量子线/铁基超导隧道结中隧道谱的研究

考虑铁基超导中能带间的相互作用和界面对每一个能带的散射作用, 利用推广的Blonder-Tinkham-Klapwijk模型, 并通过求解Bogoliubov-de Gennes 方程研究了具有不同类型双能隙系统的量子线/铁基超导隧道结中准粒子的输运系数和隧道谱. 研究表明: 1)在弹道极限时, 随着带间相互作用的增大, s_± 波隧道谱中零偏压附近的平台演变成电导峰; s₊₊ 波的平台演变成凹陷; p波的零偏压电导峰被压低. 2)界面对两个能带的散射作用不为零时, 随着带间相互作用的增大, s_± 波和s₊₊ 波两能隙处的峰值将降低, 而两峰间的凹陷值将变大; p波的零偏压电导峰被压低, 非零偏压电导增大. 3)界面对每个能带的散射, 可使其产生的电导峰变得更加尖锐, 但可压低和抹平另一个带产生的电导峰值. 这些结果对于澄清铁基超导体的能隙结构和区别不同类型铁基超导体有所帮助.     

Dynamics of quantum noise in a tunnel junction under ac excitation

We report the first measurement of the dynamical response of shot noise (measured at frequency omega) of a tunnel junction to an ac excitation at frequency omega0. The experiment is performed in the quantum regime, variant Planck's over 2piomega approximately variant Planck's over 2piomega0>kBT at very low temperature T=35 mK and high frequency omega0/2pi=6.2 GHz. We observe that the noise responds in phase with the excitation, but not adiabatically. The results are in very good agreement with a prediction based on a new current-current correlator.     

Analysis of quantum coherent semiconductor quantum dot p-i-n junction photovoltaic cells

A recent hypothesis [M.?O. Scully, Phys. Rev. Lett. 104, 207701 (2010).] indicates that the power conversion efficiency of a semiconducting p-i-n junction photovoltaic cell with an intrinsic region consisting of quantum dots can be increased by using quantum coherence to break detailed balance. The limitations of this hypothesis are shown here.     

GaInP/GaAs tandem solar cells with highly Te- and Mg-doped GaAs tunnel junctions grown by MBE

We report a GaInP/GaAs tandem solar cell with a novel GaAs tunnel junction(TJ) with using tellurium(Te) and magnesium(Mg) as n- and p-type dopants via dual-filament low temperature effusion cells grown by molecular beam epitaxy(MBE) at low temperature. The test Te/Mg-doped GaAs TJ shows a peak current density of 21 A/cm2. The tandem solar cell by the Te/Mg TJ shows a short-circuit current density of 12 m A/cm2, but a low open-circuit voltage range of1.4 V~1.71 V under AM1.5 illumination. The secondary ion mass spectroscopy(SIMS) analysis reveals that the Te doping is unexpectedly high and its doping profile extends to the Mg doping region, thus possibly resulting in a less abrupt junction with no tunneling carriers effectively. Furthermore, the tunneling interface shifts from the intended Ga As n++/p++junction to the AlGaInP/GaAs junction with a higher bandgap AlGaInP tunneling layers, thereby reducing the tunneling peak. The Te concentration of ~ 2.5 × 1020 in GaAs could cause a lattice strain of 10-3 in magnitude and thus a surface roughening,which also negatively influences the subsequent growth of the top subcell and the GaAs contacting layers. The doping features of Te and Mg are discussed to understand the photovoltaic response of the studied tandem cell.     

High efficiency solar cells using quantum interferences

In this paper, we propose a solar cell model that absorbs specific band of sunlight and investigate the effect of noise–induced quantum coherence in enhancing the output power of this cell. We numerically demonstrate that such induced coherence can increase the maximum output power from a nano structured solar cell by more than 25% as compared to the same system with no coherence. We also study the influence of increasing the bandwidth of absorption on the solar cell power and numerically show that in spite of decrease in output power due to enhancement of thermalisation loss, presence of quantum coherence effect still increases the output power of solar cell compared to non-coherence case.     

