Size-selected growth of transparent well-aligned ZnO nanowire arrays

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%.

To grow well-aligned ZnO nanowire arrays (ZNAs), various synthesis methods have been utilized, such as thermal evaporation [7], chemical vapor transport and condensation [18], and vapor-liquid-solid growth [20]; 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 wellaligned ZNAs as electroluminescent diodes and hybrid solar cells [8][9][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 wellaligned ZNAs grown via the chemical bath deposition method. However, up to now, only a few works on sizecontrolled 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.

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(CH 3 COO) 2 Á2H 2 O) 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.
Then, the ZNAs were grown in 200 mL of equimolar aqueous solution of Zn(NO 3 ) 2 Á6H 2 O and hexamethylenetetramine (C 6 H 12 N 4 ) in a conventional reaction flask. The ZnO seed layer-coated ITO substrate was immersed into the aqueous solution upside-down in the flask; the pH value of the growth solution was approximately 6.0 by adding aqueous ammonia. Various growth parameters such as concentration of precursors, growth temperature, and growth time were used in our experiments. Table 1 summarizes major experiments carried out in this work. After growth, the substrates were removed from the solution, rinsed with deionized water, and dried at 50°C in air.

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 MoO 3 was deposited on the P3HT:PCBM layers under a pressure of 3 × 10 −3 Pa as a buffer layer [24], followed by the deposition of 200-nm-thick Ag electrodes through a shadow mask.

Results and discussion
In this study, we found that the different growth temperatures strongly influenced the nanowire length and density of ZNAs. Figure 1 shows the top-view, highmagnification (shown in the insets), and cross-sectional FESEM images of samples A, B, and C. There are only wirelike features with a hexagonal cross section for all samples, which are aligned in a dense array approximately perpendicular to the substrate surface. The ZNAs uniformly covered the entire substrate surface based on the FESEM result. The mean values of the nanowire dimension and array density for samples A, B, and C were estimated from a statistical evaluation of FESEM images and are summarized in Table 2. We note that with the enhancement of growth temperature, both the average length and array density obviously increase, and the average diameter of nanowires almost displays no change.
The effect of the growth time on the morphology and density of ZNAs was also investigated, as shown in Figure 2. It can be seen that the highly uniform and densely packed arrays of ZnO nanowires are successfully formed for samples D, E, and F. The mean values of the nanowire dimension and array density for samples D, E, and F are also summarized in Table 2. Comparing the average length, diameter, and array density among samples D, C, E and F, respectively, it can be seen that both the average diameter and array density slowly increase with elongating growth time. The average length rapidly increases as the growth time elongates from 1 to 3 h. However, further elongating the growth time, the average length shows no change. Figure 3 shows top-view, high-magnification (shown in the insets), and cross-sectional FESEM images of samples G, H, and I. The results reveal that well-aligned ZNAs are also obviously formed for samples G, H, and I. Also, the mean values of the nanowire dimension and array density for samples G, H, and I are summarized in Table 2. It can be seen that both the average diameter and length of the nanowires increase with the increased concentration of precursors from 0.01 to 0.1 M, as seen in samples G, C, H, and I in Table 2. The array density rapidly increases as the concentration of precursors ranged from 0.01 to 0.05 M. However, further increasing the concentration of precursors up to 0.1 M, the array density of ZNAs sharply decreases, which is attributed  to the average diameter of the nanowires that becomes too large.
The optical transparency of all samples fabricated on ITO substrates was investigated. Figure 4 shows the optical transmittance spectra of samples A to I. With the exception of sample I, all of the samples show good optical transparency in the visible spectral range, with a transmittance above 80%. However, sample I displays a lower transmittance over a broad range of wavelengths as compared to other samples. This was due to the increase in light scattering from the large diameter of ZnO nanowires in sample I, which is consistent with the FESEM results. Figure 5 shows the XRD patterns of sample C. The XRD analysis reveals that all of the diffraction peaks of ZNAs can be indexed as those from the known wurtzite-structured (hexagonal) ZnO (JCPDS card no. 36-1451) and the (002) diffraction peak exhibits a substantially greater intensity. The result further demonstrates that as-grown ZNAs are much better aligned on the substrate and the nanowires exhibit preferred growth along the (0001) direction.
Finally, to testify the application of ZNAs for optoelectronic application, a hybrid solar cell (HSC) was built using sample C with a structure of sample C/P3HT:PCBM/ MoO 3 /Ag. Figure 6 shows the typical current densityvoltage (J-V) characteristics of the HSC in the dark and under AM 1.5 illumination with a light intensity of about 100 mW/cm 2 . The device exhibits an open-circuit voltage of 0.51 V, a short-circuit current of 4.53 mA/cm 2 , a fill factor of 39%, and a power conversion efficiency of 0.91%.

Conclusions
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, lowcost, and high-quality ZNA-based optoelectronic devices in the near future. no. 20090185110020).