The BOSCO solar cell – a both sides collecting and contacted structure

The BOSCO solar cell represents a bifacial structure with double‐sided collection. The structure allows the use of standard module interconnection technology and favours the use of material with low to medium diffusion length and low resistivity for maximum benefit towards other structures, such as Al‐BSF and PERC. Within this work, we present first results on different multicrystalline silicon materials yielding a monofacial efficiency of 17.4% on large‐area wafers from block‐cast mc‐Si. This value represents a gain of ～0.7%_abs compared to Al‐BSF cells processed in parallel. The applicability for bifacial operation is demonstrated by a significantly increased quantum efficiency for rear side illumination. These results make the BOSCO solar cell concept a promising candidate to further boost the output of utility‐scale PV plants even when using low‐cost wafers of low to medium diffusion length material.

     

Welcome to the future of solid state physics publishing

Dear readers, authors and referees of physica status solidi, With the present issue and the start of the New Year following our 50th anniversary the editorial structure of pss undergoes some important formal changes. As already announced in the previous Editorial [1] twelve months ago, our former Editor‐in‐Chief Martin Stutzmann, Technische Universität München, has retired from the Editorial Board. This happens after 16 very successful years during which the journal has been brought back to prosperity. It is with deep gratitude to Martin for his strong vision, engagement, support and cooperation over one and a half decade that we meet this moment of changing duties. The fundamental transformation that the progress of electronic publishing brought to the business and the growth of pss to its current amount of output over the last years have resulted in an immense increase of workload for the editors, requiring practically full‐time dedication. Therefore, the role of an academic Editor‐in‐Chief will not be continued, transferring the central responsibility for pss to the hands of the Editors at the Wiley‐VCH office in Berlin and colleagues worldwide. We believe that this is a necessary modification to adapt to the changing publishing environment and habits, putting pss in a position to dynamically respond to ongoing developments. In fact, we currently observe the same policy change for other large journals in our publishing house, e.g. in polymer physics or quantum chemistry, not to mention successful Wiley‐VCH titles such as Advanced Materials or Angewandte Chemie which have been led by ‘in‐house’ Editorials for many years. Despite or even because of these present changes, close contacts of our editorial team to the scientific community will be ever more important for pss. This is why we strive to maintain and further develop a strong and active Board of the journal. Moreover, regardless of the differences in scope between pss (a) and (b), the trend of overlapping basic and applied research aspects in solid state physics has further manifested. Besides, our two other journals pss (RRL) and pss (c) cover the full spectrum of the field by definition. Therefore we have decided to create, from 2012, one joint Editorial Advisory Board for the entire pss family which will embrace many members of the previous separate Boards of the four journals (see ‘Issue Information’ online and inside front cover in the print edition, respectively). We are particularly pleased to welcome, at this moment, five new members to the Advisory Board: David A. Drabold , Ohio University, OH, USA; Sebastian T. B. Gönnenwein , Walther‐Meissner‐Institut and Technische Universität München, Garching, Germany; Nicholas Grandjean , École Polytechnique Fédérale de Lausanne, Switzerland; J. Marty Gregg , Queen’s University Belfast, UK; Ferdinand Scholz , Universität Ulm, Germany; and G. Jeffrey Snyder , California Institute of Technology, Pasadena, CA, USA. We will present them with photographs and short biographies in one of the forthcoming issues, and you should also look out for additional new Board members to join in the near future.

     