Theoretical characterisation of highly efficient dye-sensitised solar cells

Molecular electronic structure calculations, employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methodologies, have been carried out to improve the performance of the synthesised dye YD2-o-C8 which is characterised by 11.9%–12.7% efficiencies. We aimed to narrow the band gap of YD2-o-C8 to extend the light-harvesting region to near-infrared (NIR). This was done by incorporating Cd instead of Zn onto the porphyrin ring and elongating the length of π-conjugation by adding ethynylene link and anthracene unit, so that the performances of the suggested cells could be expected to exceed the 11.9%–12.7% efficiencies with TiO₂, ZnO₂, and WO₃ oxide electrodes. The effects of modifying the central metal and elongating the length of π-conjugation on cell performance are confirmed in terms of frontier molecular orbital (FMO) energy gaps, density of states (DOS), molecular electrostatic potentials (MEPs), non-linear optical (NLO) properties, ultraviolet–visible (UV–vis) electronic absorption, and ¹H nuclear magnetic resonance chemical shifts. Increasing the length of π-conjugation of the D–π–A dyes leads to increasing the DOS near Fermi levels, more active NLO performance, strong response to the external electric field, delocalisation of the negative charges near the anchoring groups, deep electron injection, suppressing macrocycle aggregation, active dye regeneration, and inhibited dye recombination. The calculated band gap/eV of the present DMP-Zn is correlated with the experimental (E_1/2(oxidation)–E_1/2(reduction)/V) potentials of the identical YD2-o-C8. A co-sensitiser is suggested for NIR sensitisation (550–950 nm) to increase the power-to-conversion efficiency beyond 14%.     

Two-step annealed CdS/CdSe co-sensitizers for quantum dot-sensitized solar cells

CdS/CdSe co-sensitizers on TiO₂ films were annealed using a two-step procedure; high temperature (300 °C) annealing of TiO₂/CdS quantum dots (QDs), followed by low temperature (150 °C) annealing after the deposition of CdSe QDs on the TiO₂/CdS. For comparison, two types of films were prepared; CdS/CdSe-assembled TiO₂ films conventionally annealed at a single temperature (150 or 300 °C) and non-annealed films. The 300 °C-annealed TiO₂/CdS/CdSe showed severe coalescence of CdSe QDs, leading to the blocked pores and hindered ion transport. The QD-sensitized solar cell (QD-SSC) with the 150 °C-annealed TiO₂/CdS/CdSe exhibited better overall energy conversion efficiency than that with the non-annealed TiO₂/CdS/CdSe because the CdSe QDs annealed at a suitable temperature (150 °C) provided better light absorption over long wavelengths without the hindered ion transport. The QD-SSC using the two-step annealed TiO₂/CdS/CdSe increased the cell efficiency further, compared to the QD-SSC with the 150 °C-annealed TiO₂/CdS/CdSe. This is because the 300 °C-annealed, highly crystalline CdS in the two-step annealed TiO₂/CdS/CdSe improved electron transport through CdS, leading to a significantly hindered recombination rate.     

CdS quantum dot-sensitized ZnO nanorod-based photoelectrochemical solar cells

ZnO nanorods have been grown using ZnO seed layer onto ITO-coated glass substrates. CdS quantum dots have been deposited onto ZnO nanorods using simple precursors by chemical method and the assembly of CdS quantum dots with ZnO nanorod has been used as photo-electrode in quantum dot-sensitized solar cells. X-ray diffraction results show that ZnO seed layer, ZnO nanorods, and CdS quantum dot-sensitized ZnO nanorods exhibit hexagonal structure. The particle size of CdS nanoparticle is 5 nm. The surface morphology studied using scanning electron microscope shows that the top surface of the vertically aligned ZnO nanorods is fully covered by CdS quantum dots. The ZnO nanorods have diameter ranging from 100 to 200 nm. The absorption spectra reveal that the absorption edge of CdS quantum dot-sensitized ZnO nanorods shift toward longer wavelength side when compared to the absorption edge of ZnO. The efficiency of the fabricated CdS quantum dot-sensitized ZnO nanorod-based solar cell is 0.69% and is the best efficiency reported so far for this type of solar cells.     

An analysis of quantum coherent solar photovoltaic cells

A new hypothesis (Scully et al., Proc. Natl. Acad. Sci. USA 108 (2011) 15097) suggests that it is possible to break the statistical physics-based detailed balance-limiting power conversion efficiency and increase the power output of a solar photovoltaic cell by using “noise-induced quantum coherence” to increase the current. The fundamental errors of this hypothesis are explained here. As part of this analysis, we show that the maximum photogenerated current density for a practical solar cell is a function of the incident spectrum, sunlight concentration factor, and solar cell energy bandgap and thus the presence of quantum coherence is irrelevant as it is unable to lead to increased current output from a solar cell.     

