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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.

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