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2008, 112, 8516–8520 Published on Web 05/20/2008
Fabrication and Electrochemical Photovoltaic Response of CdSe Nanorod Arrays Martin Schierhorn,† Shannon W. Boettcher,† Anna Ivanovskaya,† Emily Norvell,‡ Jessica B. Sherman,§ Galen D. Stucky,† and Martin Moskovits†,* Department of Chemistry & Biochemistry, UniVersity of California, Santa Barbara, California 93106, Department of Materials Engineering, California Polytechnic State UniVersity, San Luis Obispo, California 93407, and Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky 40506-0055 ReceiVed: March 26, 2008; ReVised Manuscript ReceiVed: April 23, 2008
CdSe nanorod electrode arrays with the nanorods aligned normal to the substrate were fabricated using porous aluminum oxide (PAO) templates. Photovoltaic characteristics were determined electrochemically in an aqueous solution using polysulfide (Sn2-) as the redox mediator. Isolating the back electrode from the electrolyte with a 100-nm-thick TiO2 barrier layer increased the open circuit voltage from -0.23 to -0.34 V and the fill factor from 0.42 to 0.57. Depending on the electrolyte concentration, IPCE values between 2.5 and 8% were observed at an incident wavelength of 500 nm. Internal quantum efficiencies were estimated to approach 50%. This architecture could be beneficial for the fabrication of bulk-heterojunction photovoltaic devices in conjunction with solution-deposited semiconducting inorganic nanoparticles or organic semiconductors. 1. Introduction The rising demand for clean, renewable energy has led to increased interest in solar energy conversion.1,2 Much effort has been dedicated to the development of materials that are costeffective, easily processable, and/or exhibit novel opto-electronic properties3,4 such as nanoparticulate inorganic materials,5 conducting polymers,6,7 and hybrids thereof.8,9 Photovoltaic devices based on most of these materials rely on the diffusion of photogenerated excitons to an interface where charge separation occurs due to offset energy bands.7 Unfortunately, the exciton diffusion length is often significantly smaller than the absorption length, which, for planar geometries, leads to increased recombination and hence reduced efficiencies.10 This problem can be solved by forming a nanoscale interpenetrating junction network (bulk heterojunction) so that the distance from the site of photocarrier generation to a separating interface is on the order of, or less than, the exciton diffusion length. The consequential decoupling of light absorption from carrier collection then allows for optimization of both parameters independently.9 Several bulk heterojunctions have been reported previously, including polymer blends6,11 and cells based on blends between organic molecules and inorganic particles such as TiO2,12–14 ZnO,8,15,16 PbS,17 PbSe,18 and CuInS2.19 Most of these systems rely on random, isotropic blends, which result in circuitous conduction pathways, charge trapping, and decreased carrier mobilities.8 More ordered, anisotropic geometries have been shown to increase efficiencies.20,21 It was further found that both the shape of the inorganic particles and the packing of the * Corresponding author. Phone: +1(805) 893-4070; fax: +(805) 8934120; e-mail:
[email protected]. † Department of Chemistry & Biochemistry, University of California, Santa Barbara, CA 93106. ‡ Department of Materials Engineering, California Polytechnic State University, San Luis Obispo, CA 93407 § Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055
10.1021/jp802624j CCC: $40.75
polymer chains contribute to efficient charge transport.22 The ideal geometry for bulk heterojunctions consists of straight conduction pathways toward the electrodes, as would be the case for systems that contain arrays of nanorods aligned perpendicular to a conductive substrate.9,23 Several advances in fabricating nanorod arrays normal to their substrates have been reported, but the task of controlling the dimensions and exact geometry is generally difficult and/or expensive.8,24–27 Templating methods such as those using porous aluminum oxide (PAO)28–30 have been explored for the fabrication of nanorods as well as nanorod arrays. Advantages of using PAO over other fabrication routes include the following: 1. The channel diameters, and consequently the dimensions of the materials fabricated within them, can be controlled by straightforward variations in growth conditions and through postsynthesis treatments. 2. A great range of materials can be deposited into the pores of PAO either electrochemically or by other routes.31–38 3. Electrochemical deposition into PAO templates inherently results in good electrical contact between the nanorods and the back electrode, which is important for charge carrier collection. 4. The straight, easily accessible interstitial space between the nanorods could facilitate the incorporation of a secondary phase such as semiconducting inorganic nanoparticles or organic semiconductors. There are many reports on nanorod syntheses using PAO; however, only few describe their use as photovoltaic devices.27,32,39,40 Furthermore, the existing studies focus on nanorods with dimensions that are too large to be suitable for maximum solar energy conversion in conjunction with semiconducting nanocrystals or organic semiconductors.9,23 Here, we report on the fabrication of semiconductor nanorod arrays in PAO templates and demonstrate their use as electrodes for photovoltaic applications. We chose to study CdSe as a model system for two reasons. First, CdSe is a direct bandgap semiconductor with a bandgap of 1.74 eV in the bulk, which provides almost ideal absorption of the solar spectrum.41 Second, 2008 American Chemical Society
Letters
Figure 1. (A) Schematic showing the templated fabrication of CdSe nanorod arrays: (1) anodization of aluminum foil to fabricate PAO; (2a) evaporation of the Au back electrode; (2b) evaporation of a TiO2 layer followed by the evaporation of Au; (3) removal of Al and opening of the channels; (4) electrodeposition of CdSe nanorods; (5) application of a copper contact, encasing with epoxy and removal of the PAO template. Devices labeled as A consisted of CdSe nanorods in direct contact with Au, whereas devices labeled as B contain a TiO2 layer between the nanorods and the Au electrode.
