Manuel J. Mendes Website

CO2 capture and utilization (CCU) technologies are being immensely researched as means to close the anthropogenic carbon cycle. One approach known as artificial photosynthesis uses solar energy from photovoltaics (PV), carbon dioxide and water to generate hydrocarbon fuels, being methane (CH4) a preferential target due to the already in place infrastructures for its storage, distribution and consumption. Here, a model is developed to simulate a direct (1-step) solar methane production approach, which is studied in two scenarios: first, we compare it against a more conventional 2-step methane production route, and second, we apply it to address the energetic needs of concept buildings with usual space and domestic hot water heating requirements. The analysed 2-step process consists in the PV-powered synthesis of an intermediate fuel – syngas – followed by its conversion to CH4 via a Fischer–Tropsch (methanation) process. It was found that the 1-step route could be adequate to a domestic, small scale use, potentially providing energy for a single-family house, whilst the 2-step can be used in both small and large scale applications, from domestic to industrial uses. In terms of overall solar-to-CH4 energy efficiency, the 2-step method reaches 13.26{%} against the 9.18{%} reached by the 1-step method. Next, the application of the direct solar methane technology is analysed for domestic buildings, in different European locations, equipped with a combination of solar thermal collectors (STCs) and PV panels, in which the heating needs that cannot be fulfilled by the STCs are satisfied by the combustion of methane synthesized by the PV-powered electrolyzers. Various combinations of situations for a whole year were studied and it was found that this auxiliary system can produce, per m2 of PV area, in the worst case scenario 23.6 g/day (0.328 kWh/day) of methane in Stockholm, and in the best case scenario 47.4 g/day (0.658 kWh/day) in Lisbon.

One of the most promising approaches to produce industrial-compatible Transparent Conducting Materials (TCMs) with excellent characteristics is the fabrication of TCO/metal/TCO multilayers. In this article, we report on the electro-optical properties of a novel high-performing TCO/metal/TCO structure in which the intra-layer is a micro-structured metallic grid instead of a continuous thin film. The grid is obtained by evaporation of Ag through a mask of polystyrene colloidal micro-spheres deposited by the Langmuir-Blodgett method and partially dry-etched in plasma. IZO/Ag grid/IZO structures with different thicknesses and mesh dimensions have been fabricated, exhibiting excellent electrical characteristics (sheet resistance below 10 $Ømega$/□) and particularly high optical transmittance in the near-infrared spectral region as compared to planar (unstructured) TCM multilayers. Numerical simulations were also used to highlight the role of the Ag mesh parameters on the electrical properties.

{\textless}h2{\textgreater}Summary{\textless}/h2{\textgreater}{\textless}p{\textgreater}Recent trends in photovoltaics demand ever-thin solar cells to allow deployment in consumer-oriented products requiring low-cost and mechanically flexible devices. For this, nanophotonic elements in the wave-optics regime are highly promising, as they capture and trap light in the cells' absorber, enabling its thickness reduction while improving its efficiency. Here, novel wavelength-sized photonic structures were computationally optimized toward maximum broadband light absorption. Thin-film silicon cells were the test bed to determine the best performing parameters and study their optical effects. Pronounced photocurrent enhancements, up to 37{%}, 27{%}, and 48{%}, respectively, in ultra-thin (100- and 300-nm-thick) amorphous, and thin (1.5-$μ$m) crystalline silicon cells are demonstrated with honeycomb arrays of semi-spheroidal dome or void-like elements patterned on the cells' front. Also importantly, key advantages in the electrical performance are anticipated, since the photonic nano/micro-nanostructures do not increase the cell roughness, therefore not contributing to recombination, which is a crucial drawback in state-of-the-art light-trapping approaches.{\textless}/p{\textgreater}

{\textless}p{\textgreater}Opto-electronics on/with paper is fostering a novel generation of flexible and recyclable devices for sunlight harvesting and intelligent optical sensing.{\textless}/p{\textgreater}

