Lead‐free mesoscopic Cs2SnI6 perovskite solar cells using different nanostructured ZnO nanorods as electron transport layers

Lead‐free and more air‐stable perovskite Cs₂SnI₆ absorber with a direct bandgap of 1.48 eV is synthesized via a modified solution process. Different nanostructured ZnO nanorod arrays as electron transport layers and hole blocking layers are grown by controlling the seed layer and used to fabricate mesoscopic perovskite solar cells with Cs₂SnI₆ as light absorber layer. The influences of ZnO seed layers and nanorod morphology on the device photovoltaic performance were also investigated. With careful control of ZnO nanorod length and pore size to ensure high loading of the Cs₂SnI₆ absorber, we achieved power conversion efficiency of near 1%.

     

Using magnetic levitation for 2D and 3D self‐assembly of cubic silicon macroparticles

Today's micro‐ and nano‐fabrication is essentially two‐dimensional, with very limited possibilities of accessing the third dimension. The most viable way to mass‐fabricate functional structures at the nano‐scale, such as electronics or MEMS, with equal feature sizes in all directions, is by three‐dimensional self‐assembly. Up to now, three‐dimensional self‐assembly has mainly been restricted to crystals of polymer spheres. We report on two‐ and three‐dimensional self‐assembly of silicon cubes, levitated in a paramagnetic fluid. We demonstrate the benefits of templating and study the effect of a change in hydrophilicity of the cubes. These experiments bring us one step closer to three‐dimensional self‐assembly of anisotropic, semiconducting units, which is a crucial milestone in overcoming the scaling limits imposed by contemporary 2D microfabrication.

     

High‐temperature ferroelectricity and strong magnetoelectric effects in a hybrid organic–inorganic perovskite framework

A Cu‐based organic–inorganic perovskite framework exhibits high‐temperature ferroelectricity with strong magnetoelectric effects. Both electric field control of magnetization and magnetic field control of polarization are realized. Theoretical calculations suggest that a new mechanism of hybrid improper ferroelectricity arising from the Jahn–Teller distortions of magnetic metal ions and tilting of the organic cations are responsible for the peculiar multiferroic behaviors.

     

Casimir interactions in strained graphene systems

We theoretically study the strain effect on the Casimir interactions in graphene based systems. We found that the interactions between two strained graphene sheets are strongly dependent on the direction of stretching. The influence of the strain on the dispersion interactions is still strong in the presence of dielectric substrates but is relatively weak when the substrate is metallic. Our studies would suggest new ways to design next generation devices.

     

Spontaneous Ga incorporation in ZnO nanowires epitaxially grown on GaN substrate

Surface‐diffusion‐induced spontaneous Ga incorporation process is demonstrated in ZnO nanowires grown on GaN substrate. Crucially, contrasting distributions of Ga atoms in axial and radial directions are experimentally observed. Ga atoms uniformly distribute along the ～10 μm long ZnO nanowire and show a rapidly gradient distribution in the radial direction, which is attributed substantially to the difference between surface and volume diffusion. The understanding on the incorporation process can potentially modulate doping and properties in semiconductor nanomaterials.

     

High transmission nanowire contact arrays with subwavelength spacing

We demonstrate high optical transmission in solar cell contacts based on nanowire arrays with subwavelength spacing. The photocurrent results obtained from fabricated devices are compared with numerical simulations. The proposed contact design leads to optical losses significantly smaller than the fraction of the top surface taken up by the metallic contact (<10% vs. 36%). The resulting sheet resistance of the contact is 46.7 Ω/square, comparing favourably with transparent conductive oxides.

     

Photocatalytic activity of α‐PbO2‐type TiO2

The α‐PbO₂‐type TiO₂ is synthesized under high‐pressure and high‐temperature environment and it shows higher photocatalytic activity as compared to rutile and anatase under UV irradiation. The reduction in α‐PbO₂‐type TiO₂ induces visible‐light photocatalytic activity. These results indicate that α‐PbO₂‐type TiO₂ is an important candidate material for use in a photocatalytic matrix.