the positions of the band edges42 are suitable to form chargeseparating heterojunctions with several relevant organic43,44 and inorganic semiconductors5,42,45,46 (Figure S1). Electrochemical photovoltaic characteristics of the nanorod array electrodes were determined using an aqueous electrolyte,47 which provides information about their suitability as components in bulkheterojunction devices. Nanorod dimensions were selected to be appropriate for such applications.9,23 The effect of isolating the back electrode from the electrolyte via a TiO2 layer on both the open circuit voltage (VOC) as well as the fill factor (FF) was investigated. 2. Experimental Methods Preparation of PAO Templates (Figure 1). High-purity aluminum foil (99.997%, Alfa Aesar) was annealed and electropolished in EtOH/HClO4 at 45 V for 30-60 s. Highly ordered PAO was produced using a two-step anodization procedure.48 First, Al was anodized in 0.3 M oxalic acid at 15 °C and 40 V DC for 3 h with a planar graphite sheet serving as the cathode. The PAO was removed with H3PO4/H2CrO4 (6: 1.8 wt %) at 65 °C for 2 h, followed by a second anodization under identical conditions for 12 h to obtain pores of ∼80 µm in length and ∼40 nm in diameter. Next, 20 nm of Ti followed by 500 nm of Au were evaporated onto the channel openings (samples A). For samples containing a TiO2 layer (samples B), 100 nm of titania were evaporated directly onto the PAO, immediately followed by evaporation of Ti/Au (20 nm/500 nm). Next, the aluminum was removed by etching with a solution containing 0.44 M CuCl2 (EM Sciences) and 1.81 M HCl. The backside of the PAO channels was opened with a BCl3/Cl2 plasma using an ICP etcher (Panasonic E640). Templates containing a TiO2 layer were annealed at 400 °C for 1 h. Preparation of CdSe Nanorod Electrodes (Figure 1). Nanorods were deposited electrochemically from an aqueous solution containing 0.7 mM SeO2 (Sigma-Aldrich), 0.3 M CdSO4 (CdSO4 · 8/3 H2O, Sigma-Aldrich), and 0.25 M H2SO4 (Figure S2).33,49 The potential was swept between -0.357 and -0.757 V versus a Ag/AgCl counter electrode at a rate of 0.75 V/s for 2000 cycles using a potentiostat (Princeton Applied
J. Phys. Chem. C, Vol. 112, No. 23, 2008 8517 Research, Model 273A). A Pt mesh served as the counter electrode. The solution was not stirred during deposition. A current-voltage trace obtained during the cyclic electrodeposition is included in the Supporting Information (Figure S3). (Caution: both CdSO4 and SeO2 are highly toxic.) Samples were rinsed in DI water, dried in air, and annealed at 400 °C for 0.2 h in Ar (ramping rate of 2 °C/min). Next, Cu tape was attached to the Au backing using silver paste and the devices were isolated with inert epoxy (Loctite 1C Hysol). The PAO template was removed by soaking in 5 M NaOH for 2.5 h. The samples were rinsed with DI water and tested immediately thereafter. Incident-Photon-To-Current-Efficiency (IPCE) Measurements. Quantum efficiencies on samples with an active area of ∼4 mm2 were measured in a custom-built air-free electrochemical cell with an optical path length of 4 mm (Figure S4). The electrolyte contained 0.2 M NaOH, 0.2 M Na2S, and 0.2 M S and strongly absorbs at wavelengths
