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Multidimensional ZnO Architecture for Dye-Sensitized Solar Cells with High-Effi ciency up to 7.35%

Xin-Hong Lu , Yan-Zhen Zheng , Shi-Qing Bi , Yuan Wang , Xia Tao , * Liming Dai , and Jian-Feng Chen

Dr. X.-H. Lu, Dr. Y.-Z. Zheng, Dr. S.-Q. Bi, Dr. Y. Wang, Prof. X. Tao State Key Laboratory of Organic-Inorganic Composites Beijing University of Chemical Technology 15 Beisanhuan East Road , Beijing 100029 , P. R. China E-mail: taoxia@mail.buct.edu.cn Prof. L. Dai Department of Macromolecular Science and Engineering Case School of Engineering Case Western Reserve University 10900 Euclid Avenue , Cleveland , OH 44106 , USA Prof. J.-F. Chen Research Center of the Ministry of Education for High Gravity Engineering & Technology Beijing University of Chemical Technology 15 Beisanhuan East Road , Beijing 100029 , P. R. China

DOI: 10.1002/aenm.201301802

arisen from the sub-micrometer size and large surface area pro-vided by the nanocrystals. [ 8,13 ] In photoanodes constructed even with high-performance 2D ZnO NSs and 3D ZnO NAs, how-ever, it is diffi cult to simultaneously achieve the aforementioned three prerequisites for a high effi ciency since the 2D NS structure always possesses nanoscale size with poor light-scattering capa-bility while the zigzag electron transport pathway along the 3D NAs often retards the electron transport and thus infl icts electron recombination within the electrode fi lm to some extent. [ 8,10 ] Gen-erally speaking, ZnO architectures cannot provide all the above-mentioned properties required for high-performance DSCs, and hence few DSCs based on ZnO could have PCEs exceed 7%. [ 13 ] Therefore, the fabrication of photoanodes based on multidimen-sional (MD) ZnO architectures composed of different mono-dimensional nanostructures of specifi c features with synergistic effects offer a better approach to high PCEs. [ 4,15,16 ] Indeed, we have previously demonstrated that DSC based on a MD ZnO architecture consisting of a 3D-nanorod-aggregate overlayer and a 0D-nanocrystallite underlayer yielded a much higher PCE than those based on single-layered nanorod-aggregates or nanocrystal-lites. [ 4 ] Nevertheless, the overall PCE of the MD ZnO cells is still low and it is still a great challenge to fi nely design and tune the ZnO photoanode architecture to simultaneously maximize the key features for outstanding PCEs.

Here, we report the preparation of an innovative MD ZnO architecture with combination of 2D ZnO NSs and 3D ZnO NAs, in which the ZnO NSs act not only as bridges connecting the ZnO NA-framework to form favorable electron transport channels within the fi lm, but also as building blocks to enlarge the overall surface area for dye-adsorption and to provide a ran-domly-opened structure for a better concurrent electrolyte pen-etration. In this context, a key trick in this work is to synchro-nously tune the surface area and electron transport property of the MD ZnO architecture by controlling the density of the ZnO NSs with retention of relatively strong light-scattering provided by the ZnO NAs. Under optimal condition, an overall PCE of 6.66% was achieved for a photoanode based on the ZnO NA/NS composite architecture, a noticeable improvement by 64.0% in comparison with that of the corresponding cell fabricated with ZnO NA photoanode. Various analyses confi rmed that the photovoltaic performance enhancement in such a MD ZnO NA/NS composite photoanode was mainly due to the excellent com-bination of large surface area, effective electron transport, and strong light-scattering capability. To further improve the PCE, a compact layer of 0D ZnO nanocrystal was then paved on the substrate before fabricating the optimized ZnO NA/NS fi lm. This led to an impressive PCE of 7.35%, which is very close to the record PCE (7.5%) of ZnO-based DSCs ever reported. [ 17 ]

