Thin-film Junctions Research Articles

Development of aluminum-doped ZnO films for a-Si:H/μc-Si:H solar cell applications

This study deals with the optimization of direct current (DC) sputtered aluminum-doped zinc oxide (AZO) thin films and their incorporation into a-Si:H/μc-Si:H tandem junction thin film solar cells aiming for high conversion efficiency. Electrical and optical properties of AZO films, i.e. mobility, carrier density, resistivity, and transmittance, were comprehensively characterized and analyzed by varying sputtering deposition conditions, including chamber pressure, substrate temperature, and sputtering power. The correlations between sputtering processes and AZO thin film properties were first investigated. Then, the AZO films were textured by diluted hydrochloric acid wet etching. Through optimization of deposition and texturing processes, AZO films yield excellent electrical and optical properties with a high transmittance above 81% over the 380–1100 nm wavelength range, low sheet resistance of 11 Ω/□ and high haze ratio of 41.3%. In preliminary experiments, the AZO films were applied to a-Si:H/μc-Si:H tandem thin film solar cells as front contact electrodes, resulting in an initial conversion efficiency of 12.5% with good current matching between subcells.

Rietveld analysis of CdS/CdTe thin film junctions submitted to a CdCl2 heat treatment

In this work we investigate the effects of a CdCl2 heat treatment on the interface of CdTe/CdS heterojunction solar cells using Rietveld analysis of X-ray diffraction patterns. Although the Rietveld method is an important tool for the study of materials by X-ray diffraction, there have been few reports of its use in thin film analysis. The results showed the occurrence of interdiffusion in the CdS–CdTe boundary, with substitution of tellurium by sulfur in the CdTe lattice that resulted in a CdSxTe1−x alloy. The sulfur content of the alloy was greater than in previous studies. This is attributed to the low oxygen concentration during processing and the strong (111) texture of the CdTe films.

Simulation and Experimental Study of Photogeneration and Recombination in Amorphous-Like Silicon Thin Films Deposited by 27.12 MHz Plasma-Enhanced Chemical Vapor Deposition

Amorphous-like silicon (a-Si:H-like) thin films are prepared by 27.12 MHz plasma-enhanced chemical vapor deposition technique. The films are applied to p-i-n single junction thin film solar cells with varying i-layer thickness to observe the effects on the short-circuit current density, as well as the open-circuit voltage, fill factor, and conversion efficiency. The most significant experimental result is thatJschas two different behaviors with increasing the i-layer thickness, which can be related to carrier collection efficiency in the long wavelength region. Furthermore, technology computer-aided design simulation software is used to gain better insight into carrier generation and recombination of the solar cells, showing that for the i-layer thickness of 200 to 300 nm the generation dominates the carrier density and thusJsc, whereas for the i-layer thickness of 300 to 400 nm the recombination becomes the leading factor. The simulation results of cell performances are in good agreement with experimental data, indicating that our simulation has great reliability. In addition, the a-Si:H-like solar cells have low light-induced degradation, which in turn can have a great potential to be used for stable and high-efficiency solar cells.

Zero-energy bound states in tunneling conductance spectra at the interface of ans-wave superconductor and a topological insulator in NbN/Bi2Se3/Au thin-film junctions

Measurements of conductance spectra in a superconductor - topological insulator - normal metal thin film junctions of NbN-$\rm Bi_2Se_3$-Au are reported. Junctions with ex-situ and in-situ prepared $\rm NbN-Bi_2Se_3$ interfaces were studied. At low temperatures, all the ex-situ junctions showed coherence peaks in their conductance spectra, but imbedded robust zero bias conductance peaks were observed only in junctions with a metallic or a metal to insulator transition below $\rm T_c$ of the NbN electrode. The in-situ junctions which had about two orders of magnitude lower resistance at low temperatures, generally showed flat conductance spectra at low bias, with no coherence or broad Andreev peaks, since the critical current of the NbN electrode was reached first, at voltage bias below the energy gap of the superconductor. A weak zero bias conductance peak however, was observed in one of these junctions. We conclude that significant tunneling barriers, as in the ex-situ prepared junctions, are essential for the observation of coherence peaks and the zero energy bound states. The later seem to originate in the $\rm Bi_2Se_3$-NbN interface, as they are absent in reference Au-NbN junctions without the topological layer sandwiched in between.

