Orientation-Controllable ZnO Nanorod Array Using Imprinting Method for Maximum Light Utilization in Dye-Sensitized Solar Cells
- Huisu Jeong†1,
- Hui Song†1,
- Ryeri Lee1,
- Yusin Pak1,
- Yogeenth Kumaresan1,
- Heon Lee2Email author and
- Gun Young Jung1Email author
© Jeong et al. 2015
Received: 5 February 2015
Accepted: 28 May 2015
Published: 12 June 2015
We present a holey titanium dioxide (TiO2) film combined with a periodically aligned ZnO nanorod layer (ZNL) for maximum light utilization in dye-sensitized solar cells (DSCs). Both the holey TiO2 film and the ZNL were simultaneously fabricated by imprint technique with a mold having vertically aligned ZnO nanorod (NR) array, which was transferred to the TiO2 film after imprinting. The orientation of the transferred ZNL such as laid, tilted, and standing ZnO NRs was dependent on the pitch and height of the ZnO NRs of the mold. The photoanode composed of the holey TiO2 film with the ZNL synergistically utilized the sunlight due to enhanced light scattering and absorption. The best power conversion efficiency of 8.5 % was achieved from the DSC with the standing ZNL, which represented a 33 % improvement compared to the reference cell with a planar TiO2.
KeywordsDye-sensitized solar cell TiO2 patterning Scattering layer ZnO nanorods Light harvesting
The dye-sensitized solar cell (DSC) is a promising daylight-harvesting appliance because of its simple and low-cost fabrication, eco-friendly manufacturing, color modulation, and suitable building integration. The conventional DSC consists of a dye-adsorbed photoanode, electrolyte, and counter electrode, among which the photoanode is the key element for obtaining high photocurrents and high power conversion efficiency (PCE). A film of titanium dioxide (TiO2) nanoparticles with a diameter of less than 15 nm is usually used for the photoanode to adsorb more dyes, but the poor usage of incident light because of a high transparency of 20 ~ 60 % at visible wavelengths is troublesome. A thick TiO2 nanoparticulate film allows lower transmittance but induces a long charge diffusion length, which results in a higher probability of electron recombination during migration.
Currently, there are two approaches to recycle more incident light by modifying the planar TiO2 nanoparticulate photoanode. The first approach is to pattern the TiO2 film by various lithographic methods such as soft imprinting , polystyrene sphere templating [2–4], 3-dimensional holography , and glass texturing [6, 7]. The patterned TiO2 films allow more incident light to recycle inside the photoanode for enhanced photocurrents and PCE. The second approach is adding a scattering layer on top of the TiO2 nanoparticulate film. Generally, micron- or submicron-scaled metal oxide crystals are introduced in the form of spheres [8–12], rods [13–15], prisms [16, 17], sheets , cubes , tubes , and flower clusters  as the scattering layer.
Recently, maximum utilization of the incident light was achieved by combining both methods above: the patterned TiO2 film with the scattering layer on top. For example, Char et al.  introduced a micro-pyramidal TiO2 photoanode coated with a TiO2 scattering particulate film, which exhibited a higher PCE than the solitarily patterned TiO2 film. In addition, Moon et al.  reported a TiO2 nanorod (NR)-planted 3-dimensional inverse opal TiO2 film, where the NRs were incorporated as additional scattering media.
In this study, we propose a novel fabrication method to obtain a patterned TiO2 nanoparticulate film combined with an additional scattering layer simultaneously by combinatorial techniques of imprinting and transfer method. A periodically aligned vertical ZnO NR array was used as an imprint mold to pattern the TiO2 nanoparticulate film, and the ZnO NR array was transferred onto the TiO2 film as a light scattering and absorbing layer while imprinting. Therefore, patterning the TiO2 film and transferring the ZnO NR layer (ZNL) were accomplished concurrently. Furthermore, the orientation of ZNL could be controlled by altering the ZnO NR configuration on the mold, such as the pitch, size, and height.
