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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.

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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

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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.

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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).

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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.

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.