Solution-Derived ZnO Nanowire Array Film as Photoelectrode in Dye-Sensitized Solar Cells Yanfeng Gao,*,†,‡ Masayuki Nagai,† Tien-Chih Chang,§ and Jing-Jong Shyue§ AdVanced Research Laboratories, Musashi Institute of Technology, Tokyo 158-0082, Japan, Research Center for Industrial Ceramics, Shanghai Institute of Ceramics, Chinese Academy of Sciences (CAS), Dingxi 1295, Changning, Shanghai 200050, China, and Research Center for Applied Sciences, Academia Sinica, 128 Academia Road, Section 2 Nankang, Taipei 115, R. O. C.
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2467–2471
ReceiVed December 21, 2006; ReVised Manuscript ReceiVed July 16, 2007
ABSTRACT: This paper reports the effects of the aspect ratio of zinc oxide (ZnO) nanowires on the performance of ZnO-nanowirebased dye-sensitized solar cells (DSSCs). ZnO nanowire-structured photoanodes can improve the efficiency of the electron collection of DSSCs, but their performances significantly depend on the aspect ratio of component nanowires and their array structures. The aspect ratio of nanowires has been successfully regulated by controlling the supersaturation degree of solutions, that is, simply by changing the molar ratio of Zn(II)/NH3. A highly oriented, single crystalline, long ZnO nanowire with a fine aligning structure was obtained with an aspect ratio of about 100–120 (diameter: 120–150 nm, length: 14 µm). The main crystalline phase measured by X-ray diffraction and Raman scattering was proven to be wurtzite-type ZnO, whereas the appearance of another phase was also detected. The films show a transmittance of about 60% in the visible light region and optical band gaps at around 3.2 eV. An overall conversion efficiency of about 1.7% was obtained, which is almost three times of that we reported previously. The present research points out a possible way to improve ZnO-based DSSCs by engineering a nanostructured electrode. Introduction 1–3
such Low-dimensional zinc oxide (ZnO) nanostructures as nanosheets, nanowires, and nanorods have been attractive recently because of their specific optoelectronic and filedemission characteristics originating from their unique heterogeneous crystallographical structures.4–6 Films with well-aligned ZnO nanosheets, nanorods, or nanowires may exhibit large surface areas. Moreover, these superior nanoelements of ZnO single crystals are packed very densely and free of grain boundaries, enabling them to serve as a fast and effective path for electron transportation. Very recently, either dye-sensitized solar cells (DSSCs) or hybrid solar cells fabricated using ZnO nanostructured films as photoelectrodes have attracted much attention, and several pieces of pioneering work have been reported.5–7 Two-dimensional (2D) and more complex nanostructures were also proven to be promising as photoanodes in high-efficiency DSSCs.4,5 Hosono et al. prepared an upright-standing ZnO precursor from solution that subsequently transformed to ZnO while maintaining the original morphology, and a high conversion efficiency of about 3.9% was obtained.4 This achievement in the conversion efficiency is attributed to the specific microstructure of ZnO film. Minoura and Yoshida et al. have been making efforts in studies of electrodepositing ZnO photoelectrodes in the presence of organic dye molecules.5 Various kinds of ZnO/dye hybrid nanostructures with finely controlled morphologies were fabricated at low temperatures.5 The further improvement of performance of DSSCs was also possible by readsorption of organic dye molecules.5a More recently, one-dimensional (1D) ZnO nanowire arrays have gained much attention for use as photoanode materials, and a few important studies have been landmarks for the research field. Law et al. reported a solution-based seeded * Corresponding author. E-mail:
[email protected], sc.musashi-tech.ac.jp. † Musashi Institute of Technology. ‡ Chinese Academy of Sciences (CAS). § Academia Sinica.
yfgao@
process by employing a trace amount of polyethylenimine to grow long, oriented ZnO nanowire arrays and systematically investigated the performance of DSSCs.6 The nanowire photoanode should have better performance than the nanoparticlulate one because of its fast electron injection efficiency and better electron transport. The former is caused by a change in the kinetics of charge transfer. The latter is a product of high crystallinity and of an internal electric field that can assist carrier collection.6 Peiro´ et al. studied the hybrid polymer/ZnO solar cells, in which the metal oxide consists of ZnO columnar structures.7 The interaction between a seed layer and the growth solution affected the morphological and structural characteristics of columnar ZnO films. Although an oriented, continuous, and crystalline ZnO such as a nanorod/nanowire array could greatly improve the contact between ZnO and organic polymers, and increase the ability to collect both electrons and holes, the achievement of this target still needs to optimize the microstructure of ZnO electrodes and improve their interface contacts both between polymer and ZnO and between the film and the substrate. This kind of nanostructure is usually prepared by low-cost, solution-based deposition processes.3–14 The fundamentals of these methods involve a seeded substrate that is prepared by either formation of a nanoparticlate ZnO layer3,6,8–14 or directly using metallic zinc,9 and treatment of this substrate in a heated solution containing Zn2+ and a precipitation agent; the latter may be NH3, urea, amine, and others. To realize the preferable growth along a longitudinal direction, especially to regulate the aspect ratio of the produced nanorod/nanowire structures, the degree of supersaturation of the solution should be well controlled, so that the growth of the pre-existing particles is favorable compared to the formation of new nuclei. The oriented growth was also achieved by selective adsorption of modification agents to specific crystalline planes of ZnO nuclei and/or the subsequently growing particles, which prohibits the growth along these directions, resulting in heterogeneous growth.8a In previous research, we developed a method for the reproducible and easy preparation of a ZnO rod array along
10.1021/cg060934k CCC: $37.00 2007 American Chemical Society Published on Web 11/03/2007
2468 Crystal Growth & Design, Vol. 7, No. 12, 2007
Gao et al.
Experimental Procedures
Figure 1. XRD patterns of FTO substrate and ZnO nanowire films.
with other nanostructured ZnO films on transparent conductive oxide substrates and confirmed the effects of their morphologies on the performance of solar cells.15 The solar cell using nanowire arrays as the photoelectrode shows the best conversion efficiency compared to those using other microstructured ZnO films. However, the efficiency of about 0.6% at 1 sun is still low. The reasons were partially assigned to the aspect ratio of ZnO rods (about 500 nm in diameter and 10 µm in length). Increasing the aspect ratio, that is, increasing the length while decreasing the diameter of individual nanorod, may obviously enlarge the surface area of the electrodes, which could increase the amount of dye loading and therefore enhance the photoactivity. The mixture complex agent we used for the preparation of ZnO has been ammonia and amine. Zn(II) can react and/or form complexes with NH3 under different concentrations of NH3. Regulating the relative amount of Zn(II)/NH3 may alter the reaction route between them and change the supersaturation degree of Zn(II) precursors to precipitate ZnO, which could reflect the changes in aspect ratios of ZnO nanowires. In this paper, we report the success in controlling the aspect ratio of ZnO nanowires; a nanowire align film consisting of individual nanowires of about 100–120 nm in diameter and 14 µm in length has been obtained. The conversion efficiency of the DSSC using this film was measured to be 1.7% under 1 sun, representative of an increase of almost three times of that we reported previously.
Figure 2. Partially enlarged XRD patterns of Figure 1.
Electrodeposition of ZnO Seed Layers. A two-step deposition process as reported was employed in this research. For the electrodeposition of a ZnO seed layer, fluorine-doped SnO2 (FTO)-coated glass plates were used as substrates (Resistivity:
