Production of photocurrent due to intermediate-to-conduction-band transitions: a demonstration of a key operating principle of the intermediate-band solar cell

We present intermediate-band solar cells manufactured using quantum dot technology that show for the first time the production of photocurrent when two sub-band-gap energy photons are absorbed simultaneously. One photon produces an optical transition from the intermediate-band to the conduction band while the second pumps an electron from the valence band to the intermediate-band. The detection of this two-photon absorption process is essential to verify the principles of operation of the intermediate-band solar cell. The phenomenon is the cornerstone physical principle that ultimately allows the production of photocurrent in a solar cell by below band gap photon absorption, without degradation of its output voltage.     

Effect of thermal annealing on sub-band-gap absorptance of microstructured silicon in air

The optical absorption properties of femtosecond-laser-made “black silicon” as a function of the annealing conditions were investigated. We found that the annealing process changes the surface morphology and absorption spectroscopy of the “black silicon” samples, and obtained a maximum sub-band-gap absorptance value of approximately 30% by annealing at 1000 °C for 30 min. The thermal relaxation and atomic structural transformation mechanisms are used to describe the lattice recovery and the increase and decrease of the substitutional dopant atom concentration in the microstructured surface during the annealing. Our results confirm that: i) owing to the thermal relaxation, the lattice defects decrease with the increase of the annealing temperature; ii) the quasi-substitutional and interstitial configurations of the doped atoms transform into substitutional arrangements when the annealing temperature increases; iii) the quasi-substitutional and interstitial configurations with higher energies of the doped atoms transform into interstitial configurations with the lowest energy after high-temperature annealing for a long period of time, causing the deactivation or reactivation of the sub-band-gap absorptance by diffusion. The results demonstrate that the annealing can improve the properties of “black silicon”, including defects repairing, carrier lifetime lengthening, and retention of a high absorptive performance.     

Optical and dielectric properties of nanocomposites systems based on epoxy resins and reactive polyhedral oligosilsquioxanes

An epoxy network structure made of diglycidylether of bisphenol-A and diamino diphenylsulfone was modified by adding various amounts of an epoxy functionalized polyhedral oligomeric silsesquioxane. The obtained nanocomposites were characterized in terms of optical and dielectric properties. The UV-absorption spectra were collected in the wavelength range of 400–800 nm. The optical data were analyzed in terms of absorption formula for non-crystalline materials. The optical energy gap and other basic constants, such as energy tails, dielectric constants, refractive index and optical conductivity, were determined and showed a clear dependence on the POSS concentration. It was found that the optical energy gap for the neat epoxy resin is less than for nanocomposites, and it decreases with increase in the POSS content. The refractive index of nanocomposites was determined from the calculated values of absorption and reflectance. It was found that the refractive index and the dielectric constants increased with increase in the POSS concentration. The optical conductivity, which is a measure of the optical absorption, increased with the POSS content. Furthermore, it was found that the glass transition temperature and the optical energy gap correlate well with the POSS filler concentration.     

In掺杂ZnO超晶格光学性质的研究

采用基于第一性原理的线性缀加平面波方法, 计算了超晶格ZnO掺In的光学性质. 计算结果表明, 掺入In元素后Fermi能级进入导带, 介电函数、吸收系数、折射率、反射率都在Fermi面附近产生新的跃迁峰. 随着掺杂层数的增多, 跃迁峰发生红移, 当掺杂两层In时跃迁峰峰值最大, 同时吸收系数的吸收边随掺杂层数的增多而逐渐减小. 与In掺杂ZnO超晶胞相比, ZnO超晶格在可见光范围内具有高透过率的特点.     

