wiley & Sons, Ltd.     

Effects of Postproduction Treatment on Plastic Solar Cells

Efficiencies of organic solar cells based on an interpenetrating network of a conjugated polymer and a fullerene as donor and acceptor materials still need to be improved for commercial use. We have developed a postproduction treatment that improves the performance of solar cells based on poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) by means of a tempering cycle at elevated temperatures in which an external voltage is simultaneously applied, resulting in a significant increase of the short‐circuit current. Using this postproduction treatment, an enhancement of the short‐circuit current density, I_sc, to 8.5 mA cm^–2 under illumination with white light at an illumination intensity of 800 W m^–2 and an increase in external quantum efficiency (IPCE, incident photon to collected electron efficiency) to 70 % are demonstrated.     

The physical behaviour of an n+p silicon solar cell in concentrated sunlight

The performance of a simple n⁺p silicon solar cell at various illumination levels is analysed by a modified form of the Gummel and De Mari numerical algorithms. Effects of high doping, such as bandgap narrowing together with the correction to density of states are included. The effective recombination life time of the charge carriers due to both Shockley-Read-Hall recombination via traps and Auger recombination is taken into account. The base acceptor doping concentration is 10¹⁶ cm^?3. The light concentration is varied from 1 to 200 AM1. The physical mechanisms of the device at various levels of illumination are discussed by determining the cell parameters, namely, saturation current density, short circuit current density, ideality factor and fill factor. The ideality factor which is close to 1 at low illumination suggests that the cell is controlled by diffusion-recombination processes. The high value of the ideality factor, which is very much greater than 1 but less than 2, at high-illumination is attributed to high-injection effect. The efficiency reaches a maximum around 100 AM1 and starts falling beyond that. This fall is due to the high-injection and the voltage drop in the base layer. The fill factor starts falling at high-illumination.     

基于朗伯反射模型的光照估计及鲁棒人脸识别

复杂光照下的人脸识别是模式识别领域一个具有挑战性的问题.通过光照估计提取光照不变量是解决该问题的一种有效方法.在研究朗伯图像获取模型的基础上,提出一种有效的光照估计模型.该模型能够从复杂光照图像中更准确地估计光照,提取光照不变量.Yale B+复杂光照人脸库的实验结果表明所提算法能够提取更为鲁棒的光照不变量,识别性能优于当前的先进方法.     

Effect of in situ photoexcitation of n-type Si as a result of ion implantation at low doses on the formation of radiation defects

The effect of in situ photoexcitation of the electronic subsystem of a semiconductor as a result of implantation of low ion doses on the formation of complexes of radiation defects in n-type Si is investigated by the DLTS method. The n-type Si samples were irradiated with 150-keV O ₂ ⁺ and N ₂ ⁺ ions at the same dose 10¹¹ cm⁻² and Ar⁺ ions at doses 7×10¹⁰ and 2×10¹¹ cm⁻². With the exception of the latter case, the ion energy and dose were chosen so as to produce approximately the same number of initially displaced Si atoms and the same depth distribution of such atoms from the target surface. The temperature of the n-type Si samples during irradiation was 300 or 600 K. Photoexcitation of the semiconductor was performed using UV radiation with various power densities. It is shown that radiative heating of the samples during ion implantation suppresses the formation of radiation-defect complexes, while photoexcitation of n-type Si, in contrast, intensifies their formation. It is found that the effect of illumination increases with decreasing ion mass and with increasing target temperature. The effect of UV illumination on defect formation in n-type Si as a function of sample temperature during ion implantation is established. It is found that the density of divacancies in n-type Si saturates with increasing illumination intensity. Fiz. Tekh. Poluprovodn. 33, 537–541 (May 1999)     

