Application of ZnO micro-flowers as scattering layer for ZnO-based dye-sensitized solar cells with enhanced conversion efficiency

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ScienceDirect Solar Energy 101 (2014) 150–159 www.elsevier.com/locate/solener

Application of ZnO micro-flowers as scattering layer for ZnO-based dye-sensitized solar cells with enhanced conversion efficiency Jinlei Xu, Ke Fan, Wenye Shi, Kan Li, Tianyou Peng ⇑ College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, PR China Received 13 July 2013; received in revised form 14 November 2013; accepted 31 December 2013 Available online 16 January 2014 Communicated by: Associate Editor Sam-Shajin Sun

Abstract ZnO micro-flowers were synthesized by a simple solution deposition method without the need of seed or keeping fresh deposition solution. The synthesized ZnO micro-flowers were used as scattering layer for dye-sensitized solar cells (DSSCs) fabricated with ZnO nanoparticles photoanode. The UV–vis diffused reflectance absorption spectra (DRS), electrochemical impedance spectroscopy (EIS) and photoinduced open-circuit voltage decay (OCVD) measurements show that the ZnO micro-flowers/nanoparticles bilayer film-based solar cell has much higher light harvesting efficiency, lower resistance and longer electron lifetimes as compared with the ZnO nanoparticles film-based one, and consequently resulting in an improved conversion efficiency due to the scattering effect of ZnO micro-flowers layer. The simple methods of the fabrication of both the ZnO micro-flowers and the ZnO bilayer film-based solar cell, which was fabricated with photoanode consisting of ZnO nanoparticles as underlayer and ZnO micro-flowers as overlayer, are promising for the development of the low-cost, eco-friendly, and durable devices with high light-to-electricity conversion efficiency. Ó 2014 Elsevier Ltd. All rights reserved. Keywords: Dye-sensitized solar cell; ZnO micro-flower; Bilayer structure; Scattering effect

1. Introduction Due to their low cost and relatively high light-to-electricity conversion efficiency, dye-sensitized solar cells (DSSCs) have attracted intense research interests in the world since O’Regan and Gra¨tzel reported the pioneering work on the promising applications of nano-sized TiO2 porous film electrodes in DSSCs (O’Regan and Gra¨tzel, 1991). Typically, the photoanode of a DSSC is made of semiconductor nanostructured materials (such as nanoparticles, nanotubes, nanowires, nanocones, nanoleaves or their mixture) fabricated on transparent conductive glass (Mir and Salavati-Niasari, 2012; Lamberti et al., 2013; McCune et al., 2012; Chang et al., 2013; Dhas et al., 2011). For many years, TiO2 nanostuctured materials and ⇑ Corresponding author. Tel./fax: +86 27 6875 2237.

E-mail address: [email protected] (T. Peng). 0038-092X/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.12.039

the ruthenium-bipyridyl dye families such as N719, N3 and C101 are the most efficient materials for the photoanode, and have dominated the highly efficient solar cells. For example, the global conversion efficiency of the TiO2-based solar cell by using N719 as sensitizer has exceeded 10% (Ito et al., 2006; Nazeeruddin et al., 2011). Moreover, some other nanostructured semiconductor materials such as ZnO (Liu et al., 2007), SnO2 (Ramasamy and Lee, 2010), SrTiO3 (Yang et al., 2010) and ZnSnO4 (Li et al., 2011), were also exploited to apply in the field of DSSCs as photoanode materials. Among those semiconductors used, ZnO nanoparticles have attracted much more attention since its bandgap is very similar to TiO2. It indicates the possibility of the effective electron injection process in a ZnO-based solar cell (Liu et al., 2007; Chang et al., 2013; Han et al., 2010; He et al., 2010; McCune et al., 2012). More importantly, the distinct advantages of ZnO over TiO2 are its higher

