Large‐area flexible,transparent, and highly luminescent films containing lanthanide (III) complex‐doped ionic liquids for efficiency enhancement of silicon‐based heterojunction solar cell

Ionic liquid (IL)‐based lanthanide luminescent materials with luminescent downshifting properties are beneficial for enhancing the photovoltaic energy conversion efficiency (η _pv) of silicon‐based solar cells. Herein, novel lanthanide (III) complex‐doped ILs have been successfully prepared by simply dissolving the complex Ln(pybox)₃ (Ln = Eu or Tb, pybox = 2,6‐bis[(4R )‐4‐phenyl‐2‐oxazolinyl]pyridine) into a bidentate organophosphine functionalized IL (1,3‐bis‐[3‐(diphenylphosphinyl)propyl]imidazolebis (trifluoromethylsulfonyl)imide), showing improved luminescence efficiency attributed to the coordination of ILs with Ln³⁺ ions and thus occurrence of the energy transfer from ILs to Ln³⁺ ions and very high absolute quantum efficiency (up to 97.22%). Large area (17 × 17 cm²) flexible, transparent, and luminescent poly(methyl methacrylate) thin films were prepared through a simple drop‐casting method and were applied as luminescent downshifting coatings to the silicon‐based heterojunction solar cells (active area: 235 cm²). In the best case using the Tb³⁺‐containing film, the η _pv of the solar cell increases from 16.676 to 16.774%.     

Organic–Inorganic Hierarchical Self‐Assembly into Robust Luminescent Supramolecular Hydrogel

Luminescent hydrogels are of great potential for many fields, particularly serving as biomaterials ranging from fluorescent sensors to bioimaging agents. Here, robust luminescent hydrogels are reported using lanthanide complexes as emitting sources via a hierarchical organic–inorganic self‐assembling strategy. A new organic ligand is synthesized, consisting of a terpyridine unit and two flexibly linked methylimidazole moieties to coordinate with europium(III) (Eu³⁺) tri‐thenoyltrifluoroacetone (Eu(TTA)₃), leading to a stable amphiphilic Eu³⁺‐containing monomer. Synergistic coordination of TTA and terpyridine units allows the monomer to self‐assemble into spherical micelles in water, thus maintaining the luminescence of Ln complexes in water. The micelles further coassemble with exfoliated Laponite nanosheets coated with sodium polyacrylate into networks based on the electrostatic interactions, resulting in the supramolecular hydrogel possessing strong luminescence, extraordinary mechanical property, as well as self‐healing ability. The results demonstrate that hierarchical organic–inorganic self‐assembly is a versatile and effective strategy to create luminescent hydrogels containing lanthanide complexes, giving rise to great potential applications as a soft material.     

Influence of temperature on luminescent coupling and material quality evaluation in inverted lattice‐matched and metamorphic multi‐junction solar cells

Inverted metamorphic multi‐junction solar cells have reached efficiencies close to 46%. These solar cells contain very high‐quality materials that exhibit strong luminescent coupling between the junctions. The presence of luminescent coupling has a significant impact on the behavior of multi‐junction solar cells affecting the optimal design of these devices. Because of the importance of studying devices under real operating conditions, the temperature dependence of the luminescent coupling is analyzed over a range of 25–120°C. Luminescent coupling analysis results show a reduction of the luminescent coupling current as a function of temperature in two tandem components of an inverted metamorphic triple junction solar cell such as GaInP/GaAs and GaAs/GaInAs solar cells. This reduction is quantified and examined by means of luminescent coupling analysis and modeling, electroluminescence measurements and optical modeling at the device and subcell level. The results of the models are verified and discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

Aerosol‐Synthesis of Mesoporous Organosilica Nanoparticles with Highly Reactive,Superacidic Surfaces Comprising Sulfonic Acid Entities

Combining high internal surface area with tailor‐made surface properties is pivotal for granting advanced functional properties in many areas like heterogeneous catalysis, electrode materials, membranes, or also biomimetics. In this respect, organic‐inorganic hybrid nanostructures and in particular mesoporous organosilica materials are ideal systems. Here, the preparation of mesoporous solids via a new sol–gel building block comprising sulfonic acid (R‐SO₃H) is described. The degree of organic modification is not only maximal (100%), it is also proven that the novel material exhibits superacid properties. Furthermore, an aerosol assisted method is applied for generating this material in the form of mesoporous, spherical nanoparticles with substantial colloidal stability. Highly acidic, high surface area materials, like prepared here, are promising candidates for numerous future applications like in heterogeneous catalysis or for proton conducting membranes. However, first experiments addressing the antibacterial effect of the sulfonic‐acid, mesoporous organosilica materials are shown. It is demonstrated that the superacid character is required for exhibiting sufficient antifouling activity.     

Characterization of the spatial distribution of irradiance and spectrum in concentrating photovoltaic systems and their effect on multi‐junction solar cells

The irradiance and spectral distribution cast on the cell by a concentrating photovoltaic system, typically made up of a primary Fresnel lens and a secondary stage optical element, is dependent on many factors, and these distributions in turn influence the electrical performance of the cell. In this paper, the effect of spatial and spectral non‐uniform irradiance distribution on multi‐junction solar cell performance was analyzed using an integrated approach. Irradiance and spectral distributions were obtained by means of ray‐tracing simulation and by direct imaging at a range of cell‐to‐lens distances. At the same positions, I–V curves were measured and compared in order to evaluate non‐uniformity effects on cell performance. The procedure was applied to three different optical systems comprised a Fresnel lens with a secondary optical element consisting of either a pyramid, a dome, or a bare cell. Copyright © 2011 John Wiley & Sons, Ltd.     

