Showing papers in "Physica Status Solidi-rapid Research Letters in 2012"

On the mechanism of potential-induced degradation in crystalline silicon solar cells

Abstract: Multicrystalline standard p-type silicon solar cells, which undergo a potential induced degradation, are investigated by different methods to reveal the cause of the degradation. Microscopic local ohmic shunts are detected by electron-beam-induced current measurements, which correlate with the sodium distribution in the nitride layer close to the Si surface imaged by time-of-flight secondary ion mass spectroscopy. The results are compatible with a model of the formation of a charge double layer on or in the nitride, which inverts the emitter. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-temperature photoluminescence of oxide-covered single-layer MoS2

Abstract: We present a photoluminescence study of single-layer MoS2 flakes on SiO2 surfaces. We demonstrate that the luminescence peak position of flakes prepared from natural MoS2, which varies by up to 25 meV between individual flakes, can be homogenized by annealing in vacuum. We use HfO2 and Al2O3 layers prepared by atomic layer deposition to cover some of our flakes. In these flakes, we observe a suppression of the low-energy luminescence peak which appears in asprepared flakes at low temperatures. We infer that this peak originates from excitons bound to surface adsorbates. We also observe different temperature-induced shifts of the luminescence peaks for the oxide-covered flakes. This effect stems from the different thermal expansion coefficients of the oxide layers and the MoS2 flakes. It indicates that the single-layer MoS2 flakes strongly adhere to the oxide layers and are therefore strained. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface passivation of phosphorus-diffused n+-type emitters by plasma-assisted atomic-layer deposited Al2O3

Abstract: In recent years Al2O3 has received tremendous interest in the photovoltaic community for the application as surface passivation layer for crystalline silicon. Especially p-type c-Si surfaces are very effectively passivated by Al2O3, including p-type emitters, due to the high fixed negative charge in the Al2O3 film. In this Letter we show that Al2O3 prepared by plasma-assisted atomic layer deposition (ALD) can actually provide a good level of surface passivation for highly doped n-type emitters in the range of 10-100 Omega/sq with implied-V-oc values up to 680 mV. For n-type emitters in the range of 100-200 Omega/sq the implied-V-oc drops to a value of 600 mV for a 200 Omega/sq emitter, indicating a decreased level of surface passivation. For even lighter doped n-type surfaces the passivation quality increases again to implied-V-oc values well above 700 mV. Hence, the results presented here indicate that within a certain doping range, highly doped n- and p-type surfaces can be passivated simultaneously by Al2O3. (C) 2011 WILEY-VCH Verlag GmbH a Co. KGaA, Weinheim

Electronic structure of interfaces with conjugated organic materials

Abstract: The function and efficiency of most organic electronic and opto-electronic devices greatly depend on the electronic structure of the interfaces therein. Charge injection from electrical contacts into the organic semiconductor, charge extraction, or exciton dissociation at organic semiconductor heterojunctions are crucial processes that must be optimized for high device efficiency. Consequently, the energy levels at these interfaces must be matched to allow for optimal performance. The key mechanisms that determine the energy level alignment at organic/electrode and organic/organic interfaces are reviewed, and methods to adjust the levels at such interfaces are presented. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

It's not easy being green: Strategies for all-nitrides, all-colour solid state lighting

Abstract: The design strategy presently employed to obtain ‘white’ light from semiconductors combines the emission of an InGaN blue or UV light-emitting diode (LED) with that of one or more yellow-orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining ‘true’ red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1) the green gap in the internal quantum efficiency (IQE) of LEDs; (2) the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III-nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron-hole wavefunction overlap of In-rich InGaN quantum wells (QW), and provide a non-radiative shunt for electron-hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real-world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN-based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described.

