Correlation between the Morphology and Performance Enhancement of ZnO Hierarchical Flower Photoanodes in Quasi-Solid Dye-Sensitized Solar Cells

We report an effective method for mass production of ZnO hierarchical flowers (HFs) that are constructed by interlaced single crystalline ZnO nanosheets with dominant surfaces of {1120} and {1100}. The size of ZnO HFs, thickness and parking density of the nanosheets, and HF inner porosity can be tuned by changing the synthesis conditions. The HFs containing porous inner structures showed an excellent performance as the photoanode material in quasi-solid (using polymer gel electrolytes) dyesensitized solar cells (DSCs) because of their superior optical and electrical properties. The high current density (10.26 mA cm−2) and efficiency (4.93%) of the HF-based DSCs indicate a great potential for the development of high-efficient quasi-solid DSCs.


Introduction
In the past two decades, porous photoanodes have been intensively studied as one of the key components used to enhance the performance of dye-sensitized solar cells (DSCs) [1][2][3].Conventionally, different sizes of particles such as nanoparticles (∼20 nm) and submicron particles (∼200 nm) are employed to construct a double-layered photoanode structure with aims to guarantee dye-loading and improve light-scattering efficiency, respectively [1].However, the total surface area of this kind of photoanodes is inevitably reduced because of the larger size of particles introduced.In recent years, photoanodes constructed by hierarchical structured materials have been found to effectively increase the photoanode efficiency.This is because, with the advantages of nanosized primary particles and the submicron-sized secondary aggregates, the hierarchical photoanodes can offer a large surface area for sufficient dye-loading and in the meantime ensure the efficient lightscattering [4].Recently, most research work have focused on nanostructured TiO 2 as the photoanodes to which the dye is bound.However, good results have also been obtained using other materials such as ZnO, SnO 2 , and Nb 2 O 5 [5][6][7][8][9][10][11][12][13][14][15][16].ZnO is an attractive photoanode material because its electron mobility is much higher than that of TiO 2 , while the conduction band edges of both materials locate at approximately the same level.In addition, ZnO shows better flexibilities in synthesis process and morphology control.ZnO hierarchical photoanode structures based on nanocrystalline aggregates, flower-like spheres, and dendritic nanowires have been reported in recent years [17][18][19][20][21].
To increase the conversion efficiency of the DSCs, both optical (dye loading and light scattering) and electrical transport properties need to be optimized.For example, by improving the light-harvesting ability of ZnO nanostructured photoanodes, a high conversion efficiency of 5.4% (using polydispersed ZnO aggregates and liquid electrolyte) has been achieved by Zhang et al. in 2008 [22].ZnO nanowires have been suggested to be ideal nanostructures to provide direct pathways for electron collection and transportation [23][24][25][26][27].However, the insufficiency of the internal active surface area of ZnO nanowire nanostructures always limits their energy conversion performance [26].Having superior optical and electrical properties, in recent years, ZnO hierarchical nanostructures have been considered as an emerging photoanode material for developing future high efficient DSCs.Recently, we developed a facile direct precipitation method for fabricating a novel flower-like ZnO hierarchical structure from aqueous solution at room temperature [28].These ZnO flowers were mainly constructed by interlaced monocrystalline ZnO nanosheets.The photoanodes made from these flowers showed both superior optical performance and excellent electrical properties for quasisolid (using polymer gel electrolyte) DSCs.The electron diffusion coefficient in this kind of photoanodes was nearly one order of magnitude higher than that in conventional nanoparticle-(NP-) based photoanodes and thus largely improved the DSCs photovoltaic performance.The stability of DSCs is another important factor for practical application.Problems such as leakage, volatilization, and corrosion have been regarded as the critical factors limiting the long-term performance of the DSCs assembled using liquid electrolytes.Instead, quasi-solid electrolytes [29] can alleviate these problems and they is also suitable for flexible DSCs with varied geometries although the conversion efficiency of the quasi-solid DSCs are normally low due to the slow ionic transport in quasi-solid electrolytes [30].
In the paper, we report a systematic investigation of the morphology variations (e.g., flower size, thickness, and parking density of the nanosheets, inner structures, etc.) of the ZnO hierarchical flowers (HFs) formed under different experimental conditions and the relationships between flowers' morphologies and their DSCs' performances.The ZnO HFs possessing porous structures showed excellent performance due to their excellent capabilities in lightharvesting and charge transport and thus resulted in a high current density of 10.26 mA cm −2 and overall energy conversion efficiency up to 4.93% in the PEO-based polymer gel quasi-solid electrolyte system.

