Valery Shklover

Towards high-efficiency thin-film silicon solar cells with the “micromorph” concept

Tandem solar cells with a microcrystalline silicon bottom cell (1 eV gap) and an amorphous-silicon top cell (1.7 eV gap) have recently been introduced by the authors; they were designated as “micromorph” tandem cells. As of now, stabilised efficiencies of 11.2% have been achieved for micromorph tandem cells, whereas a 10.7% cell is confirmed by ISE Freiburg. Micromorph cells show a rather low relative temperature coefficient of 0.27%/K. Applying the grain-boundary trapping model so far developed for CVD polysilicon to hydrogenated microcrystalline silicon deposited by VHF plasma, an upper limit for the average defect density of around 2 × 1016/cm3 could be deduced; this fact suggests a rather effective hydrogen passivation of the grain-boundaries. First TEM investigations on μc-Si : H p-i-n cells support earlier findings of a pronounced columnar grain structure. Using Ar dilution, deposition rates of up to 9 A/s for microcrystalline silicon could be achieved.

Mesoporous thin film TiO2 electrodes

Thin layer electrodes (200–500 nm) of highly organized nanotextured anatase were prepared by hydrolysis of TiCl4 in the presence of poly(alkylene oxide) block copolymer, Pluronic P-123, acting as a structure-directing agent. Electrochemical insertion of Li was studied in LiN(CF3SO2)2 + ethylene carbonate + dimethoxyethane. Cyclic voltammograms of these electrodes showed unusual occurrence of two pairs of cathodic/anodic peaks at 1.5 and 1.6 V vs. Li/Li+. These peaks can serve as indicators of structural perfection of the mesoscopic skeleton; they diminish after mechanical or thermal destruction of the organized nanotexture.

Mesoporous Nb2O5 Films: Influence of Degree of Crystallinity on Properties

Nanocrystalline Nb2O5 films were prepared by an extended sol-gel method. The synthesis is based on the hydrolysis of a modified Nb-alkoxide precursor. Reaction of the modified precursor (Nb(OEt)5 + 2 2,4-pentanedione) with water in ethanol leads to a homogeneous hydrolyzed solution, which is stable against precipitation of niobium oxide after evaporation of the ethanol and in the whole pH-range investigated (1–10). Autoclaving leads to amorphous gels, from which homogeneous nanocrystalline niobium oxide films of up to 15 μm can be made. During annealing crystalline phases are first observed above 500°C with fully crystalline films of orthorhombic T-phase Nb2O5 attained at 600°C. The microstructural, crystallographic, optical and photoelectrical properties of the films were characterized by means of SEM, XRD, UV-VIS spectroscopy and surface photovoltage spectroscopy, respectively.

Terahertz photoconductivity of Pb1−xSnxTe(In)

We have analyzed photoconductivity in Pb1−xSnxTe(In) under the action of ∼100 ns long terahertz laser pulses with the wavelength varying from 90 to 280 μm in the temperature range 4.2–300 K. Strong photoresponse has been observed at all laser wavelengths used. Two types of photoresponse have been detected. Positive persistent photoconductivity, which is observed at T<10 K is due to photoexcitation of impurity states, whereas negative nonpersistent photoresponse prevailing at higher temperatures T∼25 K results from free carrier heating. Specific features of photoconductivity mechanisms are discussed.

Controlled nucleation of thin microcrystalline layers for the recombination junction in a-Si stacked cells

In high-efficiency a-Si : H based stacked cells, at least one of the two layers that form the internal n/p junction has preferentially to be microcrystalline so as to obtain sufficient recombination at the junction [1–6]. The crucial point is the nucleation of a very thin μc-Si : H layer on an amorphous (i-layer) substrate [2, 4]. In this study, fast nucleation is induced through the treatment of the amorphous substrate by a CO2 plasma. The resulting n-layers with a high crystalline fraction were, however, found to reduce the Voc when incorporated in tandem cells. The reduction of the Voc could be restored only by a precise control of the crystalline fraction of the n-layer. As a technologically more feasible alternative, we propose a new, combined n-layer, consisting of a first amorphous layer for a high Voc, and a second microcrystalline layer, induced by CO2 treatment, for a sufficient recombination at the n/p junction. Resulting tandem cells show no Voc losses compared to two standard single cells, and an efficient recombination of the carriers at the internal junction as proved by the low series resistance (15 Ωcm2) and the high FF (⪰ 75%) of the stacked cells.