Silicon heterojunction solar cell with amorphous silicon oxide buffer and microcrystalline silicon oxide contact layers

Abstract: This Letter reports on the fabrication and characterization of silicon heterojunction solar cells with silicon oxide based buffer (intrinsic amorphous silicon oxide) and contact layers (doped microcrystalline silicon oxide) on flat p-type wafers. The critical dependency of the cell performance on the front and rear buffer layer thickness reveals a trade-off between the open circuit voltage Voc and the fill factor FF. At the optimum, the highest efficiency of 18.5% (active area = 0.67 cm2) was achieved with Voc = 664 mV, short circuit current Jsc = 35.7 mA/cm2 and FF = 78.0%. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Host structure dependence of light yield and proportionality in scintillators in terms of hot and thermalized carrier transport

Abstract: Several outstanding questions, including why complex halide scintillator host structures allow higher light yield and flatter electron energy response than simple monovalent metal halides, have remained unanswered by current models of luminescence in dense ionization tracks. Our measurements of nonlinear quenching kinetic order, recent literature on hot-electron transport in scintillators, and calculations presented here of hot-electron velocity from band structure of SrI2 and NaI, lead us to expand our previously described diffusion and nonlinear quenching model to include hot-electron transport. Trends in multivalent versus monovalent metal halides, heavier versus lighter halides, and halides versus oxides versus semiconductors can be predicted based on optical phonon frequency, thermalized band edge mobilities, velocity in the upper conduction bands, and hole self-trapping. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structure, stability and electronic properties of tricycle type graphane

Abstract: We propose a new allotrope of graphane, named as tricycle graphane, with a 4up/2down UUUDUD hydrogenation in each hexagonal carbon ring, which is different from previously proposed allotropes with UUDUUD (boat-1) and UUUUDD (boat-2) types of hydrogenation. Its stability and electronic structures are systematically studied using first-principles method. We find that the tricycle graphane is a stable phase in between the previously proposed chair and stirrup allotropes. Its electronic properties are very similar to those of chair, stirrup, boat-1 , boat-2, and twist-boat allotropes. The negative Gibbs free energy of tricycle graphane is -91 meV/atom, which very close to that of the most stable chair one (-103 meV/atom). Thus, this new two-dimensional hydrocarbon may be produced in the process of graphene hydrogenation with a relative high probability compared to other conformers.

Piezoelectric properties of PbHfO3–PbTiO3–Pb(Mg1/3Nb2/3)O3 ternary ceramics

Abstract: (1 – x)Pb(Hf1–yTiy)O3–x Pb(Mg1/3Nb2/3)O3 (x = 01 ∼ 025, y = 0555) ternary piezoelectric ceramics were prepared using the two-step precursor method Morphotropic phase boundary (MPB) compositions, located at x = 018 ∼ 022, were confirmed using X-ray diffraction and by their dielectric, piezoelectric and ferroelectric properties The optimum dielectric and piezoelectric properties were achieved for the MPB composition 08Pb(Hf0445Ti0555)O3–02Pb(Mg1/3Nb2/3)O3, with dielectric permittivity er, piezoelectric coefficient d33, planar electromechanical coupling kp and Curie temperature TC being on the order of 2800, 680 pC/N, 70% and 276 °C, respectively Of particular significance is that the new ternary ceramics exhibit comparable piezoelectric and electromechanical properties to commercial PZT5H ceramics, but with much improved TC, showing a potential for applications at elevated temperature (© 2012 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim)

Intrinsic spin Hall effect in silicene: transition from spin Hall to normal insulator

Abstract: An intrinsic contribution to the spin Hall effect in two-dimensional silicene is considered theoretically within the linear response theory and Green's function formalism. When an external voltage normal to the silicene plane is applied, the spin Hall conductivity is shown to reveal a transition from the spin Hall insulator phase at low bias to the conventional insulator phase at higher voltages. This transition resembles the recently reported phase transition in bilayer graphene. The spin–orbit interaction responsible for this transition in silicene is much stronger than in graphene, which should make the transition observable experimentally. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Auger carrier leakage in III‐nitride quantum‐well light emitting diodes

