Abstract:
This thesis presents the design, optimization, and performance analysis of three axial junction nanowire solar cells based on cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and copper zinc tin sulfide (CZTS) with significant improvement in their optical and electrical characteristics compared to their planar counterparts. It is shown that the performance of these NW SCs can be further improved by incorporating hemispherical indium doped tin oxide (ITO) forward scatterers on top of the ITO front contact of the solar cells. The comparison between forward scatterer incorporated NW SCs, and forward scatterer incorporated planar solar cells (PSCs) has also been presented. It is shown that the forward scatterers significantly enhance optical absorption in both cases. The optimum size and arrangement of ITO hemispheres that result in improved photocurrent have been analyzed. In optimum cases, the incorporation of forward scatterers leads to absorption enhancement of 7.8%, 5.36%, and 8.8% in PSCs, and 21.4%, 7.36%, and 6.02% in NW SCs, respectively, for CdTe, CIGS, and CZTS absorbers in the same order.
From the absorption profile at various wavelengths, it is found that forward scatterers enhance absorption in the 450-600 nm wavelength range, while nanowires improve absorption in the 600-800 nm range, and their combination results in an improved absorption profile for the entire visible wavelength range. The omnidirectionality of NW SCs has been highlighted through the study of absorption enhancement at various angles of the incident radiation. Nanowire solar cells have been observed to maintain their superior absorption characteristics even for oblique irradiance.
The electron-hole-pair (EHP) generation rate profiles due to the incorporation of nanowires and forward scatterers have been analyzed, from which significant light trapping and consequent increased EHP generation is observed at the center of the nanowires below forward scattering hemispheres. This has resulted in 46%, 32%, and 82.5% improvement in ultimate power conversion efficiency for the three absorber layers, respectively. These promising findings encourage further exploration into forward scatterer incorporated axial junction NW SCs in terms of design, material and processing techniques, with the ultimate goal of attaining a future powered by sustainable and clean solar energy.