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electron mobility, simpler and more convenient fabrication processes for variable nanostructures and morphologies, and therefore, different morphologies of ZnO nanostructured materials, such as nanoparticles (Lu et al., 2010), nanowires (Weintraub et al., 2010; McCune et al., 2012), nanotubes (Han et al., 2010), aggregated beads (Zheng et al., 2011), hollow spheres (He et al., 2010), tetrapods (Chen et al., 2010; Lee et al., 2010), nanotrees (Ko et al., 2011), and nanocones (Chang et al., 2013) as well as ZnO/ZnO core–shell nanowire array (Guille´n et al., 2013), have been used as photoanode materials for the ZnO-based solar cells. Although various strategies have been applied to improve the ZnO-based solar cell’s performance, the global energy conversion efficiencies are still relatively lower than the TiO2 counterparts, mainly due to the low electrically conducting Zn2+-dye complexes formed on the ZnO surfaces. It leaves lots of room for further improvement of the ZnO-based solar cell’s performances via modifying the nanostructures of ZnO. Very recently, ZnO nanocones with exposed {1 0 1 1} facets were prepared and used as photoanode material in a N719-sensitized solar cell, which displayed a better conversion efficiency (3.84%) than the devices fabricated with commonly used ZnO nanorods with predominantly exposed {1 0 1 0} facets due to the significantly less dye aggregation on the ZnO {1 0 1 1} facets as compared to the other ZnO facets. Alternatively, another effective way to improve the performance of the TiO2-based DSSCs is to coat a scattering layer over the TiO2 nanoparticle film to fabricate bilayer photoanode. Following this strategy, submicrometer/ micrometer-scale particles with diameter of 200–400 nm were used as light scattering materials because the optical absorption can be enhanced to a large extent when TiO2 nanoparticle films were combined with large particles. Many groups have succeeded in improving the TiO2-based solar cell’s efficiency by using light-scattering materials (Fan et al., 2011a,b; Koo et al., 2008; Nishimura et al., 2003). As for the ZnO-based solar cells, micro/nano-textured ZnO thick film was fabricated by one-step self-assembly electrodeposition process, and the corresponding solar cell sensitized with eosin Y exhibited an efficiency of 2.0% (Hosono et al., 2004). Cao’s group developed a nanostructural ZnO film consisting of monodisperse or polydisperse aggregates for the efficient dye adsorption and the enhanced light absorption (Zhang et al., 2008; Chou et al., 2007). In the above systems, the light-scatter function is mainly stemmed from the submicrometer-scale aggregates of ZnO nanoparticles. Nevertheless, ZnO-based solar cell fabricated with photoanode consisting of ZnO nanoparticles as underlayer and ZnO submicrometer/micrometer-scale particles as light-scattering overlayer is seldom reported to the best of our knowledge. Since the shape-controlled syntheses of ZnO can be more easily realized as compared to TiO2 as-mentioned above, and the obtained ZnO submicrometer/micrometerscale materials with variable morphologies have many advantages over the above-mentioned big TiO2 particles

