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Journal Article
Menda, U. D., G. Ribeiro, J. Deuermeier, E. López, D. Nunes, S. Jana, I. Artacho, R. Martins, I. Mora-Seró, MJ Mendes, and I. Ramiro. "Thermal-Carrier-Escape Mitigation in a Quantum-Dot-In-Perovskite Intermediate Band Solar Cell via Bandgap Engineering." ACS Photonics. 10 (2023): 3647-3655. AbstractWebsite
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Pimentel, A., A. Araújo, B. J. Coelho, D. Nunes, M. J. Oliveira, MJ Mendes, H. Águas, R. Martins, and E. Fortunato. "{3D ZnO/Ag surface-enhanced Raman scattering on disposable and flexible cardboard platforms}." Materials. 10 (2017). Abstract

© 2017 by the authors. In the present study, zinc oxide (ZnO) nanorods (NRs) with a hexagonal structure have been synthesized via a hydrothermal method assisted by microwave radiation, using specialized cardboard materials as substrates. Cardboard-type substrates are cost-efficient and robust paper-based platforms that can be integrated into several opto-electronic applications for medical diagnostics, analysis and/or quality control devices. This class of substrates also enables highly-sensitive Raman molecular detection, amiable to several different operational environments and target surfaces. The structural characterization of the ZnO NR arrays has been carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical measurements. The effects of the synthesis time (5-30 min) and temperature (70-130 °C) of the ZnO NR arrays decorated with silver nanoparticles (AgNPs) have been investigated in view of their application for surface-enhanced Raman scattering (SERS) molecular detection. The size and density of the ZnO NRs, as well as those of the AgNPs, are shown to play a central role in the final SERS response. A Raman enhancement factor of 7 × 105was obtained using rhodamine 6 G (RG6) as the test analyte; a ZnO NR array was produced for only 5 min at 70 °C. This condition presents higher ZnO NR and AgNP densities, thereby increasing the total number of plasmonic "hot-spots", their volume coverage and the number of analyte molecules that are subject to enhanced sensing.

Vicente, A. T., PJ Wojcik, MJ Mendes, H. Águas, E. Fortunato, and R. Martins. "{A statistics modeling approach for the optimization of thin film photovoltaic devices}." Solar Energy. 144 (2017). Abstract

© 2017 The growing interest in exploring thin film technologies to produce low cost devices such as n-i-p silicon solar cells, with outstanding performances and capability to address the highly relevant energy market, turns the optimization of their fabrication process a key area of development. The usual one-dimensional analysis of the involved parameters makes it difficult and time consuming to find the optimal set of conditions. To overcome these difficulties, the combination of experimental design and statistical analysis provides the tools to explore in a multidimensional fashion the interactions between fabrication parameters and expected experimental outputs. Design of Experiment and Multivariate Analysis are demonstrated here for the optimization of: (1) the low temperature deposition (150 °C) of high quality intrinsic amorphous silicon (i-a-Si:H); and (2) the matching of the n-, i-, and p-silicon layers thickness to maximize the efficiency of thin film solar cells. The multiple regression method applied, validated through analysis of variance and evaluated against exact numerical simulations, is shown to predict the overall intrinsic layer properties and the devices performance. The results confirm that experimental design and statistical data analysis are effective approaches to improve, within a minimum time frame and high certainty, the properties of silicon thin films, and subsequently the layer structure of solar cells.

Gaspar, D., AC Pimentel, MJ Mendes, T. Mateus, BP Falcão, JP Leitão, J. Soares, A. Araújo, A. Vicente, SA Filonovich, H. Águas, R. Martins, and I. Ferreira. "{Ag and Sn Nanoparticles to Enhance the Near-Infrared Absorbance of a-Si:H Thin Films}." Plasmonics. 9 (2014): 1015-1023. AbstractWebsite
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Chapa, Manuel, Miguel F. Alexandre, Manuel J. Mendes, Hugo Águas, Elvira Fortunato, and Rodrigo Martins. "{All-Thin-Film Perovskite/C-Si Four-Terminal Tandems: Interlayer and Intermediate Contacts Optimization}." ACS Applied Energy Materials. 2 (2019): 3979-3985. Abstract

Combined perovskite/crystalline-silicon four-terminal tandem solar cells promise {\textgreater}30{%} efficiencies. Here we propose all-thin-film double-junction architectures where high-bandgap perovskite top cells are coupled to ultrathin c-Si bottom cells enhanced with light trapping. A complete optoelectronic model of the devices was developed and applied to determine the optimal intermediate layers, which are paramount to maximize the cells' photocurrent. It was ascertained that by replacing the transparent conductive oxides by grid-based metallic contacts in the intermediate positions, the parasitic absorption is lowered by 30{%}. Overall, a 29.2{%} efficiency is determined for ∼2 um thick tandems composed of the optimized interlayers and improved with Lambertian light trapping.