The p recombination layer in tunnel junctions for micromorph tandem solar cells

A new tunnel recombination junction is fabricated for n–i–p type micromorph tandem solar cells. We insert a thin heavily doped hydrogenated amorphous silicon (a-Si:H) p + recombination layer between the n a-Si:H and the p hydrogenated nanocrystalline silicon (nc-Si:H) layers to improve the performance of the n–i–p tandem solar cells. The effects of the boron doping gas ratio and the deposition time of the p-a-Si:H recombination layer on the tunnel recombination junctions have been investigated. The current-voltage characteristic of the tunnel recombination junction shows a nearly ohmic characteristic, and the resistance of the tunnel recombination junction can be as low as 1.5 ·cm 2 by using the optimized p-a-Si:H recombination layer. We obtain tandem solar cells with open circuit voltage V oc = 1.4 V, which is nearly the sum of the V oc s of the two corresponding single cells, indicating no V oc losses at the tunnel recombination junction.     

GaAs量子阱太阳能电池量子效率的研究

将量子阱结构引入到单结GaAs太阳能电池中能够有效扩展吸收光谱.为了研究量子阱结构在GaAs太阳能电池中的作用机理,本文采用实验和理论的方法研究了InGaAs/GaAsP量子阱结构对电池量子效率的影响.实验结果表明,量子阱结构的窄带隙阱层材料将电池的吸收光谱从890 nm扩展到1000 nm.同时,量子阱结构的引入提高了680—890 nm波长范围内的量子效率,降低了波长在680 nm以下的量子效率.通过计算得到的量子阱结构和GaAs材料的光吸收系数,可以用来解释量子阱结构对太阳能电池量子效率的影响.     

Optical and continuous-wave characteristics of V-grooved quantum well wire lasers confined by a p-n junction array

Role of the side-wall quantum wells in a V-grooved quantum well wire (QWW) is briefly reviewed by temperature-dependent photoluminescence, and then continuous-wave (cw) characteristics of QWW lasers confined by a p-n junction array are reported. In terms of the effectiveness in current confinement, very high power operation (over 11 mW) and a single longitudinal mode operation up to 8mW are achieved. Room-temperature threshold currents are measured to be 32.5 mA (pulsed) and 47.8 mA (cw) for a 200 m long uncoated cavity. The current- and temperature-tuning rates of the oscillation wavelength are as low as 0.038 nm/mA and 0.17 nm/°C, respectively.     

Development of gel polymer electrolytes for application in quantum dot-sensitized solar cells

A methylcellulose–polysulfide gel polymer electrolyte has been prepared for application in quantum dot-sensitized solar cells (QDSSCs) having the configuration FTO/TiO₂/CdS/ZnS/SiO₂/electrolyte/Pt(cathode). The electrolyte with the composition of 30.66 wt.% methylcellulose, 67.44 wt.% Na₂S, and 1.90 wt.% sulfur exhibits the highest conductivity of 0.183 S cm^?1 with the lowest activation energy of 6.14 kJ mol^?1. CdS quantum dot sensitizers have been deposited on TiO₂ film via the successive ionic layer absorption and reaction (SILAR) method. The QDSSC fabricated using the highest conducting electrolyte and CdS QD prepared with five SILAR cycles exhibits a power conversion efficiency (PCE) of 0.78%. After deposition of zinc sulfide (ZnS) and silicon dioxide SiO₂ passivation layers, the PCE of the QDSSC with photoanode arrangement of TiO₂/CdS(5)/ZnS(2)/SiO₂ increased to 1.42%, an improvement in performance by 82%.     

A superconducting tunnel junction receiver for 345 GHz

In this paper we discuss the design, fabrication, and testing of a quasiparticle tunnel junction receiver for use at 345 GHz. The design employs small area Nb/Nb-oxide/PbInAu edge junctions in order to keep the device capacitance small and maintain a modest value for R_NC. For optimura noise performance and beam properties the mixer is contained in a waveguide mounting structure. Our best sensitivity was obtained at 312 GHz where we measured a double sideband (DSB) noise temperature of 275 K. Noise temperatures of 400 K (DSB) or better were obtained out to 350 GHz.     

