Publications

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2021
Oliveira, R. D., A. Mouquinho, P. Centeno, M. Alexandre, S. Haque, R. Martins, E. Fortunato, H. Águas, and MJ Mendes. "Colloidal lithography for photovoltaics: An attractive route for light management." Nanomaterials. 11 (2021). AbstractWebsite
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Grey, P., M. Chapa, M. Alexandre, T. Mateus, E. Fortunato, R. Martins, MJ Mendes, and L. Pereira. "Combining Soft with Hard Condensed Matter for Circular Polarized Light Sensing and Logic Operations." Advanced Optical Materials. 9 (2021). AbstractWebsite
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Alishah, H. M., F. P. G. Choi, U. D. Menda, C. Kahveci, M. C. Rodop, MJ Mendes, and S. Gunes. "Effect of Bathocuproine Concentration on the Photovoltaic Performance of NiOx-Based Perovskite Solar Cells." Journal of the Mexican Chemical Society. 65 (2021): 149-160. AbstractWebsite
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Menda, U. D., G. Ribeiro, D. Nunes, T. Calmeiro, H. Águas, E. Fortunato, R. Martins, and MJ Mendes. "High-performance wide bandgap perovskite solar cells fabricated in ambient high-humidity conditions." Materials Advances. 2 (2021): 6344-6355. AbstractWebsite
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Alexandre, M., H. Águas, E. Fortunato, R. Martins, and MJ Mendes. "Light management with quantum nanostructured dots-in-host semiconductors." Light: Science and Applications. 10 (2021). AbstractWebsite
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Boane, Jenny L. N., Pedro Centeno, Ana Mouquinho, Miguel Alexandre, Tomás Calmeiro, Elvira Fortunato, Rodrigo Martins, Manuel J. Mendes, and Hugo Águas. "Soft-Microstructured Transparent Electrodes for Photonic-Enhanced Flexible Solar Cells." Micro. 1 (2021): 215-227. AbstractWebsite

Microstructured transparent conductive oxides (TCOs) have shown great potential as photonic electrodes in photovoltaic (PV) applications, providing both optical and electrical improvements in the solar cells’ performance due to: (1) strong light trapping effects that enhance broadband light absorption in PV material and (2) the reduced sheet resistance of the front illuminated contact. This work developed a method for the fabrication and optimization of wavelength-sized indium zinc oxide (IZO) microstructures, which were soft-patterned on flexible indium tin oxide (ITO)-coated poly(ethylene terephthalate) (PET) substrates via a simple, low-cost, versatile, and highly scalable colloidal lithography process. Using this method, the ITO-coated PET substrates patterned with IZO micro-meshes provided improved transparent electrodes endowed with strong light interaction effects—namely, a pronounced light scattering performance (diffuse transmittance up to  50%). In addition, the photonic-structured IZO mesh allowed a higher volume of TCO material in the electrode while maintaining the desired transparency, which led to a sheet resistance reduction (by  30%), thereby providing further electrical benefits due to the improvement of the contact conductance. The results reported herein pave the way for a new class of photonic transparent electrodes endowed with mechanical flexibility that offer strong potential not only as advanced front contacts for thin-film bendable solar cells but also for a much broader range of optoelectronic applications.