Abstract: Auger induced leakage is shown to be a contributing factor for the internal quantum efficiency (IQE) droop in III-nitride quantum-well light emitting diodes (LEDs). The mechanism is based on leakage current from carrier spill-out of the well originating from energy transfer during Auger recombination. Adding this leakage reduces the Auger coefficient by 50% when compared to a standard Auger model with cubic density dependence. As reference, experimental data of a green quantum-well LED are taken. Direct leakage due to non-ideal carrier capture and re-emission out of the well affects the IQE at current densities much larger than the maximum IQE point. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Excellent organic/inorganic transparent thin film moisture barrier entirely made by hot wire CVD at 100 °C

Abstract: We present a silicon nitride/polymer hybrid multilayer moisture barrier for flexible electronics made entirely by hot wire chemical vapor deposition (HWCVD) at substrate temperatures below 100 °C. Using the initiated CVD (iCVD) variant of HWCVD for the polymer layers, these can be extremely thin, while efficiently decoupling the defects in consecutive inorganic layers. Although a single layer of low temperature SiNx is more prone to have pinholes than its state-of-the-art high temperature equivalent, we have achieved a simple three-layer structure consisting of two low-temperature SiNx layers with a polymer layer in between, which is pinhole free and shows a water vapor transmission rate (WVTR) as low as 5 × 10–6 g/m2/day at a temperature of 60 °C and a relative humidity of 90%. This WVTR is low enough for organic devices. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Organic magnetoresistance and spin diffusion in organic semiconductor thin film devices

Abstract: We review recent work in the field of organic spintronics, focusing on our own contributions to this field. There are two principle magnetoresistance effects that occur in organic devices. (i) Organic magnetoresistance (OMAR), which occurs in nonmagnetic organic semiconductor devices. For example, in devices made from the prototypical small molecule Alq3 OMAR reaches values of 10% or more at room temperature. (ii) Organic spin-valve effects that occur in devices that employ ferromagnetic electrodes for spin-polarized current injection and detection. We undertake an analysis of these two types of magnetoresistance with the goal of identifying the dominant spin-scattering mechanism. Analysis of OMAR reveals that hyperfine coupling is the dominant spin-coupling mechanism. Spin–orbit coupling, on the other hand, is important only in organic semiconductor materials containing heavy atoms. We explore the reasons why spin–orbit coupling is relatively unimportant in hydrocarbon materials. Next, we present a theory for spin diffusion in disordered organic semiconductors based on hyperfine coupling, taking into account a combination of incoherent carrier hopping and coherent spin precession in the random hyperfine magnetic fields. We compare our findings with experimental values for the spin-diffusion length. Finally, we demonstrate a criterion that allows the determination whether the organic spin-valves operate in the tunneling or injection regimes. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Efficient second harmonic generation by birefringent phase matching in femtosecond‐laser‐inscribed KTP cladding waveguides

Abstract: We report on the fabrication of nonlinear KTiOPO4 (KTP) cladding waveguides by using femtosecond-laser direct inscription. The produced guiding structures with a circular cross section of ∼100 μm diameter support good light confinement both for the visible and infrared, for TE and TM polarization. Under the 1064 nm pulsed fundamental pump beam, guided-wave second harmonic generation (SHG) at 532 nm has been realized with an optical conversion efficiency as high as 45.6% (∼0.076% W–1 cm–2), which is three times the magnitude of the normalized efficiency obtained for “double-line” KTP waveguides (fabricated with the aim of comparison), thus showing an intriguing potential as integrated frequency converters. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Solution‐processible polymer solar cells fabricated on a papery substrate

Abstract: 1 School of Materials Science and Engineering, Heeger Center for Advanced Materials (HCAM), Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Korea 2 Korea Institute of Science and Technology (KIST), Institute of Advanced Composite Materials, Jeollabuk-do 565-902, Korea 3 Department of Nano & Chemical Engineering, Kunsan National University, 753-701, Kunsan, Jeollabuk-do, 753-701, Korea 4 School of Nanobio Materials and Electronics, Research Institute for Solar and Sustainable Energies (RISE), GIST, Gwangju 500-712, Korea

