Theoretical Study Of Absorption Enhancemnt In Silicon Thin Film Solar Cell

The light trapping scheme is an indispensable element in high efficient thin film solar cell. The absorption of thin film solar cell can be enhanced by light trapping structures which prolong optical path length of the light in the absorbing layer. The optical characterizations for different active materials in solar cells are different, which leads to construct different trapping structures. To find out the optimal light trapping structures, it is necessary to make theoretical analysis of the absorption limit in different solar cells. It can provide the aim to design appropriate light trapping structures. This thesis includes the contents as follows:For the materials with weak absorption, the absorption limit was analyzed using the coupled guided mode theory. The theoretical analyses associated with maximal enhancement ratio were made for different light trapping schemes. The design principle for light trapping structures was proposed, by which an optimal structure was given and numerically demonstrated by using the rigorous coupled wave analysis(RCWA). The absorption enhancement of a thin film crystalline silicon solar cell as an example validates the effectiveness of this light trapping structure at the wavelengths between 600 nm and 1100 nm.For the absorbing materials with substantial absorption, such as amorphous silicon, gallium arsenide(GaAs), and crystalline silicon at the short wavelengths, these absorption limits were studied using the coupled leaky mode theory(C LMT). The relation between the refractive index of the materials and the radiative loss of the leaky mode was analyzed. The design principle for light trapping structure in such substantial absorbing materials was also proposed, by which an optimal light t rapping structure was also obtained.Finally, the design principle, termed effective refractive index method, of light trapping structures for all the absorbing materials in entire solar spectrum was proposed by combining the above two design principles of the weak and substantial absorption materials. Two optimal light trapping structures, pyramid and nanocone, for the crystalline silicon thin film solar cell were presented. The numerical simulations demonstrate that the absorptions of such two thin film solar cells are very close to the theory limit for the entire solar spectrum from 300 nm to 1100 nm and insensitive to the indecent angle of the light. The absorption is always very well within the angle varies from 0 to 70 degree.