Millimeter-Wave Radiation From Misaligned $ \hbox{Tl}_{2}\hbox{Ba}_{2}\hbox{CaCu}_{2}\hbox{O}_{8}$ Thin-Film Intrinsic Josephson Junctions

We have measured the current-voltage characteristics of misaligned Tl2Ba2CaCu2O8 thin-film intrinsic Josephson junctions (IJJs) embedded in a quasi-optical resonator at 78.5 K. The substrate is regarded as a dielectric resonator to improve high-frequency electromagnetic coupling between the IJJs. The experimental result is fitted well with the electromagnetic simulation. Radiation power larger than 10 pW has been detected at 75.6 GHz. Moreover, the phenomenon of jump voltage is explored, and a possible explanation is discussed.

A study of ZnO:B films for thin film silicon solar cells

Boron doped zinc oxide (ZnO:B) films with different thicknesses were prepared with low pressure chemical vapor deposition (LPCVD) technique and implemented in thin film silicon solar cells as front and back electrodes. It is found that thick back ZnO:B film electrode in thin film silicon solar cells leads to a high fill factors (FF), which is attributed to an improvement of the electrical properties of the thick ZnO:B films, and in the meanwhile a slightly low short circuit currents (Jsc) due to a high light absorption in the thick back ZnO:B films. Differently, the thicker front ZnO:B film electrodes result in a high Jsc but a low FF of solar cells compared to the thinner ones. The low FF of the solar cells may be caused by the local shunt originated from the pinholes or by the cracks (zones of non-dense material) formed in particular in microcrystalline silicon materials deposited on rough front ZnO:B films. As to the high Jsc, it is expected to be due to a good light trapping effect inside solar cells grown on rough front ZnO:B films. Moreover, the application of high reflective polyvinyl butyral (PVB) foils effectively enhances the utilization of incident light in solar cells. By optimizing deposition process of the ZnO:B films, high efficiencies of 8.8% and 10% for single junction thin film amorphous silicon solar cells (a-Si:H, intrinsic layer thickness<200nm) and amorphous/microcrystalline silicon tandem solar cells (a-Si:H/μc-Si:H, intrinsic amorphous silicon layer thickness<220nm), respectively, are achieved.

Bismuth-Catalyzed and Doped Silicon Nanowires for One-Pump-Down Fabrication of Radial Junction Solar Cells

Silicon nanowires (SiNWs) are becoming a popular choice to develop a new generation of radial junction solar cells. We here explore a bismuth- (Bi-) catalyzed growth and doping of SiNWs, via vapor-liquid-solid (VLS) mode, to fabricate amorphous Si radial n-i-p junction solar cells in a one-pump-down and low-temperature process in a single chamber plasma deposition system. We provide the first evidence that catalyst doping in the SiNW cores, caused by incorporating Bi catalyst atoms as n-type dopant, can be utilized to fabricate radial junction solar cells, with a record open circuit voltage of V(oc) = 0.76 V and an enhanced light trapping effect that boosts the short circuit current to J(sc) = 11.23 mA/cm(2). More importantly, this bi-catalyzed SiNW growth and doping strategy exempts the use of extremely toxic phosphine gas, leading to significant procedure simplification and cost reduction for building radial junction thin film solar cells.

