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In this work, we report a degradation study of high-efficiency conventional polymer solar cells (PSCs). The high performance of the PM6:Y7-based device is achieved with a power conversion efficiency (PCE) of up to 15.64% in the first measurement. The article describes the results of the measurements and analysis of the degradation process. The electrical parameters during the degradation process were extracted from the current density – voltage characteristics curves (J–V) under light and dark conditions. Were observed the specific parameters of the selected cells: open-circuit voltage (VOC), short circuit current density (JSC), fill factor (FF), PCE, series (RS), and shunt (RSH) resistance values The PCE of the device decreased down to 64% of the initial PCE after 1056 h., En este trabajo, nosotros reportamos un estudio de celdas solares poliméricas (PSCs) convencionales de alta eficiencia. El alto desempeño del dispositivo basado en PM6:Y7 se logró con una eficiencia de conversión de potencia (PCE) de hasta el 15.64 % en la primera medición. El artículo describe los resultados de las mediciones y el análisis del proceso de degradación. Los parámetros eléctricos durante el proceso de degradación fueron extraídos de las curvas características de densidad de corriente-voltaje (J–V) en condiciones de luz y bajo oscuridad. Se observaron los parámetros específicos de las celdas seleccionadas: voltaje de circuito abierto (VOC), densidad de corriente de cortocircuito (JSC), factor de llenado (FF), PCE, serie (RS) y resistencia en paralelo (RSH). La PCE del dispositivo disminuyó hasta el 64% del su PCE inicial después de 1056 h.Palabras claveAnálisis de degradación; PM6:Y7; celdas solares orgánicas; aceptores no fulerenos; celdas encapsuladas.

Several MgAl-LDHs (HT) and CaAl-LDHs (HC) were synthesized in different conditions and later calcined at 723 K, resulting in catalysts (cHT and cHC), with different acid base properties. Calcined samples were tested in the etherification reaction of glycerol towards short-chain oligomers (di- to pentaglycerol). All catalysts were characterised by XRPD, ICP, N-2,physisorption and TEM techniques. The basicity was evaluated using the CO2-TPD technique and the acidity by cyclohexylamine adsorption. Catalytic results were correlated with catalysts properties. Catalysts with higher acidity showed higher conversion (96% for a cHT and 75% for a cHC) but also higher selectivity to other products, with main contribution of acrolein (88% for a cHT and 58% for a cHC). In contrast, catalysts with lower acidity resulted in lower conversion (24% for a cHT and 40% for a cHC) but higher selectivity towards di- and triglycerol (100% and 64%, respectively). The formation of triglycerol and other low-weight polyglycerols (tetra- and pentaglycerol) was favoured in catalysts with low acidity and strong basic sites. When the temperature of calcination was raised to 1073 K for one cHC, the acidity decreased and the number of strong basic sites increased, resulting in higher selectivity to triglycerol (20%), the formation of tetra- and pentaglycerol (15% and 6%, respectively) and the decrease of the acrolein amount. (C) 2014 Elsevier B.V. All rights reserved., Postprint (published version)

We revisited two different strategies to fabricate 1D photonic crystals of nonlinear optical dielectric materials based on ultrafast laser ablation of the surface of an RbTiOPO4 crystal, and selective etching of ferroelectric domains of the surface of a periodically poled LiNbO4 crystal. We evaluated their behaviour as Bragg diffraction gratings. We also presented the recent advances we developed in a new procedure of fabrication of 2D and 3D photonic crystals of KTiOPO4 (KTP) grown on the surface of a KTP substrate by liquid phase epitaxial means within the pores of a silicon macroporous template. Optical, structural, morphological, and compositional characterization for the photonic crystals produced through this technique are presented., Peer Reviewed, Postprint (published version)

The deposition of specific materials into ordered pores arranged in a regular lattice allows the fabrication of ordered fiber arrays. Porous silicon is an attractive candidate for use as a template because the porosity and pore size can be tuned. Polymer microfibers using macroporous silicon with various pore depths as template system is reported here. The method entails vacuum infiltration of a polymer solution into the pores of the template. Two different types of polymer were selected in order to obtain ordered arrays of fibers made from Poly(methyl) methacrylate) or Poly(dimethylsiloxane). Microtubes were obtained when we used a silicon membrane as a template, whereas microfibers were obtained when we used a silicon template with pores opened only at one end., Peer Reviewed, Postprint (published version)

