Células solares: princípios e investigação do uso de redes de nanofios de SnO2 para aumento de eficiência

Abstract

In this work, a review was carried out on the functioning of commercially available solar cells and methods used to improve their efficiency, focusing in the final part on the proposal to apply the use of a network of SnO2 nanowires as a covering layer. Currently on the market there is a prevalence of silicon cells and thin films, so that several techniques have already been developed to increase light absorption in these devices. Since the operation of the aforementioned solar cells is based on the junction of two doped semiconductors, recombination processes aim to be avoided, prioritizing the greatest possible generation of free electron-hole pairs. In order to increase the optical path inside the device and consequently the photogeneration rate, surface texturing processes were developed that retain the incident photon for a longer time. As an example, the use of pyramidal structures in Si crystalline cells is applied. However, the texturing with the best theoretical performance is the so-called Lambertian surface, which is considered to be perfectly random. In addition to the interest in increasing the photon path, there is also a concern to avoid reflective losses at cell interfaces. To get around this problem, anti-reflective (AR) layers are used to improve the absorption of a certain band of the electromagnetic spectrum. Therefore, an AR layer is usually used together with surface texturing. Among the materials that have been highlighted in the photovoltaic industry, there is the class of materials TCOs (transparent conductive oxides) that have high transmission in the visible spectrum combined with high conductivity. TCOs are usually used as contacts in solar cells, however reflective losses of up to 30%-40% show the need to decrease this percentage. Several studies have already investigated the use of nanowire network with application to increase the performance of photovoltaic devices. Techniques are found that use nanostructures from an AR layer to a layer on an already textured surface with known geometries. Thus, the use of a network of SnO2 nanowires proves to be a hitherto unprecedented alternative in this scenario, but with potential applicability. Through the synthesis via the VLS method (vapor-liquid-solid) the growth of a network containing nanobelts is guaranteed. This type of synthesis stands out for its simplicity in obtaining high purity samples. As it is a chemical vapor deposition where the growth of nanowires is oriented on nanodrops of a catalyst metal, only the end of the wire contains the metal, resulting in a high purity nanowire. By changing the synthesis parameters, such as the thickness of the metallic layer and the concentration of Sn used, aspects such as the diameter of the nanowire and the thickness of the mesh can be indirectly modified. Therefore, when applying to cells already used in industry, it is possible to investigate which are the best synthesis conditions that lead to the most satisfactory results.