Here we demonstrate a more effective use of III−V photoconversion material to achieve an ultrahigh power-per-weight ratio from a solar cell utilizing an axial p-i-n junction GaAs/AlGaAs nanowire (NW) array grown by molecular beam epitaxy on a Si substrate. By analyzing single NW multicontact devices, we first show that an n-GaAs shell is self-formed radially outside the axial p-and i-core of the GaAs NW during n-core growth, which significantly deteriorates the rectification property of the NWs in the axial direction. When employing a selective-area ex situ etching process for the n-GaAs shell, a clear rectification of the axial NW p-i-n junction with a high on/off ratio was revealed. Such a controlled etching process of the self-formed n-GaAs shell was further introduced to fabricate axial p-i-n junction GaAs NW array solar cells. Employing this method, a GaAs NW array solar cell with only ∼1.3% areal coverage of the NWs shows a photoconversion efficiency of ∼7.7% under 1 Sun intensity (AM 1.5G), which is the highest achieved efficiency from any single junction GaAs NW solar cell grown on a Si substrate so far. This corresponds to a power-perweight ratio of the active III−V photoconversion material as high as 560 W/g, showing great promise for high-efficiency and lowcost III−V NW solar cells and III−V NW/Si tandem solar cells.

The purpose of this chapter was to study the concept of topological structure formed by soft multi-sets. The notion of relative complement of soft multiset, soft multi-point, soft multi-open set, soft multi-closed set, soft multi-basis, soft multi-sub-basis, neighbourhoods and neighbourhood system, interior and closure of a soft multi-set, etc., is to be introduced, and their basic properties are also to be investigated. It is seen that a soft multi-topological space gives a parameterised family of topological spaces. Lastly, the concept of soft multi-compact space is also introduced.

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We developed a new technique to fabricate single nanowire devices with reliable graphene/nanowire contacts using a position-controlled microtransfer and an embedded nanowire structure in a planar junction configuration. A thorough study of electrical properties and fabrication challenges of single p-GaAs nanowire/graphene devices was carried out in two different device configurations: (1) a graphene bottom-contact device where the nanowire–graphene contact junction is formed by transferring a nanowire on top of graphene and (2) a graphene top-contact device where the nanowire–graphene contact junction is formed by transferring graphene on top of an embedded nanowire. For the graphene top-contact devices, graphene–nanowire–metal devices, where graphene is used as one electrode and metal is the other electrode to a nanowire, and graphene–nanowire–graphene devices, where both electrodes to a nanowire are graphene, were investigated and compared with conventional metal/p-GaAs nanowire devices. Conventional metal/p-GaAs nanowire contact devices were further investigated in embedded and nonembedded nanowire device configurations. A significantly improved current in the embedded device configuration is explained with a “parallel resistors model” where the high-resistance parts with the metal–semiconductor Schottky contact and the low-resistance parts with noncontacted facets of the hexagonal nanowires are taken into consideration. Consistently, the nonembedded nanowire structure is found to be depleted much easier than the embedded nanowires from which an estimation for a fully depleted condition has also been established.

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