Formation of 3D Ge quantum dots array for advanced photovolatics in layer-cake technique
Placement of quantum dots (QDs) and insight into QD's basic internal structure and optical properties lay nature cornerstones for advanced photonic devices. We report a manageable growth method for placing dense three-dimensional Ge QD arrays in a uniform or a grading size distribution, using thermal oxidation of poly-SiGe in layer-cake techniques. The QD size and spatial density in each stack could be well modulated by Ge content in poly-Si1−xGex, oxidation and underlay buffer layer conditions. Size-dependent internal structure, strain, and photoluminesce properties of Ge QDs are systematically investigated. Optimization of processing conditions was carried out for producing dense Ge QD arrays for maximizing photovoltaic efficiency.
Third generation photovoltaics: solar cells for 2020 and beyond
Abstract Many working in the field of photovoltaics believe that ‘first generation’ silicon wafer-based solar cells sooner or later will be replaced by a ‘second generation’ of lower cost thin-film technology, probably also involving a different semiconductor. Historically, CdS, a-Si, CuInSe 2 , CdTe and, more recently, thin-film Si have been regarded as key thin-film candidates. Since any mature solar cell technology is likely to evolve to the stage where costs are dominated by those of the constituent materials, be it silicon wafers or glass sheet, it is argued that photovoltaics will evolve, in its most mature form, to a ‘third generation’ of high-efficiency thin-film technology. By high efficiency, what is meant is energy conversion values double or triple the 15–20% range presently targeted. Tandem cells provide the best-known example of such high-efficiency approaches, where efficiency can be increased merely by adding more cells of different band gap to a stack. However, a range of other better-integrated approaches are possible that offer similar efficiency to an infinite stack of such tandem cells.
To Devolve or Not To Devolve? Welfare Reform in the States
This article analyzes variation in the degree to which states have responded to the devolution of welfare at the federal level by devolving authority over welfare policy to local government. I find that to the extent that states have devolved authority to lower levels of government, they tend to be states that already had a high degree of involvement of local government in welfare provision. In states without this record of local government involvement, the devolution that has occurred has not generated greater involvement of local government, but rather responsibility has been devolved to regional entities with ties to workforce development and with a substantial degree of business involvement. As states gain increasing authority over a redistributive policy, they may begin to treat it as a subset of a larger developmental policy—workforce development.