Mendes, MJ, S. Morawiec, T. Mateus, A. Lyubchyk, H. Águas, I. Ferreira, E. Fortunato, R. Martins, F. Priolo, and I. Crupi. "{Broadband light trapping in thin film solar cells with self-organized plasmonic nanocolloids}." Nanotechnology. 26 (2015). Abstract

© 2015 IOP Publishing Ltd. The intense light scattered from metal nanoparticles sustaining surface plasmons makes them attractive for light trapping in photovoltaic applications. However, a strong resonant response from nanoparticle ensembles can only be obtained if the particles have monodisperse physical properties. Presently, the chemical synthesis of colloidal nanoparticles is the method that produces the highest monodispersion in geometry and material quality, with the added benefits of being low-temperature, low-cost, easily scalable and of allowing control of the surface coverage of the deposited particles. In this paper, novel plasmonic back-reflector structures were developed using spherical gold colloids with appropriate dimensions for pronounced far-field scattering. The plasmonic back reflectors are incorporated in the rear contact of thin film n-i-p nanocrystalline silicon solar cells to boost their photocurrent generation via optical path length enhancement inside the silicon layer. The quantum efficiency spectra of the devices revealed a remarkable broadband enhancement, resulting from both light scattering from the metal nanoparticles and improved light incoupling caused by the hemispherical corrugations at the cells' front surface formed from the deposition of material over the spherically shaped colloids.

Morawiec, Seweryn, Manuel J. Mendes, Sergej A. Filonovich, Tiago Mateus, Salvatore Mirabella, Hugo Águas, Isabel Ferreira, Francesca Simone, Elvira Fortunato, Rodrigo Martins, Francesco Priolo, and Isodiana Crupi. "{Broadband photocurrent enhancement in a-Si:H solar cells with plasmonic back reflectors}." Optics Express. 22 (2014): A1059-A1070. AbstractWebsite

Plasmonic light trapping in thin film silicon solar cells is a promising route to achieve high efficiency with reduced volumes of semiconductor material. In this paper, we study the enhancement in the opto-electronic performance of thin a-Si:H solar cells due to the light scattering effects of plasmonic back reflectors (PBRs), composed of self-assembled silver nanoparticles (NPs), incorporated on the cells{&}{\#}x2019; rear contact. The optical properties of the PBRs are investigated according to the morphology of the NPs, which can be tuned by the fabrication parameters. By analyzing sets of solar cells built on distinct PBRs we show that the photocurrent enhancement achieved in the a-Si:H light trapping window (600 {&}{\#}x2013; 800 nm) stays in linear relation with the PBRs diffuse reflection. The best-performing PBRs allow a pronounced broadband photocurrent enhancement in the cells which is attributed not only to the plasmon-assisted light scattering from the NPs but also to the front surface texture originated from the conformal growth of the cell material over the particles. As a result, remarkably high values of Jsc and Voc are achieved in comparison to those previously reported in the literature for the same type of devices.

Mendes, Manuel J., Howard K. Schmidt, and Matteo Pasquali. "{Brownian dynamics simulations of single-wall carbon nanotube separation by type using dielectrophoresis}." Journal of Physical Chemistry B (2008). AbstractWebsite
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Mendes, MJ, S. Morawiec, F. Simone, F. Priolo, and I. Crupi. "{Colloidal plasmonic back reflectors for light trapping in solar cells}." Nanoscale. 6 (2014). Abstract

A novel type of plasmonic light trapping structure is presented in this paper, composed of metal nanoparticles synthesized in colloidal solution and self-assembled in uniform long-range arrays using a wet-coating method. The high monodispersion in size and spherical shape of the gold colloids used in this work allows a precise match between their measured optical properties and electromagnetic simulations performed with Mie theory, and enables the full exploitation of their collective resonant plasmonic behavior for light-scattering applications. The colloidal arrays are integrated in plasmonic back reflector (PBR) structures aimed for light trapping in thin film solar cells. The PBRs exhibit high diffuse reflectance (up to 75{%}) in the red and near-infrared spectrum, which can pronouncedly enhance the near-bandgap photocurrent generated by the cells. Furthermore, the colloidal PBRs are fabricated by low-temperature ({\textless}120 °C) processes that allow their implementation, as a final step of the cell construction, in typical commercial thin film devices generally fabricated in a superstrate configuration. © 2014 the Partner Organisations.