Highly conductive GaN anti-reflection layer at transparent conducting oxide/Si interface for silicon thin film solar cells

Highly conductive GaN film was prepared by magnetron sputtering and this was applied as an anti-reflection layer (ARL) between a transparent conducting oxide and microcrystalline silicon (μc-Si:H) in order to decrease optical reflection. The efficiency (8.81%) of μc-Si:H single junction thin film solar cell with the proposed GaN ARL exceeded that of the cell (8.36%) with the widely used TiO2/ZnO bilayer ARL. Moreover, the proposed GaN ARL requires no protection layer against hydrogen plasma such as ZnO overcoating (∼10nm) in case of the TiO2/ZnO bilayer. GaN ARL can replace the TiO2/ZnO bilayer ARL in terms of high performance and simple fabrication process.

Low frequency dielectric loss of metal/insulator/organic semiconductor junctions in ambient conditions

The complex admittance of metal/oxide/pentacene thin film junctions is investigated under ambient conditions. At low frequencies, a contribution attributed to proton diffusion through the oxide is seen. This diffusion is shown to be anomalous and is believed to be also at the origin of the bias stress effect observed in organic field effect transistors. At higher frequencies, two dipolar contributions are evidenced, attributed to defects located one at the organic/oxide interface or within the organic, and the other in the bulk of the oxide. These two dipolar responses show different dynamic properties that manifest themselves in the admittance in the form of a Debye contribution for the defects located in the oxide, and of a Cole–Cole contribution for the defects related to the organic.

Radial junction amorphous silicon solar cells on PECVD-grown silicon nanowires

Constructing radial junction hydrogenated amorphous silicon (a-Si:H) solar cells on top of silicon nanowires (SiNWs) represents a promising approach towards high performance and cost-effective thin film photovoltaics. We here develop an all-in situ strategy to grow SiNWs, via a vapour–liquid–solid (VLS) mechanism on top of ZnO-coated glass substrate, in a plasma-enhanced chemical vapour deposition (PECVD) reactor. Controlling the distribution of indium catalyst drops allows us to tailor the as-grown SiNW arrays into suitable size and density, which in turn results in both a sufficient light trapping effect and a suitable arrangement allowing for conformal coverage of SiNWs by subsequent a-Si:H layers. We then demonstrate the fabrication of radial junction solar cells and carry on a parametric study designed to shed light on the absorption and quantum efficiency response, as functions of the intrinsic a-Si:H layer thickness and the density of SiNWs. These results lay a solid foundation for future structural optimization and performance ramp-up of the radial junction thin film a-Si:H photovoltaics.

Planar hybrid superconductor-normal metal-superconductor thin film junctions based on BaFe1.8Co0.2As2

To investigate the transport properties of iron based superconductors, we prepared planar hybrid superconductor-normal metal-superconductor (SNS’) thin film junctions with BaFe1.8Co0.2As2 as base electrode. As counter electrode we used a lead indium alloy, while the normal metal layer was formed by thin gold films. The junction design allows us to characterize the electrodes as well as the junction itself. Temperature dependent Andreev reflection studies of the junction show gap-like features but were also strongly influenced by the properties of the electrodes.

Radiation from Intrinsic Josephson Junctions in Misaligned Tl2Ba2CaCu2O8 Thin Films

We measure the current-voltage (I-V) curves of intrinsic Josephson junctions (IJJs) in misaligned Tl2Ba2CaCu2O8 thin films embedded in a Fabry-P,rot (F-P) resonator at liquid nitrogen temperature. Regarding the substrate as a dielectric resonator, electromagnetic coupling between the intrinsic Josephson junctions is improved. By adjusting the location of the substrate in the F-P resonator, the critical current of the IJJs under microwave irradiation (75.6 GHz) is suppressed almost to zero. Two pairs of symmetrical radiation (75.6 GHz) peaks are detected. Possible explanations for this experimental result are discussed.

