A device physics model has been developed for radial p-n junction nanorod solar cells, in which densely packed nanorods, each having a p-n junction in the radial direction, are oriented with the rod axis parallel to the incident light direction. High-aspect-ratio ͑length/diameter͒ nanorods allow the use of a sufficient thickness of material to obtain good optical absorption while simultaneously providing short collection lengths for excited carriers in a direction normal to the light absorption. The short collection lengths facilitate the efficient collection of photogenerated carriers in materials with low minority-carrier diffusion lengths. The modeling indicates that the design of the radial p-n junction nanorod device should provide large improvements in efficiency relative to a conventional planar geometry p-n junction solar cell, provided that two conditions are satisfied: ͑1͒ In a planar solar cell made from the same absorber material, the diffusion length of minority carriers must be too low to allow for extraction of most of the light-generated carriers in the absorber thickness needed to obtain full light absorption. ͑2͒ The rate of carrier recombination in the depletion region must not be too large ͑for silicon this means that the carrier lifetimes in the depletion region must be longer than ϳ10 ns͒. If only condition ͑1͒ is satisfied, the modeling indicates that the radial cell design will offer only modest improvements in efficiency relative to a conventional planar cell design. Application to Si and GaAs nanorod solar cells is also discussed in detail.

Single-nanowire solar cells were created by forming rectifying junctions in electrically contacted vapor-liquid-solid-grown Si nanowires. The nanowires had diameters in the range of 200 nm to 1.5 microm. Dark and light current-voltage measurements were made under simulated Air Mass 1.5 global illumination. Photovoltaic spectral response measurements were also performed. Scanning photocurrent microscopy indicated that the Si nanowire devices had minority carrier diffusion lengths of approximately 2 microm. Assuming bulk-dominated recombination, this value corresponds to a minimum carrier lifetime of approximately 15 ns, or assuming surface-dominated recombination, to a maximum surface recombination velocity of approximately 1350 cm s(-1). The methods described herein comprise a valuable platform for measuring the properties of semiconductor nanowires, and are expected to be instrumental when designing an efficient macroscopic solar cell based on arrays of such nanostructures.

Arrays of vertically oriented Si wires with diameters of 1.5 m and lengths of up to 75 m were grown over areas Ͼ1 cm 2 by photolithographically patterning an oxide buffer layer, followed by vapor-liquid-solid growth with either Au or Cu as the growth catalyst. The pattern fidelity depended critically on the presence of the oxide layer, which prevented migration of the catalyst on the surface during annealing and in the early stages of wire growth. These arrays can be used as the absorber material in novel photovoltaic architectures and potentially in photonic crystals in which large areas are needed. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2779236͔Photovoltaic devices designed to achieve high cell efficiency with low-quality materials must have optically thick absorber layers, yet must simultaneously allow efficient collection of low diffusion length charge carriers. An attractive approach involves an array of vertically aligned semiconducting wires to enable carrier collection in the wires' radial direction, a distance that is short relative to their optical thickness ͑i.e. length͒.1 Well-defined wire arrays have been produced using lithographic patterning followed by anisotropic etching, 2,3 but such methods require large areas of high-quality substrate materials. In contrast, wires of various materials 4 have also been grown 'bottom up' by the vaporliquid-solid ͑VLS͒ process.5 Control of the size and position of VLS-grown wires has been demonstrated, 6,7 particularly in the case of Si by patterning of a surface oxide.8-10 Wire array growth, however, has only been achieved over relatively small areas, unless a template is used.11 We demonstrate herein the VLS growth of arrays of Si wires having diameters of 1.5 m and lengths of Ͼ70 m, with very low defect densities, over areas Ͼ1 cm 2 , without the use of a template.Attempts to grow Si wire arrays did not yield high pattern fidelity when the catalyst was not confined. Wires were grown by photolithographically patterning S1813 photoresist ͑Microchem͒ on a clean Si͑111͒ wafer, then exposing it for 5 s to buffered HF͑aqueous͒ ͑Transene, Inc., 9% HF, 32% NH 4 F͒, followed by evaporation of 500 nm of Au and lift-off of the resist. This produced a square array of 3 m diameter Au islands with a center-to-center pitch of 7 m. Samples were then annealed in a tube furnace at 900-1000°C for 20 min under 1 atm of H 2 at a flow rate of 1000 SCCM ͑SCCM denotes cubic centimeters per minute at STP͒, followed by wire growth under 1 atm of H 2 and SiCl 4 , at flow rates of 1000 and 20 SCCM, respectively. This produced arrays of low fidelity, with no control over the wire diameter or wire position ͑not shown͒. Examination of the samples after a 20 min H 2 anneal only revealed that this behavior was due to substantial agglomeration of the catalyst ͑Fig. 1͒.The successful production of large-area Si wire arrays involved the use of an oxide buffer layer to confine the VLS catalyst to the desired areas in the pattern. To implement this approach, a 300 nm oxide was ther...

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Over the past decade, the global cumulative installed photovoltaic (PV) capacity has grown exponentially, reaching 591 GW in 2019. Rapid progress was driven in large part by improvements in solar cell and module efficiencies, reduction in manufacturing costs and the realization of levelized costs of electricity that are now generally less than other energy sources and approaching similar costs with storage included. Given this success, it is a particularly fitting time to assess the state of the photovoltaics field and the technology milestones that must be achieved to maximize future impact and forward momentum. This roadmap outlines the critical areas of development in all of the major PV conversion technologies, advances needed to enable terawatt-scale PV installation, and cross-cutting topics on reliability, characterization, and applications. Each perspective provides a status update, summarizes the limiting immediate and long-term technical challenges and highlights breakthroughs that are needed to address them. In total, this roadmap is intended to guide researchers, funding agencies and industry in identifying the areas of development that will have the most impact on PV technology in the upcoming years.