Size-selected growth of transparent well-aligned ZnO nanowire arrays
© Yu et al.; licensee Springer. 2012
Received: 24 August 2012
Accepted: 15 September 2012
Published: 21 September 2012
This paper reports the effect of precursor concentration, growth temperature, and growth time on the size and density of ZnO nanowire arrays (ZNAs). The well-aligned ZNAs were grown on indium tin oxide substrate using a facile chemical bath deposition method. The results showed that the ZnO nanowires could be tailored to the desired sizes with a simple variation of the growth parameters. Optical transmission spectra revealed a sufficient transparency of the ZNAs, qualifying them for photovoltaic and other optoelectronic applications. An inverted hybrid solar cell was fabricated using the ZNAs as the electron collecting layer, and the solar cell exhibited a power conversion efficiency of 0.91%.
KeywordsZnO nanowire arrays Chemical bath deposition Optical transmission Hybrid solar cell
As an important wide-bandgap semiconductor, ZnO possesses remarkable optical, electrical, and optoelectronic properties, thus being of immense research interest[1–4]. Recently, well-aligned ZnO nanowire or nanorod arrays have been extensively studied as a promising candidate for applications in electroluminescent devices[5, 6], field emission devices, solar cells[8–14], nanogenerators[15–17], and chemical sensors[18–20].
To grow well-aligned ZnO nanowire arrays (ZNAs), various synthesis methods have been utilized, such as thermal evaporation, chemical vapor transport and condensation, and vapor–liquid-solid growth; the complex process, sophisticated equipment, and high temperatures make it hard to use them on a wide range of substrates. In contrast, the chemical bath deposition method shows its great advantages due to its much easier operation, very low temperature (≤ 95°C) growth, high potential for scale-up, and low-cost. The facile and low-temperature growths favor the applications of well-aligned ZNAs as electroluminescent diodes and hybrid solar cells[8–10]. Previous reports[21, 22] indicate that the sizes (diameter and length) of ZnO nanowires or nanorods play an important role on the performance of building ZNA hybrid solar cells. In light of our experimental observations, several factors, including concentration of precursors, growth temperature, and growth time, had great effect on the sizes and density of well-aligned ZNAs grown via the chemical bath deposition method. However, up to now, only a few works on size-controlled and transparent well-aligned ZNAs are available[21, 23].
In this work, we present size-controlled and transparent ZNAs by a facile chemical bath deposition method. It is shown that the nanowires can be tailored to the desired sizes with a simple variation of the growth parameters. The optical transmission of the obtained well-aligned ZNAs in the visible wavelength region was also discussed. Exemplarily, a hybrid solar cell was constructed and fabricated, consisting of the as-prepared ZNAs and polymer hybrid bulk heterojunction architecture.
Materials and substrates
Regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT, 99.5%) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM, 99.5%) were purchased from Lumtec (Hsin-Chu, Taiwan) and used as received. Other chemicals (Kelong Chemical Agent, Chengdu, China) used in our experiments were of analytical reagent grade without further purification. All the aqueous solutions were prepared using distilled water (resistivity = 18.2 MΩ cm). The indium tin oxide (ITO)-coated glasses (10 Ω/sq) were used as the substrate and first cleaned by ultrasonic agitation in detergent, then deionized water, acetone, and ethanol, successively. The cleaned substrates were then blown dry using nitrogen gas and treated with O2 plasma for 5 min.
Preparation of ZNAs
ZNAs were prepared by the following two-step process, and the growth of the ZNAs was conducted following our previous work[24, 25]. Briefly, zinc acetate dehydrate (Zn(CH3COO)2·2H2O) was dissolved in ethanol with a concentration of 5 mM. A droplet of the solution was coated onto treated ITO substrates, rinsed with clean ethanol after 10 s, and then blown dry with a stream of nitrogen gas. This coating step was repeated several times. The coated substrates were dried at room temperature and then annealed at 350°C for 20 min in air to yield the multilayers of ZnO seed. The zinc acetate deposition and decomposition procedure were carried out twice to ensure a complete and uniform coverage of ZnO seeds.
Growth parameters of ZnO nanowire arrays
Growth temperature (°C)
Growth time (h)
Concentration of precursors (M)
Preparation of ZNA/polymer hybrid solar cell
A hybrid solar cell was fabricated using the following procedure. The organic layers consisted of P3HT:PCBM (1:1 by weight ratio) were spin-coated on the top of sample C from a chlorobenzene solution with a concentration of 30 mg/ml in air, then annealed at 110°C for 15 min under vacuum (≤133 Pa). The 3-nm-thick MoO3 was deposited on the P3HT:PCBM layers under a pressure of 3 × 10−3 Pa as a buffer layer, followed by the deposition of 200-nm-thick Ag electrodes through a shadow mask.
The morphology of as-grown ZNAs was analyzed using a field-emission scanning electron microscope (FESEM, Hitachi S-4800, Chiyoda-ku, Japan); X-ray diffractometry (XRD) data were obtained on a Philips X'pert Pro MPD diffractometer (CuKα radiation, λ = 1.54056 Å, Amsterdam, The Netherlands). Optical transmission was measured by a UV–vis spectrophotometer (SHIMADZU UV1700, Kyoto, Japan). The current–voltage characteristics of as-prepared solar cells in the dark and under illumination were recorded with a Keithley 4200 programmable voltage–current source (Cleveland, OH, USA)[26–28]. A solar simulator equipped with a xenon lamp with an illumination power of 500 W (CHF-XM35, Beijing Trusttech Co. Ltd., Beijing, China) was used as the light source. All measurements were performed under ambient conditions without device encapsulation.
Results and discussion
Mean values of the nanowire diameter, length, and array density of the samples
In summary, well-aligned ZNAs were grown on ITO substrates by a facile chemical bath deposition method. Size-selected ZnO nanowires were achieved by changing the precursor concentration, growth temperature, and growth time. The well-aligned ZNAs showed good optical transparency in the visible spectral range, and the nanowires exhibited an excellent crystal quality. A hybrid solar cell composed of the investigated ZNAs with a reasonable power conversion efficiency were realized. This work will be beneficial to develop large-area, low-cost, and high-quality ZNA-based optoelectronic devices in the near future.
This work was partially supported by the National Natural Science Foundation of China (NSFC) via no. 61177032, the Foundation for Innovation Groups of NSFC via no. 61021061, the Fundamental Research Funds for the Central Universities (grant no. ZYGX2010Z004), SRF for ROCS, SEM (grant no. GGRYJJ08-05), and Doctoral Fund of Ministry of Education of China (grant no. 20090185110020).
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