A long road to a bright future

Dear pss readers, “This year's Nobel Prize in Physics recognizes the pioneering work of Professors Isamu Akasaki, Hiroshi Amano and Shuji Nakamura for the breakthrough developments in GaN materials and devices that enabled the field of solid state lighting. Blue LEDs have revolutionized display technology and enabled high efficiency white light sources.” said our Editorial Advisory Board member Jim Speck in response to our enthusiastic congratulations to his colleague at University of California, Santa Barbara. It was certainly an exceptional moment for a journal that has extensively documented GaN‐related research for decades. Isamu Akasaki and Hiroshi Amano, then both Meijo University and Nagoya University, published in pss on dislocations and stress in AlGaN/GaN thin films, e.g. [1, 2], Hiroshi Amano repeatedly acted as a guest editor [3–5], and all three laureates together have more than a 150 articles in pss and continue to publish on the topic. Obviously, though these contributions have gathered more than 1400 citations to date, not all are “breakthroughs”. Most are documenting continuous achievements in an important and highly specialized area. But together they witness the exceptional persistence required to master the growth of GaN to the degree of perfection that enabled blue LEDs. It is an example for the amount of patient, high‐level work that is necessary to turn a brilliant idea into a widespread invention, and pss is proud to accompany such progress along the way. Regarding today's promising trends in materials physics, we would like to showcase some of the special issues and topical sections that appeared in pss sister journals in 2014: “Oxide materials might form the basis for the next technological revolutions” according to J. M. Knaup, T. Frauenheim, P. Broqvist, and S. Ramanathan, the guest editors of the pss (RRL) Focus issue “Functional Oxides” [6]. These materials' enhanced complexity proposes both a greater challenge and greater benefits for future electronics and energy applications, and the 3 Reviews and 20 Letters offer computational and experimental perspectives on the current understanding. A topical section “Nanoscaled Magnetism and Applications”, guest‐edited by A. Zhukov, features contributions on magnetocaloric materials, spin relaxation, magnetization processes and magnetic nanostructures in pss (a) [7]. Another bustling topic in pss (a) are the recurring annual diamond and nanocarbon special issues, this time focused on “Advances on Diamond Surfaces and Devices” edited by B. Rezek, M. Nesládek, and K. Haenen [8], and kicked‐off by a Feature Article “Photoelectron emission from lithiated diamond” by K. M. O'Donnell, L. Ley, and others [9].

The logo of our journal assembled with blue GaN‐on‐Si LEDs (courtesy of Armin Dadgar, Otto‐von Guericke‐Universität, Magdeburg, Germany)     

Evidence of two‐photon absorption in strain‐free quantum dot GaAs/AlGaAs solar cells

We report the fabrication procedure and the characterization of an Al_0.3Ga_0.7As solar cell containing high‐density GaAs strain‐free quantum dots grown by droplet epitaxy. The production of photocurrent when two sub‐bandgap energy photons are absorbed simultaneously is demonstrated. The high quality of the quantum dot/barrier pair, allowed by the high quality of nanostructured strain‐free materials, opens new opportunities for quantum dot based solar cells.

     

Excellence – energy – ethics

Dear pss readers, As the previous year approached its end, news on three excellent prize winnings and nominations of pss authors and editors reached us: Gerhard Abstreiter of TU Munich will receive the Stern–Gerlach Medal 2014, the highest prize of the German Physical Society for experimental physics, honoring his work on low‐dimensional electron systems in semiconductor hetero‐ and nanostructures. His Review@RRL on InGaAs nanowires on silicon is opening the 2014 volume of pss (RRL) [1]. It is a welcome addition to our recent successful Focus Issue on Semiconductor Nanowires [2]. Our long‐term Editorial Advisory Board member, Wiley author and Guest Editor, Rainer Waser of RWTH Aachen and Research Centre Jülich, is one of the 11 winners of the highly prestigious Leibniz Prize for his outstanding research on nanoelectronics, especially oxides, ferroelectrics and resistive switching [3, 4]. Last but not least, one of the three nominated teams for the German Future Prize has been led by Wolfgang Schnick , LMU Munich, and Peter J. Schmidt , Philips Lumileds Aachen. Their groundbreaking work on new phosphor materials in white light emitting diodes (LEDs) for solid‐state lighting [5] goes back to a highly‐cited pss (a) article from 2005 [6] (see figure). The technology is now being commercialized and expected to enable energy savings on a grand scale in the coming years. Speaking of energy, research results related to this global challenge have been important throughout the year, touching areas such as thermoelectrics [7], efficiency of organic LEDs [8] and photovoltaics [9]. The latter field is even better represented since the recent introduction of our section rrl solar, covering solar cell materials or device development and characterization (see Editorial [10]). The full‐paper sister journals pss (a) and (b) presented an unprecedented number of high‐profile special issues in 2013 [11–15]. With heartfelt gratitude we look back onto fruitful collaboration with highly engaged guest editors, who helped bring to light issues such as the “Advanced Concepts for Silicon Based Photovoltaics” [11], the “Quantum Criticality and Novel Phases” [12], the “Disorder in Order: A special issue on amorphous materials” [13], the “Substrate Interactions in Heterogeneous Catalysis” [14], and the “Quantum Transport at the Molecular Scale” [15] among other interesting topical issues and sections. Both contributors and fine articles are too numerous to do justice to all of them here. We must restrict ourselves to a general invitation to browse this content, only hinting a few possible starting points, such as topological insulators [16], molecular electronics [17] and quantum phase transitions [18].