We report the experimental characterization of two-dimensional (2D) macroporous silicon photonic crystals using angular-dependent reflectance spectroscopy in the mid-IR region. We have investigated different sample structures and we have shown that an adequate post-processing of the measured data is crucial in order to recognize the photonic bands and to achieve a good agreement of the measured data with the theoretical predictions for the studied structures., Peer Reviewed, Postprint (published version)

In this work, we study the changes in the optical properties of 300-nm-thick hydrogenated amorphous silicon carbide layers after an annealing process. Both intrinsic and phosphorus-doped amorphous silicon carbide layers (a-SiCx:H) were deposited on silicon wafers by plasma enhanced chemical vapour deposition (PECVD) at 400 °C and annealed in a quartz furnace at 800 °C. The presence of randomly oriented silicon nanocrystals was confirmed by X-ray diffraction (XRD) measurements after the partial recrystallization process only in the doped layers. The presence or the absence of the nanocrystals clearly changes the Fourier transform infrared (FTIR) spectra. From the fitting of the experimental curves with the model of Lorentz oscillators, the refractive index and the extinction coefficient of the different layers were obtained., Peer Reviewed, Postprint (published version)

Macroporous silicon structures have been fabricated by electrochemical etching. Such fabrication process is known to result in the presence of a thin microporous Si layer at the walls of the macropores and at the surface. Photoluminescence measurements conducted in plan-view and cross-section exhibit a wide emission peak around 650 nm which can be attributed to the microporous Si. The combination of a photonic crystal and a light emitter in one structure represents a potential for applications that has not been studied previously. We initiate this study by investigating the influence of the main fabrication parameters, namely the current density and the etchant solution, on the emission properties of the microporous Si layer., Peer Reviewed, Postprint (published version)

2D KTiOPO4 photonic crystals are fabricated using a silicon membrane of air holes as a template. Columns of KTiOPO4 are grown by liquid-phase epitaxy from a substrate into such air holes. The crystallographic orientation of the columns is the same as that of the substrate. The photonic-crystal properties of the fabricated structures and their capability to generate second-harmonic light are experimentally demonstrated., Peer Reviewed, Postprint (published version)

This article describes the benefits of including cross-layer information in the scheduling mechanism of a UMTS downlink channel. In particular, the information obtained from the fast power control algorithm is used to properly schedule transmissions. A prioritization function that exploits the short-term channel variations is proposed. This strategy is shown to be a feasible approach to improve system performance in terms of capacity and delay. This enhancement is obtained as a benefit of intrinsic multi-user diversity. The proposal is applicable within the current UMTS radio resource management framework., Peer Reviewed, Postprint (published version)

This paper presents a novel technique for silicon dioxide (SiO2) microneedle fabrication. Microneedles are hollow microcapillaries with tip diameters in the range of micrometers. They can be used in the fabrication of microsyringes. These structures can be of high interest in medical and biological applications, such as DNA injection, antibody manipulation and drug delivery, and cell manipulation. Fabrication process is based on electrochemical etching of n-type silicon in hydrofluoric acid (HF) solutions. Basic process flow and etching conditions that ensure a stable pore growth are described. These conditions also determine the geometry of the resulting microneedle structure. Microneedle arrays of different dimensions can be fabricated in a single run on the same wafer. In this work, microneedle arrays with pore diameters ranging from 2 to 5 µm, pore lengths from 30 to 140 µm and wall thicknesses in the range of 70–110 nm are reported., Peer Reviewed, Postprint (published version)

This paper reports on the fabrication of two-dimensional macropore arrays by electrochemical etching of silicon. We describe some critical effects accompanying the etching process that can generate imperfections in the etched pore pattern. We also give the solutions to improve the pore uniformity and to produce high-aspect-ratio macropores with good structural characteristics. The obtained macroporous structures can be further processed to produce high-aspect-ratio pillars, instead of pores. By modulating the applied etching current we have also induced periodicity in the third dimension., Peer Reviewed, Postprint (published version)

