Manuel J. Mendes Website

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.

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.

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.

© 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.

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.

The inexorable increase of energy demand and the efficiency bottleneck of monocrystalline silicon solar cell technology is promoting the research and development of alternative photovoltaic materials. Copper-arsenic-sulfide (CAS) compounds are still rather unexplored in the literature, yet they have been regarded as promising candidates for use as p-type absorber in solar cells, owing to their broad raw material availability, suitable bandgap and high absorption coefficient. Here, a comprehensive study is presented on the structural and optoelectronic properties of CAS thin-films deposited via radio-frequency magnetron co-sputtering, using a commercial Cu target together with a Cu-As-S target with material obtained from local resources, specifically from mines in the Portuguese region of the Iberian Pyrite Belt. Raman and X-ray diffraction analysis confirm that the use of two targets results in films with pronounced stoichiometry gradients, suggesting a transition from amorphous CAS compounds to crystalline djurleite (Cu31S16), with the increasing proximity to the Cu target. Resistivity values from 4.7 mΩ·cm to 17.4 Ω·cm are obtained, being the lowest resistive films, those with pronounced sub-bandgap free-carrier absorption. The bandgap values range from 2.20 to 2.65 eV, indicating promising application as wide-bandgap semiconductors in third-generation (e.g., multi-junction) photovoltaic devices.

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.

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]

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.

© 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.