Dye-sensitized solar cells (DSCs) are important low-cost photovol-taic devices that show great promise to solve many environmental and energy problems. [ 1 ] DSC is typically a sandwich-structure solar cell consisting of a porous photoanode fi lm coated with dye molecules, an iodide/triiodide electrolyte and a platinum counter electrode. [ 2 ] Apart from the search for more effi cient and stable dyes, the design and modifi cation of the photoanode architecture has also attracted great attention. [ 3 ] In general, high specifi c sur-face area, fast electron transport and outstanding light-scattering capability are prerequisites for an excellent photoanode in high-effi ciency DSCs. [ 4 ] In the past decade, ZnO has attracted extensive interest owning to its much higher electronic mobility in compar-ison with the traditional TiO 2 , which may facilitate more effective electron transport in DSCs. [ 5 ] More importantly, its unique merit, namely ease of crystallization and anisotropic growth, opens wide possibilities to accommodate all of the above favorable charac-teristics through control of morphology and structure. [ 5–9 ] To date, various ZnO nanostructures, including zero-dimensional (0D) nanoparticles, 1D nanowires, nanotubes and nanobelts, 2D nanosheets, 3D hollow spheres, nanocrystalline aggregates and nanoforests, have been fabricated. [ 5–14 ] Among them, sev-eral exceptional structures, such as 2D nanosheets (NSs) and 3D nanocrystalline aggregates (NAs), are of particular interest in performance enhancement for DSCs. For DSCs based on ZnO NSs, high photo-to-current conversion effi ciencies (PCEs) above 6% have been reported mainly due to their high surface area, effective electron transport, and high porosity. [ 10–12 ] Aside from NSs, the 3D ZnO NAs assembled with 0D ZnO nanocrystals have also been one of the hotspot nanostructures to yield high PCEs of 5.3−6.3% because of their superior light-scattering effect

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The MD ZnO architecture was fabricated via suspending the ZnO NA fi lm in an aqueous solution of zinc nitrate and urea at 90 °C for 2 h (see Experimental Section for details). The scan-ning electron microscopy (SEM) images in Figure 1 a–c clearly show the variation of the ZnO NA fi lm after hydrothermal treatment in a 0.20 M Zn-precursor solution (designated as ZnO NA/NS−020 for convenience, similarly hereinafter) to pro-duce a 9 μm-thick vertically-grown ZnO NS layer on top of the ZnO NA layer. Note that the NSs atop the NA layer are accumu-lated forming many large gaps, which could benefi t the pen-etration of electrolyte into the photoanode. A magnifi ed SEM image (inset of Figure 1 b) reveals 2D morphological character-istic, featuring individual NSs with a thickness of 20−30 nm. Figure 1 d shows a transmission electron microscopy (TEM) image of a single NS, showing numerous mesoscopic pores. Such morphological characteristics are favorable to possess large surface area for dye adsorption. [ 18 ] Further, a high-reso-lution TEM image (Figure 1 e) and the corresponding selected area electron diffraction pattern (inset) clearly reveals that the ZnO NS is a single-crystalline wurtzite structure, which may serve as a highway for electron transport. [ 6,19 ]

Instead of just growing on top of the ZnO NA fi lm, ZnO NSs tended to also grow within the ZnO NA fi lm framework (Figure 1 g,h), in which a small amount of NS bridges between ZnO NAs were formed as illustrated in Figure 1 f. Since NSs within the NA fi lm suffered much greater spatial hindrance to grow, the NSs would preferentially grow on top of the NA fi lm. Nevertheless, the NS bridges thus formed throughout the whole

NA fi lm could provide direct pathways, rather than a zigzag way in the ZnO NA photoanode, for electron transport. [ 10 ] Although these NS bridges could inevitably cause disruption of the closed light loops locally to weaken the light scattering of the NA photo anode, the large-scale spherical morphology of ZnO NA was preserved after the hydrothermal process, leading to the retention of strong light scattering capability (>70%) (Figure S1, Supporting Information) and good crystallinity of the ZnO NA fi lm (see Figure S2, Supporting Information). As such, the well-crystallized ZnO NA/NS−020 fi lm with a dense NA underlayer and porous NS overlayer, interparticle connecting NS bridges, and sub-micron size of NAs possesssed all the aforementioned morphological properties required for high-effi ciency DSCs, as discussed in more details below.