     

Probing Dy3+ magnetic moments in multiferroic perovskite DyMnO3 by optical spectroscopy

We present a detailed temperature‐dependent (4–300 K) spectroscopic study of DyMnO₃ single crystals with distorted perovskite structure. Energies of 36 crystal‐field levels of Dy³⁺ in paramagnetic DyMnO₃ were determined. The Dy³⁺ ground Kramers doublet does not split at and splits below T_lock = 18 K. The splitting grows fast at temperatures near and reaches Δ₀ ≈ 11 ± 2 cm^–1 at 4 K. Using the experimental temperature dependence Δ₀(T), we calculate the dysprosium magnetic moment m_Dy(T) and the dysprosium contribution into specific heat and magnetic susceptibility. Analysing all the experimental data, we conclude that the Dy–Mn interaction is of the Dzyaloshinskii–Moriya type.

Intensity map in the temperature–wave number coordinates for a spectral line corresponding to the f–f transition of Dy³⁺ in DyMnO₃ and a scheme of the splitting of the Dy ground Kramers doublet. Arrows represent Dy magnetic moments.     

Impact of strain induced by polymer curing in benzocyclobutene embedded semiconductor nanostructures

Polymers such as benzocyclobutene are commonly used as embedding materials for semiconductor nanostructures. During the curing process of the polymer up to 250 °C, a significant impact of strain can be induced on the embedded semiconductor material due to different thermal expansion coefficients. This strain has been revealed by X‐ray diffraction in free‐standing GaAs nanowires grown on a silicon substrate, embedded in a polymer matrix. It will be shown that this strain is released during the X‐ray irradiation if additionally an external static electric field is applied.

     

Numerical modeling of photovoltaic efficiency of n‐type GaN nanowires on p‐type Si heterojunction

In this Letter, we investigate the photovoltaic properties of heterojunction solar cells based on n‐GaN nanowire (NW)/ p‐Si substrate heterostructures by means of numerical modeling. Antireflection properties of the NW array on the top of Si substrate were studied theoretically to show an order of magnitude enhancement in antireflection properties in comparison to the pure Si surface (2.5% vs. 33.8%). In order to determine the optimal morphology and doping levels of the structure with maximum possible efficiency we simulated its properties. The carried out simulation showed that the maximum efficiency should be more than 20% under AM1.5D illumination. The proposed design opens new perspectives and opportunities in the field of heterojunction tandem solar cell researches.

     

Al‐density variation as one driving force for void formation in PERC solar cells

The reduction of void formation in local Al contact structures is of high interest in studies dealing with passivated emitter and rear contact (PERC) solar cells. So far, several processing parameters and their impact on local contact formation were investigated in detail. However, up to now density variation of Al in dependence on temperature and Si content in the melt were not taken into account as a principal reason for void formation. In this context the current assumption of a constant volume of the Al paste particles is discussed in more detail. Based on the results of energy dispersive X‐ray spectroscopy, void formation implies either an expansion of paste particles or their burst during contact formation.

     

Fabrication of transparent and flexible carbon‐doped ZnO field emission display on plastic substrate

Transparent and flexible carbon doped ZnO (C:ZnO) field emission device was successfully fabricated on an arylite substrate. Excellent adhesion of deposited C:ZnO on the flexible substrate was achieved with low sputtering power and Ar flow rate. In the fabricated device, nanostructured C:ZnO and as‐deposited thin films were used as field emitter and phosphor screen, respectively. The C:ZnO thin film showed a transparency of about 80% at 550 nm wavelength and average sheet resistance of 1.96 kΩ/□. The C:ZnO phosphor screen emitted red light during the field emission measurement, correlating the dominant cathodoluminescence peak at 646 nm. Thus, a promising transparent and flexible field emission display can be realized with C:ZnO based material.

Transparent and flexible C:ZnO film phosphor screen (anode) and nanocone emitters (cathode) for field emission device.     

Monolithic circuits with epitaxial graphene/silicon carbide transistors

A concept is presented that uses epitaxial graphene on silicon carbide (SiC) for digital circuits. It uses graphene as a metal and the underlying substrate SiC as semiconductor. On the base of transistors with excellent switching behavior, Inverter and NAND operation is demonstrated.

