Fundamentals of Solar Cell Design. Rajender Boddula

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resonance frequency in free space can be given as Equation (2.4).

      2.2.2 Mechanism of Plasmonic Solar Cells

      Plasmonic nanostructures are used as light harvesting antennas in thin film solar cells. Plasmonic nanostructures such as metallic nanoparticles or thin films are used to trap or concentrate light. They can be introduced at top or back surface as antireflecting coating in solar cells. Various fabrication techniques can be used to deposit metal nanoparticles. A simple technique is the evaporation followed by heat treatment for making arrays of metal nanoparticles. The lithography is also used for making metal nanoparticles [36, 37].

      The plasmonic nanostructures are mainly used for increasing light absorption through scattering in plasmonic solar cells. The plasmonic nanostructures are affecting to optical properties such as light trapping, absorption, scattering, resonance wavelength range, and energy levels.

      2.3.1 Trapping of Light

      The metal nanoparticles are normally placed at a particular distance for trapping enough sunlight between nanoparticle and substrate in solar cell. The illuminated light intensity decreases with distance from the substrate in the case of embedded plasmonic nanoparticle. The top deposited plasmonic nanoparticle design is advantageous for illuminated light into the substrate. The more light escapes from device if there is no distance between the substrate and particle. The absorption is enhanced by folding light in absorber using plasmonic nanoparticles. The folding of light is mainly depending on the structural properties of metallic nanoparticles. The absorption is large in small nanoparticles because of increased near-field. However, very smaller metal nanoparticles go through ohmic loss. Hence, the surface plasmons can be utilized to improve the electric and optical behavior of solar cells. SPRs have ability to collect about 95% incident light where as conventional solar cell can collect about 30% of sunlight [41].

      2.3.2 Scattering and Absorption of Sunlight

      The absorption and scattering of incident sunlight are main affecting parameters on the efficiency of solar cells. The utilization of plasmonic nanostructures at the top surface of the PV device improves the overall absorption through the scattering of light. For example, silicon does not absorb much light and hence more lights need to be scattered to increase the absorption. The deposited silver metal nanoparticles at the surface increases the absorption and scattering due to surface plasmons is about 10 times more than the nanoparticle. A broader plasmon resonance can be achieved in metal nanoparticles with a large scattering [42].

      2.3.3 Multiple Energy Levels

Material Energy band gap (eV) at 300K λmax (λ = hc/E = 1242/E) nm
Ge 0.66 1,881
Si 1.11 1,118
InP 1.27 977
CIGS 1.42 874
GaAs 1.43 868
CdTe 1.44 862
CZTS 1.5 828
CuO 1.7 730
CdSe 1.74 714
BiFeO3 2.09 594
Fe2O3 2.1 591
GaP 2.25 552
CdS 2.25 552
PbO 2.9 428
TiO2 3.2 388
SrTiO3

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