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as the light scattering materials. For example, it has been reported that a double light-scattering-layer ZnO (DLZnO) film consisting of ZnO monodisperse aggregates (MA-ZnO) as underlayer and submicrometer-sized platelike ZnO (SP-ZnO) as overlayer was prepared and used as photoanodes, and an efficiency of 3.44% was achieved by the formation of DL-ZnO film, which is 47% higher than that (2.34%) formed by MA-ZnO alone (Zheng et al., 2009). This result indicates the great role of ZnO scattering layer in the improvement of the ZnO-based DSSC’s performance. However, the syntheses of different morphologies of ZnO micro/nano-structured materials are usually complicated. For example, needing careful preparation of the seed layer and/or renewing the deposition solution to keep the ZnO material growing (Ko et al., 2011; Zheng et al., 2009), which usually limit its wide usage. Herein, we use a simple and facile solution deposition method to prepare ZnO micro-flowers without need of seed or keeping fresh deposition solution. By using the ZnO micro-flower as the scattering layer of a ZnO nanoparticle film-based solar cell, the fabricated ZnO micro-flowers/ nanoparticles bilayer film-based solar cell has much higher light harvesting efficiency, lower resistance and longer electron lifetime as compared with the ZnO nanoparticles film-based one, and consequently resulting in improved conversion efficiency. 2. Experimental 2.1. Materials preparation A typical preparation process of ZnO nanoparticles as follows: 11.74 g of Zn(NO3)26H2O were dissolved into 100 mL anhydrous ethanol, 2.13 g of polyethylene glycol 400 (PEG 400) were added slowly into the resultant solution under violent magnetic stirring, and then 7.36 g of NH4HCO3 were dissolved into a mixture of 100 mL distilled water and 50 mL anhydrous ethanol. The above Zn(NO3)2 solution was added dropwise into the NH4HCO3 solution under violent stirring. The resulting white precipitate was recovered by centrifugation and washed with 1.0 M NH4HCO3 solution and ethanol several times, and then dried at 80 °C for 6 h to produce the white precursor. The dried precursor was further calcined at 400 °C for 2 h to obtain ZnO nanoparticles as described in our previous report (Lu et al., 2010). A typical preparation process of ZnO micro-flowers as follows: 19.36 g of KOH and 11.96 g of Zn(NO3)26H2O was dissolved in 80 mL water, respectively. The resultant two solutions were mixed by adding Zn(NO3)26H2O solution to KOH solution dropwise. A glass slide as the substrate for deposition stood vertically in the above mixture solution, and a piece of zinc-foil as the Zn source was also inserted in the solution with an inclination angle as shown in Scheme 1. After heat treatment at 60 °C for 1 h, plenty of white powder deposited on the glass slide substrate surface, which was taken out from the deposition solution and

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between the photoanode and the counter electrode to avoid short-circuiting. At least five devices fabricated with each kind of ZnO film were prepared in order to obtain statistically results. 2.3. Material characterization and photoelectrochemical measurements

Scheme 1. Schematic diagram of the preparation process of the ZnO micro-flowers via a solution deposition method.

rinsed by water and alcohol, and then dried in the air. Afterward, the white powder was collected by scratching from the substrate to obtain the ZnO micro-flowers powder (Maiti et al., 2011). 2.2. Fabrication of ZnO photoanodes and ZnO-based solar cells 1 g of the obtained ZnO nanoparticles (or ZnO micro-flowers), 5 mL of ethanol, 0.2 mL of acetic acid, 3 g of terpinol and 0.5 g of ethylcellulose were ballmilled for 12 h to obtain a homogeneous paste. The obtained paste was spread on a clean FTO glass (15 X sq1) by using a doctor blading technique. The thicknesses of the films (6 cm  6 cm) were controlled by adhesive tape (Scotch, 50 lm) serving as spacers. After drying in atmosphere, the film was sintered at 500 °C for 1 h to remove the binders in the paste. As for the scattering overlayer, the ZnO micro-flowers paste was spread on the above ZnO nanoparticles film electrode and was sintered at 500 °C for 1 h again. Those resulting ZnO nanoparticles film and the bilayer film were incised into smaller pieces to obtain ZnO film with the same thickness, which was measured with TalyFormvS4C-3D profilometer (U.K.). The thicknesses of the ZnO nanoparticles film and the bilayer film were measured to be 5.1 lm and 10.8 lm, respectively. The thickness of the scattering overlayer should be 5.7 lm by subtracting the ZnO nanoparticles film thickness from the bilayer film thickness. Dye sensitization was achieved by immersing the above electrodes with or without scattering layer in a 0.3 mM N719 dye (Solaronix) in ethanol solution for 8 h, followed by rinsing in ethanol and drying in air. The dye-sensitized electrode was assembled in a typical sandwichtype cell. The identical platinized FTO counter electrode was placed over the dye-sensitized electrode, and the electrolyte, containing 0.5 M LiI, 0.05 M I2, and 0.1 M 4-tert-butylpyridine in 1:1 acetonitrile propylene carbonate, was sandwiched between the photoanode and the platinized counter electrode by firm press. Adhesive tape (approximately 50 lm) was placed

