The schematic structure of flexible DSC is shown in Fig. 1.

Fig. 1

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	Fig. 1 Schematic structure of flexible DSC1. ITO-PET; 2. ZnO seeded layers; 3. Dye-sensitized ZNRs and electrolyte solution; 4. Platinum electrode; 5. ITO-PET or the FTO-glass

ZnO nanorod arrays (ZNRs) were grown by a typical two-step method^{[ 5, 6, 7, 8, 9, 10, 11]}, which comprised of ZnO seed layer deposited on ITO-PET (15 Ω/□, Peccell) by spin-coating and ZNRs grew on the seed layer by hydrothermal method. Firstly, ZnO transparent colloidal solution was obtained by a Sol-Gel method. Zinc acetate dehydrate (ZAC) (0.01 mol/L) was dissolved in methanol at 60℃ under vigorous stirring. Then a sodium hydroxide (NaOH) solution in methanol (0.03 mol/L) was dropwisely added and stirred at 60℃ for 2 h. The volume ratio of ZAC solution of methanol and NaOH solution of methanol was 2:1. After cooling to room temperature, the coating solution was obtained. Then the coating solution was spin-coated onto ITO-PET at 1500 r/min for five times according to the expected thickness of ZnO seed layers, between every two layers, the seeded ITO-PET was subsequently baked for about 10 min in an oven at 60℃. At last, it was baked in an oven at 120℃. The second step was the growth of ZNRs on the seeded ITO-PET by hydrothermal method. In this experiment, two growth conditions including the reaction time and the precursor concentration were adjusted to investigate their influence on the morphology of ZNRs. The precursor solution was prepared by mixing the zinc nitrate (Zn(NO₃)₂) with methenamine (C₆H₁₂N₄) while keeping their volume ratio at 1:1. The hydrothermal growth was carried out at 95℃ in a general purpose autoclave (80 mL) by immersing the seeded substrates in the precursor solutions (60 mL). After several hours’ reaction, the substrates were taken out and rinsed with deionized water and then dried at 60℃ in air.

Flexible DSCs were assembled by a simple process as follows. The dye-sensitized photoanodes were prepared by adsorbing dye N-719 (bis-tetrabutylammonium cis-bis (isothiocyanato) bis(2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium(II)) onto ZNRs. The platinized FTO glass used as counter electrode was placed on the top of the photoanode and sealed with 30 μm thickness thermal adhesive films (Bynel, DuPont, USA). The electrolyte solution (0.6 mol/L tetrapropylammonium iodide, 0.1 mol/L iodine, 0.1 mol/L lithium iodide, 0.5 mol/L 4-tertbutylpyridine (TBP) in 3-methoxpropiponitrile) was filled into the space between the photoanode and the counter electrode from a hole made on the counter electrode using capillary action. The active electrode area was 0.25 cm².

The images of ZNRs were characterized using a field- emission scanning electron microscope (SEM) (FESEM, FEI Sirion-200, USA). The performance of flexible DSCs was measured with a Keithley 2420 digital source meter under irradiation of a solar simulator (Newport Oriel 94043A, USA, AM1.5, 100 mW/cm²). All the measurements were conducted at room temperature.

ZNRs with different morphologies were obtained by controlling the reaction time. Figure 2 shows the SEM images of ZNRs prepared with the 0.05 mol/L precursor solution at different reaction times. The diameters of nanorods are different on the same image. By selecting the nanorods in a certain area on the statistics, the distribution density of ZnO nanorods diameters is in line with normal distribution and the diameter of nanorods is selected in the peak of the normal distribution. The diameter of nanorod is varied from 70 nm to 150 nm while the density of rods decreases rapidly from 4.5×10⁹ rods/cm² to 1.4 ×10⁹ rods/cm². The thickness is changed from 1.1 μm to 2.7 μm.

Fig. 2

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	Fig. 2 SEM images of ZnO nanorods at different reaction time(a) 4 h; (b) 8 h; (c) 10 h; (d) 12 h; (e) 14 h

As shown in Fig. 3, the growth of nanorods is divided into two stages with the reaction time. When the reaction time is less than 10 h, the diameter and length increase rapidly and the density of rods decreases fast. When the reaction time is more than 10 h, the change becomes slower. The change rate in the diameter and the density is inversely proportional relationship, because two or more nanorods are integrated into a new nanorod leading to the decrease of density, and the increase of diameter^{[ 11]}.