Experiments
2.1.Synthesis of ZnO HFs.ZnO HFs containing hierarchical structures were synthesized by a direct precipitation method, and all reactants were used without further purification.Firstly, zinc nitrate hexahydrate and sodium hydroxide were dissolved by 1 L distilled water separately.Then the zinc nitrate hexahydrate aqueous solution was added into the sodium hydroxide aqueous solution continuously with a flow rate of about 20 mL s −1 .After stirring for 20 min, the resulting white product was filtered and washed with distilled water for several times.

Assembly of the DSCs.
In our experiment, in order to enhance the mechanical strength of the ZnO layer and the electrical connection between the ZnO hierarchical structures and the FTO glass electrode, the ZnO HFs were mixed with about 20 wt% of ZnO nanoparticles (20 nm in diameter).The ZnO paste was prepared by mixing 0.5 g ZnO power (0.4 g ZnO HFs and 0.1 g ZnO nanoparticles) with 0.75 g absolute alcohol and 0.5 g distilled water, then followed by ultrasonic treatment until the particles were dispersed homogeneously.Doctor blade technique was used to prepare the porous ZnO layers with their thickness being controlled by a 120 μm adhesive tape.The ZnO photoanode was sintered at 200 • C for 1.5 h.After cooling down to 100 • C, the ZnO films were sensitized in 0.3 mM N719 absolute ethanol solution at 50 • C for 60 min, followed by cleaning with absolute ethanol.A chemically platinized conductive glass served as the counter electrode.The polymer gel electrolyte in this study was a quasi-solid mixture containing 0.1 M LiI, 0.1 M I 2 , 0.6 M 1,2-dimethyl-3-propyl imidazolium iodide, 0.45 M N-methyl-benzimidazole in 3-methoxypropionitrile solvent, and 15% (weight ratio) gelator (PEO, M w = 2 × 10 6 g mol −1 ).When assembling DSCs, a sensitized ZnO photoanode and a counter electrode sandwiching the polymer gel electrolyte were fixed by two clips, and an adhesive tape with a thickness of 50 μm is used to avoid the contact between two electrodes.Finally, the DSCs were baked at 90 • C for 20 min to ensure the polymer gel electrolyte can penetrate into the nanoporous electrodes.

Characterization.
The morphologies of the ZnO HFs and the photoanodes were characterized by scanning electron microscopy (Philips XL30) and high-resolution transmission electron microscopy (JEOL, JEM-2010F).The diffuse-reflectance spectra were characterized with a UV-vis spectrophotometer (Lambda 20).BET surface area of the samples was characterized using Coulter SA 3100 surface area analyzer.The photovoltaic performances of the DSCs were measured by KEITHLEY 2420 source-meter under AM 1.5, 100 mW illumination (Xenon lamp solar simulator, Newport, Class A).

The Structure and Performance of ZnO HFs in DSCs.
In this work, we employed the direct precipitation method [28] to synthesize the ZnO HF reference sample (sample 1).As shown in Figure 1(a), the sizes of the HFs in sample 1 are uniform and most of the spherical HFs possess diameters in the range of 5-6 μm.The enlarged image in Figure 1(b) reveals that the HF is constructed with densely interlaced nanosheets.In addition to the {1100} dominated planes we reported in a previous publication [28], {1120} planes have also been found in ZnO nanosheets (see Figure S.1). Figure 1(c) presents a typical morphology of the inner parts of ZnO HFs by breaking the HFs through ultrasonic treatment.Obviously, the core of the HF in sample 1 is nearly solid.As shown in the enlarged SEM images in Figure S.2a, the core structure consists of tiny closely stacked nanoplates.The HFs from sample 1 were then assembled into quasisolid DSCs and the current density (J)-voltage (V) curve was measured under illumination of AM 1.5, 100 mW cm −2 simulated sunlight.As shown in Figure 1(d) and summarized in Table 1, the DSCs based on sample 1 can generate a short-circuit density (J SC ) of 8.23 mA cm −2 and an opencircuit voltage (V OC ) of 0.688 V, resulting in a conversion efficiency (η) of 3.92%.We believe that because of their wellcrystallized nanosheets and the microsized particles, the HFbased photoanodes can efficiently minimize massive electron hoping which normally happens in nanoparticle basedphotoanodes [3].However, the low Brunauer-Emmett-Teller (BET) specific internal area (18.82 m 2 g −1 ) of this sample reveals that the solid core of the HFs is not utilized since this will cause insufficient adsorption of the dye molecules.

