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Chapa, M, Alexandre MF, Mendes MJ, Águas H, Fortunato E, Martins R.  2019.  All-Thin-Film Perovskite/C–Si Four-Terminal Tandems: Interlayer and Intermediate Contacts Optimization. ACS Applied Energy Materials. 2:3979-3985., Number 6 AbstractWebsite

Combined perovskite/crystalline-silicon four-terminal tandem solar cells promise >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.

Torrisi, G, Luis JS, Sanchez-Sobrado O, Raciti R, Mendes MJ, Águas H, Fortunato E, Martins R, Terrasi A.  2019.  Colloidal-structured metallic micro-grids: High performance transparent electrodes in the red and infrared range. Solar Energy Materials and Solar Cells. 197:7-12. AbstractWebsite

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 Ω/□) 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.

Sanchez-Sobrado, O, Mendes MJ, Haque S, Mateus T, Aguas H, Fortunato E, Martins R.  2019.  Lightwave trapping in thin film solar cells with improved photonic-structured front contacts. J. Mater. Chem. C. 7:6456-6464.: The Royal Society of Chemistry AbstractWebsite

Photonic microstructures placed at the topside of photovoltaic cells are currently one of the preferred light management solutions to obtain efficiency enhancement due to the increment of the optical absorption produced in the active medium of the devices. Herein{,} we present the results concerning a practical{,} low-cost and scalable approach to integrate metal-oxide based light trapping microstructures on the front contact of amorphous silicon thin film solar cells. A colloidal lithography method was used to pattern the wavelength-sized pyramidal-like features composing the structures{,} made of two different transparent materials{,} TiO2 and IZO{,} allowing the detailed study of the influence of their geometrical parameters on the optoelectronic properties of the devices. These top coating structures are deposited as a post-process after the solar cell fabrication{,} thus facilitating and broadening their industrial applicability. Measurements of the light absorption{,} external quantum efficiency and I–V curves revealed that the structured coatings provide strong broadband improvements in the generated current{,} due to the suppression of reflected light at short wavelengths and the increment of the optical path length of the longer wavelengths (via light scattering){,} within the amorphous silicon layer. As a result{,} in the four types of structures analyzed in this study{,} remarkable increments were achieved in the cells’ efficiencies (up to 14.4%) and generated currents (up to 21.5%){,} with respect to the flat reference cells.

Alexandre, M, Chapa M, Haque S, Mendes MJ, Águas H, Fortunato E, Martins R.  2019.  Optimum Luminescent Down-Shifting Properties for High Efficiency and Stable Perovskite Solar Cells. ACS Applied Energy Materials. 2:2930-2938., Number 4 AbstractWebsite

In recent years, the discovery of the excellent optical and electrical properties of perovskite solar cells (PSCs) made them a main focus of research in photovoltaics, with efficiency records increasing astonishingly fast since their inception. However, problems associated with the stability of these devices are hindering their market application. UV degradation is one of the most severe issues, chiefly caused by TiO2’s photogenerated electrons that decompose the perovskite absorber material, coupled with the additional intrinsic degradation of this material under UV exposure. The solution presented here can minimize this effect while boosting the cells’ generated photocurrent, by making use of combined light-trapping and luminescent down-shifting effects capable of changing the harmful UV radiation to higher wavelengths that do not affect the stability and can be effectively “trapped” in the cell. This work focuses in the optimization of the photocurrent gains that can be attained by emulating the changed spectrum resulting from applying down-shifting media as encapsulant in photonic-enhanced PSCs, as well as the reduction in the harmful effects of UV radiation on the devices. Such optimized photonic solution allows current enhancement while reducing the harmful UV photocarrier generation both in the TiO2 (by 1 order of magnitude) and in the perovskite (by 80%) relative to a standard PSC without light management.

Haque, S, Mendes MJ, Sanchez-Sobrado O, Águas H, Fortunato E, Martins R.  2019.  Photonic-structured TiO2 for high-efficiency, flexible and stable Perovskite solar cells. Nano Energy. 59:91-101. AbstractWebsite

Optical solutions are promising for Perovskite solar cell (PSC) technology, not only to increase efficiency, but also to allow thinner absorber layers (higher flexibility) and improve stability. This work optimized the combined anti-reflection and scattering properties of two types of light trapping (LT) structures, based on TiO2 semi-spheroidal geometries with honeycomb periodicity, for application in PSCs with substrate configuration and different perovskite layer thicknesses. Their optically lossless material (TiO2) allows the structures to be patterned in the final processing steps, integrated in the cells’ top n contact, therefore not increasing the surface area of the PV layers and not degrading the electric performance via recombination. Therefore, this strategy circumvents the typical compromise of state-of-the-art LT approaches between optical improvements and electrical deterioration, which is particularly relevant for PSCs since their main recombination is caused by surface defects. When patterned on the cells’ front, the wave-optical micro-features composing the LT structures yield up to 21% and 27% photocurrent enhancement in PSCs with conventional (500 nm thick) and ultra-thin (250 nm) perovskite layers, respectively; which are improvements close to those predicted by theoretical Lambertian limits. In addition, such features are shown to provide an important encapsulation role, preventing the cells’ degradation from UV penetration.