Self‐formed Schottky barrier induced selector‐less RRAM for cross‐point memory applications

Abstract: We propose a selector-less Pr0.7Ca0.3MnO3 (PCMO) based resistive-switching RAM (RRAM) for high-density cross-point memory array applications. First, we investigate the inhomogeneous barrier with an effective barrier height (Φeff), i.e., self-formed Schottky barrier. In addition, a scalable 4F2 selector-less cross-point 1 kb RRAM array has been successfully fabricated, demonstrating set, reset, and read operation for high cell efficiency and high-density memory applications. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Epitaxial phase-change materials

Abstract: Epitaxial thin layers were successfully grown in the metastable cubic phase on both slightly lattice-mismatched (GaSb) and highly lattice-mismatched (Si) templates. The higher quality of the films grown on (111)-oriented substrates is attributed to the tendency to form layered structures in the stable bulk phase as well as to the nature of distortion in the metastable cubic phase. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Real-time analysis of the microstructural evolution and optical properties of Cu(In,Ga)Se2 thin films as a function of Cu content

Abstract: Thin films of Cu(In,Ga)Se2 with various copper contents were deposited by co-evaporation onto thermally oxidized silicon substrates. Characterization by real-time spectroscopic ellipsometry reveals clear similarities among the samples, as well as key variations with Cu content. Although all films exhibit a Volmer–Weber nucleation and similar fundamental critical point energies in the analysis of optical properties, Cu-rich films exhibit enhanced coalescence, smoother surfaces, larger grain sizes, as well as a sub-bandgap absorption which is absent in Cu-poor films. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Band alignment at sputtered ZnSx O1–x/Cu(In,Ga)(Se,S)2 heterojunctions

Abstract: Valence band offsets ΔEVBM at ZnSx O1–x/Cu(In,Ga)(Se,S)2 (CIGSSe) heterojunctions have been studied by photoemission spectroscopy (XPS, UPS) as a function of composition x in sputtered ZnSx O1–x films. In the composition range from ZnO to ZnS we found ΔEVBM between –(2.1 ± 0.3) eV and –(0.8 ± 0.4) eV, respectively. Considering the optical band gaps, the conduction band offsets ΔECBM range from –(0.1 ± 0.3) eV to +(1.4 ± 0.4) eV. These results suggest that sputtered ZnSx O1–x is suitable as substitution for the CdS buffer and ZnO window layers in standard chalcopyrite-based solar cells. Current–voltage characteristics of the solar cells have been investigated as a function of the composition x. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thin macroporous silicon heterojunction solar cells

Abstract: We demonstrate the processing of a heterojunction solar cell from a purely macroporous silicon (MacPSi) absorber that is generated and separated from a monocrystalline n-type Cz silicon wafer by means of electrochemical etching. The etching procedure results in straight pores with a diameter of (4.7 ± 0.2) µm and a distance of 8.3 µm. An intrinsic amorphous Si (a-Si)/p+-type a-Si/indium tin oxide (ITO) layer stack is on the front side and an intrinsic a-Si/n+-type a-Si/ITO layer stack is on the rear side. The pores are open when depositing the layers onto the 3.92 cm2-sized cell. The conductive layers do not cause shunting through the pores. A silicon oxide layer passivates the pore walls. The energy-conversion efficiency of the (33 ± 2) µm thick cell is 7.2%. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Piezoelectric power generation from zinc oxide nanowires grown on paper substrate

Abstract: In this study, we demonstrate piezoelectric power generation from zinc oxide (ZnO) nanowires grown on paper substrate. Vertically aligned ZnO nanowires are deflected by an atomic force microscopy ( ...