Reliability studies of pentacene based thin film transistors

ABSTRACTThe complex admittance of the Si+/SiO2/Pentacene/Au (metal/oxide/pentacene) thin film junctions is investigated under ambient conditions. The results are compared with the ones obtained for the corresponding Si+/SiO2/Au junctions (i.e. a small part of the surface left free from pentacene) which constitutes the “reference” of our samples. This allows us to extract the “organic” part of the dielectric response from the whole spectrum. Our data clearly show that the admittance is decomposed in three main contributions. At low frequencies, a contribution attributed to proton diffusion through the oxide is seen. This diffusion is shown to be anomalous and is believed to be also at the origin of the bias stress effect observed in organic field effect transistors. At higher frequencies, two dipolar contributions are evidenced, attributed to defects located one at the organic/oxide interface or within the organic, and the other in the bulk of the oxide. These two dipolar responses show different dynamic properties that manifest themselves in the admittance in the form of a Debye contribution for the defects located in the oxide, and of a Cole-Cole contribution for the defects related to the organic.

Niobium dc SQUIDs with Nanobridge Junctions

Abstract DC SQUIDs with outer loop size ∼10 μmx 10 μm have been fabricated with nanobridge Josephson junctions in thin Niobium films (25, 100 and 200 nm) using either electron beam or focused ion beam techniques. Such SQUIDs are ideally suited to incorporate into the centre strip of a superconducting coplanar waveguide. The current-voltage characteristics and magnetic field modulation of the SQUIDs are compared for the two methods. The temperature dependence of the critical current is quite linear for the e-beam devices whilst the focused ion beam devices fit an exponential. Here we also investigate trimming methods to reduce the critical currents of the nanojunctions fabricated in the thicker films, in order to improve the SQUID parameters at lower temperatures.

ISS2011 Development of iron-based superconducting devices

For the investigation of the superconducting properties of iron-based superconductors Josephson junctions as well as Andreev reflection studies are potential tools. Therefore, we prepared different kinds of junctions, based on transport in direction of c-axis and ab-plane, respectively. The first one was a planar hybrid superconductor-normal metal-superconductor (SNS’) thin film junction for transport studies along the c-axis. We used a BaFe1.8Co0.2As2 (Ba-122) thin film (thickness ≈ 80 nm) prepared by pulsed laser deposition (PLD) as base electrode and a lead indium alloy as counter electrode, while the normal metal (barrier) layer was formed by thin gold films. By tuning the thickness of the gold layer and the area of the tunneling window, we were able to switch the behavior of our junction. For a thin gold layer (≈ 5 nm) and greater areas, RCSJ-like Josephson behavior can be observed with a radiation dependent excess current and nearly linear temperature dependence of the critical current. Increasing the thickness of the gold layer to 10 nm leads to conductance spectra, which are dominated by Andreev reflection and which show no remaining sign of Josephson effects. Secondly, an edge-type junction for transport measurements in the ab-plane of the pnictide layer, was prepared. It also is realized with PbIn as counter electrode, but with no artificial thin film. The temperature dependent conductance spectra showed some interesting features, which cannot be described within usual theoretical models. We introduced a third geometry, phase-sensitive corner junctions, which could ultimately test the pairing symmetry of Ba-122, if it is s± type.

Microcrystalline silicon oxide (μc-SiOx:H) alloys: A versatile material for application in thin film silicon single and tandem junction solar cells

We report on the development and application of n-type hydrogenated microcrystalline silicon oxide (μc-SiOx:H) alloys in single and tandem junction thin film silicon solar cells. Single junction microcrystalline silicon (μc-Si:H) solar cells are prepared in n-i-p deposition sequence where n-type μc-SiOx:H films serve as window layers. In tandem solar cells, μc-SiOx:H layers are placed between amorphous (a-Si:H) and μc-Si:H component cells, serving as an intermediate reflector. The requirements for μc-SiOx:H layer depending on its application are discussed. Our results show that the optical, electrical and structural properties of μc-SiOx:H can be conveniently tuned over a wide range to fulfil various requirements for applications in both types of cells. Additionally, the properties of μc-SiOx:H layers appear to be substrate dependent, which should be taken into account when layers are utilized in cells. The advantages of low refractive index and high optical band gap allow to achieve high efficiencies of 9.2% and 12.6% for single junction and tandem solar cells, respectively.