三维传感中衍射型结构照明光场的应用

在基于结构照明的三维传感系统中，产生结构照明的方法和装置对正确和有效的三维重建具有重要的意义。本文提出了一种新颖的产生结构照明的方法－基于衍射，特别是菲涅耳衍射原理的结构照明方法，利用角谱理论，通过分析在激光光刀照明条件下，两种不同正弦型光栅的菲涅耳衍射光场的强度分布情况，提出了基于衍射原理的结构照明的原理和方法，并且给出了适用于三维传感的结构照明方案，并通过对实物模型的三维面形测量证实，本文提出的原理和方法是正确的有效的。     

A Method for Fabricating an Ultrathin Multilayer Film Composed of Poly(p‐phenylenevinylene) and Reduced Graphene Oxide on a Plastic Substrate for Flexible Optoelectronic Applications

The photoconductive properties of a uniform ultrathin multilayer film composed of alternating poly(p‐phenylene vinylene) (PPV) and reduced graphene oxide (RGO) layers, fabricated on a poly(ethylene terephthalate) (PET) sheet are reported. The assembly of the two electron‐rich layer components on the temperature‐sensitive substrate is realized using a layer‐by‐layer‐deposition technique under mild conditions and HI/H₂O vapor treatment at 100 °C. This protocol is established to simultaneously convert the layer components to their conjugated counterparts, PPV and RGO in the multilayer films, whose total thicknesses shrinks to 50% of their original values due to lattice contraction. Furthermore, the surface roughness decreases significantly, in contrast to the results obtained from general chemical treatments. The PET sheets coated with (PPV/RGO)₁₅ films exhibit a photocurrent of 115 μA at an illumination intensity of 1.1 mW and a photoresponsivity of 111.1 mA W^?1 at an illumination intensity of 0.5 mW; these are among the best values yet achieved in carbon‐based materials. The establishment of a method for fabricating (PPV/RGO) films on a temperature‐sensitive transparent flexible sheet is crucial for the development of organic‐based portable electronic devices.     

Bifacial silicon solar cells with 21·3% front efficiency and 19·8% rear efficiency

We introduced a triode structure with p–n junctions on both sides into single‐crystalline bifacial silicon solar cells in order to improve solar cell performance. These fabricated bifacial silicon solar cells have an energy conversion efficiency of 21·3% under front 1 sun illumination (the standard 1 kW/m² AM 1·5 global spectrum at 25°C) and 19·8% under rear 1 sun illumination tested at the Japan Quality Assurance Organization. The total of the front and rear conversion efficiencies is the highest ever reported for bifacial silicon solar cells. Copyright © 2000 John Wiley & Sons, Ltd.     

Sun position estimation and tracking for virtual object placement in time-lapse videos

Realistic illumination of virtual objects placed in real videos is important in terms of achieving visual coherence. We propose a novel approach for illumination estimation on time-lapse videos and seamlessly insert virtual objects in these videos in a visually consistent way. The proposed approach works for both outdoor and indoor environments where the main light source is the Sun. We first modify an existing illumination estimation method that aims to obtain sparse radiance map of the environment in order to estimate the initial Sun position. We then track the hard ground shadows on the time-lapse video by using an energy-based pixel-wise method. The proposed method aims to track the shadows by utilizing the energy values of the pixels that forms them. We tested the method on various time-lapse videos recorded in outdoor and indoor environments and obtained successful results.     

TheI–V characteristic analysis of schottky amorphous silicon solar cells

 The present paper introduces an experimental method for measuring the width of illuminated and short-circuited Ni/a-Si:H Schotthy barriers.The current-voltage curves for the Schottky barrier solar cells under an AM1 illumination of 100 mW/cm~2 are calculated by using the parameters determined by experiments.The diffusion length of holes in a-Si:H obtained from the illuminated Ⅰ-Ⅴ curve is consistent with the results measured by the author with the surface photovoltage method in 1983.The factors affecting the fill factor are analysed on the basis of the calculated results.A comparison of the calculated results to the experimental ones reveals that the very low fill factor of the solar cells measured is due to series and shunt resistances rather than the low diffusion length of the Loles.