Stoichiometry Control for the Tuning of Grain Passivation and Domain Distribution in Green Quasi‐2D Metal Halide Perovskite Films and Light‐Emitting Diodes

Quasi‐2D metal halide perovskite films are promising for efficient light‐emitting diodes (LEDs), because of their efficient radiative recombination and suppressed trap‐assisted quenching compared with pure 3D perovskites. However, because of the multidomain polycrystalline nature of solution‐processed quasi‐2D perovskite films, the composition engineering always impacts the emitting properties with complicated mechanisms. Here, defect passivation and domain distribution of quasi‐2D perovskite films prepared with various precursor compositions are systematically studied. As a result, in perovskite films prepared from stoichiometric quasi‐2D precursor compositions, large organic ammonium cations function well as passivators. In comparison, precursor compositions of simply adding large organic halide salt into a 3D perovskite precursor ensure not only the defect passivation but also the effective formation of quasi‐2D perovskite domains, avoiding unfavorable appearance of low‐order domains. Quasi‐2D perovskite films fabricated with a well‐designed precursor composition achieve a high photoluminescence quantum yield of 95.3% and an external quantum efficiency of 14.7% in LEDs.     

Manipulation of the Open‐Circuit Voltage of Organic Solar Cells by Desymmetrization of the Structure of Acceptor–Donor–Acceptor Molecules

The synthesis of acceptor–donor–acceptor (A–D–A) molecules based on a septithiophene chain with terminal electron acceptor groups is reported. Using a dicyanovinyl‐ (DCV) substituted molecule as reference, another symmetrical A–D–A donor containing thiobarbituric (TB) groups is synthesized and these two acceptor groups are combined to produce the unsymmetrical A–D–A′ compound. The electronic properties of the donors are analyzed by cyclic voltammetry and UV‐Vis absorption spectroscopy and their photovoltaic properties are characterized on bilayer planar heterojunction cells that include spun‐cast donor films and vacuum‐deposited C₆₀ as acceptor. Optical and electrochemical data show that replacement of DCV by TB leads to a small increase of the HOMO level and to a larger decrease of the LUMO, which result in a reduced band‐gap. The desymmetrized compound presents the lowest oxidation potential in solution but the highest oxidation onset in the solid state, which leads to a significant increase of the open‐circuit voltage of the resulting solar cells.     

Copper(I) Iodide as Hole‐Conductor in Planar Perovskite Solar Cells: Probing the Origin of J–V Hysteresis

Organic–inorganic lead halide perovskite solar cells are promising alternatives to silicon‐based cells due to their low material costs and high photovoltaic performance. In this work, thin continuous perovskite films are combined with copper(I) iodide (CuI) as inorganic hole‐conducting material to form a planar device architecture. A maximum conversion efficiency of 7.5% with an average efficiency of 5.8 ± 0.8% is achieved which, to our knowledge, is the highest reported efficiency for CuI‐based devices with a planar structure. In contrast to related planar 2,2′,7,7′‐tetrakis‐(N,N ‐di‐4‐methoxyphenylamino)‐9,9′‐spirobifluorene (spiro‐OMeTAD)‐based devices, the CuI‐based devices do not show a pronounced hysteresis when tested by scanning the potential in a forward and backward direction. The strong quenching of photoluminescence (PL) signal and comparatively fast decay of open‐circuit voltage demonstrates a more rapid removal of positive charge carriers from the perovskite layer when in contact with CuI compared to spiro‐OMeTAD. A slow response on a timescale of 10–100 s is observed for the spiro‐OMeTAD‐based devices. In comparison, the CuI‐based device displays a significantly faster response as determined through electrochemical impedance spectroscopy (EIS) and open‐circuit voltage decays (OCVDs). The characteristically slow kinetics measured through EIS and OCVD are linked directly to the current–voltage hysteresis.     

H2 plasma treatment at the p/i interface of a hydrogenated amorphous Si absorption layer for high‐performance Si thin film solar cells

Plasma treatment (PT) of the buffer layer for highly H₂‐diluted hydrogenated amorphous silicon (a‐Si:H) absorption layers is proposed as a technique to improve efficiency and mitigate light‐induced degradation (LID) in a‐Si:H thin film solar modules. The method was verified for a‐Si:H single‐junction and a‐Si:H/microcrystalline silicon (µc‐Si:H) tandem modules with a size of 200 × 200 mm² (aperture area of 382.5 cm²) under long‐term light exposure. H₂ PT at the p/i interface was found to eliminate non‐radiative recombination centers in the buffer layer, and plasma‐enhanced chemical vapor deposition at low radio‐frequency power was found to suppress the generation of defects during the growth of a‐Si:H absorption layers on the treated buffer layers. With optimized H₂ PT of the a‐Si:H single‐junction module, the stabilized short circuit current and fill factor increased, and the stabilized open circuit voltage moves beyond its initial value. The results demonstrate 7.7% stabilized efficiency and 10.5% LID for the a‐Si:H single‐junction module and 10.82% stabilized efficiency and 7.76% LID for the a‐Si:H/µc‐Si:H tandem module. Thus, the growth of an a‐Si:H absorption layer on a H₂ PT buffer layer can be considered as a practical method for producing high‐performance Si thin film modules. Copyright © 2012 John Wiley & Sons, Ltd.