Photon Management in Si-Based Solar Cells
Date Issued
2011
Date
2011
Author(s)
Chang, Hung-Chih
Abstract
In this thesis, first, the nanowire array (NWA) layers with controlled structure profiles fabricated by maskless galvanic wet etching on Si substrates are found to exhibit extremely low specular reflectance (< 0.1 %) in the wavelengths of 200-850 nm. The significantly suppressed reflection is accompanied with other favorable antireflection (AR) properties, including omnidirectionality and polarization-insensitivity. The NWA layers are also effective in suppressing the undesired diffuse reflection. These excellent AR performances benefit from the rough interfaces between air/NWA layers and NWA layers/substrate and the decreased nanowire densities, providing the gradient of effective refractive indices. The Raman intensities of Si NWAs were enhanced by up to 400 times as compared with the signal of the polished Si, confirming that the NWA layers enhance both insertion and extraction efficiencies of light. This study provides an insight into the interaction between light and nanostrucutres, and should contribute to the structural optimization of various optoelectronic devices.
Second, wafer-scale nanowire arrays (NWAs) with hierarchical structure, combined the nanowire and interface micro-roughness were fabricated by single process of costless wet etching. The NWA based solar cells with designed hierarchical structure demonstrate excellent light-harvesting characteristics, such as broadband working ranges and omnidirectionality in external quantum efficiency and reflectance measurement. Compared to the polished Si and conventional NWAs, the solar cell with hierarchical structure exhibits significantly superior photovoltaic characteristics, i.e., short-circuit current of 32.7 mA/cm2 and conversion efficiency of 11.25 %. The enhanced photovoltaic performances agree with the theoretical analysis based on a finite-difference time-domain method. A viable scheme for light harvesting using the hierarchical structure employing micro-roughness/nanoscale surface textures on single crystalline Si solar cells has been demonstrated.
Third, antireflective Si/oxide core-shell nanowire arrays (NWAs) were fabricated by galvanic etching and subsequent annealing process. The excellent light-harvesting characteristics of the core-shell NWAs, such as broadband working ranges, omnidirectionality, and polarization-insensitivity, ascribed to the smooth index transition from air to the substrates, have been demonstrated. By tuning core-shell volume ratios, we obtained enhanced light trapping regions implemented in either the planar Si underneath NWAs or the core regions of NWAs, greatly benefiting the geometry design of planar and radial p-n junction cell structures, respectively. This photon management scheme indicates the potential use in nanostructured photovoltaic applications.
Finally, rough AZO films were employed to enhance the internal scattering and consequent optical absorption of thin film (amorphous/polycrystalline Si) tandem solar cells. Through the optimization work by simulations, the matched current densities from the top and the bottom cells were obtained with the device structure containing 1.5-μm roughened polycrystalline Si layer, which produces the efficiencies comparable to those of the 3.5-μm layer without roughening. The simulation results were supported by the device performances measured experimentally. The significantly enhanced light scattering in the thin rough active region was revealed by the calculation results based on finite-difference time-domain method. The concept and technique presented in this study should benefit the development of next generation of thin film solar cells.
Subjects
Si
nanowire
thin film
solar cell
SDGs
Type
thesis
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