2020
Haque, S., M. Alexandre, MJ Mendes, H. Águas, E. Fortunato, and R. Martins. "Design of wave-optical structured substrates for ultra-thin perovskite solar cells." Applied Materials Today. 20 (2020). AbstractWebsite
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Li, K., S. Haque, A. Martins, E. Fortunato, R. Martins, MJ Mendes, and C. S. Schuster. "Light trapping in solar cells: Simple design rules to maximize absorption." Optica. 7 (2020): 1377-1384. AbstractWebsite
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Sanchez-Sobrado, O., MJ Mendes, T. Mateus, J. Costa, D. Nunes, H. Aguas, E. Fortunato, and R. Martins. "Photonic-structured TCO front contacts yielding optical and electrically enhanced thin-film solar cells." Solar Energy. 196 (2020): 92-98. AbstractWebsite
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Centeno, P., M. F. Alexandre, M. Chapa, JV Pinto, J. Deuermeier, T. Mateus, E. Fortunato, R. Martins, H. Águas, and MJ Mendes. "Self-Cleaned Photonic-Enhanced Solar Cells with Nanostructured Parylene-C." Advanced Materials Interfaces. 7 (2020). AbstractWebsite
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Mendes, Manuel J., Olalla Sanchez-Sobrado, Sirazul Haque, Tiago Mateus, Hugo Águas, Elvira Fortunato, and Rodrigo Martins. "{Wave-optical front structures on silicon and perovskite thin-film solar cells}." Solar Cells and Light Management. Elsevier, 2020. 315-354. Abstract
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2019
Morawiec, S., MJ Mendes, F. Priolo, and I. Crupi {Plasmonic nanostructures for light trapping in thin-film solar cells}. Vol. 92. Materials Science in Semiconductor Processing, 92. Elsevier Ltd, 2019. Abstract

The optical properties of localized surface plasmon resonances (LSPR) sustained by self-assembled silver nanoparticles are of great interest for enhancing light trapping in thin film photovoltaics. First, we report on a systematic investigation of the structural and the optical properties of silver nanostructures fabricated by a solid-state dewetting process on various substrates. Our study allows to identify fabrication conditions in which circular, uniformly spaced nanoparticles are obtainable. The optimized NPs are then integrated into plasmonic back reflector (PBR) structures. Second, we demonstrate a novel procedure, involving a combination of opto-electronic spectroscopic techniques, allowing for the quantification of useful and parasitic absorption in thin photovoltaic absorber deposited on top of the PBR. We achieve a significant broadband useful absorption enhancement of 90{%} for 0.9 µm thick $μ$c-Si:H film and demonstrate that optical losses due to plasmonic scattering are insignificant below 730 nm. Finally, we present a successful implementation of a plasmonic light trapping scheme in a thin film a-Si:H solar cell. The quantum efficiency spectra of the devices show a pronounced broadband enhancement resulting in remarkably high short circuit current densities (Jsc).

Brites, Maria João, Maria Alexandra Barreiros, Victoria Corregidor, Luis C. Alves, Joana {V. Pinto}, Manuel J. Mendes, Elvira Fortunato, Rodrigo Martins, and João Mascarenhas. "{Ultrafast Low-Temperature Crystallization of Solar Cell Graded Formamidinium-Cesium Mixed-Cation Lead Mixed-Halide Perovskites Using a Reproducible Microwave-Based Process}." ACS Applied Energy Materials. 2 (2019): 1844-1853. 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.

Vieira, F., B. Sarmento, A. S. Reis-Machado, J. Facão, M. J. Carvalho, MJ Mendes, E. Fortunato, and R. Martins. "{Prediction of sunlight-driven CO2 conversion: Producing methane from photovoltaics, and full system design for single-house application}." Materials Today Energy. 14 (2019). Abstract

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.

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.

Alexandre, Miguel, Manuel Chapa, Sirazul Haque, Manuel J. Mendes, Hugo Águas, Elvira Fortunato, and Rodrigo Martins. "{Optimum Luminescent Down-Shifting Properties for High Efficiency and Stable Perovskite Solar Cells}." ACS Applied Energy Materials. 2 (2019): 2930-2938. 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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Haque, Sirazul, Manuel J. Mendes, O. Sanchez-Sobrado, H. Aguas, E. Fortunato, and R. Martins. "{Photonic-structured TiO2 for high-efficiency, flexible and stable Perovskite solar cells}." Nano Energy. 59 (2019): 91-101. AbstractWebsite
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2018
Araújo, Andreia, Manuel J. Mendes, Tiago Mateus, João Costa, Daniela Nunes, Elvira Fortunato, Hugo Águas, and Rodrigo Martins. "{Ultra-fast plasmonic back reflectors production for light trapping in thin Si solar cells}." Solar Energy. 174 (2018): 786-792. Abstract