To investigate morphological changes of the as-prepared multidimensional fi lms, ZnO NA/NS fi lms were also fabricated in other two Zn-precursor growth solutions (i.e., ZnO NA/NS−010: 0.10 M; ZnO NA/NS−015: 0.15 M). For comparison, ZnO NA fi lm without further treatment was also prepared as a reference. The top-view SEM images of the as-prepared fi lms (Figure 1 , also see Figure S3 in the Supporting Information) clearly show that the packing density of the ZnO NSs on top of the ZnO NA fi lm increased with increasing Zn-precursor concentration. When the Zn-precursor concentration is up to 0.20 M, the NA fi lm is uniformly covered with ZnO NS assem-blies, as shown in Figure 1 b. Meanwhile, the top ZnO NS layer also increased in thickness from about 5 to 9 μm (see Figure 1 c and Figure S3b,d, Supporting Information). In addition, the

Adv. Energy Mater. 2014, 4, 1301802

Figure 1. Top-view SEM images a) ZnO NA and b) ZnO NA/NS−020 fi lms (the scale bar of the inset is 100 nm). c) Cross-sectional SEM image of the ZnO NA/NS−020 fi lm. d) TEM and e) high-resolution TEM images together with the corresponding selected area electron diffraction pattern (inset) of a typical NS in the ZnO NA/NS−020 fi lm. f) Schematic diagram of typical NS assembly and NS bridges between NA particles. g,h) Magnifi ed cross-section SEM images taken from the regions in (c) labeled with the green and orange rectangles, respectively.

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enlarged cross-sectional SEM images (Figure S3c,f, Supporting Information) of the NA/NS fi lms also revealed that the number of the interparticle NS bridges increased with increasing Zn-precursor concentration too. It was noted that further increase in the Zn-precursor concentration to 0.25 M caused the top NS layer to become so dense and thick that the whole ZnO NA/NS−025 fi lm easily fell off from the substrate to be inapplicable for the DSC application, and hence no further discussion on ZnO NA/NS−025.

Figure 2 a shows the current density−voltage ( J − V ) curves for the DSCs based on the abovementioned ZnO NA/NS fi lms (i.e., ZnO NA/NS−010, ZnO NA/NS−015, ZnO NA/NS−020) and ZnO NA fi lm under AM 1.5 sunlight illumina-tion (100 mW cm −2 ), and the photoelectric characteristics are summarized in Table 1 . As can be seen, the ZnO NA cell exhibited only a low PCE of 4.06% with a short-circuit current density ( J SC ) of 12.92 mA cm −2 and an open-circuit voltage

( V OC ) of 0.603 V, respectively. In comparison with that of the ZnO NA cell, the J SC and V OC of the ZnO NA/NS cells both increased with increasing Zn-precursor concentration, leading to a continuous enhancement of PCE. Impressively, a high J SC of 17.90 mA cm −2 and V OC of 0.631 V were obtained for the ZnO NA/NS−020 cell, leading to an encouraging PCE of 6.66%, a 64.0% increment from that of the ZnO NA cell. The observed photovoltaic performance enhancement is attributable to the unique morphology change (i.e., tailored density and thickness of the NS layer as well as ameliorative interparticle connec-tions) with the Zn-precursor concentration shown in Figures 1 and Figure S3 (Supporting Information).

To reveal the infl uence of the NSs in the ZnO NA/NS com-posite fi lms on the DSC performance, the dye loading amounts into the photoanode fi lms were fi rst characterized (Table 1 ). Compared with the ZnO NA fi lm (1.58 × 10 −7 mol cm −2 ), the ZnO NA/NS fi lms exhibited much better capabilities in dye

Table 1. Characteristics of the ZnO NA/NS and ZnO NA photoanodes together with the photovoltaic parameters of the corresponding DSCs.