Fabrication of ZnO NR Mold
A mold with periodically aligned vertical ZnO NRs on a GaN substrate was achieved by polymer-templated hydrothermal growth [24, 25]. The GaN substrate is appropriate for vertical ZnO NR growth because of the well-matched epitaxial growth between GaN and ZnO . A 10-nm-thick ZnO film was deposited by sputtering onto the GaN substrate as a seed layer, and the polymer hole template was obtained by nanoimprint lithography. An UV-curable imprint resin composed of polydimethylsiloxane material (Gelest), ethylene glycol dimethacrylate (Aldrich), and Irgacure 184 (Ciba) was prepared with a weight ratio of 87:10:3, respectively. This nanoimprint resin was spin-coated at 6000 rpm for 200 s on the ZnO seed/GaN substrate and subsequently UV-imprinted using a transparent stamp with periodic nanopillars at 7 bar for 8 min. After detaching the stamp from the UV-cured polymer resin, a dry etching process was performed using a CF4 gas plasma (50 sccm, 20 mTorr, 20 W, 30 s) to remove any residual layer under the hole trenches until the underlying ZnO seed layer was exposed.
Next, the ZnO NRs were grown by hydrothermal process. The polymer-templated GaN substrate was immersed into a prepared nutrient solution, which consisted of zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O) and hexamethylenetetramine (C6H12N4) (both had concentrations of 5 mM in DI water) . The ZnO NR growth was proceeded in an oven at 92 °C, and the growth time was determined based on the ZnO NR height. The fabrication scheme and relevant scanning electron microscope (SEM) images are illustrated for easy understanding in Additional file 1: Figure S1.
A TiO2 nanoparticulate paste was prepared by the previously reported recipe and doctor-bladed on a cleaned fluorine-doped tin oxide (FTO) glass (Pilkington, TEC-7, 8 Ω sq−1) . A semi-dried TiO2 film was prepared by baking on a hot plate at 80 °C for 30 min. After imprinting and transferring the ZnO NRs, the sample was fully sintered at 450 °C for 30 min under air condition. The photoanodes were finalized by immersing into a solution of N719 dye (0.3 mM in ethanol) for 3 h at 80 °C . The dye-adsorbed photoanode was assembled with a catalytic platinum counter electrode at a gap of 30 μm using a Surlyn film (Dupont), which was filled later with an electrolyte (Solaronix, Iodolyte AN-50).
Results and Discussion
The imprint process between the ZnO NR mold and the semi-dried TiO2 film was performed using an embossing machine at 10 bar for 15 s. After detaching the mold, the well-aligned ZnO NRs were successfully transferred on top of the semi-dried TiO2 film, and then the photoanode substrate was completely sintered at 450 °C for 30 min to remove the remaining organic binders. Figure 1c represents the transferred ZnO NRs on the TiO2 film. Highly ordered ZnO NRs are tilted and lean on each other, and the root part of the ZnO NRs is facing upward after transferring. A dye-adsorbed ZNL/TiO2 photoanode is shown in the inset of Fig. 1c, which was made in an area of 4 cm2 on a FTO glass. Iridescent colors (red-green) appear because of light reflection at the periodic ZnO NR array surface, which demonstrates a faithful transfer of the ZNL uniformly over the entire area. Figure 1d reveals an area where the ZnO NRs were not transferred onto the TiO2 film defectively. Imprinted holes by the ZnO NRs appeared on the TiO2 film, in which the ZnO NRs should be embedded if properly transferred. A holey TiO2 film without the ZNL was also possible after removing the ZNL by chemical wet etching using a hydrochloric acid solution before dye adsorption, as shown in Fig. 1e.
The best light-scattering effect appeared in the tilted ZNL photoanode because the tilted NRs occupied the entire TiO2 film tightly without any empty space, as shown in the inset of Fig. 2b(ii). The closely packed tilted ZnO NR array works better as a light-scattering layer than the laid and standing ZNLs, which have voids between the neighboring NRs as shown in the insets of Fig. 2b(i) and (iii), respectively. However, in terms of light absorption (Fig. 3b), the standing ZNL photoanode exhibits the best light absorbance because the vertically standing dye-adsorbed ZnO NRs can trap more incident light than those in tilted state (see Additional file 1: Figure S6) . Since more efficient light utilization can be achieved by absorbing and scattering more light within the photoanode, a lower transmittance of the photoanode is a prerequisite for high-efficiency DSCs, which can be adjusted by controlling the orientation of the transferred ZnO NRs in this experiment. Overall, the standing ZNL photoanode has the lowest transmittance in the entire visible range (Fig. 3c).
Photovoltaic properties of the DSCs with different photoanodes
J sc (mA cm−2)
V oc (V)
Laid ZNL + TiO2
Tilted ZNL + TiO2
Standing ZNL + TiO2
An electrochemical impedance spectroscopy (EIS) analysis was performed to understand the charge transfer resistance in the different photoanodes. Figure 4b presents the Nyquist plots of the five DSCs in the frequency range of 105–10−1 Hz under the dark at −0.7 V. Normally, three semi-circles appear in the Nyquist plots from the left to the right with decreasing frequencies, which correspond to the reduction at the counter electrode (105–103 Hz range), electron recombination resistance at the photoanode (103–1 Hz range), and Nernst diffusion in the electrolyte (1–10−1 Hz range) . However, in this study, only two semi-circles appeared because the electrolyte impedance was overlapped with the electron recombination resistance of the photoanode due to the relatively fast electron diffusion through the 30-μm-gap-distance electrolyte . The second semi-circle expanded when the ZNL was applied, demonstrating that the recombination resistance of the photoanode increased [30–32]. The higher recombination resistance signifies that the photo-generated electrons can diffuse for a longer time prior to trapping or charge recombination. Thinner nanoparticulate TiO2 films [33, 34] and ZnO NR-embedded TiO2 film  can lead to lower surface state traps and thus a lower probability of electron recombination. As the cells with the ZNL have a thinner TiO2 film along with the ZnO NRs on top, they are beneficial for reducing the electron recombination.
where k is Boltzmann’s constant, T is the absolute temperature, q is the electron charge, V oc is the open-circuit voltage of each cell, and t is the time. The calculated electron lifetimes are 39.5 ms (standing ZNL cell), 36.7 ms (tilted ZNL cell), 40.6 ms (laid ZNL cell), 24.3 ms (holey TiO2 cell), and 26 ms (planar TiO2 cell). These results are consistent with the electron recombination resistance in the Nyquist plots, where the higher recombination resistance signifies the longer electron lifetime. The laid ZNL cell shows a slightly better electron lifetime than the other ZNL cells. However, the superior light utilization (higher light absorption and scattering) of the standing ZNL cell overwhelms the slightly shorter electron lifetime and leads to the highest photovoltaic performance among the cells.
In summary, periodically transferred ZNLs were successfully fabricated simultaneously onto a TiO2 film by imprinting and transferring of ZnO NRs. The orientation of the ZNL could be controlled by the ZnO NR configuration on the mold, such as the pitch and the height. Laid, tilted, and standing ZnO NR-embedded TiO2 photoanodes were generated along with a normal flat TiO2 film and a holey TiO2 film for references. The highest PCE of 8.5 % was achieved from the standing ZnO NR-embedded cell, which represented a 33 % improvement compared to the planar TiO2 cell and an 18 % improvement compared to the holey TiO2 cell. The improved photovoltaic properties were attributed to the combined effects of superior light utilization and longer electron lifetime, which were evidenced by optical spectroscopy, EIS and OCVD measurements.
This work was supported by the Basic Science Research programs through the National Research Foundation (NRF) of Korea and funded by the Pioneer Research Center Program (No. 2014M3C1A3016468) and the GIST Specialized Research Project provided by GIST.
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