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.
Improvement of PV Performance by Using Multi-Stacked High Density InAs Quantum Dot Molecules
InAs quantum dot molecules (QDMs) are prepared by thin-capping-and-regrowth MBE process. The dot density can be varied between 1010 cm-2, for as-grown quantum dots (QDs), to 1012 cm-2, for multi-stack QDMs. Photocurrent measurements on 1-and 5-stack high-density QDM layers show that these InAs QDMs when embedded inside a GaAs bulk structure extend photon absorption beyond the 850-nm bandedge limited by GaAs. The results also indicate that the higher the number of stacks the higher the resulting current. The presence of high-density QDMs in solar cells thus extends the absorption region and at the same time increase the output current. Electrical characterisations on homojunction (p-n) solar cells with 1-and 5-stack high-density QDMs embedded between the junction show that the 5-stack sample provides a higher short-circuit current density of Jsc=14.4 mA/cm2 compared to 9.6 mA/cm 2 provided by the 1-stack sample. The increase is due entirely to the difference in absorptive dot volume accounted for by the difference in the number of stacks of high-density InAs QDMs. The efficiency of homo-structure 5-stack high-density QDM solar cell is 5.1%