Area-selective electrodeposition of micro islands for CuInSe2-based photovoltaics

For mass fabrication of highly-efficient photovoltaic modules based on Cu(In,Ga)Se 2 (CIGSe) absorber layers the availability and cost of the critical raw materials In and Ga present a potential bottleneck. The micro-concentrator solar cell concept provides a solution by using micro lenses to concentrate incoming sun light on an array of micro-sized CIGSe solar cells. The challenge is to fabricate CIGSe micro islands in exactly the desired positions using only the required material. Here, we analyze the area-selective electrodeposition of CuInSe 2 into holes in an insulating SiO 2 template layer as a material-efficient fabrication approach. We observe that the deposition process shows a strong dependence on the hole size, with a faster deposition around the hole perimeter. Based on a model developed for electrochemical reactions at ultra-micro electrodes, we develop numerical simulations for the electrochemical deposition process. The simulations consider the changing micro-electrode geometry throughout the deposition process, and provide a reasonable fit to the experimental data. Finally, it is shown that CuInSe 2 micro solar cells fabricated by electrodeposition reach efficiencies of 4.8% under 1 sun, providing a proof-of-concept demonstration meriting further development

Experimental study of linear and non linear optical properties of thin graphene layers

Non linear optical properties of few grapheme layers were examined by Z-scan technique. We had prepared thin graphene layers by the exfoliation method. Then, we used an atomic force microscope (AFM) to verify that our prepared samples contain only very few layers. Finally, the study of linear optical properties allowed us to deduce that the graphene is completely transparent. Whereas, the study of its non linear optical properties showed that its Kerr refractive index is about 107cmW-1.Peer Reviewe

InAs nanostructures grown by droplet epitaxy directly on InP(001) substrates

This work deals with the development of growth processes by droplet epitaxy to obtain InAs quantum dots directly on InP (001) surfaces (without any InGaAs or InAlAs intermediate layer). The indium atoms for droplet formation were deposited at different substrate temperatures, TS, below 300 °C in a solid source molecular beam epitaxy system. From the evolution of the size and shape of the nanostructures with TS, values of magnitudes related with indium atoms diffusivity have been extracted. The photoluminescence signal is investigated for ensemble and single InAs nanostructures emitting around 1.3–1.5 μm. The emission properties drastically change with thermal annealing processes that improve the crystalline quality.The authors wish to acknowledge the financial support by MINECO (ENE2012-37804-C02-02, AIC-B-2011-0806) and Comunidad de Madrid (S2013/MAE-2780).Peer reviewe

Effect of vanadium doping on ZnO sensing properties synthesized by spray pyrolysis

Semiconductor oxides with high sensing capacity remain a real challenge from long ago. Several attempts were considered to enhance the sensor's performance factors and achieve high quality requirements. In the present work, we operated a chemical sensor using intrinsic and vanadium doped zinc oxide. These samples were prepared and deposited using low cost spray pyrolysis technique. The structural data revealed good surface morphology and roughness, confirming the existence of ideal environment for oxidizing/reduction reactions. The addition of 4% vanadium minimized the grain size with a diameter lower than 9 nm. The gas testing measurements showed that vanadium doped ZnO presented higher response compared to pure ZnO. V-doped ZnO confirmed an improvement of the optimal operating temperature which varies from 350 degrees C to 300 degrees C at 100 ppm of acetone, 50 ppm of ethanol and 500 ppm of H-2. Furthermore, V-doped ZnO showed a maximum response reaching 100 at 100 ppm, for 450 degrees C. This high response is attributed to the effect of vanadium impurities that altered ZnO structure which was confirmed by structural data

CuInSe2 quantum dots grown by molecular beam epitaxy on amorphous SiO2 surfaces

The currently most efficient polycrystalline solar cells are based on the Cu(In,Ga)Se2 compound as a light absorption layer. However, in view of new concepts of nanostructured solar cells, CuInSe2 nanostructures are of high interest. In this work, we report CuInSe2 nanodots grown through a vacuum-compatible co-evaporation growth process on an amorphous surface. The density, mean size, and peak optical emission energy of the nanodots can be controlled by changing the growth temperature. Scanning transmission electron microscopy measurements confirmed the crystallinity of the nanodots as well as chemical composition and structure compatible with tetragonal CuInSe2. Photoluminescence measurements of CdS-passivated nanodots showed that the nanodots are optoelectronically active with a broad emission extending to energies above the CuInSe2 bulk bandgap and in agreement with the distribution of sizes. A blue-shift of the luminescence is observed as the average size of the nanodots gets smaller, evidencing quantum confinement in all samples. By using simple quantum confinement calculations, we correlate the photoluminescence peak emission energy with the average size of the nanodots

