English abstract
Thin-film silicon technology is a major candidate to react with the ever-increasing global energy demand. The small thickness of silicon allows high industrial output and low material usage and subsequently opens new paths to mass-production of low-cost solar cells. Thin film amorphous silicon (a-Si) solar cells suffer from weak absorption of long wavelength rays, which have absorption lengths that are far greater than the absorber layer thickness, hence there is a reduction in the efficiency in contrast to bulk Si cells. Light trapping schemes utilizing surface plasmons nanostructures (in addition to guided wave mode excitations) can strongly diffract long wavelength photons and increase their optical path length, causing improving the absorption in the active layer. Surface plasmons (SPs) are a light induced phenomenon resulting in desirable electromagnetic (EM) field enhancements concentrated at the surface of an appropriate dielectric-metal (or highly doped dielectric) interface. Free electrons in metals (or highly doped dielectric) can strongly interact with incident light to form Surface-Plasmon-Polaritons (SPPs). The resulting electromagnetic waves are trapped within sub-wavelength dimensions and can propagate along the interface. To efficiently use SPP propagation, their relatively short propagation length has to be enhanced. Hence, the SPs need to be optically excited using the diffraction grating method.
In this thesis, both top and bottom grating structures of solar cell designs are studied using commercial finite element analysis tool, COMSOL Multiphysics® software. The aim of this thesis is to investigate novel architecture combining two grating structure, leading to enhance absorption in thin film solar cells. The proposed model here is an active layer of amorphous silicon (a-Si), structured with two grating layers, one with the silicon dioxide (SiO2) on the top of it, the other is with titanium nitride (Tin) underneath of it. Numerical simulations by calculating the photocurrent enhancement were conducted to obtain the optimal geometry parameters of the proposed model. The double grating structure exhibits an improvement in absorption up to 3.4 times comparing with that of the planar structure over a wide range of incident light wavelength. This directly causes an enhancement of 1.34 of the short circuit current density. The parametric studies also indicated that the absorption enhancement of such architecture is a little sensitive to the angle of incidence. Extensive numerical simulations of various designs with changed materials and gratings shape may done to achieve better performance and lower the cost of the solar cell design.