The copper zinc tin sulfide (CZTS) compound is a promising candidate as an alternative absorber material for thin-film solar cells. In this study, we investigate the direct formation of Cu1.92ZnSnx(Sb1-x)S4 compounds [CZT(A)S], with x=1, 0.85, 0.70, and 0.50, via a mechanochemical synthesis (MCS) approach, starting from powders of the corresponding metals, zinc sulfide, and sulfur. The thermal stability of the CZT(A)S compounds was evaluated in detail by in situ synchrotron high-energy x-ray diffraction measurements up to 700 °C. The CZT(A)S compounds prepared via MCS revealed a sphalerite-type crystal structure with strong structural stability over the studied temperature range. The contribution of the MCS to the formation of such a structure at room temperature is analyzed in detail. Additionally, this study provides insights into the MCS of CZTS-based compounds: the possibility of a large-scale substitution of Sn by Sb and the production of single phase CZT(A)S with a Cu-poor/Zn-poor composition. A slight increase in the band gap from 1.45 to 1.49-1.51 eV was observed with the incorporation of Sb, indicating that these novel compounds can be further explored for thin-film solar cells.

The employment of printing techniques as cost-effective methods to fabricate low cost, flexible, disposable and sustainable solar cells is intimately dependent on the substrate properties and the adequate electronic devices to be powered by them. Among such devices, there is currently a growing interest in the development of user-oriented and multipurpose systems for intelligent packaging or on-site medical diagnostics, which would greatly benefit from printable solar cells as their energy source for autonomous operation. This chapter first describes and analyzes different types of cellulose-based substrates for flexible and cost effective optoelectronic and bio devices to be powered by printed solar cells. Cellulose is one of the most promising platforms for green recyclable electronics and it is fully compatible with large-scale printing techniques, although some critical requirements must be addressed. Paper substrates exist in many forms. From common office paper, to packaging cardboard used in the food industry, or nanoscale engineered cellulose (e.g. bacterial cellulose). However, it is the structure and content of paper that determines its end use. Secondly, proof-of-concept of optoelectronic and bio devices pro-duced by inkjet printing are described and show the usefulness of solar cells as a power source or as a chemical reaction initiator for sensors.

© 2017 Elsevier B.V. Ultrasonic spray pyrolysis deposition of ZnO-based materials offers an attractive high-throughput low-cost route towards industrial production of high-quality transparent conductive oxide (TCO) thin-films. In this work, undoped and aluminium-doped ZnO films have been grown employing ultrasonic spray pyrolysis at relatively low-temperate (300 °C), followed by a post-annealing treatment. The role of Al concentration in the starting solution, as well as the rapid thermal annealing (RTA) atmosphere, were investigated and correlated to the morphological, structural, electrical and optical properties of the films. The remarkable enhancement of electrical conductivity attained here is mainly ascribed to the combined effects of: (1) homogenous incorporation of Al3+into the ZnO matrix, which enhances crystal quality providing higher electronic mobility; and (2) the RTA which releases the localized electrons caused by oxygen absorption and thereby increases the free carrier density. Under optimum deposition conditions, a low resistivity and a high optical transmittance around 4 × 10−3$Ømega$ cm and 87{%}, respectively, were obtained. The application of the RTA post-process after low temperature growth has several advantages relative to the direct growth at high temperature (usually 400–575 °C), such as shorter growth time and lower cost associated to the spray pyrolysis equipment requirements and usage. The results suggest that the electrical and optical properties of the ZnO:Al films can be further improved for solar cell applications by controlling the temperature of the post-deposition annealing in reducing atmosphere.

© 2016 WILEY-VCH Verlag GmbH {&} Co. KGaA, Weinheim The post-deposition modification of ZnO-based transparent conductive oxides (TCOs) can be the key to produce thin films with optoelectronic properties similar to indium tin oxide (ITO), but at a much lower cost. Here, we present electro-optical results achieved for post-deposition annealing of Al–Zn–O (AZO), AZO:H, Ga–Zn–O:H (GZO:H), and Zn–O:H (ZNO:H) thin films deposited by RF sputtering at room temperature. These studies comprise results of thermal annealing at atmospheric pressure, vacuum, forming gas, H2and Ar atmospheres, and H2and Ar plasmas, which lead to significant enhancement of their electro-optical properties, which are correlated to morphological and structural improvements. The post-deposition annealing leads to an enhancement in resistivity above 40{%} for AZO, AZO:H, and GZO:H, reaching $\rho$ ≈ 2.6–3.5 × 10−4$Ømega$cm, while ZnO:H showed a lower improvement of 13{%}. The averaged optical transmittance in the visible region is about 89{%} for the investigated TCOs. Such results match the properties of state-of-art ITO ($\rho$ ≈ 10−4$Ømega$cm and transmittance in VIS range of 90{%}) employing much more earth-abundant materials.