Double‐shell metamaterial coatings for plasmonic cloaking

Abstract: We study a double-shell cloak for hiding objects with the dimensions comparable with the radiation wavelength. We demonstrate that the structure consisting of a dielectric layer and a layer of an epsilon-near-zero material can suppress sub- stantially the scattering from a sphere and at the same time shield its interiors. The double-layer coating allows to cloak different objects with various material and geometrical parameters. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

One‐step synthesis of a highly conductive graphene–polypyrrole nanofiber composite using a redox reaction and its use in gas sensors

Abstract: A reduced graphene oxide (RGO) and polypyrrole (PPY) nanofiber composite was fabricated using a one-step redox reaction under UV illumination at room temperature. Graphite oxide (GO) was effectively reduced to RGO without any chemical agent during the formation of the polypyrrole nanofibers. The RGO–PPY nanofiber composite showed four times higher NO2 sensitivity than that of RGO alone. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of detached back reflector designs for thin-film silicon solar cells

Abstract: We present a precise and flexible method to investigate the impact of diverse detached reflector designs on the optical response of p–i–n thin-film silicon solar cells. In this study, the term detached reflectors refers to back reflectors that are separated from the silicon layers by an intermediate rear dielectric of several micrometers. Based on the utilization of a highly conductive n-doped layer and a local electrical contact scheme, the method allows the use of non-conductive rear dielectrics such as air or transparent liquids. With this approach, diverse combinations of back reflector and rear dielectric can be placed behind the same solar cell, providing a direct evaluation of their impact on the device performance. We demonstrate the positive effect of a rear dielectric of low refractive index on the light trapping and compare the performance of solar cells with an air/Ag and a standard ZnO/Ag back reflector design. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Preparation, luminescence and structural properties of rare-earth-doped RbLuS2 compounds

Abstract: Rare-earth (RE = Ce, Pr, Sm, Tb) doped ternary sulfides of the general formula RbLu0.99RE0.01S2 and undoped RbLuS2 were synthesized in the form of crystalline hexagonal platelets by chemical reaction in the electric resistance furnace under the flow of hydrogen sulfide. Only a single crystalline phase of the rhombohedral lattice system (space group was detected by X-ray powder diffraction. Absorption and luminescence characteristics were measured. The band edge of RbLuS2 is found at 310 nm, and characteristic Pr3+, Sm3+ and Tb3+ 4f–4f emission lines in the visible spectral range are observed. A charge transfer transition in the Pr3+ excitation spectrum in the near UV spectral region is revealed and an efficient energy transfer from the host to the emission centers is found. The application potential for white LED or X-ray phosphors is discussed. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

3D hopping conduction in SnO2 nanobelts

Abstract: The temperature dependence of the electrical transport of an individual tin oxide nanobelt was measured in darkness from 400 to 5 K. We found four intrinsic electrical transport mechanisms through the nanobelt. It starts with thermally activated conduction between 400 K and 314 K, followed by nearest-neighbor hopping conduction between 268 K and 115 K and variable range hopping conduction below 58 K, with a crossover from the 3D Mott to the 3D Efros–Shklovskii regime at 16 K. We claim that this sequence reveals the three-dimensional nature of the electrical transport in the SnO2 nanobelts, even though they are expected to behave as one-dimensional systems. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Au/TiO2 nanorod-based Schottky-type UV photodetectors

Abstract: TiO2 nanorods (NRs) were synthesized on fluorine-doped tin oxide (FTO) pre-coated glass substrates using hydrothermal growth technique. Scanning electron microscopy studies have revealed the formation of vertically-aligned TiO2 NRs with length of ∼2 µm and diameter of 110–128 nm, homogenously distributed over the substrate surface. 130 nm thick Au contacts using thermal evaporation were deposited on the n-type TiO2 NRs at room temperature for the fabrication of NR-based Schottky-type UV photodetectors. The fabricated Schottky devices functioned as highly sensitive UV photodetectors with a peak responsivity of 134.8 A/W (λ = 350 nm) measured under 3 V reverse bias. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)