The Photovoltaic Performance of ZnO HFs.
To investigate the performance variation caused by different HF morphologies and synthesis conditions, four kinds of samples were selected and assembled into DSCs for further study.The detailed synthesis conditions for synthesizing the HFs are summarized in Table 2. Figure 3 shows the J-V curves of these four DSCs and the corresponding parameters are summarized in Table 3.Since there are only insignificant differences in the open-circuit voltage (V OC ) and fill factor (FF), the differences in these DSCs conversion efficiencies are mainly due to the changes of the short-circuit densities (J SC ).Obviously, the DSC based on sample 2 displays a much better photoelectric conversion capacity than the other three samples.It has a higher J SC of 10.26 mA cm −2 , and this eventually leads to a high conversion efficiency (η) of 4.93%.The performances of samples 3 and 4 are not good due to their small effective surface area in these two samples.The BET results (Table 3) clearly show that the specific area of sample 2 (21.13 m 2 g −1 ) is higher than that of other samples.This is in good agreement with the SEM results.The extended ZnO bundles in sample 3 and the defective thick nanosheets in sample 4 are main factors resulting in small surface area.Although the loosely packing of nanosheets may cause some loss of internal specific area, the porous inner structure in sample 2 is believed to largely extend the total surface area.The light-scattering performances of these unsensitized photoanodes were then characterized.As shown by the solid lines in the UV-vis diffuse-reflectance spectra in Figure 5(a), all photoanodes exhibit excellent light-scattering capability.
From 400 to 900 nm, the slow decline in light reflectance of these four samples illuminates that the photoanodes can effectively prevent most of the incident light with wavelength in this range from transmitting out directly and therefore result in a high chance for photons to be captured by dye molecules.Among these four un-sensitized photoanodes, the one based on sample 2 shows the best light-scattering capability.This should be attributed to not only the smaller size (2 to 4 μm) of the HFs that are more comparable to the wavelength of incident light than the other three samples, but also the broad and deep channels between adjacent nanosheets which can extend the pathway into the porous inner architecture of HFs for photon localization enhancement [31,32].Diffuse-reflectance spectra (dashed lines) of corresponding sensitized photoanodes are also given  in Figure 5(a), from which we can see that the range of the incident light absorbed by the dye molecules is from about 400 to 750 nm, nearly covering the whole spectrum of visible light.The optical adsorption spectra of these four dye-sensitized photoanodes (see Figure 5(b)) are obtained by subtracting the reflectance spectra of the un-sensitized photoanodes from that of the sensitized ones.As anticipated, the photoanode based on sample 2 exhibits better light adsorption capability than the other three photoanodes.Within the wavelength range from 400 to 600 nm, over 50% of the incident light can be captured by this photoanode.

Charge Transport Properties of the DSCs.
In order to estimate the resistance of electron transport and charge recombination processes in ZnO-HF-based DSCs, we have measured the electrochemical impedance spectra (EIS) under fixed illumination of 100 mW cm −2 .The Nyquist plots of the DSCs under open-circuit condition are shown in Figure 6 in which the equivalent circuit of the DSCs is exhibited.As seen from the equivalent circuit, the series resistance (R S ) involves the FTO electrode and external circuit while other three impedance units represent resistances (R) and constant phase elements (CPE).Typically, the diameters of the semicircles (see Figure 6) at high, intermediate, and low frequency ranges represent R 1 , R 2 , and R 3 , corresponding to the resistances for the charge transfer at the counter electrode, the electron transport/charge recombination at the ZnO/electrolyte interface, and the diffusion of I − 3 ions in the electrolyte, respectively [33,34].According to Figure 6, R S and R 1 are nearly the same for all four DSC devices, indicating a better repeatability in fabrication of our devices.Because the electron transport resistance is negligible under full sunlight illumination, R 2 mainly represents the electron recombination resistance at the ZnO/electrolyte interface.The photoanode made from sample 2 shows the smallest charge recombination resistance R 2 .As mentioned above, HFs in sample 2 have loosely packed nanosheets, porous inner structures, and a large surface area.These structural advantages favor the penetration of the polymer gel electrolyte and then ensure the sufficient contact between the ZnO surface and electrolyte.Subsequently, the charge transfer at this interface is facilitated and a small R 2 value is obtained for sample 2.