Cells J SC [mA cm −2 ]

V OC [V]

FF [%]

PCE [%]

Film thickness [µm]

Dye uptake [×10 −7 mol

cm −2 ]

S BET [m 2 g −1 ]

R ct2 [ohm]

τ e [ms]

IPCE [%]

NA 12.92 0.603 52.1 4.06 13 1.58 36.9 22.7 2.38 56.5

NA* 13.06 0.607 50.9 4.04 22 2.65 − 40.4 0.60 −

NA/NS−010 15.13 0.611 49.7 4.61 18 2.63 47.0 17.9 3.80 65.6

NA/NS−015 16.64 0.617 51.6 5.30 21 3.60 55.0 13.8 4.87 72.9

NA/NS−020 17.90 0.631 59.0 6.66 22 3.87 56.3 12.8 6.32 87.0

NA/NS−020/

CL

20.72 0.622 57.0 7.35 − − − − − −

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Figure 2. Photovoltaic performances: a) J − V curves, b) Nyquist plots, c) Bode phase plots, and d) IPCE spectra of the ZnO NA/NS and ZnO NA cells.

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adsorption with the dye loading amount increased from 2.63 × 10 −7 mol cm −2 to as high as 3.87 × 10 −7 mol cm −2 by increasing the content of NSs in the ZnO NA/NS fi lms from the NA/NS−010 to NA/NS−020. This signifi cant enhancement in the dye loading can be attributed to the concurrent increment of the density and thickness of the NS layer as well as the aug-ment of overall surface area of the ZnO NA/NS fi lms associated with the newly-formed high-surface-area NSs. [ 10 ] The increase in surface area has also been well verifi ed by the Brunauer–Emmett–Teller (BET) specifi c surface area ( S BET ) measure-ments as summarized in Table 1 , in which a distinct increase in S BET from 36.9 to 56.3 m 2 g −1 is observed with increasing the content of NSs in the ZnO NA/NS fi lms. A larger dye loading amount could ensure that the incident photons are suffi ciently absorbed, resulting in a larger photocurrent and thus the high overall PCE. [ 10 ]

We further performed electrochemical impedance spectros-copy (EIS) to study the impact of the NSs in the ZnO NA/NS composite fi lms on the electric behaviors of the DSCs. [ 18 ] As shown in Figure 2 b, the Nyquist plots spectra obtained from the ZnO NA/NS and ZnO NA cells exhibit three distinguish-able semicircles, which are related to the electrochemical reac-tion at the Pt counter electrode in the 10 3 Hz range ( R ct1 ), at the ZnO fi lm/dye/electrolyte interface in the 10 0 −10 3 Hz ( R ct2 ), and the Warburg diffusion process of I − /I 3 − at low fre-quency range <1 Hz ( R diff ) from the left to the right, respec-tively. [ 20 ] The extent of electron transport in the photoanode can be judged by R ct2 , which is defi ned by the diameter of the middle semicircle. [ 15,16 ] According to the fi tted values of R ct2 listed in Table 1 , it can be seen that the electron trans-port resistance decreases dramatically from 22.7 ohm for the ZnO NA cell to 12.8 ohm for the ZnO NA/NS−020 cell with the increasing content of NS, implying an easier and faster electron transport in the ZnO NA/NS fi lms with a higher NS content. This observation can be ascribed to the interparticle NS bridges serving as transport highway for electrons in the photoanodes to reduce the charge recombination, and hence a higher electron diffusion and collection effi ciency in ZnO NA/NS cells with a higher NS content. [ 6 ] The corresponding Bode phase plots (Figure 2 c) can be drawn from the Nyquist plots under illumination in Figure 2 b. According to the reported EIS model, the lifetime ( τ e ) of the injected electrons in photo-anodes can be acquired from the following equation: τ e = 1/ ω max = 1/(2π f max ), where f max is the maximum frequency of the intermediate frequency regime. [ 21 ] As listed in Table 1 , the τ e is remarkably increased from 2.38 ms for the ZnO NA cell to 6.32 ms for the ZnO NA/NS−020 cell. A prolonged electron lifetime and lower electron transport resistance are directly related to the improved J SC and V OC of the ZnO NA/NS cells. To better understand the signifi cance of NSs to electron trans-port, we also fabricated ZnO NA photoanode with an identical fi lm thickness as that of the ZnO NA/NS−020 fi lm (denoted as ZnO NA*). As expected, the photovoltaic performance of the ZnO NA* cell is not improved with increasing of the fi lm thickness and adsorbed dye because the thicker NA fi lm causes a great increase of R ct2 up to 40.4 ohm and a much lower τ e of 0.60 ms (Table 1 , also see Figure S4, Supporting Information), which fully proves the advantage of NSs on elec-tron transport over NAs.

