dc.description.abstract | Thin film solar cells, which require much less energy for fabrication, have recently begun to attract attention. Among the present photovoltaic devices, silicon-based thin film solar cells show the greatest potential, because the material is plentiful, stable, robust and nontoxic. Additionally, silicon-based cells are compatible with existing processes in the well-developed semiconductor fabrication industry. Large area hydrogenated amorphous silicon (a-Si:H) thin film solar cells can be fabricated on various substrates. In this thesis, we focus on the investigations of amorphous silicon thin film solar cells, both in optics and electronics. The analysis of microelectronic and photonic structures (AMPS-1D) program simulates the electrical properties and the rigorous coupled-wave analysis (RCWA) and finite-difference time-domain (FDTD) methods simulate the optical properties.
The AMPS-1D program is applied to investigate the thickness effect of an a-Si:H p-i-n thin film solar cell. The filling factor (FF) in J-V curve increases to 0.547 from 0.388 when the thickness of intrinsic layer decreases to 100nm from 300nm. Higher FF implies the higher internal quantum efficiency (IQE) and this improvement can be explained by the increasing of the build-in electric field and the shorting of collection distance of excited carriers. Therefore, for high efficiency, the requirement of thinner solar cells is proposed. However, a thinner solar cell always encounters the light absorption problem leading to lower external quantum efficiency (EQE). The solar cells with dielectric grating with various grating periods and depths are studied for enhancement through its far field scattering effect (Haze effect). The Haze effect only has benefit with an active layer thicker than 400nm. When the active layer is thinner than 300nm, the anti-reflection effect from the dielectric grating becomes important. For enhancing light absorption in active layer thinner than 100nm, a metal grating is applied. The metal grating and original solar cell forms a plasmonic multilayer structure (PMS). The surface plasmon resonances of the PMS are analyzed by the rigorous coupled-wave analysis (RCWA) and finite-difference time-domain (FDTD) methods. The surface plasmon (SP) resonances including the surface plasmon polaritons (SPPs) and the localized surface plasmon (LSP) in the PMS are recognized through angle-resolved spectrums and corresponding field distributions. The coupling between them makes a border absorption band than traditional SPPs on a flat surface; this is benefit for the broad solar spectrum. The absorbed photon numbers can increase 28.7% when the PMS is applied. The spatial distribution and separation of absorptioin in plasmonic solar cells is further studied and we show a usually overestimation of absorption in the active layer. The active layer only absorbs 50% of incident energy while the metallic material absorbs 15%, which can not contribute to the external quantum efficiency (EQE). When the IQE is assumed to be 100% everywhere inside the a-Si layer, the slight enhancement 12.2% of short-circuit current (Jsc) is achieved through the excitation of SPPs resonance. The concentration of energy dissipation around the corners of metal grating and the blocking effect of metal grating are shown. We demonstrate that the spatial-dependent IQE (SDIQE) in ITO/a-Si:H/Ag solar cell through the AMPS-1D program. The correlation between absorption distribution and SDIQE influences a lot in the effective IQE. In other words, the position of absorbing photons is critical. The EQE of plasmonic solar cells is theoretically investigated through the combination of the optics and electronics simulations. The surface plasmon resonances might fail to enhance the light absorption of active layer when the surface plasmon exciter is far from the active layer.
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