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The global energy consumption is increasing rapidly. To meet these demands, extensive research has started to develop different kinds of materials for photovoltaic application.
Silicon and its different nanostructures are already being extensively used in the field of electronics, optics, and photovoltaics. The various nanostructures of silicon create unique opportunities for enhanced performance in photovoltaics, sensors, CMOS, and optics.
Photovoltaic cells are an energy harvesting technology, which convert solar energy into electrical energy. The nanowire based solar cells are being investigated, to try and provide high energy efficiency with low fabrication cost. Some of the properties of nanowire solar cells are more desirable than the traditional wafer-based solar cells particularly around the charge separation mechanism and cost.
Materials like silicon nanowires, cadmium selenide nanorods, titania and zinc oxide nanowires are currently under focus for the photovoltaic applications . Some of the important criteria for material selection are the cost, whether it is environmentally friendly, and abundance . It should have a band-gap which can maximize the solar absorption and give high energy efficiency . Silicon nanostructures satisfy most of the important criteria needed in a material for photovoltaic applications.
Silicon Nanowires (SiNWs) are one-dimensional materials with a diameter in the range of 1 to 50 nanometers, and lengths ranging from hundreds of nanometers to few centimeters. Some of the fabrication methods used are Vapour Liquid Solid (VLS) method, Chemical Vapour Deposition (CVD), evaporation, Molecular Beam Epitaxy (MBE), laser ablation and metal-assisted chemical etching (MACE) . Out of these, metal-assisted chemical etching is one of the simplest and cost-effective methods. In MACE, by controlling the concentration of the metal and the etching time, we can obtain high aspect-ratio nanowires .
Silicon Nanowires in Photovoltaic Application
A number of properties of SiNWs make them a potential material for photovoltaic applications. First and foremost is the anti-reflecting property. SiNWs have a very low reflectance, i.e., they absorb most of the incident light. Additionally, there is no need for additional light-tracking holders as SiNWs are independent of the light incident angle. This reduces the cost of the photovoltaic modules.
The second beneficial property is the high aspect-ratio of SiNWs. This helps them to absorb more sunlight with thinner silicon structures, thus reducing the material cost. In traditional solar cells, for complete absorption, a thicker material is required due to the indirect band-gap of the bulk silicon .
Thirdly is the self-cleaning property of SiNWs. Some unique structures of SiNWs make it hydrophobic in nature, exhibiting a property known as the self-cleaning effect. In a real environment, dust particles gets collected on the surface of the photovoltaic modules. Due to this the sunlight is then blocked which reduces the power efficiency. The nanowires solar cells overcome this problem using the self-cleaning effect .
Another key requirement for photovoltaic applications is that the total amount of light absorbed should be converted into free charge carriers. This is known as the light-harvesting capacity, which determines the total number of available photons for photocurrent conversion. As SiNWs are good absorbers, their light harvesting capacity is also high . In commercial solar cells, surface texturing is carried out to decrease the reflection but this is not required for nanowire solar cells.
The research on silicon nanowire solar cells is still under process. For example, recently scientists have found that if the orientation of the nanowires is changed from axial p-n junction to radial p-n junction, the efficiency of the solar cells increases. They are also planning to build up a procedure to contact nanowire-arrays in a substrate-free mode to make the devices more flexible and commercially attractive.
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