     

Na‐site energy of P2‐type NaxM O2 (M = Mn and Co)

P2‐type Na_xM O₂ (M = Mn and Co) is a promising cathode material for low‐cost sodium ion secondary batteries. In this structure, there are two different crystallographic Nai (i = 1 and 2) sites with different Coulomb potential $ (\varphi _i)$ provided by M^4–x and O^2–. Here, we experimentally determine a difference ${(\rm \Delta }\varepsilon \equiv \varepsilon _1 - \varepsilon _2)$ of Na‐site energies ${(}\varepsilon _i \equiv e\varphi {\kern 1pt} _i)$ based on the temperature dependence of the site occupancies. We find that ${\rm \Delta }\varepsilon \;{=}\;56\;{K}$ for Na_0.52MnO₂ is significantly smaller than 190 K for Na_0.59CoO₂. We interpret the suppressed ${\rm \Delta }\varepsilon $ in Na_0.52MnO₂ in terms of the screening effect of the Na⁺ charge. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ru‐doped TiO2 nanotubes: Improved performance in dye‐sensitized solar cells

In the present work we use a series of Ti–Ru alloys, with minor amounts of Ru (0.01, 0.02, 0.05 and 0.2 at%) to grow anodic self‐organized Ru‐doped TiO₂ nanotube layers. When used in dye‐sensitized solar cells (DSSCs), the nanotube layers with an optimum amount of Ru (0.02 at% Ru in the alloy) show a considerable increase in solar cell efficiency (η = 5.2%) under AM1.5 (100 mW/cm²) conditions compared with non‐doped TiO₂ nanotubes (η = 4.3%).

     

Nature of the transient species formed in the pulse radiolysis of 4‐hydroxybenzyl alcohol in aqueous solutions: observation of equilibrium in the reaction of OH‐adducts with HPO ions

Reactions of ^. OH/O ^.? radicals, H‐atoms as well as specific oxidants such as N and Cl radicals with 4‐hydroxybenzyl alcohol (4‐HBA) in aqueous solutions have been investigated at various pH values using the pulse radiolysis technique. At pH 6.8, ^. OH radicals were found to react with 4‐HBA (k = 6 × 10⁹ dm³ mol^?1 s^?1) mainly by contributing to the phenyl moiety and to a minor extent by H‐abstraction from the ? CH₂OH group. ^. OH radical adduct species of 4‐HBA, i.e., ^. OH‐(4‐HBA) formed in the addition reaction were found to undergo dehydration to give phenoxyl radicals of 4‐HBA. Decay rate of the adduct species was found to vary with pH. At pH 6.8, decay was very much dependent on phosphate buffer ion concentrations. Formation rate of phenoxyl radicals was found to increase with phosphate buffer ion concentration and reached a plateau value of 1.6 × 10⁵ s^?1 at a concentration of 0.04 mol dm^?3 of each buffering ion. It was also seen that ^. OH‐(4‐HBA) adduct species react with HPO ions with a rate constant of 3.7 × 10⁷ dm³ mol^?1 s^?1 and there was no such reaction with H₂PO ions. However, the rate of reaction of ^. OH‐(4‐HBA) adduct species with HPO ions decreased on adding KH₂PO₄ to the solution containing a fixed concentration of Na₂HPO₄ which indicated an equilibrium in the H⁺ removal from ^. OH‐(4‐HBA) adduct species in the presence of phosphate ions. In the acidic region, the ^. OH‐(4‐HBA) adduct species were found to react with H⁺ ions with a rate constant of 2.5 × 10⁷ dm³ mol^?1 s^?1. At pH 1, in the reaction of ^. OH radicals with 4‐HBA (k = 8.8 × 10⁹ dm³ mol^?1 s^?1), the spectrum of the transient species formed was similar to that of phenoxyl radicals formed in the reaction of Cl radicals with 4‐HBA at pH 1 (k = 2.3 × 10⁸ dm³ mol^?1 s^?1) showing that ^. OH radicals quantitatively bring about one electron oxidation of 4‐HBA. Reaction of ^. OH/O ^.? radicals with 4‐HBA by H‐abstraction mechanism at neutral and alkaline pH values gave reducing radicals and the proportion of the same was determined by following the extent of electron transfer to methyl viologen. H‐atom abstraction is the major pathway in the reaction of O ^.? radicals with 4‐HBA compared to the reaction of ^. OH radicals with 4‐HBA. At pH 1, transient species formed in the reactions of H‐atoms with 4‐HBA (k = 2.1 × 10⁹ dm³ mol^?1 s^?1) were found to transfer electrons to methyl viologen quantitatively. Copyright © 2009 John Wiley & Sons, Ltd.     