CuInSe2 quantum dots grown by molecular beam epitaxy on amorphous SiO2 surfaces

The currently most efficient polycrystalline solar cells are based on the Cu(In,Ga)Se compound as a light absorption layer. However, in view of new concepts of nanostructured solar cells, CuInSe nanostructures are of high interest. In this work, we report CuInSe nanodots grown through a vacuum-compatible co-evaporation growth process on an amorphous surface. The density, mean size, and peak optical emission energy of the nanodots can be controlled by changing the growth temperature. Scanning transmission electron microscopy measurements confirmed the crystallinity of the nanodots as well as chemical composition and structure compatible with tetragonal CuInSe. Photoluminescence measurements of CdS-passivated nanodots showed that the nanodots are optoelectronically active with a broad emission extending to energies above the CuInSe bulk bandgap and in agreement with the distribution of sizes. A blue-shift of the luminescence is observed as the average size of the nanodots gets smaller, evidencing quantum confinement in all samples. By using simple quantum confinement calculations, we correlate the photoluminescence peak emission energy with the average size of the nanodots.The authors would like to acknowledge the CAPES (CAPESINL 04/14), CNPq, and FAPEMIG funding agencies for financial support. We acknowledge the collaboration project with IMM-CSIC (AIC-B-2011-0806). P. M. P. S. acknowledges financial support from EU through the FP7 Marie Curie IEF 2012 Action No. 327367. The projects RECI/FIS-NAN/0183/ 2012 (COMPETE: FCOMP-01-0124-FEDER-027494) and UID/CTM/50025/2013 from the Fundação para a Ciência e a Tecnologia (Portugal) and the project Nanotechnology Based Functional Solutions (NORTE-01-0145-FEDER-000019) are also acknowledged

Micro-sized thin-film solar cells via area-selective electrochemical deposition for concentrator photovoltaics application.

Micro-concentrator solar cells enable higher power conversion efficiencies and material savings when compared to large-area non-concentrated solar cells. In this study, we use materials-efficient area-selective electrodeposition of the metallic elements, coupled with selenium reactive annealing, to form Cu(In,Ga)Se2 semiconductor absorber layers in patterned microelectrode arrays. This process achieves significant material savings of the low-abundance elements. The resulting copper-poor micro-absorber layers' composition and homogeneity depend on the deposition charge, where higher charge leads to greater inhomogeneity in the Cu/In ratio and to a patchy presence of a CuIn5Se8 OVC phase. Photovoltaic devices show open-circuit voltages of up to 525 mV under a concentration factor of 18 ×, which is larger than other reported Cu(In,Ga)Se2 micro-solar cells fabricated by materials-efficient methods. Furthermore, a single micro-solar cell device, measured under light concentration, displayed a power conversion efficiency of 5% under a concentration factor of 33 ×. These results show the potential of the presented method to assemble micro-concentrator photovoltaic devices, which operate at higher efficiencies while using light concentration

Synthesis and formation mechanism of CuInSe2 nanowires by one-step self-catalysed evaporation growth

High-quality CuInSe2 (CISe) nanowires have been prepared by a one-step evaporation process. The presented growth process results in a composite material consisting of CISe NWs on top of a polycrystalline CISe base layer. The nanowires were extensively characterized by transmission electron microscopy, confirming their composition and atomic-scale crystal structure with a very low number of structural defects. From these analyses, we infer that the growth axis is along the [111] direction. The polycrystalline base layer has a tetragonal chalcopyrite structure and is optically active as confirmed by X-ray diffraction and photoluminescence (PL) analysis, respectively. Potential applications of this composite CISe NW/base-layer material for photovoltaic energy conversion are supported by the reduced reflectivity of the material and its strong PL intensity. The presented growth method is based on elemental evaporation under vacuum conditions, which makes the process compatible with the fabrication of photovoltaic devices.The authors would like to acknowledge the CAPES (CAPES-INL 04/14), CNPq, and FAPEMIG funding agencies for financial support. We acknowledge the collaboration project with IMM-CSIC (AIC-B-2011-0806). P. M. P. S. acknowledges financial support from EU through the FP7 Marie Curie IEF 2012 Action No. 327367. The projects RECI/FIS-NAN/0183/2012 (COMPETE: FCOMP-01-0124-FEDER-027494) and UID/CTM/50025/2013 from the Fundação para a Ciência e a Tecnologia (Portugal) are also acknowledged.Peer reviewe