Structure phase analyses with X-ray diffraction (XRD) method were performed on a D8-advance X-ray diffractometer (Bruker) with Cu Ka radiation (k = 0.15418 nm). The microstructures were explored by a high-resolution transmission electron microscope (HRTEM; JEM 2100F). The morphologies of the films were investigated by scanning electron microscope (SEM; JSM-6700F). UV–vis absorption and diffuse reflectance absorption spectra (DRS) were obtained with a Shimadzu UV-3600 UV– vis-NIR spectrophotometer equipped with an integrating sphere. The DSSC was illuminated by light with energy of 100 mW cm2 (AM 1.5) from a 300 W solar simulator (Newport, 91160). The light intensity was determined using a reference monocrystalline silicon cell system (Oriel, US). Computer-controlled Keithley 2400 sourcemeter was employed to collect the photocurrent–voltage (I–V) curves of DSSCs. The active area was 0.16 cm2. To estimate the dye-adsorbed amount on the film electrode, the dye-sensitized electrode was separately immersed into a 0.1 M NaOH solution in a mixed solvent (Vwater: Vethanol = 1:1), which resulted in desorption of N719. The absorbance of the resulting solution was measured by a UV-3600 UV–vis spectrophotometer (Shimadzu, Japan), and the dye-adsorbed amount was determined by the molar extinction coefficient of 1.41  104 dm3 mol1 cm1 at 515 nm as reported previously (Wang et al., 2004). The electrochemical impedance spectroscopy (EIS) measurements were carried out by applying bias of the opencircuit voltage (VOC) without electric current under 100 mW cm2 illumination and were recorded over a frequency range of 0.05–105 Hz with ac amplitude of 10 mV. For the photoinduced open-circuit voltage decay (OCVD) measurements, the illumination was turned off using a shutter after the solar cell was first illuminated to a steady voltage, and then the OCVD curve was recorded. The above measurements were carried out on a CHI-604C electrochemical analyzer. The photon to electricity conversion efficiency was calculated according to Eq. (1). g¼

V OC J SC FF P in

ð1Þ

where g is the global conversion efficiency, VOC, JSC, and FF are the open-circuit voltage, short-circuit current density, and fill factor, respectively. Pin is the incident light energy (100 mW cm2).

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3. Results and discussion 3.1. Microstructure and crystal phase analyses of the obtained materials Fig. 1 shows the SEM images of the obtained ZnO nanoparticles and ZnO micro-flowers. As can be seen from Fig. 1a, the ZnO nanoparticles has relatively uniform particle sizes with an average diameter of 20 nm, and the ZnO micro-flowers deposited on the glass slide substrate by the solution deposition method show micro-flower-like morphology clearly, which is composed of several pencilshaped ZnO nanorods with uniform morphologies and sizes as shown in Fig. 1b. From the SEM images (Fig. 1c) with high magnifications, it can be observed that the diameter of ZnO micro-flower is 5 lm, and the pencillike nanorods have hexagonal cross sections with diameter of 300 nm and length of 3 lm (insets of Fig. 1c). These pencil-like nanorods towards different directions compose of the flower-like ZnO microstructures. As can be seen from Fig. 2, all diffraction peaks in the XRD pattern of the obtained ZnO micro-flowers can be readily indexed as hexagonal wurtzite ZnO (JCPDS, No. 75-576), indicating the high crystallinity of the ZnO micro-flowers derived from the present facile solution deposition process. Fig. 3 shows the TEM and HRTEM images of the obtained ZnO micro-flowers. As can be seen from Fig. 3a, flower-like ZnO microstructures can still be observed, and the lattice fringes of ZnO nanocrystalline can be seen clearly from the HRTEM image (Fig. 3b),

Fig. 2. XRD pattern of the obtained ZnO micro-flowers.

and the spacing of 0.248 nm corresponds to the interplanar distance of (1 0 1) planes of wurtzite ZnO. This observation keeps consistent with the above XRD pattern. Although the obtained sample is not treated by high temperature, the above investigations indicate that a high crystallinity has successfully formed, which is beneficial for the improvement of photoelectrochemical properties of its corresponding film electrode. According to Chattopadhyay’s suggestion (Maiti et al., 2011), Zn(NO3)2 aqueous solution being mixed with KOH solution produces transparent zinc tetrahydroxide 2 (ZnðOHÞ4 , zincket) solution, and the zinc foil in the high concentration of alkaline solution will release sufficient Zn2+ to form ZnðOHÞ2 4 through a continuous dissolution process. Thus overall ZnðOHÞ2 concentration in the 4

Fig. 1. SEM images of the obtained ZnO nanoparticles (a) and ZnO micro-flowers (b and c).