K s →π+π−γ: a laboratory for meson dynamics

We discuss the decayK _{s +–} in the framework of chiral perturbation theory. The direct emission in this decay can be an useful test of meson dynamics.     

Crystal structures of 3‐methyl‐2(1H)‐quinoxalinone and three substituted derivatives

3‐Methyl‐2(1H)‐quinoxalinone and three derivatives (3,7‐dimethyl‐2(1H)‐quinoxalinone, 3‐methyl‐6,7‐dichloro‐2(1H)‐quinoxalinone and 3‐methyl‐7‐nitro‐2(1H)‐quinoxalinone) have been synthesised and analysed by ¹H NMR and IR spectral spectroscopies. The crystal structures have been determined at room temperature from X‐ray single crystal diffraction data for three of them and from powder diffraction data for the nitro derivative. 3‐Methyl‐2(1H)‐quinoxalinone crystallises in the P2₁/c monoclinic system, 3,7‐dimethyl‐2(1H)‐quinoxalinone in the Pbca orthorhombic system and the two others compounds in the P$\overline {1} $ triclinic system. For the nitro derivative, C? H$\cdots $ N short contacts are established between the carbon of the methyl and the double bounded nitrogen of the ring. For the three other compounds N? H$\cdots $ O hydrogen bonds involve the atoms of the heterocyclic ring. Copyright © 2011 John Wiley & Sons, Ltd.     

Natural boundaries for area-preserving twist maps

We consider KAM invariant curves for generalizations of the standard map of the form (x, y)=(x+y, y+f(x)), wheref(x) is an odd trigonometric polynomial. We study numerically their analytic properties by a Padé approximant method applied to the function which conjugates the dynamics to a rotation +. In the complex plane, natural boundaries of different shapes are found. In the complex plane the analyticity region appears to be a strip bounded by a natural boundary, whose width tends linearly to 0 as tends to the critical value.     

Investigation of defect luminescence from multicrystalline Si wafer solar cells using X‐ray fluorescence and luminescence imaging

Multicrystalline silicon wafer solar cells reveal performance‐ reducing defects by luminescence. X‐ray fluorescence spectra are used to investigate the elemental constituents from regions of solar cells yielding reverse‐bias or sub‐bandgap luminescence from defects. It is found that a higher concentration of metals is present in regions yielding reverse‐bias electroluminescence than in regions yielding sub‐bandgap electroluminescence. This suggests, dislocations do not create strong breakdown currents in the absence of impurity precipitates.

     

Kinetic Study of the Emission of the First Negative System in an Argon-Nitrogen Plasma at Atmospheric Pressure

Quantitative optical spectroscopy measurements of the emission spectra of the N(B²∑_u,)ν′→X²∑_gν″ transition (first negative system) in an Ar-N₂ microwave discharge at atmospheric pressure have allowed determination of the rate coefficient of the production of N molecules in the B² ∑_u, state with vibrational level ν′ = 0. The N(B²∑_u, ν′) molecules are produced by the reaction in a surface-wave-induced microwave discharge (2450 MHz) sustained in an open-ended dielectric tube. The rate coefficient K (T) has been obtained for ν′ν″ = 0 for different gas temperatures by varying the incident microwave power. The K₀₀(T) values are between 7.10^?10 and 4.10^?10 cm³ s^?1 for the temperature range 2500 to 3450K.     