Internal degrees of freedom, long-range interactions and nonlocal effects in perturbed Klein–Gordon equations

We investigate different mechanisms for the excitation of soliton internal degrees of freedom and for the existence of long-range interactions between solitons. We will study a nonlocal Klein–Gordon equation that is used as a model for Josephson junctions in thin films. We will show the connections between nonlocality, nonlinearity, internal degrees of freedom, long-range interactions and power-law behaviors.

Simulation Model for High Efficiency of Solar Cells

Simulation model of solar cell device is developed to investigate the optimization of conversion efficiency as a function of its geometrical and materials parameters. An arbitrary profile of impurity distribution in the base region is taken into account by using the method of piece wise integration by exponential approximations .The spatially varying built-in field and mobility are explicitly taken into account. Illustrative numerical computations of quantum & collection efficiencies as well as of solar conversion parameters are presented. Simulation conversion efficiency of mono- crystalline silicon solar cell is in conformity with the 24% efficiency reported in Australia for the PERL structure when reasonably effective (about 2-3%) light trapping is taken into account. The model also supports the structure of multi junction thin film solar cells for ultra high efficiency. Key Words : Solar cell; Mono-crystalline; Simulation model; Ultrahigh efficiency

Electric transport across Sr1−xLaxCuO2/Au/Nb planar tunnel junctions and x-ray photoelectron and Auger-electron spectroscopy on Sr1−xLaxCuO2thin films

Thin-film planar tunnel junctions with the electron-doped infinite-layer superconductor Sr${}_{1\ensuremath{-}x}$La${}_{x}$CuO${}_{2}$ (SLCO) with $x\ensuremath{\sim}0.15$ as bottom electrode, a thin Au interlayer, and Nb as top electrode were fabricated and characterized. Measurements of electric transport across these junctions provide information on the interface and surface properties of the SLCO thin films. No Cooper pair tunneling is observed; however, nonlinear current-voltage characteristics give evidence for quasiparticle (QP) tunneling across a thin insulating SLCO barrier at the SLCO/Au interface, with a single gap value $\ensuremath{\sim}$$1.4\phantom{\rule{0.28em}{0ex}}$meV, originating from superconducting Nb. The absence of a superconducting SLCO gap in the QP conductance curves indicates a thin normal-conducting SLCO layer below the insulating SLCO barrier. To examine its origin, x-ray photoelectron spectroscopy (XPS) and x-ray Auger-electron spectroscopy (XAES) on SLCO thin films were performed. We observe a Cu valence of +1 in the SLCO surface layer (within $\ensuremath{\sim}$$3\phantom{\rule{0.28em}{0ex}}$nm thickness) and of +2 in deeper regions, as expected for fully oxidized CuO${}_{2}$ planes in the bulk. Hence, the XPS and XAES results for the SLCO films are consistent with the QP tunneling spectra observed for our planar SLCO/Au/Nb junctions.

Intrinsic Josephson Junctions in Bi-2212 Thin Films Fabricated by Metal-Organic Decomposition

We fabricated a mesa <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$10\ \mu{\rm m}\times 10\ \mu{\rm m}$</tex></formula> in area and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\sim$</tex></formula> 150 nm in height on a single grain in a 240-nm <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm Bi}_{2}{\rm Sr}_{2}{\rm CaCu}_{2}{\rm O}_{8+\delta}/{\rm MgO}$</tex></formula> thin-film grown by metal-organic decomposition (MOD) with face-to-face annealing. A multi-branch structure peculiar to intrinsic Josephson junctions (IJJs) was observed in the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$I-V$</tex> </formula> characteristics of the fabricated mesa, and this structure was characterized by approximately 100 quasiparticle branches with critical current densities of 1 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm kA/cm}^{2}$</tex> </formula> and voltage-jump intervals of 20 mV. Moreover, the temperature dependence of the critical current was in good agreement with a theoretical result based on <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$c$</tex> </formula> -axis Josephson tunneling in <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$d$</tex></formula> -wave superconductors. These results imply that the thin films grown by MOD have sufficient quality to fabricate IJJs for practical terahertz applications.