Enhancement Of Light Absorption In Thin Film Silicon Solar Cells With Two Dimensional Light Trapping

Sub-wavelength gratings are the structures whose grating period is shorter than the wavelength of incident light and own the properties of anti-reflection and provide polarization birefringence. Metallic nanostructures can induce the excitation of surface plasmon modes which are applied to enhance the absorption rate and reduce the bulk volume and the thickness of the active layer. Photonic crystals, which have a periodic refractive index change and show a unique dispersion relation, can be used to realize slow light. In this thesis, several optical designs, based on the diffraction characteristics of photonic crystal and sub-wavelength grating, have been proposed and they can enhance the absorption of thin-film solar cells.In this thesis,2D Rigorous Coupled Wave Analysis and Plane Wave Expansion method have been derived in detail, which can solve the diffraction efficiency, field distribution of sub-wavelength structures and band structure of photonic crystals.Firstly, as for anti-reflection coating of thin-film solar cells, two sub-wavelength grating designs have been studied:hemispherical sub-wavelength grating and cylinder sub-wavelength grating. The grating period, depth, duty cycle and thickness of them are optimized using Rigorous Coupled Wave Analysis method. Then the average transmittance of all these gratings are more than 82%in the incident angle range of 0°-89° and wavelength range of 300 nm-1100 nm. They have been demonstrated as an effective light trapping structure to boost the optical absorption, especially between incident angle of 40° and 80°.Combining the hemispherical sub-wavelength grating on top of the device as anti-reflection coating, I explore a thin-film silicon solar cell design using 2D PC structure with a square lattice as an absorbing layer based on the slow light effect. This thin-film solar cell show a significant enhancement of absorption up to 90.47% in the wavelength range of 300 nm-110O nm and incident angle range of 0°-80°; the average absorption is 86%at the range of 0°-89°. This structure can not only increase the absorption all over the incident angle, but reduce the bulk volume and the thickness of the active layer.Finally, based on properties of diffractive gratings, which can enhance optical path length, and metallic nanostructures, which could enhance the absorption by the excitation of surface plasmon modes, I propose a 1.45 μm thick thin-film solar cell design on a 1 μm thick absorbing amorphous silicon layer, which incorporates the anti-reflection coating with light cone photonic crystal and the back surface field layer with rectangular pyramid metal grating. According to our simulation, the average absorption of this thin-film solar cell is up to 92%in the incident angle range of 0°-75° and the wavelength between 300 nm and 750 nm. The enhancement of average absorption has increased by 11.54%when anti-reflection coating is between 300 nm and 600 nm, and by 3.75%when back surface field layer between 600 nm and 850 nm respectively.