Magnesium Insertion in Vanadium Oxides: A Structural Study

X-ray powder diffraction patterns of V205-related electrode materials [V205, NaV308 and Mg(V308)2] as well as of poly(tetrafluoroethylene)-bonded composite electrodes, containing chemically and electrochemically inserted Mg2 +, were analysed before and after Mg2+ insertion. Transmission electron microscopy and both wavelength and energy dispersive electron probe microanalysis were applied for the determination of the composition and structure of different types of microparticles found in the electrode mass.

Efficient Multiterminal Spectrum Splitting via a Nanowire Array Solar Cell

Nanowire-based solar cells opened a new avenue for increasing conversion efficiency and rationalizing material use by growing different III–V materials on silicon substrates. Here, we propose a multiterminal nanowire solar cell design with a theoretical conversion efficiency of 48.3% utilizing an efficient lateral spectrum splitting between three different III–V material nanowire arrays grown on a flat silicon substrate. This allows choosing an ideal material combination to achieve the proper spectrum splitting as well as fabrication feasibility. The high efficiency is possible due to an enhanced absorption cross-section of standing nanowires and optimization of the geometric parameters. Furthermore, we propose a multiterminal contacting scheme that can be fabricated with a technology close to standard CMOS. As an alternative we also consider a single power source with a module level voltage matching. These new concepts open avenues for next-generation solar cells for terrestrial and space applications.

Hard wear-resistant coatings with anisotropic thermal conductivity for high thermal load applications

High thermal load applications such as high speed dry cutting lead to high temperatures in the coated tool substrate that can soften the tool and high temperature gradients that can put stress on the coating/tool interface. In this work, theoretical considerations are presented for multilayer and graded protective coatings that can induce a significant anisotropy in their thermal conductivity. Solution of the heat equation shows that anisotropy of thermal conductivity has the potential to reduce temperatures and temperature gradients arising due to brief, localized heat at the coating surface (“hot-spots”). Experimental realization of high anisotropy is demonstrated in TiN/AlCrN multilayer coatings with variable layer thickness. In the coating with 50 nm bilayer periodicity, the highest anisotropy was obtained with a value of κ||/κ⊥=3.0±0.9. Time-domain thermoreflectance is used to measure in-plane and cross-plane thermal conductivity of fabricated coatings. The observed high values of anisotropy of therm...

Tris(4,4'-dimethoxy-2,2'-bipyridine)osmium(II), amperometric properties and crystal structure

Reference LPI-ARTICLE-1998-002doi:10.1016/S0020-1693(97)05989-6View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

CRYSTAL STRUCTURE OF TRIS(4,4'-DIMETHOXY-2,2'-BIPYRIDINE)IRON(II) BIS(HEXAFLUOROPHOSPHATE), SOLVATE WITH N-METHYL-2-PYRROLIDONE

Abstract The crystal study of tris(4,4′-dimethoxy-2,2′-bipyridine)iron(II)bis(hexafluorophosphate) (crystal solvate with N -methyl-2-pyrrolidone) was performed at 295 K. The crystals are triclinic, a = 11.721(4), b = 12.864(6), c =15.664(7) A , α = 77.14(4),β = 85.03 (3), γ = 81.76(3)° , space group P 1 , Z = 2, R = 0.0679 for 4694 reflections. The average FeN distances of 1.967 A agrees well with those observed in other tris(2,2-bipyridine) complexes of hexacoordinated Fe(II) with a large low-spin fraction. A small difference of the individual FeN distances in the dication (which are shorter for one of the bipyridine ligands) may be explained by crystal packing effects. A very specific orientation of dications [Fe(DMO-bpy) 3 ] 2+ in the crystal is worthy of note (the vector of one of the moieties N…(Fe)…N of the dication coincides in fact with direction [011] of the crystal). The possible interaction of the trication of tris(4,4′-dimethoxy-2,2′-bipyridine)iron (III) with the active site of glucose oxidase (i.e. the metal complex acting as an oxidant/electron-transfer mediator) is discussed.