Sanchez-Sobrado, Olalla, Manuel J. Mendes, Sirazul Haque, Tiago Mateus, Andreia Araujo, Hugo Aguas, Elvira Fortunato, and Rodrigo Martins. "{Colloidal-lithographed TiO2 photonic nanostructures for solar cell light trapping}." J. Mater. Chem. C (2017). Abstract
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Torrisi, Giacomo, João S. Luis, Olalla Sanchez-Sobrado, Rosario Raciti, Manuel J. Mendes, Hugo Águas, Elvira Fortunato, Rodrigo Martins, and Antonio Terrasi. "{Colloidal-structured metallic micro-grids: High performance transparent electrodes in the red and infrared range}." Solar Energy Materials and Solar Cells. 197 (2019): 7-12. Abstract

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.

Mendes, Manuel J., Andreia Araújo, António Vicente, Hugo Águas, Isabel Ferreira, Elvira Fortunato, and Rodrigo Martins. "{Design of optimized wave-optical spheroidal nanostructures for photonic-enhanced solar cells}." Nano Energy. 26 (2016): 286-296. AbstractWebsite

The interaction of light with wavelength-sized photonic nanostructures is highly promising for light management applied to thin-film photovoltaics. Several light trapping effects come into play in the wave optics regime of such structures that crucially depend on the parameters of the photonic and absorbing elements. Thus, multi-parameter optimizations employing exact numerical models, as performed in this work, are essential to determine the maximum photocurrent enhancement that can be produced in solar cells.Generalized spheroidal geometries and high-index dielectric materials are considered here to model the design of the optical elements providing broadband absorption enhancement in planar silicon solar cells. The physical mechanisms responsible for such enhancement are schematized in a spectral diagram, providing a deeper understanding of the advantageous characteristics of the optimized geometries. The best structures, composed of TiO2 half-spheroids patterned on the cells' top surface, yield two times higher photocurrent (up to 32.5 mA/cm2 in 1.5 $μ$m thick silicon layer) than the same devices without photonic schemes.These results set the state-of-the-art closer to the theoretical Lambertian limit. In addition, the considered light trapping designs are not affected by the traditional compromise between absorption enhancement versus current degradation by recombination, which is a key technological advantage.

Araújo, A., A. Pimentel, M. J. Oliveira, MJ Mendes, R. Franco, E. Fortunato, H. Águas, and R. Martins. "{Direct growth of plasmonic nanorod forests on paper substrates for low-cost flexible 3D SERS platforms}." Flexible and Printed Electronics. 2 (2017). Abstract

Paper substrates, coated with ZnO nanorods (NRs) decorated with Ag nanoparticles (NPs), allowed the production of inexpensive, highly-performing and extremely reproducible three-dimensional (3D) SERS platforms. The ZnO NRs were synthesized by a simple, fast and low-temperature hydrothermal method assisted by microwave radiation and made SERS-active by decorating them with a dense array of silver nanoparticles deposited via a single-step thermal evaporation technique. Using Rhodamine 6G (R6G) as a probe molecule, with an amount down to 10-9 M, the SERS substrates allowed a Raman signal enhancement of 107. The contribution of the inter-Ag-NPs gaps for 3D geometry, ZnO NRs orientation and the large sensing area allowed by theNRscaffolds, were determinant factors for the significant Raman enhancement observed. The results demonstrate that plasmonic nanorod forests, covered with Ag NPs, are efficient SERS substrates with the advantages of being recyclable, flexible, lightweight, portable, biocompatible and extremely low-cost.

