Abstract
A material with varied geometry structures can exhibit distinct material properties. Therefore in this study, we will employ this concept to enhance the performance of metamaterials and Si-based solar cells by manipulating their geometry structures. The concept of metamaterials describes sub-wavelength media whose collective responses arise mainly from their structures rather than their constitutions. In the last decade, metamaterials attract numerous attentions because of its rare and even unprecedented electromagnetic (EM) properties, such as, negative refractive index, inverse Snell’s law, superlensing effect, slowing-light effect and perfect absorber. However, the strong dependence of incident EM wave polarization of the metamaterials will hinder them from the practical applications. For example, split-ring resonators and plasmonic wires exhibit the negative permeability and permittivity respectively only under grazing-angle incidence while the H-shaped metallic wires, short-wire pairs and two-handed metamaterials reveal negative refractive index only under the normal incidence. In this study, to ease the limitation of polarization to the conventional metamaterials, we propose a novel structure to present the negative permittivity and permeability as well as negative refractive index under multi-angle incidence, which is verified theoretically and experimentally in the microwave region. In conclusion, by manipulating the morphology of the conventional metamaterials, we have successfully designed a negative refractive index medium operating at a variety of incident angles to eases the burden of strong anisotropic responses in conventional metamaterials and widely increased the feasibility of practical applications. Besides, we also exploit a low-cost wet etching method, statistic electroless metal deposition (SEMD) to manipulate the morphology of Si for enhancing the conversion efficiency (CE) of Si-based solar cells (SCs) . This is because currently, the efficiency-to-cost ratio among the diverse photovoltaic techniques remains too low to compete with fossil energy. To boost this ratio, the CE can be improved or the cost of SCs can be lowered. Silicon nanowires (SiNWs), which have excellent antireflective properties, have the potential to facilitate these changes. However, freestanding SiNWs are fragile, and this might hinder the use of SiNWs in practical devices. In addition, complicated procedures, such as chemical vapor deposition, which is often used to fabricate SiNWs, add to the manufacturing cost. Fortunately, silicon nanoholes, which also have excellent antireflective properties, are more robust than SiNWs and are better candidates for use in SCs. In this study, we employ a series of wet and room-temperature processes to fabricate low-cost Si nanoholes (SiNHs) and SiNWs for photovoltaic applications. SiNH-based SCs possess excellent antireflective properties, resulting in an enhanced CE 48% greater than single-side polished Si-based SCs. Moreover, the SiNH-based SCs have an additional advantage over SiNW-based SCs—the flatness of the SiNH surface supports a larger fill factor than SiNWs do. To summarize, such a low-cost fabrication process enables us to fabricate low-cost, high-efficiency SiNH SCs. Furthermore, we also exploit the SEMD method to fabricate Si-micrograting SC (SiMG-SC). This is because that if one applies Si nano- or micro-structured on the SCs, they will exhibit the enhanced optical absorption properties and the ability of decoupling minority carrier diffusion and light absorption paths. Therefore, we monolithically fabricate SiMG-SC with vertical sidewall electrodes and vertical multijunction, which enable to minimize the reflection losses from conventional planar metallic electrodes and to increase minority carrier collection probability, respectively. Based on the SiMG-SC, we consequently intensify the photovoltaic properties of the current density, fill factor, and power conversion efficiency, by 11.2%, 23.7%, and 52.9%, respectively compared to those of the control SCs