We report the use of Te as an n-type dopant in GaAs core-shell p-n junction nanowires for use in photovoltaic devices. Te produced significant change in the morphology of GaAs nanowires grown by the vapor-liquid-solid process in a molecular beam epitaxy system. The increase in radial growth of nanowires due to the surfactant effect of Te had a significant impact on the operating characteristics of photovoltaic devices. A decrease in solar cell efficiency occurred when the Te-doped GaAs growth duration was increased.

1 Introduction Global energy demand is predicted to exceed 30 TW by 2050, about double the present value [1]. This predicament, known as the TeraWatt challenge, and concern over anthropogenic climate change, resource availability ("peak energy") and energy security have all increased the interest in renewable energy (hydroelectric, wind, solar, geothermal and biomass) [2]. One of the most promising renewable energy technologies is solar photovoltaics (PV) which convert sunlight directly into electrical energy. Although the resource potential of PV is enormous, it currently constitutes a small fraction (<1%) of global energy supply [2]. One of the main factors limiting the widespread adoption of PV is its low energy density, low efficiency, and relatively high cost in comparison to other energy technologies. This means that current PV technology can only compete in areas of high insolation or by government incentives such as feed-in tariff programs.One of the most relevant metrics for PV devices is the power conversion efficiency (PCE); that is, the efficiency with which sunlight can be converted to electrical power. A significant effort in PV research today aims to improve PCE while simultaneously reducing (or, at least, not significantly impacting) production cost. The vast majority of

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The growth of III-V nanowires from metal seed particles is described in an analytical manner within the framework of a material conservation model. Direct impingement of growth species on the particle, coupled to their diffusion from the sidewall and the substrate surface, are considered in the derivation of expressions for the time evolution of both axial and radial growths. Two regimes are distinguished: the structure originally grows in a purely axial manner until its length exceeds the diffusion length of adatoms incoming from the substrate, at which point sidewall nucleation is triggered, resulting in a shell expanding radially in the lower part of the wire. Factors that take into account the nonunity probability of inclusion of group III adatoms in the axially growing crystal are introduced. Moreover, a step-mediated growth is included to describe the axial evolution of the shell. The numerical values of the various parameters were assessed by fitting the model to experimental data on the morphology evolution of molecular-beam-epitaxy-grown GaAs and InAs nanowires.

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The morphology and crystal structure of Au-seeded GaAs nanowires (NWs) grown by molecular beam epitaxy were investigated as a function of the temperature, V/III flux ratio, and Ga flux. Low and intermediate growth temperatures of 400 and 500 °C resulted in a strongly tapered morphology, with stacking faults occurring at an average rate of 0.1 nm(-1). NWs with uniform diameter and the occurrence of crystal defects reduced by more than an order of magnitude were achieved at 600 °C, a V/III flux ratio of 2.3, and a Ga impingement rate on the surface of 0.07 nm s(-1). Comparison of nanowire densities on the various post-growth surfaces suggests a possible incubation time between the moment the Ga shutter is opened and when nanowire growth is initiated. Increasing the flux ratio favored uniform sidewall growth, making the process suitable for the fabrication of core-shell structures.

Numerical simulation of current-voltage (J-V) characteristics of III-V nanowire core-shell p-n junction diodes under illuminated conditions is presented with an emphasis on optimizing the nanowire design for photoconversion efficiency. Surface recombination and depletion effects are found to play a dominant role in the J-V characteristics. The impact of surface charge density, surface recombination velocity, doping concentration, and nanowire geometry are investigated. Investigation of contacting methodology indicated that solar cell efficiency is degraded with electrical contacts on the sidewalls of the nanowire due to Fermi level pinning at the metal/semiconductor interface. On the other hand, contacts on the top of nanowires with sidewall passivation provide solar cell performance close to the detailed balance efficiency limit of ∼30%. Elimination of the thin film between nanowires produces a smaller dark current and improved cell performance.

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