In addition, the incident photon-to-current conversion effi -ciency (IPCE) spectra provide further evidence for the improved electron transport property of the ZnO NA/NS cells than the ZnO NA cell. Figure 2 d shows IPCE spectra of the ZnO NA/NS cells and ZnO NA cell as a function of the illuminated wave-length, in which strong photoactivity for all the cells is observed in the visible region from 470 to 620 nm that coincides with the absorption maximum wavelength of the D205 dye used in this work. In contrast to the ZnO NA cell (56.5%), the ZnO NA/NS cells show higher photoresponse (>65%) in the entire visible wavelength region with the highest IPCE value up to 87.0% at around 500 nm ± 20 nm observed for the ZnO NA/NS−020 cell. IPCE is known to predominantly determined by the harvesting effi ciency of the incident light, the injection effi -ciency and the electron collection effi ciency; the fi rst of which is proportional to dye loading of the photoanodes. [ 8 ] Since it is rational to assume the same injection effi ciency for the given ZnO/dye system, the improvement of IPCE can be deduced to originate from enhancement in light harvesting effi ciency and/or charge collection effi ciency, which are mainly determined by the dye-loading amount and electron transport property. [ 7,8 ] In this work, the variation of dye uptakes for the ZnO NA cell, ZnO NA/NS−010 cell and ZnO NA/NS−015 cell is remark-able. Therefore, the augment of IPCE for the ZnO NA/NS−010 cell and ZnO NA/NS−015 cell can be mainly attributed to the increased adsorbed dye molecules on the photoanodes despite of their slightly lowered light-scattering capability with respect to the ZnO NA cell (Figure S1, Supporting Information). In view of the slight discrepancy of dye loading amounts between the ZnO NA/NS−015 cell and ZnO NA/NS−020 cell and even a little weaker light scattering for the ZnO NA/NS−020 cell (Figure S1, Supporting Information), however, the higher IPCE for the ZnO NA/NS−020 cell could be mainly resulted from the enhanced electron transport (i.e. electron collection effi ciency) attributed to the incorporation of more ZnO NSs into the elec-trode fi lm, as discussed above.

To further improve the overall PCE, we introduced an ultrathin compact layer between the photoanode fi lm and the conductive FTO substrate, which has been proved to be effec-tive in suppressing the charge recombination and thus boosting PCE of DSCs. [ 22 ] Specifi cally, a ZnO nanocrystallite compact layer with thickness of around 100 nm was subsequently deposited on the substrate via a spin-coating method before doctor-blading the ZnO NA fi lm. The photovoltaic performance of ZnO NA/NS−020 cell with compact layer (denoted as NA/NS−020/CL cell) is given in Figure 3 , which shows a noticeable improvement of J SC from 17.90 to 20.72 mA cm −2 , leading to an outstanding PCE up to 7.35%. This is very close to the record PCE of 7.5% ever reported for DSCs based on ZnO. According to the EIS analysis and J − V measurement performed in dark condition (Figure S5, Supporting Information), such enhance-ment of J SC and thus PCE can be mainly ascribed to the sup-pressed charge recombination, and hence improved electron collecting effi ciency.

