Published: May 23, 2011 r2011 American Chemical Society 2490 dx.doi.org/10.1021/nl200965j | Nano Lett. 2011, 11, 2490–2494 LETTER pubs.acs.org/NanoLett Patterned Radial GaAs Nanopillar Solar Cells Giacomo Mariani,* ,† Ping-Show Wong, † Aaron M. Katzenmeyer, § Francois L  eonard, § Joshua Shapiro, † and Diana L. Huffaker †,‡ † Electrical Engineering Department and ‡ California NanoSystems Institute, University of California at Los Angeles, Los Angeles, California 90095, United States § Sandia National Laboratories, Livermore, California 94551, United States N anostructured solar cells have gained much attention due to light trapping effects that drastically reduce the portion of reflected photons, therefore enhancing the optical absorption. Nanodomes, 1 nanocones, 2 nanoparticles, 3 and nanowires 4À9 (NWs) have the potential to improve performance compared to standard solar cells. The high surface-to-volume ratio increases the photoactive junction area and facilitates the carrier collection, enhancing the power conversion efficiency (PCE) with respect to bulk solar cells. In particular, compared to top-down 10 ap- proaches, a bottom-up growth mode may lead to high-quality composition, crucial for any photovoltaic device. Furthermore, by exploiting radial junctions in each single NW, the light absorption (vertical direction) is decoupled from the carrier collection (radial direction), an unresolved issue in conventional planar solar cells. Recently, silicon microwire solar cells 9 grown as ordered arrays have achieved PCE of 7.9%. However, due to the low optical absorption coefficient of silicon, long nanowires (∼60 μm) are required to absorb the above-band-gap photons. IIIÀV, direct band gap technology requires ∼1 μm of material to efficiently absorb the photons and integration of radial multi- junctions becomes possible too. In addition, GaAs crystalline single junction solar cells 11 have recently demonstrated the highest PCE (up to 26.4%) in monojunction photovoltaics. Despite that, only one reported work regarding radial GaAs nanostructures, 12 based on Au-catalyzed vaporÀliquidÀsolid (VLS) growth, demonstrates coreÀshell NW growth for photo- voltaic applications. These photovoltaic devices exhibit low PCEs (0.83%), possibly due to the random distribution in terms of height, diameter, and position. Furthermore, the midgap trap states introduced by diffusion of Au catalyst into the NW 13 degrade minority carrier lifetime and diffusion length on which photovoltaic devices rely. For successful devices, NW diameter, length, and filling ratio have to be carefully chosen and kept constant by the adoption of a regular geometry. 14 The less semi- conductor material required to absorb the same amount of photons with respect to thin films in conjunction with the possibility to peel off the NWs from the substrate and reuse it in subsequent growths, open up promising routes to low-cost, flexible solar cells. One approach to realizing a patterned architecture is to take advantage of lithography techniques to precisely define radius and center-to-center pitch in a mask from which the NWs can be grown. Such a mask translates into a uniform NW growth, allowing ease of processing/fabrication and avoiding adjacent NW junctions from contacting each other during growth. In addition, the small NW cross sections that can be realized allow the growth of junctions of dissimilar materials with high lattice-constant mismatch, permitting a prompt inte- gration of heterogeneous material platforms. Device structure and geometry, nonetheless, are not sufficient to ensure high-efficiency nanostructured photovoltaics: each interface must be carefully designed, including the nature of the contacts. This is especially true for degenerately doped semi- conducting oxides such as indium tin oxide (ITO) or aluminum zinc oxide (AZO). Their properties can profoundly modify carrier extraction/injection mechanisms once they come into contact with p-doped and n-doped semiconductors. This study discusses in detail the performance, in terms of electrical and Received: March 23, 2011 Revised: May 18, 2011 ABSTRACT: Photovoltaic devices using GaAs nanopillar radial pÀn junctions are demonstrated by means of catalyst-free selective-area metalÀorganic chemical vapor deposition. Dense, large-area, lithographically defined vertical arrays of nanowires with uniform spacing and dimensions allow for power conver- sion efficiencies for this material system of 2.54% (AM 1.5 G) and high rectification ratio of 213 (at (1 V). The absence of metal catalyst contamination results in leakage currents of ∼236 nA at À1 V. High-resolution scanning photocurrent microscopy measurements reveal the independent functioning of each nanowire in the array with an individual peak photocurrent of ∼1 nA at 544 nm. External quantum efficiency shows that the photocarrier extraction highly depends on the degenerately doped transparent contact oxide. Two different top electrode schemes are adopted and characterized in terms of Hall, sheet resistance, and optical transmittance measurements. KEYWORDS: Nanowire, catalyst-free growth, solar cell, transparent contact, ITO, AZO