μSR studies in the heavy fermion systems UNi2Al3, UPd2Al3 and U2PtC2

We have performed Muon Spin Relaxation (SR) measurements of several heavy fermion systems: UNi₂Al₃ (single crystal), UPd₂Al₃ (single crystal) and U₂PtC₂ (polycrystal). ZF -SR measurements of UNi₂Al₃ show magnetic order below the Néel temperatureT _N=4.6K, with the muon precession frequency(T 0)=2.5MHz at low temperatures. The local field distribution obtained at low temperatures in UNi₂Al₃ resembles that of the incommensurate spin density wave (SDW) system (TMTSF)₂PF₆, suggesting an incommensurate spin arrangement in UNi₂Al₃. TF -SR measurements give the penetration depth(T 0)=6100Å for UPd₂Al₃ and(T 0)=7000Å for U₂PtC₂ respectively. Since UPd₂Al₃ and U₂PtC₂ are clean superconductors (/l-0), we can combine n _s/m^* with the Sommerfeld constant n _s ^1/3 m^* to calculate the Fermi temperatureT _F ^3/4–1/4, which represents the characteristic energy scale of the superconducting carriers. We obtainT _C/T_F 0.01 for both UPd₂Al₃ and U₂PtC₂, comparable to other exotic superconductors, but quite different from simple metal BCS superconductors, whereT _C/T_F 0.01.     

Note on the susceptibility exponent for two-dimensional percolation

Analyses of mean site content, mean bond content, mean perimeter and related quantities give a susceptibility diverging as (p _c –p)^–, with 2.41±0.025. This exponent disagrees with some earlier estimates but it is consistent with the (p _c –p)^–2.388... divergence predicted by the formulas of den Nijs, Nienhuis et al. and Pearson.     

Percolation and Minimal Spanning Trees

Consider a random set of points in the box [n, –n) ^d , generated either by a Poisson process with density p or by a site percolation process with parameter p. We analyze the empirical distribution function F _n of the lengths of edges in a minimal (Euclidean) spanning tree on . We express the limit of F _n, as n , in terms of the free energies of a family of percolation processes derived from by declaring two points to be adjacent whenever they are closer than a prescribed distance. By exploring the singularities of such free energies, we show that the large-n limits of the moments of F _n are infinitely differentiable functions of p except possibly at values belonging to a certain infinite sequence (p _c(k): k 1) of critical percolation probabilities. It is believed that, in two dimensions, these limiting moments are twice differentiable at these singular values, but not thrice differentiable. This analysis provides a rigorous framework for the numerical experimentation of Dussert, Rasigni, Rasigni, Palmari, and Llebaria, who have proposed novel Monte Carlo methods for estimating the numerical values of critical percolation probabilities.     

Self‐assembly of poly(3‐hexylthiophene) nanowire networks by a mixed‐solvent approach for organic field‐effect transistors

Organic field‐effect transistors (OFETs) based on interconnected nanowire networks of P3HT have been successfully fabricated by using a mixed‐solvent method. The nanowire network density can be tuned by controlling the anisole/chlorobenzene ratio of mixed solvents. The obtained field‐effect mobility, threshold voltage and the ratio of on‐state current and off‐state current (I_on/I_off) was 0.0435 cm²/V s, –10 V and 1.75 × 10⁴, respectively. The three‐dimensional and interconnected nanowire structure of the networks can enhance the charge transport in P3HT.

     

Enhanced dielectric permittivity of fluorine polyimide matrix with embedded graphene

A polymer nanohybrid material with enhanced dielectric permittivity was prepared using the fluorine‐containing polyimide (PI) 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride/4,4′‐oxydianiline (6FDA/ODA) as matrix and graphene as conductive filler in our present work. Studies on the dielectric properties of the 6FDA/ODA–graphene nanohybrid films show that the dielectric permittivity (ε) can be significantly enhanced by the layer‐by‐layer structure of graphene and the presence of fluorine also has an important influence on the improvement of ε. The percolation theory and microcapacitor model are used to explain the change of dielectric properties and a percolation threshold f_c = 0.0152 (2.45 wt%) was obtained by a linear‐fit calculation.