In conclusion, a class of novel ZnO photoanodes with multidimensional architecture composed of 2D ZnO NSs and 3D ZnO NAs has been developed for DSCs to show a much improved PCE of 6.66% under optimal condition. Var-ious materials and device characterization demonstrated that

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Received: November 25, 2013 Revised: February 3, 2014

Published online: March 5, 2014

Adv. Energy Mater. 2014, 4, 1301802

the ZnO NSs played a key role in remarkably enhancing the PCE by connecting ZnO NA-framework for favorable elec-tron transport within the fi lm and simultaneously enlarging the overall surface area for effi cient dye loading with reten-tion of the superior light-scattering provided by ZnO NAs. A further enhanced PCE up to 7.35%, which is among the top records of the ZnO-based DSCs, was achieved by utilizing a ZnO compact layer to suppress possible charge recombina-tion in the newly developed photoanode. This work provides a facile and feasible way to attain outstanding DSC cell effi -ciencies by rational design and tune the ZnO photoanode architecture.

Experimental Section Preparation of ZnO NA Films : All chemicals were received from

Sigma-Aldrich and used as received. The synthesis of spherical ZnO NAs were achieved by hydrolysis and condensation of zinc acetate dehydrate (0.1 M) in diethylene glycol at 160 °C for 8 h under refl ux, followed by repeating centrifugation/sonication/dispersion in ethanol. [ 8 ] After drying at 60 °C in air overnight, the NAs were then mixed with ethyl cellulose and terpineol with a weight ratio of 1:0.2:1.5 to form a viscous ZnO paste. [ 23 ] The ZnO NA fi lm was sequentially fabricated by doctor-blading the as-prepared ZnO paste on the FTO/glass substrate (2.3 mm thickness, 8 ohm sq −1 , GEAO, China), following a programmed heating of 350 °C for 1 h and then stepwise rising to 450 °C at 10 °C min −1 . [ 24,25 ]

Preparation of ZnO NA/NS Photoanode Films : The ZnO NA/NS fi lm was prepared via suspending the above-synthesized NA fi lm upside-down in a sealed bottle containing an aqueous solution of zinc nitrate with certain concentration (0.10 M, 0.15 M and 0.20 M) and 16.7 wt% of urea at 90 °C for 2 h to form layered hydroxide zinc carbonate (Zn 4 CO 3 (OH) 6 ·H 2 O), which further transforms into ZnO NS in the following annealing step (300 °C) with loss of H 2 O and CO 2 . [ 18 ] For comparison, the ZnO NA photoanode without hydrothermal treatment was also prepared as reference.

DSC Fabrication : The ZnO photoanode with an active area of 0.16 cm 2 was immersed into a 0.5 mM D205 solution in a mixture of acetonitrile and tert-butyl alcohol (V : V = 1 : 1) containing 1.0 mM chenodeoxycholic acid for 50 min at 60 °C. Then, the photoanode was assembled with a Pt-coated photocathode into a sandwich cell using a 60 µm-thick surlyn sheet, followed by fi lling of electrolyte composed of 0.03 M I 2 , 0.5 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII) and 0.5 M tertbutylpyridine in acetonitrile.

Characterization : The morphology and structure of ZnO photoanode fi lms were observed by SEM (JEOL, JSM−6701F) and TEM (JEOL, JEM−3010). The photoelectrochemical characterization of DSCs was performed using electrochemical station (CHI660C, ShangHai) under simulated AM 1.5 sunlight illumination with 100 mW cm −2 light output. A 1000 W xenon lamp (Thermo Oriel, America) served as the light source. The electrochemical impedance spectroscopy (EIS) was performed on an electrochemical workstation (Zennium Zahner, Germany). The EIS measurement was recorded at V OC with ac amplitude of 10 mV and a frequency ranging from 10 −1 to 10 5 Hz. Monochromatic light in the range of 400−800 nm was obtained by using a series of fi lters and the IPCE measurements were performed on a Keithley Model 2000 Source Meter.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors are grateful for the fi nancial support from National Natural Science Foundation of China (Nos. 21121064, 21176019, 21377011), and 863 project (2013AA031901).

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Figure 3. J − V curve of the ZnO NA/NS−020/CL cell.

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