We investigate the absolute photonic band gap (PBG) formation in two-dimensional (2-D) photonic crystals designed using symmetry reduction approach. The lattice symmetry, shape and orientation of dielectric scatterers affect the photonic gap parameters. We use symmetry reduction, achieved either by including additional rods into the lattice unit cell or by reorienting noncircular scatterers to engineer the photonic band gaps in 2-D square and triangular structures. The case of air rods drilled into silicon background is considered. We show that for square structures symmetry reduction can be an effective way to enlarge the absolute PBG, but for triangular lattices any modification of the crystal structure considerably reduces the absolute PBG width. We also discuss the practical technological feasibility of the proposed structures., Peer Reviewed, Postprint (published version)

We investigate different aspects of the absolute photonic band gap (PBG) formation in two-dimensional photonic structures consisting of rods covered with a thin dielectric film. Specifically, triangular and honeycomb lattices in both complementary arrangements, i.e., air rods drilled in silicon matrix and silicon rods in air, are studied. We consider that the rods are formed of a dielectric core (silicon or air) surrounded by a cladding layer of silicon dioxide (SiO2), silicon nitride (Si3N4), or germanium (Ge). Such photonic lattices present absolute photonic band gaps, and we study the evolution of these gaps as functions of the cladding material and thickness. Our results show that in the case of air rods in dielectric media the existence of dielectric cladding reduces the absolute gap width and may cause complete closure of the gap if thick layers are considered. For the case of dielectric rods in air, however, the existence of a cladding layer can be advantageous and larger absolute PBG’s can be achieved., Peer Reviewed, Postprint (published version)

In this work we investigate the effects of typical thermal treatments on c-Si surface passivation by intrinsic a-SiCx:H films. First, a forming gas anneal (FGA), (400 °C, 30 min in H2/N2) is applied resulting in an improvement in surface passivation. The best result is a surface recombination velocity lower than 23 cm s-1. A second thermal step (730 °C, 30 s) is studied to try and simulate the firing step for screen-printed contacts. This annealing has no effect in surface passivation indicating that a-SiCx:H films are promising candidates for c-Si surface passivation in the photovoltaic industry. In order to analyze the improvement of surface passivation after FGA, a physical model of surface recombination at the a-SiCx:H/c-Si interface is developed. This improvement is directly connected to a reduction in fundamental recombination velocities of electrons and holes at the interface. A possible explanation could be the reduction in the interface state density due to hydrogen saturation of dangling bonds., Peer Reviewed, Postprint (published version)

The stability under gate bias stress of unpassivated thin film transistors was studied by measuring the transfer and output characteristics at different temperatures. The active layer of these devices consisted of in nanocrystalline silicon deposited at 125°C by Hot-Wire Chemical Vapour Deposition. The dependence of the subthreshold activation energy on gate bias for different gate bias stresses is quite different from the one reported for hydrogenated amorphous silicon. This behaviour has been related to trapped charge in the active layer of the thin film transistor., Peer Reviewed, Postprint (published version)

Silicon–carbon emitters were deposited by plasma enhanced chemical vapour deposition (PECVD) on a conventional silicon transistor base. The resulting transistors were annealed after the emitter had been deposited which led to the dopant diffusing from the amorphous layer into the crystalline base. This paper analyses the effect of the annealing temperature on the properties of the amorphous layer and on the electrical characteristics of the transistors. First, after isochronal annealing between 740°C and 880°C, the physical properties of PECVD a-Si0.8C0.2:H(n) layers were measured. Then, the base current of the transistors was analysed as a function of the annealing temperature to gain deeper insight into its physical origin., Peer Reviewed, Postprint (published version)

The fabrication and characterization of in situ-doped amorphous Si/sub 0.8/C/sub 0.2/ emitter transistors are presented. Emitter Gummel numbers exceeding 10/sup 14/ s/cm/sup 4/ are reported for the first time in this type of structure. The high values obtained for G/sub E/ are believed to be due to the valance band discontinuity between the Si/sub 0.8/C/sub 0.2/ layer and the crystalline part of the emitter, which effectively blocks the minority carrier injection from the base into the noncrystalline part of the emitter., Peer Reviewed, Postprint (published version)

Space charge Shockley-Read-Hall recombination currents in the presence of discrete or continuous distributions of recombination centres are analysed. For a single level trap, depending on its position inside the forbidden band, asymptotic values both for the ideality factor of the current-voltage characteristic and for the activation energy of the saturation current are obtained. The analysis is extended to continuous trap distributions and the current-voltage characteristics obtained are explained in terms of the simple theory developed for single level traps., Peer Reviewed, Postprint (published version)