PbS纳米颗粒复合的溶胶凝胶薄膜的飞秒非线性光学特性

通过光Kerr效应研究了PbS半导体纳米颗粒的三阶光学非线性响应。在较低的激发强度下,单激子态的饱和吸收和双激子效应的激发态吸收对非线性的贡献同时存在;而在高激发强度下,双激子效应是主要的。计算了高激发强度下(4mJ/cm²)PbS半导体纳米颗粒的三阶非线性光学极化率为5.1×10^-10esu。     

Annealing-induced optical and sub-band-gap absorption parameters of Sn-doped CdSe thin films

Thin films of Sn-doped CdSe were prepared by thermal evaporation onto glass substrates in an argon gas atmosphere and annealed at different temperatures. Structural evaluation of the films was carried out using X-ray diffraction and their stoichiometry studied by energy-dispersive X-ray analysis. The films exhibit a preferred orientation along the hexagonal direction of CdSe. The optical transmittance of the films shows a red shift of the absorption edge with annealing. The fundamental absorption edge corresponds to a direct energy gap with a temperature coefficient of 3.34 × 10^?3 eV K^?1. The refractive index, optical conductivity and real and imaginary parts of the dielectric constants were found to increase after annealing. The sub-band gap absorption coefficient was evaluated using the constant photocurrent method. It varies exponentially with photon energy. The Urbach energy, the density of defect states, and the steepness of the density of localized states were evaluated from the sub-band-gap absorption.     

Fe, S共掺杂SnO2材料第一性原理分析

本文采用基于第一性原理的全电势线性缀加平面波(FP-LAPW)法, 计算了Fe, S两种元素共掺杂SnO₂材料的电子结构和光学性质. 结果表明: 材料仍为直接禁带半导体, 体系呈现半金属性; Fe, S共掺可以窄化带隙, 且随S浓度增加, 态密度向低能方向移动, 带隙减小; 共掺体系电荷密度重新分布, 随S浓度增加, Fe原子极化程度增强, 原子间键合能力增强. 共掺后介电函数虚部谱与光学吸收谱各峰随S浓度增加而发生红移, 光学吸收边减小.     

Fabrication and sub-band-gap absorption of single-crystal Si supersaturated with Se by pulsed laser mixing

Selenium supersaturated silicon layers were fabricated by pulsed excimer laser induced liquid-phase mixing of thin Se films on Si(001) wafers. Sufficiently low Se coverage avoids destabilization of rapid epitaxial solidification, resulting in supersaturated solid solutions free of extended defects, as shown by transmission electron microscopy. The amount of retained Se depends on the original film thickness, the laser fluence, and the number of laser pulses irradiating the same spot on the surface. Using this method, Se has incorporated into the topmost 300 nm of the silicon with a concentration of 0.1 at.%. Channeling Rutherford backscattering spectrometry measurements show that the substitutional fraction can be as high as 75% of the total retained Se. These alloys exhibit strong sub-band-gap absorption with optical absorption coefficient ranging up to about 10⁴ cm⁻¹, thus making them potential candidates for applications in Si-based optoelectronic devices.     

Coherently photoinduced ferromagnetism in diluted magnetic semiconductors

Ferromagnetism is predicted in undoped diluted magnetic semiconductors illuminated by intense sub-band-gap laser radiation. The mechanism for photoinduced ferromagnetism is coherence between conduction and valence bands induced by the light which leads to an optical exchange interaction. The ferromagnetic critical temperature T(C) depends both on the properties of the material and on the frequency and intensity of the laser and could be above 1K.     

Electronic Structure and Optical Properties of Antimony-Doped SnO_2 from First-Principle Study

A first-principles study has been performed to calculate the electronic and optical properties of the SbxSn1xO system.The simulations are based upon the method of generalized gradient approximations with the Perdew-Burke-Ernzerhof form in the framework of density functional theory.The supercell structure shows a trend from expanding to shrinking with the increasing Sb concentration.The increasing Sb concentration induces the band gap narrowing.Optical transition has shifted to the low energy range with increasing Sb concentration.Other important optical constants such as the dielectric function,reflectivity,refractive index,and electron energy loss function for Sb-doped SnO2 are discussed.The optical absorption edge of SnO2 doped with Sb also shows a redshift.     

镧系元素掺杂TiO_2的电子和光学特性的第一性原理计算

基于密度泛函理论的第一性原理局部密度近似法,结合Hubbard U校正(LDA+U),研究了镧系元素掺杂的锐钛矿型TiO_2的取代能、热力学电荷跃迁能级和光学性质.除La之外的所有镧系元素,在掺杂时向主带隙中引入杂质态.所得取代能表明了镧系元素掺杂TiO_2的可行性.同时预测了最佳掺杂比例约为3%,由Ce、Nd、Sm、Gd或Tm掺杂引入的掺杂能级具有负U特性.另外,所得热力学跃迁能级预测了Lu在主带隙内不发生电荷跃迁.算出的光吸收系数表明镧系元素掺杂可使TiO_2具有可见光吸收性能.     