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Fig. 3. TEM (a) and HRTEM (b) images of the obtained ZnO micro-flowers.

solution will enhance with the reaction time due to a continuous release of Zn2+ ions into the solution. When the system reaches a suitable supersaturation level sufficient to overcome the nucleation energy barrier, a number of ZnO nuclei heterogeneously formed on the substrate. Those heterogeneously formed ZnO nucleus on the substrate quickly forms nuclei aggregates by capturing nuclei from the solution under the driving force of surface energy, electrostatic force and so on. In order to minimize the surface energy of the system, higher surface energy facets should have a smaller area as compared to lower energy facets. As a result, ZnO tends to grow as a one dimensional nanostructure elongated along the polarized c-axis with (0 0 0 1) being the top surface and the non-polar surfaces as the side surfaces (Maiti et al., 2011; Ghoshoal et al., 2008), and this anisotropic growth behavior leads to the formation of the present pencil-like nanorods, which might be attributed to continuous diameter shrinkage along the growth direction because the simultaneous growth of ZnO nanostructure on the same would limits the reaction of OH with zinc, and then resulting in the continuous reduction in the supply of zinc precursor from the Zn foil as reported by a similar study previously (Maiti et al., 2011). Comparing with the other method for preparing ZnO nanocrystal, this solution deposition method has the merits of high crystallinity without need of sintering or keeping fresh deposition solution, which can simplify significantly the synthesis process of the ZnO scattering materials with the present micro/nanostructures. 3.2. Microstructure and light reflection analyses of the film electrodes Fig. 4 shows the SEM images of the fabricated ZnO nanoparticles film and ZnO micro-flower scattering layer film. As can be seen the ZnO nanoparticles film displays a morphorlogy with smooth and porous surface, while the ZnO micro-flower scattering layer displays a stack of lager micro-flowers with high roughness. It can be observed that several fragments and aggregation of ZnO

micro-flowers exist in the layer as shown in Fig. 5b, where the fragments may be resulted from the scratching for the collection of ZnO micro-flowers from the slide substrate, and the aggregation should be attributed to the high temperature sintering process of the photoanode. Fig. 5 shows the diffused reflection absorption spectra (DRS) of the obtained ZnO nanoparticles and ZnO micro-flowers. It is clearly observed that in the visible-light region, the light reflection of the ZnO micro-flowers is stronger than that of the ZnO nanoparticles, indicating that ZnO micro-flowers have higher light scattering effect due to its relatively lager particle size and complicated morphology as compared with the ZnO nanoparticles. Therefore, it is expected that this light scattering effect can be used in the ZnO nanoparticles photoanode to enhance the light harvesting efficiency because of the prolonged optical path in the film electrode, which is similar to the situation in the TiO2-based solar cell as reported previously (Fan et al., 2011a,b). Fig. 6 displays UV–vis absorption spectra of the fabricated ZnO nanoparticles film and the ZnO micro-flowers/ nanoparticles bilayer film without dye sensitization. As can be seen, the ZnO bilayer film has higher light absorption than the bare nanoparticles film, indicating that the ZnO micro-flowers can enhance the light scattering, thus it can be imagined that the ZnO micro-flowers film can be used as scattering layer to reflect the penetrating incident light from the ZnO nanoparticles film to compensate the loss of the light harvesting and to prolong the optical path in the film electrode, and then improve the light harvesting efficiency as-mentioned previously (Fan et al., 2011a,b). Moreover, the ZnO micro-flowers scattering layer can also enhance the dye-adsorbed amount of the ZnO nanoparticles films as can be seen from Table 1. Although the bare ZnO micro-flowers electrode only shows limited N719-adsorbed amount (6.4  109 mol cm2), the ZnO micro-flowers/nanoparticles bilayer electrode has a dye-adsorbed amount of 3.4  108 mol cm2, which is higher than that (3.0  108 mol cm2) of the bare ZnO nanoparticles electrode. In other words, the ZnO nanopar-