     

Metal‐assisted chemical etching of CIGS thin films for grain size analysis

Grain size of the CIGS absorber is an important monitoring factor in the CIGS solar cell manufacturing. Electron backscatter diffraction (EBSD) analysis is commonly used to perform CIGS grain size analysis in the scanning electron microscope (SEM). Although direct quantification on SEM image using the average grain intercept (AGI) method is faster and simpler than EBSD, it is hardly applicable on CIGS thin films. The challenge is that, not like polycrystalline silicon, to define grain boundaries by selective chemical etching is not easily realizable for the multi‐component CIGS alloy. In this Letter, we present direct quantification of CIGS thin film grain size using the AGI method by developing metal‐assisted wet chemical etching process to define CIGS grain boundaries. The calculated value is similar to EBSD result.

The CIGS thin film surface morphology before and after the wet chemical etching. Grain boundaries are well defined after the processing.     

Ag fractals formed on top of a porous TiO2 thin film

This work demonstrates the formation of Ag fractals on top of a Ag:TiO₂ thin film. The dendrite‐type objects emerged from a homogeneous and highly transparent Ag:TiO₂ nanocomposite, via the mechanism of diffusion‐limited‐aggregation of Ag atoms, during heat‐treatment at 500 °C. A porous TiO₂ matrix was also formed during this process, opening a wide range of possible applications, namely in sensing‐based ones, together with surface enhanced spectroscopies. Furthermore, fractals incorporate a wide range of shapes and spatial scales, inducing a potentially interesting optical response, over the whole visible range, presumably related with localized surface plasmon modes with very broad spectral distribution.

     

P‐type tin‐oxide thin film transistors for blue‐light detection application

We reported the characteristics of p‐type tin‐oxide (SnO) thin film transistors (TFTs) upon illumination with visible light. Our p‐type TFT device using the SnO film as the active channel layer exhibits high sensitivity toward the blue‐light with a high light/dark read current ratio (I_light/I_dark) of 8.2 × 10³ at a very low driven voltage of <3 V. Since sensing of blue‐light radiation is very critical to our eyes, the proposed p‐type SnO TFTs with high sensitivity toward the blue‐light show great potential for future blue‐light detection applications.

     

A high efficiency 3D photovoltaic microwire with carbon nanotubes (CNT)‐quantum dot (QD) hybrid interface

An innovative hybrid QD sensitized photovoltaic carbon nanotubes microyarn has been developed using thermally‐stable and highly conductive carbon nanotubes yarns (CNYs). These CNYs are highly inter‐aligned, ultrastrong and flexible with excellent electrical conductivity, mechanical integrity and catalytic properties. The CNYs are coated with a QD‐incorporated TiO₂ microfilm and intertwined with a second set of CNYs as a counter electrode (CE). The maximum photon to current conversion efficiency (η_AM1.5) achieved with prolonged‐time stability was 5.93%. These cells are capable of efficiently harvesting incident photons regardless of direction and generating photocurrents with high efficiency and operational stability.

     

Solvable potentials with exceptional orthogonal polynomials

Classes of solvable potentials are presented within an standard application of supersymmetric quantum mechanics. Sets of exceptional orthogonal polynomials generated by these solvable potentials are introduced and examined in detail. Several properties of these polynomials including orthogonality conditions, weight functions, differential equations, the Wronskains, possible recurrence relations are also investigated.

     

First‐principle investigations of the peculiar magnetic behavior of Sr2FeOsO6

Structural, electronic and magnetic properties of Sr₂FeOsO₆ have been revisited by using the first‐principle calculations. Semiconducting behavior is reproduced. The band gap is 0.09 eV from generalized gradient approximation (GGA) and 0.30 eV by considering both SOC and U, a bit larger than the experimental observed 0.125 eV. In the C‐type antiferromagnetic configuration, spin frustration is found by analysing the magnetic exchange parameters, explaining the experimental observed magnetic complexity.