Journal of Crystal Growth 304 (2007) 80–85 Control of ZnO nanorod array alignment synthesized via seeded solution growth Yun-Ju Lee à , Thomas L. Sounart 1 , David A. Scrymgeour, James A. Voigt, Julia W.P. Hsu Sandia National Laboratories, P.O. Box 5800, MS-1082, Albuquerque, NM, USA Received 3 October 2006; received in revised form 7 January 2007; accepted 8 February 2007 Communicated by K. Nakajima Available online 5 March 2007 Abstract The critical factors that determine the degree of alignment of dense ZnO nanorod (NRAs) arrays synthesized via a two-step seeding and solution growth process were systematically examined, with a goal of optimizing the array density and surface area for hybrid organic–inorganic photovoltaic (PV) devices. Unexpectedly, we found that the degree of alignment of ZnO nanorod arrays depended strongly on the ambient humidity level during the seeding step. Close-packed ZnO nanorod arrays with [0 0 1] axis perpendicular to the substrate were obtained only when seeded at 420% relative humidity (RH) at room temperature (RT), according to data from scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) spectroscopy. In addition, when the seeding RH was raised to 460%, the polydispersity in the diameter and length of the ZnO nanorods increased, although good alignment was maintained. As a result, we determined an optimal seeding humidity of 30–40% for the synthesis of ordered ZnO nanorod arrays. Atomic force microscopy and contact angle measurements revealed that the density of ZnO seeds was significantly lower below the 20% relative humidity threshold, indicating that water vapor plays a critical role in generating insoluble zinc hydrolysis products that act as nucleation sites for the subsequent growth of ZnO nanorods. Finally, we found that the alignment of the ZnO nanorod arrays was strongly disrupted as the roughness of the underlying fluorinated tin oxide substrate increased. The improved control of the morphology of ZnO nanorod arrays may lead to improved carrier collection of conducting polymer-ZnO nanorod array hybrid PV devices. r 2007 Elsevier B.V. All rights reserved. PACS: 81.07.b; 68.65.k; 78.67.n; 71.55.Gs; 81.15.Lm Keywords: A1. Crystal morphology; A1. Nanostructures; A3. Liquid phase epitaxy; B1. Zinc compounds; B2. Semiconducting II–IV materials; B3. Solar cells 1. Introduction Hybrid polymer-inorganic photovoltaic (PV) devices, consisting of a hole transporting conjugated polymer and an electron transporting inorganic material in close contact, represent a promising opportunity for fabricating low-cost, large-area solar cells. When all processing steps are conducted at or near room temperature, it may be possible to integrate the PV fabrication with flexible plastic substrates [1], further reducing cost and enabling novel applications. Current hybrid PV devices utilize a variety of electron transporting materials, including fullerene deriva- tives [2,3], chalcogenide nanorods [4], and oxide nanopar- ticles [5] and extended nanostructures [6], with a current maximum solar polar conversion efficiency of 4.4% [3]. Compared to the alternatives, oxides have the advantages of better thermal and environmental stability, tunable band structures, and higher electron mobilities. In particular, highly aligned ZnO nanorod arrays (NRAs) can be grown at low temperature in dilute aqueous solutions and may serve as the electron transporter in conjugated polymer– oxide hybrid PV devices due to low fabrication cost, high surface area, and opportunities for bandgap engineering. ARTICLE IN PRESS www.elsevier.com/locate/jcrysgro 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.02.011 à Corresponding author. Tel.: +1 505 284 4397; fax: +1 505 844 8985. E-mail address: ylee@sandia.gov (Y.-J. Lee). 1 Current address: Intel Corporation, 4500 Dobson Road, Chandler, AZ 85248, USA.