A fast method is presented to fabricate plasmonic light trapping structures in just ten minutes ({\textgreater}5 × faster than the present state of art), with excellent light scattering properties. The structures are composed of silver nanoparticles (Ag NPs) deposited by thermal evaporation and self-assembled using a rapid thermal annealing (RTA) system. The effect of the RTA heating rate on the NPs production reveals to be crucial to the decrease of the annealing process. The Ag NPs are integrated in thin film silicon solar cells to form a plasmonic back reflector (PBR) that causes a diffused light reflectivity in the near-infrared (600–1100 nm wavelength region). In this configuration the thicknesses of the AZO spacer/passivating layers between NPs and rear mirror, and between NPs and silicon layer, play critical roles in the near-field coupling of the reflected light towards the solar cell absorber, which is investigated in this work. The best spacer thicknesses were found to be 100 and 60 nm, respectively, for Ag NPs with preferential sizes of about 200 nm. The microcrystalline silicon ($μ$c-Si:H) solar cells deposited on such improved PBR demonstrate an overall 11{%} improvement on device efficiency, corresponding to a photocurrent of 24.4 mA/cm2 and an efficiency of 6.78{%}, against 21.79 mA/cm2 and 6.12{%}, respectively, obtained on flat structures without NPs.

Mendes, Manuel J., Sirazul Haque, Olalla Sanchez-Sobrado, Andreia Araújo, Hugo Águas, Elvira Fortunato, and Rodrigo Martins. "{Optimal-Enhanced Solar Cell Ultra-thinning with Broadband Nanophotonic Light Capture}." iScience. 3 (2018): 238-254. AbstractWebsite

{\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}

{T. Vicente}, António, Andreia Araújo, Manuel J. Mendes, Daniela Nunes, Maria J. Oliveira, Olalla Sanchez-Sobrado, Marta P. Ferreira, Hugo Águas, Elvira Fortunato, and Rodrigo Martins. "{Multifunctional cellulose-paper for light harvesting and smart sensing applications}." Journal of Materials Chemistry C. 6 (2018): 3143-3181. AbstractWebsite

{\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}

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.

{Ben Wannes}, H., Benabderrahmane R. Zaghouani, R. Ouertani, A. Araújo, MJ Mendes, H. Aguas, E. Fortunato, R. Martins, and W. Dimassi {Study of the stabilizer influence on the structural and optical properties of sol-gel spin coated zinc oxide films}. Vol. 74. Materials Science in Semiconductor Processing, 74. Elsevier Ltd, 2018. Abstract

In this work, we highlight the influence of three different sol stabilizers, namely diethanolamine (DEA), Ammonium Hydroxide (NH4OH), and Nitric Acid (HNO3), on the optical and structural properties of spin-coated zinc oxide (ZnO) thin films. The XRD patterns related to all films exhibit a hexagonal crystal structure with a preferential orientation along the (0 0 2) direction. However an additional {\textless}100{\textgreater} peak arises when the films are prepared with DEA and NH4OH showing a better crystallinity than that displayed by HNO3-prepared films. The elaborated films show a high transparency reaching 80{%} for DEA-prepared films. The analysis of the transmittance and the reflectance measurements confirms a direct band-to-band transition. Depending on the sol stabilizer, the optical band gap energy is varying from 3.16 to 3.22 eV. The relatively wide band-gap of DEA-prepared ZnO films is correlated to their high crystallinity. Room temperature photoluminescence spectra indicate strong UV emission at around 377 nm originated from nearby band-edge transitions. Yet, the use of DEA as a stabilizer leads to a net intensity increase of the blue peak emission.