Enhanced Light Absorption in Thin Crystalline Silicon Solar Cells With Silica Nanoparticles

In this paper, numerical simulations are performed to investigate the effects of different configurations of dielectric SiO₂ particles on the improvement of light absorption in 2-μm single crystal silicon photovoltaic solar cells. The numerical model is developed on the basis of the FDTD solution of the transient Maxwell equations and checked with analytical solutions for simple configurations and against experimental measurements of light absorption in bare Si films. The numerical model is also checked for mesh sensitivity such that the computed data are approximately mesh-insensitive. Computed results are analyzed and the short circuit current of the Si films is used as a measure of the efficiency for light trapping in Si films. Results show that with SiO₂ nanoparticles closely packed atop the Si film, good improvement in light absorption efficiency is achieved if the particle is 700 nm in diameter. This is considered to be attributed to the anti-reflection effect of the particle layer and the whispering gallery mode of SiO₂ particles excited by the incident light. If the closely arranged SiO₂ nanoparticles are embedded half-way into a Si film through its top surface, the light absorption is enhanced by ∼120%, approaching to the Yablonovitch limit. The structured surface of the Si film can almost realize 100% anti-reflection of incident, because the use of the half embedded SiO2 particles in the top layer of the Si film creates a graded transition of the effective refractive index along the direction of incident; and as a result almost all the light with the wavelength below or near 500nm are absorbed due to the higher imaginary part of the refractive index. The improvement in light absorption with the wavelength greater than 500nm comes, however, from the resonance behavior of the SiO₂ nanoparticles. Experiments are now planned and measurements of light absorption will be conducted with a photospectrometer to validate the above calculations.