Morawiec, S, Mendes MJ, Priolo F, Crupi I.  2019.  Plasmonic nanostructures for light trapping in thin-film solar cells. Materials Science in Semiconductor Processing. 92:10-18. AbstractWebsite

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 μc-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, MJ, Barreiros MA, Corregidor V, Alves LC, V. Pinto J, Mendes MJ, Fortunato E, Martins R, Mascarenhas J.  2019.  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. 2:1844-1853., Number 3 AbstractWebsite

The control of morphology and crystallinity of solution-processed perovskite thin-films for solar cells is the key for further enhancement of the devices’ power conversion efficiency and stability. Improving crystallinity and increasing grain size of perovskite films is a proven way to boost the devices’ performance and operational robustness, nevertheless this has only been achieved with high-temperature processes. Here, we present an unprecedented low-temperature (<80 °C) and ultrafast microwave (MW) annealing process to yield uniform, compact, and crystalline FA0.83Cs0.17Pb(I(1–x)Brx)3 perovskite films with full coverage and micrometer-scale grains. We demonstrate that the nominal composition FA0.83Cs0.17PbI1.8Br1.2 perovskite films annealed at 100 W MW power present the same band gap, similar morphology, and crystallinity of conventionally annealed films, with the advantage of being produced at a lower temperature (below 80 °C vs 185 °C) and during a very short period of time (∼2.5 min versus 60 min). These results open new avenues to fabricate band gap tunable perovskite films at low temperatures, which is of utmost importance for mechanically flexible perovskite cells and monolithic perovskite based tandem cells applications.

Neves, F, Stark A, Schell N, Mendes MJ, Aguas H, Fortunato E, Martins R, Correia JB, Joyce A.  2018.  Investigation of single phase Cu2ZnSn(x)Sb(1−x)S4 compounds processed by mechanochemical synthesis, Jul. Phys. Rev. Materials. 2:075404.: American Physical Society AbstractWebsite


Mendes, MJ, Haque S, Sanchez-Sobrado O, Araújo A, Águas H, Fortunato E, Martins R.  2018.  Optimal-Enhanced Solar Cell Ultra-thinning with Broadband Nanophotonic Light Capture, 2018. iScienceiScience. 3:238-254.: Elsevier AbstractWebsite
T. Vicente, A, Araujo A, Mendes MJ, Nunes D, Oliveira MJ, Sanchez-Sobrado O, Ferreira MP, Aguas H, Fortunato E, Martins R.  2018.  Multifunctional cellulose-paper for light harvesting and smart sensing applications. J. Mater. Chem. C. 6:3143-3181.: The Royal Society of Chemistry AbstractWebsite

A novel generation of flexible opto-electronic smart applications is now emerging{,} incorporating photovoltaic and sensing devices driven by the desire to extend and integrate such technologies into a broad range of low cost and disposable consumer products of our everyday life and as a tool to bring together the digital and physical worlds. Several flexible polymeric materials are now under investigation to be used as mechanical supports for such applications. Among them{,} cellulose{,} the most abundant organic polymer on the Earth{,} commonly used in the form of paper{,} has attracted much research interest due to the advantages of being recyclable{,} flexible{,} lightweight{,} biocompatible and extremely low-cost{,} when compared to other materials. Cellulose substrates can be found in many forms{,} from the traditional micro-cellulose paper used for writing{,} printing and food/beverage packaging (e.g. liquid packaging cardboard){,} to the nano-cellulose paper which has distinct structural{,} optical{,} thermal and mechanical properties that can be tailored to its end use. The present article reviews the state-of-the-art related to the integration and optimization of photonic structures and light harvesting technologies on paper-based platforms{,} for applications such as Surface Enhanced Raman Scattering (SERS){,} supporting remarkable 107 signal enhancement{,} and photovoltaic solar cells reaching [similar]5% efficiency{,} for power supply in standalone applications. Such paper-supported technologies are now possible due to innovative coatings that functionalize the paper surfaces{,} together with advanced light management solutions (e.g. wave-optical light trapping structures and NIR-to-visible up-converters). These breakthroughs open the way for an innovative class of disposable opto-electronic products that can find widespread use and bring important added value to existing commercial products. By making these devices ubiquitous{,} flexible and conformable to any object or surface{,} will also allow them to become part of the core of the Internet of Things (IoT) revolution{,} which demands systems{'} mobility and self-powering functionalities to satisfy the requirements of comfort and healthcare of the users.

Araújo, A, Mendes MJ, Mateus T, Costa J, Nunes D, Fortunato E, Águas H, Martins R.  2018.  Ultra-fast plasmonic back reflectors production for light trapping in thin Si solar cells. Solar Energy. 174:786-792. AbstractWebsite

A fast method is presented to fabricate plasmonic light trapping structures in just ten minutes (>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 (μc-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.

Sanchez-Sobrado, O, et al.  2017.  Colloidal-lithographed TiO2 photonic nanostructures for solar cell light trapping. J. Mater. Chem. C. 5:6852-6861.
Marouf, S, et al.  2017.  Low-temperature spray-coating of high-performing ZnO:Al lms for transparent electronics. J. Anal. Appl. Pyrolysis. 127:299-308.
Vicente, AT, et al.  2017.  Optoelectronics and Bio Devices on Paper Powered by Solar Cells. Nanostructured Solar Cells. (Narottam Das, Ed.).: InTech Open
Vicente, AT, et al.  2017.  A statistics modeling approach for the optimization of thin film photovoltaic devices. Solar Energy. 144:232-243.