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Fig. 4. SEM images of the fabricated ZnO nanoparticles film electrode (a) and the ZnO micro-flowers scattering film (b).

specific surface area resulting from the much smaller diameter of the ZnO nanoparticles, while the micro-flowers scattering layer only contributes little to the dye-adsorbed amount in the ZnO bilayer film electrode. 3.3. Electrochemical impedance spectrum (EIS) analyses

Fig. 5. DRS spectra of the obtained ZnO nanoparticles and ZnO microflowers powders.

Fig. 6. UV–vis absorption spectra of the ZnO nanoparticles film electrode and the ZnO bilayer film electrode.

ticles film has much higher dye adsorption capability than the ZnO micro-flowers scattering layer due to the larger

Fig. 7 shows the Nyquist diagrams of the electrochemical impedance spectra (EIS) of the solar cells fabricated with the bare ZnO nanoparticles electrode and the ZnO micro-flowers/nanoparticles bilayer electrode under AM1.5 sun illumination (95 mW cm2). Normally, the Nyquist diagram features three semicircles that in the order of increasing frequency are attributed to the Nernst diffusion within the electrolyte, the electron transfer at the semiconductor oxide/electrolyte interface, and the redox reaction at the platinum counter electrode (Wang et al., 2005). In our EIS diagrams, two obvious semicircles are detected in the Nyquist plots. The semicircle attributed to the Nernst diffusion within the electrolyte is featureless due to the relatively fast diffusion of the electrolyte in the porous films (Zhao et al., 2008). As mentioned above, the semicircle in middle frequency region is related to the resistance (R2) of the accumulation/transport of the injected electrons within ZnO film and the charge transfer across either the ZnO/redox electrolyte interface or the FTO/ ZnO interface, therefore it should be investigated in details as follows (Fan et al., 2011a,b). According to the EIS model reported previously (Park et al., 2005), the corresponding R2 values for the ZnObased solar cells were obtained by fitting with Z-View software (v2.1b, Scribner Associate, Inc.) (Fan et al., 2011a,b), and shown in Table 1. As can be seen from Fig. 7 and

Table 1 The photoelectrochemical parameters of the ZnO-based solar cells fabricated with ZnO nanoparticles film or the ZnO micro-flowers/nanoparticles bilayer film photoanode. Solar cell

R2/X

JSC/mA cm2

VOC/V

FF

g/%

Dye-loading/108 mol cm2

ZnO nanoparticles film-based cell ZnO bilayer film-based cell

55.85 42.43

8.03 10.30

0.49 0.53

0.56 0.55

2.31 ± 0.16 3.20 ± 0.20

3.0 3.4

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Fig. 7. EIS spectra of the ZnO-based solar cells fabricated with the ZnO nanoparticles film or the ZnO micro-flowers/nanoparticles bilayer film photoanode.