Indium tin oxide (ITO) is the current standard state-of-the-art transparent conductive oxide (TCO), given its remarkable optical and electrical properties. However, the scarcity of indium carries an important drawback for the long-term application due to its intensive use in many optoelectronic devices such as displays, solar cells, and interactive systems. Zinc oxide-based TCOs can be a cost-effective and viable alternative, but the limitations imposed by their transmittance versus resistivity tradeoff still keep them behind ITO. In this work, an in-depth study of the structural and compositional material changes induced by specific postannealing treatments is presented, based on aluminum zinc oxide (AZO) and hydrogenated AZO (AZO:H) thin films grown by rf-magnetron sputtering at room temperature that allows an extensive understanding of the films' electrical/structural changes and the ability to tune their physical parameters to yield increasingly better performances, which put them in line with the best ITO quality standards. The present investigation comprises results of thermal annealing at atmospheric pressure, vacuum, forming gas, H2 and Ar atmospheres and plasmas. Overall the study being performed leads to a decrease in resistivity above 40{%}, reaching $\rho$ ≈ 3 × 10−4 $Ømega$ cm, with an average optical transmittance in the visible region around 88{%}. Such results are equivalent to the properties of state-of-the-art ITO.

Metal nanoparticles and diffractive nanostructures are widely studied for enhancing light trapping efficiency in thin-film solar cells. Both have achieved high performance enhancements, but there are very few direct comparisons between the two. Also, it is difficult to accurately determine the parasitic absorption of metal nanoparticles. Here, we assess the light trapping efficiencies of both approaches in an identical absorber configuration. We use a 240 nm thick amorphous silicon slab as the absorber layer and either a quasi-random supercell diffractive nanostructure or a layer of self-assembled metal nanoparticles for light trapping. Both the plasmonic and diffractive structures strongly enhance the absorption in the red/near-infrared regime. At longer wavelengths, however, parasitic absorption becomes evident in the metal nanoparticles, which reduces the overall performance of the plasmonic approach. We have formulated a simple analytical model to assess the parasitic absorption and effective reflectivity of a plasmonic reflector and to demonstrate good agreement with the experimental data.

Nowadays there is a strong demand for intelligent packaging to provide comfort, welfare and security to owners, vendors and consumers by allowing them to know the contents and interact with the goods. This is of particular relevance for low cost, fully disposable and recyclable products, such as identification tags and medical diagnostic tests, and devices for analysis and/or quality control in food and pharmaceutical industries. However, the increase of complexity and processing capacity requires continuous power and can be addressed by the combined use of a small disposable battery, charged by a disposable solar cell, which is able to work under indoor lighting. Herein, we show a proof-of-concept of the pioneering production of thin-film amorphous silicon (a-Si:H) solar cells with an efficiency of 4{%} by plasma enhanced chemical vapour deposition (PECVD) on liquid packaging cardboard (LPC), which is commonly used in the food and beverage industries. Such accomplishment put us one step closer to this revolution by providing a flexible, renewable and extremely cheap autonomous energy packaging system. Moreover, such Si thin films take advantage of their good performance at low-light levels, which also makes them highly desirable for cheap mobile indoor applications.

The present development of non-wafer-based photovoltaics (PV) allows supporting thin film solar cells on a wide variety of low-cost recyclable and flexible substrates such as paper, thereby extending PV to a broad range of consumer-oriented disposable applications where autonomous energy harvesting is a bottleneck issue. However, their fibrous structure makes it challenging to fabricate good-performing inorganic PV devices on such substrates. The advances presented here demonstrate the viability of fabricating thin film silicon PV cells on paper coated with a hydrophilic mesoporous layer. Such layer can not only withstand the cells production temperature (150 °C), but also provide adequate paper sealing and surface finishing for the cell's layers deposition. The substances released from the paper substrate are continuously monitored during the cell deposition by mass spectrometry, which allows adapting the procedures to mitigate any contamination from the substrate. In this way, a proof-of-concept solar cell with 3.4{%} cell efficiency (41{%} fill factor, 0.82 V open-circuit voltage and 10.2 mA cm−2 short-circuit current density) is attained, opening the door to the use of paper as a reliable substrate to fabricate inorganic PV cells for a plethora of indoor applications with tremendous impact in multi-sectorial fields such as food, pharmacy and security.