Morawiec, S., J. Holovský, MJ Mendes, M. Müller, K. Ganzerová, A. Vetushka, M. Ledinský, F. Priolo, A. Fejfar, and I. Crupi. "{Experimental quantification of useful and parasitic absorption of light in plasmon-enhanced thin silicon films for solar cells application}." Scientific Reports (2016). AbstractWebsite
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Araújo, A., C. Caro, MJ Mendes, D. Nunes, E. Fortunato, R. Franco, H. Águas, and R. Martins. "{Highly efficient nanoplasmonic SERS on cardboard packaging substrates}." Nanotechnology. 25 (2014). Abstract

© 2014 IOP Publishing Ltd. This work reports on highly efficient surface enhanced Raman spectroscopy (SERS) constructed on low-cost, fully recyclable and highly reproducible cardboard plates, which are commonly used as disposable packaging material. The active optical component is based on plasmonic silver nanoparticle structures separated from the metal surface of the cardboard by a nanoscale dielectric gap. The SERS response of the silver (Ag) nanoparticles of various shapes and sizes were systematically investigated, and a Raman enhancement factor higher than 106for rhodamine 6G detection was achieved. The spectral matching of the plasmonic resonance for maximum Raman enhancement with the optimal local electric field enhancement produced by 60 nm-sized Ag NPs predicted by the electromagnetic simulations reinforces the outstanding results achieved. Furthermore, the nanoplasmonic SERS substrate exhibited high reproducibility and stability. The SERS signals showed that the intensity variation was less than 5{%}, and the SERS performance could be maintained for up to at least 6 months.

Lyubchyk, A., A. Vicente, P. U. Alves, B. Catela, B. Soule, T. Mateus, MJ Mendes, H. Águas, E. Fortunato, and R. Martins. "{Influence of post-deposition annealing on electrical and optical properties of ZnO-based TCOs deposited at room temperature}." Physica Status Solidi (A) Applications and Materials Science. 213 (2016). Abstract

© 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.

Araujo, Andreia, Manuel J. Mendes, Tiago Mateus, Antonio Vicente, Daniela Nunes, Tomas Calmeiro, Elvira Fortunato, Hugo Aguas, and Rodrigo Martins. "{Influence of the Substrate on the Morphology of Self-Assembled Silver Nanoparticles by Rapid Thermal Annealing}." Journal of Physical Chemistry C (2016). AbstractWebsite
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Neves, F., A. Stark, N. Schell, MJ Mendes, H. Aguas, E. Fortunato, R. Martins, J. B. Correia, and A. Joyce. "{Investigation of single phase Cu2ZnSnxSb1-xS4 compounds processed by mechanochemical synthesis}." Physical Review Materials. 2 (2018). Abstract

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.

HB, Wannes, Dimassi WR, Zaghouani B, and Mendes MJ. "{Li-doped ZnO Sol-Gel Thin Films: Correlation between Structural Morphological and Optical Properties}." Journal of Textile Science {&} Engineering. 08 (2018). Abstract
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Luque, A., A. Marti, MJ Mendes, and I. Tobias. "{Light absorption in the near field around surface plasmon polaritons}." Journal of Applied Physics. 104 (2008): 8. Abstract

A semiclassical method is developed to calculate the energy absorption of an electronic system located in the near field of a metal nanoparticle sustaining surface plasmons. The results are found to be similar to those of photon absorption from ordinary transversal radiation. However, they are affected by a geometrical factor that can increase the absorption by several orders of magnitude. As example, we investigate ellipsoidal-shaped metal nanoparticles which, under favorable conditions, may provide near field aborption enhancements almost as large as 10(4), and in many cases above 10. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3014035]

Mendes, Manuel J., Ignacio Tobías, Antonio Martí, and Antonio Luque. "{Light concentration in the near-field of dielectric spheroidal particles with mesoscopic sizes}." Optics Express. 19 (2011): 16207-16222. AbstractWebsite
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Sanchez-Sobrado, Olalla, Manuel J. Mendes, Sirazul Haque, T. Mateus, H. Aguas, E. Fortunato, and R. Martins. "{Lightwave trapping in thin film solar cells with improved photonic-structured front contacts}." J. Mater. Chem. C. 7 (2019): 6456-6464. Abstract
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Marouf, S., A. Beniaiche, K. Kardarian, MJ Mendes, O. Sanchez-Sobrado, H. Águas, E. Fortunato, and R. Martins. "{Low-temperature spray-coating of high-performing ZnO:Al films for transparent electronics}." Journal of Analytical and Applied Pyrolysis. 127 (2017). Abstract

© 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.

Lyubchyk, Andriy, António Vicente, Bertrand Soule, Pedro Urbano Alves, Tiago Mateus, Manuel J. Mendes, Hugo Águas, Elvira Fortunato, and Rodrigo Martins. "{Mapping the Electrical Properties of ZnO-Based Transparent Conductive Oxides Grown at Room Temperature and Improved by Controlled Postdeposition Annealing}." Advanced Electronic Materials. 2 (2016): 1500287. AbstractWebsite

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.