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New ZnO-based core-shell nanostructures for perovskite solar cells
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Perovskite solar cells are the emerging thin-film photovoltaics that has been most studied in the last decade, reaching record power conversion efficiencies close to those exhibited by silicon solar cells, which means a considerable breakthrough in photovoltaic technology. However, it has been found that in methylammonium lead halide perovskite devices, MAPbX₃, either the electron transport material or hole transport material, affects the perovskite material’s stability, compromising the perovskite device performance. Therefore, other alternatives should be considered to avoid the perovskite layer’s degradation, namely, the electron transport material employed in the perovskite solar cells.
This PhD project aimed to develop nanostructures alternatives to the standard titanium dioxide (TiO₂) through ZnO-based nanostructures, using a low-cost and versatile technique such as pulsed electrodeposition, and applying them as the electron transport layer within perovskite solar cells.
Well-aligned arrays of ZnO nanorods were produced by pulsed potentiostatic electrodeposition in aqueous media, under mild reaction conditions. Several modified substrates were evaluated for the growth of nanorods to optimise the nanorod diameter and vertical orientation. Using a TiO₂ intermediate layer as a template for the ZnO nanorods growth successfully allowed a decrease of the nanorod diameter, increased their spatial density, and increased the ZnO films’ chemical stability with time and under illumination. Also, it was verified that the pulsed electrodeposition conditions at which the nanorods grow, namely the pulse operational parameters, had a very appreciable impact on its optoelectronic properties.
Several ZnO thin films prepared using different deposition media were studied to assess the influence of ZnO nature on the thermal stability of the MAPbI₃ perovskite. Some chemical groups attached to the ZnO surface affected the crystallization of the perovskite layer and accelerated its thermal degradation at the ZnO/perovskite interface.
The ZnO@TiO₂ core-shell nanostructures were considered to prevent the perovskite instability issues when in intimate contact with ZnO. Despite the slight improvement in device performance using ZnO@TiO₂ core-shell nanorods, compared to ZnO nanorods, the morphological reproducibility of a TiO₂ shell that completely covers the ZnO surface is crucial to obtain higher photovoltaic performances.
Descrição
Palavras-chave
nanopilares de ZnO electrodeposição pulsada células solares de perovskita perovskita de MAPbI₃ nanopilares core-shell de ZnO@TiO₂ ZnO nanorods pulsed electrodeposition perovskite solar cells MAPbI₃ perovskite ZnO@TiO₂ core-shell nanorod