Fabrication of Subsurface Metallic Nanoparticles For Enhanced Carrier Generation in Silicon-based Photovoltaics

Abstract

Due to the low absorption coefficient of silicon in the bulk of the solar spectrum, the majority of silicon-based photovoltaic cells are at least 300 micrometers thick, limiting their economic feasibility. To achieve cost parity with conventional sources of energy, silicon-based photovoltaics have begun to move towards thinner substrates, on the order of a few micrometers; such thinner cells necessitate the use of light-trapping methods to increase the optical path length. Much research has begun investigating the use of trapped electromagnetic waves, or surface plasmons, to increase light scattering and interband carrier transition rates in the surrounding material. In one scheme, plasmonic modes can be supported through polarization of metallic nanoparticles. Past research has focused on the deposition of silver nanoparticles on the surface, and has shown that light absorbance can be increased markedly for certain bandwidths. Here, the relevant mechanism is increased lateral light scattering, which tends to guide the light into directions that are then totally internally reflected. When such metallic nanoparticles are polarized by incoming radiation, in addition to increased light scattering, the electric field in the vicinity of the nanoparticle is highly magnified. Such high fields can increase the carrier transition rates, and therefore the absorbance, in the surrounding silicon by orders of magnitude. The enhancement however decreases rapidly with the radial distance and is virtually diminished by 10 nm from the nanoparticle surface. Deposition on the surface of the solar cell, therefore, cannot exploit this effect, being isolated from the silicon by the passivating layer. It has been suggested that instead, such particles might be embedded into the silicon itself. To that end, we have developed a method to create subsurface silver nanospheres, using a combination of ion implantation, thermal deposition, and subsequent thermal annealing. By tailoring the implantation parameters, we can localize the layer of nanospheres and even create various bands at various depths. Through Rutherford backscattering and secondary ion mass spectroscopy characterization, we have found that the Ag has indeed annealed into the desired location. To ensure that the Ag has agglomerated to nanoparticles, we have confirmed this with TEM images and selected area diffraction patterns that indicate that such nanoparticles are indeed bulk phase silver and have diameters of 20-35 nanometers. This method can help realize more efficient surface plasmon-enhanced Si-based photovoltaics.