Keywords: plasmonic, plasmon, nanoparticle, solar cell, silicon, semiconductor, light trapping, photovoltaic 1. Plasmons Plasmons may be described as the oscillations of free electrons which arise from the action of electromagnetic (light) waves to form a dipole in a material (1-3). They thus constitute a density wave in an electron gas, in analogy with a sound wave, which is a density wave in a molecular gas. A plasmon is a collective wave, where billions of electrons oscillate in synchrony (4). The tendency of the system is to revert to its initial state by electron migration; however, due to the oscillation of the waves, a constantly shifting dipole is induced, with which the electrons are driven to oscillate in phase. The radiation frequency must be equal to, or below, the plasma frequency for this electromagnetic coupling to take place, which is strongest when the two frequencies match, i.e. at the resonance condition. Plasmons are implicit to the optical behaviour of both conducting (metals) and semiconducting materials, i.e. those frequencies of light that are greater than the plasma frequency pass through (transmission) the material because its electrons are unable to respond sufficiently quickly to screen them, while those frequencies at or below the plasma frequency are reflected, due to the screening effect of the electronic response. The reason that most metals have a shiny appearance is that the frequencies of visible light are lower than their plasma frequencies (which lie in the ultraviolet region), and are hence reflected. Copper and gold, and some semiconductors, have electronic inter-band transition frequencies in the visible region, which accounts for their distinct colours3. Figure 1 shows a Gothic stained glass rose window of Notre-Dame de Paris, whose colours were achieved using colloids of gold nanoparticles. 2. Plasmonic nanoparticles A plasmonic nanoparticle has the property that wavelengths of light far larger than the particle itself can couple electromagnetically with its electron density, as a consequence of the particular dielectric-metal interface between the medium and the particles. This is in contrast with a pure metal, where the dimension of the particle determines the upper limit of wavelength that can couple effectively with it (3). Clusters of nanoparticles may form plasmonic molecules, and according to their mutual symmetry and the electronic distribution within them, styles of bonding or antibonding character may be attained which are reminiscent of molecular orbitals (5). According to their geometries and relative positions, nanoparticle clusters may be induced5 to confer novel properties of scattering, absorbance, and coupling of light radiation, such that they are of potential interest in various applications (7,8) including solar cells, spectroscopy, imaging technologies, and cancer treatment. 2.1 Plasmonic solar cells Since plasmonic nanoparticles can scatter light back into the photovoltaic structure and have a low degree of light absorption, forcing more light to be absorbed by the dielectric, they are being investigated as a means to increase the efficiency of solar cells. A plasmonic solar cell (PSC) may be regarded as a version of the thin-film solar cell, which is normally thinner than 2 pm (theoretical considerations suggest that they could be made (2) as thin as 100 nm). While thin-film solar cells have the advantage that they can be made (9) using far smaller quantities of semiconducting materials and far cheaper substrates (e.g. plastic, steel or glass) than is the case for conventional silicon PV-cells, they absorb less light energy than their thicker counterparts made with materials of a similar coefficient of absorption (Figure 2). Thus, means the enhancement of light trapping is an important consideration for thin-film cells, in which regard, plasmonic cells employ metal nanoparticles excited at their surface plasmon resonance, to raise the overall absorption of light energy (1-3). …
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Oct 8, 2024
Yue Zeng + 3
Open Access