Magnetic tunnel junction thermocouple for thermoelectric power harvesting

The thermoelectric power generated in magnetic tunnel junctions (MTJs) is determined as a function of the tunnel barrier thickness for a matched electric circuit. This study suggests that lower resistance area product and higher tunnel magnetoresistance will maximize the thermoelectric power output of the MTJ structures. Further, the thermoelectric behavior of a series of two MTJs, a MTJ thermocouple, is investigated as a function of its magnetic configurations. In an alternating magnetic configurations the thermovoltages cancel each other, while the magnetic contribution remains. A large array of MTJ thermocouples could amplify the magnetic thermovoltage signal significantly.     

高效率钙钛矿太阳电池发展中的关键问题

钙钛矿太阳电池的迅速发展为解决未来能源问题带来一线曙光. 但是, 钙钛矿太阳电池在高效率电池器件的可重现性、稳定性以及性能评估等方面还面临着很多问题, 严重制约其今后的发展. 本文综述了钙钛矿太阳电池面世以来发生的重要进展, 以及存在的几个关键性问题. 从器件基本结构和基本工作原理出发, 本文重点讨论了光吸收层的光谱和形貌等性质对器件性能和可重现性的影响, 阐明了电子传输层和空穴阻隔层的重要作用, 论述了空穴传输层的相关进展以及其对器件稳定性的影响. 通过对以上关键问题的讨论和总结, 本文对钙钛矿太阳电池未来的研究发展进行了展望.     

预先合成量子点组装制备高效量子点太阳电池

量子点太阳电池现已成为极具潜力的“第三代” 光伏器件, 其优点体现在材料成本低廉, 制备工艺简便, 以及其敏化剂特有的多激子效应(MEG) 潜能和吸光范围可方便调节等方面. 但是与染料分子敏化剂相比, 量子点敏化剂粒径更大、表面缺乏具有与TiO₂结合的官能团, 这导致其在TiO₂介孔中渗透阻力大、难以在TiO₂表面吸附沉积, 所以量子点沉积手段在电池组装过程中尤为重要. 本文综述了电池组装过程中量子点的沉积方法, 分类阐述了直接生长量子点方法: 化学浴沉积(CBD)和连续离子层吸附生长(SILAR), 以及采用预先合成量子点的沉积方法: 连接分子辅助法(LA)、直接吸附法(DA)和电泳沉积(EPD)方法, 陈述了各沉积方法的发展过程及相应电池性能的改善, 对比了这些沉积方法的优缺点. 突出介绍了预先合成量子点的沉积方法, 特别是近年来不断优化而凸显优势的连接分子辅助法(LA). 总结了此方法快速、均匀沉积以及实现器件高性能的特点, 介绍了此方法沉积表面缺陷更少、结构更完善、材料更“绿色化”的量子点敏化剂的最新研究成果.     

Effective microscopic theory of quantum dot superlattice solar cells

We introduce a quantum dot orbital tight-binding non-equilibrium Green’s function approach for the simulation of novel solar cell devices where both absorption and conduction are mediated by quantum dot states. By the use of basis states localized on the quantum dots, the computational real space mesh of the Green’s function is coarse-grained from atomic resolution to the quantum dot spacing, which enables the simulation of extended devices consisting of many quantum dot layers.     

Recent progress of colloidal quantum dot based solar cells

Colloidal quantum dot(CQD) solar cells have attracted great interest due to their low cost and superior photo-electric properties. Remarkable improvements in cell performances of both quantum dot sensitized solar cells(QDSCs) and PbX(X = S, Se) based CQD solar cells have been achieved in recent years, and the power conversion efficiencies(PCEs) exceeding 12% were reported so far. In this review, we will focus on the recent progress in CQD solar cells. We firstly summarize the advance of CQD sensitizer materials and the strategies for enhancing carrier collection efficiency in QDSCs, including developing multi-component alloyed CQDs and core-shell structured CQDs, as well as various methods to suppress interfacial carrier recombination. Then, we discuss the device architecture development of PbX CQD based solar cells and surface/interface passivation methods to increase light absorption and carrier extraction efficiencies. Finally, a short summary, challenge, and perspective are given.