Fig. 3

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	Fig. 3 The change of length, diameter and density of nanorods with the reaction time from 4 h to 14 h

From Fig. 4, the aspect ratio increases from 16 to 20, and then reduces to 18 after reacted for 10 h. As the reaction time increases, the decrease of aspect ratio is mainly due to the constant length after reacted for 10 h.

Fig. 4

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	Fig. 4 The relationship between aspect ratio and the reaction time

The performance of flexible DSCs changes as shown in Fig. 5. As the reaction time increases, the open circuit voltage ( V_OC) and fill factor ( FF) change slightly, but the short-circuit current density ( J_SC) and cell conversion efficiency ( E_FF) change obviously. Because the trend of the current density and the conversion efficiency is consistent, the photocurrent determines the performance of flexible DSCs. The photocurrent of the DSCs depends on the total surface area^{[ 12, 13]}. Since the aspect ratio changes with the reaction time, there is a maximum point of the total surface area. This implies that the aspect ratio is the most important factor for the performance of flexible DSCs in this experiment.

Fig. 5

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	Fig. 5 Photoelectric performance parameters of flexible DSCs based on ZNRs at different reaction time

Figure 6 shows the different morphologies of the ZNRs on the ITO-PET obtained by changing the precursor con- centration at 10 h reaction time. As shown in Fig. 7, the diameter and the length increase obviously with the increase of precursor concentration. The change rate in the diameter and the density are also inversely proportional relationship, and the reason is the same as the mentioned before. The density of rods reduces in two stages. At low precursor concentration, the density reduces quickly from 4.2×10⁹ rods/cm² to 1.5 ×10⁹ rods/cm². When the precursor concentration is higher than 0.03 mol/L, the density decreases slowly from 1.5×10⁹ rods/cm² to 1.1 ×10⁹ rods/cm². The possible reason is that the effective reaction time is short at low concentration.

Fig. 6

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	Fig. 6 SEM images of ZnO nanorods prepared at different growth concentrations(a) 0.02 mol/L; (b) 0.03 mol/L; (c) 0.04 mol/L; (d) 0.05 mol/L; (e) 0.06 mol/L; (f) 0.08 mol/L

Fig. 7

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	Fig. 7 Changes of length, diameter and density of nanorods with the precursor concentration from 0.01 mol/L to 0.08 mol/L

As shown in the Fig. 8, the aspect ratio increases obviously from 15 to 23. When the concentration is greater than 0.06, the aspect ratio then reduces to 19. It is also demonstrated that concentration has a significant role on aspect ratio of nanorods by changing growth speed. From analysis of Fig. 3 and Fig. 7, when the precursor concen tration is lower than 0.06 mol/L, the higher concentration can effectively promote the growth of (002) crystal surface to achieve the purpose of increasing aspect ratio^{[ 7, 12]}. The aspect ratio decreases to 19 at 0.08 mol/L, and an obvious reason is that 10 h is longer than the optimum reaction time. As shown in Fig. 8, the conversion efficiency and the current density increase because of the increase of aspect ratio. This result is similar with the reports in Ref^{[ 12, 13]}that the morphology of the ZNRs has great influence on the performance of the DSC.

Fig. 8

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	Fig. 8 The relationship between the aspect ratio and the reaction concentration

Fig. 9

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	Fig. 9 Photoelectric performance parameters of flexible DSCs based on ZNRs at different precursor concentrations

A simple method was applied in the production of flexible DSCs. One-dimensional ZNRs were synthesized by mild hydrothermal method utilizing Zn(NO₃)₂•6H₂O and C₆H₁₂N₄ at 95℃ on the seeded ITO-PET. Morphology parameters such as diameter and length of nanorods were changed by controlling the reaction time or concentration. In our experiment, the reaction time and the precursor concentration have important influence on the aspect ratio. When the precursor concentration is lower than 0.06 mol/L, increasing precursor concentration will be good for growth of (002) crystal surface. The advantage growth of (002) crystal surface is favorable for the improvement of the total surface area because of the change in the aspect ratio and the density of rods. Therefore, the precursor concentration is the most important factor for the performance of flexible DSCs. Though the maximum conversion efficiency of flexible DSCs is only 0.18%, it is believed that the aspect ratio could be further improved by controlling the reaction conditions, and the performance will be accordingly improved.