The optical properties and energy transition process in nanocomposite of polyvinyl-pyrrolidone polymer and Mn-doped ZnS

This study has been carried out on the optical properties of polyvinyl-pyrrolidone (PVP), the energy transition process in nanocomposite of PVP capped ZnS:Mn nanocrystalline and the influence of the PVP concentration on the optical properties of the PVP capped ZnS:Mn nanocrystalline thin films synthesized by the wet chemical method. The microstructures of the samples were investigated by X-ray diffraction, the atomic absorption spectroscopy, and transmission electron microscopy. The results showed that the prepared samples belonged to the sphalerite structure with the average particle size of about 2–3 nm. The optical properties of samples are studied by measuring absorption, photoluminescence (PL) spectra and time-resolved PL spectra in the wavelength range from 200 to 700 nm at 300 K. From data of the absorption spectra, the absorption edge of PVP polymer was found about of 230 nm. The absorption edge of PVP capped ZnS:Mn nanoparticles shifted from 322 to 305 nm when the PVP concentration increases. The luminescence spectra of PVP showed a blue emission with peak maximum at 394 nm. The luminescence spectra of ZnS:Mn–PVP exhibits a blue emission with peak maximum at 437 nm and an orange–yellow emission of ion Mn²⁺ with peak maximum at 600 nm. While the PVP coating did not affect the microstructure of ZnS:Mn nanomaterial, the PL spectra of the PVP capped ZnS:Mn samples were found to be affected strongly by the PVP concentration.     

不同浓度Mg掺杂单层Janus WSSe的第一性原理研究

二维Janus WSSe作为一种新型过渡金属硫族化合物(TMDs)材料由于其独特的面外非对称结构及众多新颖的物理特性,在自旋电子器件中具有巨大的应用潜力.本文基于密度泛函理论的第一性原理平面波赝势方法,通过构建四种掺杂模型W_9-xMg_xS₉Se₉(x=0、1、2、3),分别计算了不同浓度Mg掺杂单层WSSe的电子结构和光学性质.结果表明：掺杂使得WSSe由直接带隙半导体变为间接带隙半导体,并且随着掺杂浓度的增加,带隙逐渐减小,费米能级穿过价带,使得掺杂体系变成P型半导体,当x=3时,掺杂体系呈现金属性.此外,掺杂体系的静态介电常数随着掺杂浓度的增加而变大,极化程度显著增强,介电函数虚部和光吸收峰都发生了红移,说明掺杂有利于可见光的吸收.并且,静态折射率随着掺杂浓度的增加而呈现上升趋势,同时消光系数的峰值也与Mg原子的掺杂浓度呈现正相关.     

Mn2+、N2-共掺杂Zn2GeO4晶体电子结构和光学性质的研究

采用基于密度泛函理论的平面波超软赝势方法对本征Zn2GeO4,Mn2+掺杂Zn2GeO4,Mn2+/N2-共掺杂Zn2GeO4超晶胞进行了几何结构优化,计算了掺杂前后体系的晶格常数、能带结构、态密度和光学性质。结果表明,Mn离子掺入后,Mn离子3d轨道与O离子2p轨道之间有强烈的轨道杂化效应,掺杂系统不稳定,而Mn/N离子共掺后,Mn离子和N离子之间的吸引作用克服了Mn离子之间的排斥作用,能够明显地提高掺杂浓度和体系的稳定性。光学性质计算结果表明,Mn离子与N离子共掺杂能改善Zn2GeO4电子在低能区的光学跃迁特性,增强电子在可见光区的光学跃迁;吸收谱计算结果显示,Mn离子与N离子掺入后体系对低频电磁波吸收增加。     

Defects related emission and nanosecond optical power limiting in CuS quantum dots

We report optical and nonlinear optical properties of CuS quantum dots and nanoparticles prepared through a nontoxic, green, one-pot synthesis method. The presence of surface states and defects in the quantum dots are evident from the luminescent behavior and enhanced nonlinear optical properties measured using the open aperture Z-scan, employing 5 ns laser pulses at 532 nm. The quantum dots exhibit large effective third order nonlinear optical coefficients with a relatively lower optical limiting threshold of 2.3 J cm⁻², and the optical nonlinearity arises largely from absorption saturation and excited state absorption. Results suggest that these materials are potential candidates for designing efficient optical limiters with applications in laser safety devices.     