Summary
We have synthesized novel ZnO HFs in large scale by a facile direct precipitation method at room temperature.The ZnO HFs are constructed by interlaced single crystalline nanosheets.The hierarchical structures can be tuned by changing synthesis parameters.We demonstrate that the quasi-solid DSCs fabricated by ZnO HFs with porous inner structure show the best photovoltaic performance.We have evidenced that the porous inner construction played an important role for enhancing the DSC's performance because this structure can (1) enrich the internal surface area of the HFs for dye adsorption, (2) efficiently trap incident light inside each individual HF, leading to a better light harvesting capability, and (3) form a ZnO network with open channels which can largely facilitate electrolyte penetration for the quasi-solid electrolytes.

Figure 1 :
Figure 1: (a), (b), and (c) SEM images of the HFs in sample 1.(d) J-V curve of the DSCs based on sample 1.
Changing Synthetic Conditions 3.2.1.Reactant Concentrations.Comparing with sample 1, we observed obvious changes on the HF morphology by increasing the concentrations of NaOH and Zn(NO 3 ) 2 solutions (keeping the ratio of NaOH and Zn(NO 3 ) 2 unchanged).As shown in Figure 2(a), the size of the HFs became small (2 to 4 μm) and nonuniform when the reactant concentration was doubled.The interlaced nanosheets become loosely packed (see Figure 2(b)).In the inner parts of the HFs, we observed that the density of the stacked nanoplates decreased (see Figure S.2b) , leading to a porous structure as presented in Figure 2(c).On the other hand, the morphology of the HFs formed in a dilute solution (e.g., 0.5 time of the concentration used for sample 1) did not show significant changes in comparison with sample 1 (see Figure S.3a).By decreasing the concentration of Zn(NO 3 ) 2 (e.g., from 0.1 M to 0.05 M, while keeping NaOH concentration (0.5 M) unchanged) a large number of irregular HFs (see Figure2(d)) were obtained.The typical morphology of the HFs in this sample is illustrated in Figure2(e), from which different structures have been observed in comparison with sample 1.Firstly, ZnO nano ribbons forming thick bundles extend out from the HFs.Secondly, the nanosheets that construct the HFs become much thicker than those in sample 1. Thirdly, substructures such as small nanorods and nanoparticles form on the surfaces of the HF nanosheets.The core of the HFs is solid, similar to that in sample 1 (Figure2(f)).We have confirmed that by increasing the concentration of Zn(NO 3 ) 2 (e.g., from 0.1 M to 0.15 M); however, the morphology of the HFs was not significantly changed.Instead, a small amount of by-products such as fan-shaped ZnO structures (see Figure S.3b) have been generated.3.2.2.Temperature Effects.By increasing the reaction temperature and keeping other synthesis conditions unchanged (same as sample 1), the size of the HFs decreased (4∼5 μm at 40 • C) as shown in Figure 2(g), and the thickness of the nanosheets increased (Figure 2(h)).The nanosheets contain more defects.Similar to samples 1 and 3, the inner parts of the HFs are solid (Figure 2(i)).Further increasing of the reaction temperature to 60 • C, the products are no longer spherical HFs, instead small-sized (1 to 2 μm) star-like (see Figure S.3c) products were obtained.

Figure 4 :
Figure 4: (a) The cross-sectional and (b) top view SEM images of the photoanode based on sample 2.

Table 1 :Table 2 :3. 4 .
Summary of the properties and the performance of ZnO-HF-based DSCs (sample 1Synthetic parameters for different samples.Sample Temperature ( • C) C NaOH (M) C Zn(NO3) 2 Light Scattering and Absorption Capability.

Figure 4
(a)   shows a typical morphology of the photoanode films based on sample 2. To enhance the link between HFs (see Figure4(b)) and the mechanical strength of the photoanode films, we have added ZnO small particles (about 20 nm in diameter) into different HF samples and fabricated HF photoanode films with thicknesses of about 35.5-38.6 μm.

Figure 6 :
Figure 6: Electrochemical impedance spectra of the DSC devices based on the four samples.

Table 3 :
Property and performance of different photoanode samples in DSCs.