Intersubband resonant enhancement of the nonlinear optical properties in asymmetric (CdS/ZnSe) based quantum wells

In this work, the linear and nonlinear optical properties are studied theoretically in asymmetric (CdS/ZnSe/BeTe)/(ZnSe/BeTe) quantum wells. The electronic states are calculated using the envelope wave function approximation and the intersubband transition energies are studied as a function of CdS and ZnSe well thicknesses as well as doping concentration. The optimum parameters carrying out the transition energy 0.8 eV (1.55 μm wavelength) are given. Results are presented for the linear, the third order nonlinear optical absorption and the refractive index changes in the studied heterostructure. Results show that the changes in the linear and the third order nonlinear optical absorption as well as refractive index change are as important as the temperature is high, the nonlinear terms must be taken into consideration especially near the resonance.     

Optical and electrical studies of as-prepared and annealed Se-Te-Bi thin films

The effect of thermal annealing on the electrical and optical properties of a thin film of Se-Te-Bi alloy has been studied. It has been found that the mechanism of optical absorption follows a non-direct transition. The optical band-gap decreases with an increase in the Bismuth concentration. The electrical conductivity of the as-deposited and annealed films has been found to be of Arrhenius type. The results are discussed on the basis of rearrangements of defects and disorders in the chalcogenide systems. PACS 71.23.An; 73.61.-r; 73.61Jc; 74.25Gz; 78.66Jg     

空位浓度对纤锌矿BN电子结构和光学性质影响的第一性原理研究

本文采用了第一性原理研究了空位缺陷纤锌矿BN的电子结构和光学性质.通过对能带结构、态密度分析发现：缺陷体系由于B、N的缺失,费米能级附近出现杂质能级.相较于本征体系,随着空位浓度增加,杂质能级变多,跃迁能量减小. N空位缺陷的纤锌矿BN态密度总体向低能区移动,且能级相较于B空位缺陷的纤锌矿BN明显增多.从复介电函数和光学吸收谱分析中发现,随着空位浓度的增加,缺陷体系纤锌矿BN在可见光区域的吸收逐渐增强.尤其是B₂₂N₂₄在可见光区域出现的吸收效果更好.     

基于光学浮标的赤潮生消过程半分析监测方法

赤潮爆发时水体叶绿素a质量浓度升高,引起浮游植物吸收系数、光束总吸收系数等水体固有光学性质(IOP)的变化,并导致水体表观光学性质(AOP)的改变。海洋光学浮标可实现水体表观光学性质的定点连续时间序列观测,基于此发展相应的模型方法有望实现赤潮生消全过程的监测。利用一次赤潮生消过程的海洋光学浮标数据,发展了一种赤潮半分析监测方法。该方法首先由光学浮标数据得到的水体光谱漫衰减系数Kd(λ)和遥感反射率rrs(λ),结合经验确定的水下光场平均余弦进行水体光束总吸收系数a(λ)的半分析估算,然后再半分析反演浮游植物吸收系数aph(λ)和叶绿素a质量浓度。经检验,该方法估算a(675),aph(675)和叶绿素a质量浓度的中值相对误差分别为8.6%,34.9%和38.9%。将本方法与半分析方法(QAA)和统计回归方法进行了对比分析,本方法的优势在于反演精度较高,所采用的经验参数大都源自辐射传输理论计算、不依赖于浮标数据且对反演结果的影响有限。     

高压下AlN的电子结构和光学性质

运用密度泛函理论体系下的平面波赝势(PWP)和广义梯度近似(GGA)方法,利用第一性原理计算了不同压强下六方纤锌矿结构AlN晶体的晶格常数、总能量、电子态密度、能带结构、光反射系数与吸收系数。通过比较能带结构的变化行为,得出 AlN在16.7Gpa附近存在等结构相变,即由直接带隙结构转变为间接带隙结构。同时,结合高压下的态密度分布图和能级移动情况,分析了AlN在高压下的光学性质,吸收谱有向高能端移动(蓝移) 的趋势。