Table 1, the semicircle in the middle frequency region of the ZnO bilayer electrode is smaller than that of the bare ZnO nanoparticles electrode. The fitted R2 values of the ZnO nanoparticles electrode and the ZnO bilayer electrode are 55.85 X and 42.43 X, respectively. The smaller semicircle of the ZnO bilayer electrode exhibits the lower R2 value, indicating more efficient charge-transfer process at the dye-coated ZnO bilayer/electrolyte interface. The lower resistance of the ZnO bilayer electrode is favorable for the electron transport through a longer distance with less diffusive hindrance to some extent, and thus probably leading to the reduction of electron recombination and the capture of more effective electrons as compared with the bare ZnO nanoparticles electrode (Zheng et al., 2009). 3.4. Open-circuit voltage decay (OCVD) curve analyses Fig. 8a shows the photoinduced open-circuit voltage decay (OCVD) curves of the solar cells fabricated with the bare ZnO nanoparticles film and the ZnO bilayer film. To estimate the electron lifetime (sn) of the solar cell, the corresponding electron lifetime–voltage curves (Fig. 8b) can be obtained from Fig. 8a by Eq. (2).  1 k B T dV OC sn ¼  ð2Þ e dt where kB is the Boltzmann constant, T is the temperature, e is the electron charge (Bisquert et al., 2004). As can be seen from Fig. 8a, the VOC values of both solar cells decay significantly when the illumination is interrupted, indicating the recombination processes of the photoexcited electrons in the solar cells. After decaying for 60 s, the VOC value of the ZnO bilayer film-based solar cell still maintains 0.1 V, while the VOC value of the bare ZnO nanoparticles film-based one decays near to 0. It implies the ZnO bilayer electrode possesses less recombination probability and longer lifetime of the photoexcited electrons than the bare ZnO nanoparticles electrode. Especially in the low VOC region (<0.2 V), the linear dependence

Fig. 8. OCVD (a) and electron lifetime-VOC (b) curves of the ZnO-based solar cells fabricated with ZnO nanoparticles film or the ZnO microflowers/nanoparticles bilayer film photoanode.

of the electron lifetime on VOC turned into a curved one because the charge transfer process is mainly governed by the distribution of surface traps (Fan et al., 2010). As stated in the above EIS analyses, more efficient charge-transfer process occurs at the dye-coated ZnO bilayer/electrolyte interface than that of the bare ZnO electrode, which can result in less charge recombination on the surface traps and then the prolonged electron lifetime. The UV–vis and DRS spectra shown in Figs. 5 and 6 also reveal that the ZnO micro-flowers layer can reflect the incident light to the ZnO nanoparticles film and enhance the light harvesting efficiency, which can lead to more photoexcited electrons. Therefore, the ZnO bilayer electrode-based solar cell possesses faster electron transfer and lower charge recombination probability as compared with the bare ZnO nanoparticles film-based one. 3.5. Photovoltaic performance analyses A series of five parallel solar cells sandwiched with each kind of ZnO film were fabricated for the photovoltaic measurement. Fig. 9 shows the typically dark current–voltage curves and the photocurrent density–voltage curves of the solar cells fabricated with the bare ZnO nanoparticles film and the ZnO bilayer film after N719 sensitization. The corresponding values of short-circuit current (Jsc), open-circuit voltage (Voc) and fill factor (FF) of the solar cells are listed

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in Table 1. To show the error analysis results, the average conversion efficiencies (g) of the parallel solar cells with the respective standard deviations are also listed in Table 1. As can be seen, the reproducibility of the efficiency was confirmed by measuring five such parallel solar cells within an uncertainty of ±7%. As can be seen from Fig. 9a, the dark current of the ZnO bilayer film-based solar cell shows much slower decreasing trends than the ZnO nanoparticle film-based one. It can be attributed to the scattering effect of the ZnO micro-flowers layer over the ZnO nanoparticles film, which leads to the enhanced light harvesting efficiency and the reduced charge recombination probability as mentioned above, and therefore the dark current density of the ZnO bilayer film-based solar cell is decreased consequently. Since it has been reported that the suppression of dark current by introducing a compact TiO2 underlayer between the FTO and the TiO2 nanocrystals increased the VOC by 27 mV in a DSSC using I =I 3 as a redox couple (Ito et al., 2005), it can be expected that the present reduced dark current by the ZnO micro-flowers overlayer would also enhance the VOC of this ZnO film-based solar cell. This conjecture can be validated by the corresponding data shown in Table 1. According to Zaban’s opinion (Zaban et al., 2003), VOC of a solar cell under constant illumination corresponds to the increase of the quasi-Fermi level (EFn) of the semiconductor with respect to the dark value (EF0), which equals

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the electrolyte redox energy (EF0 = Eredox), and therefore VOC can be written as Eq. (3):   EFn  EF0 k B T n ð3Þ V OC ¼ ¼ ln e n0 e where kBT is the thermal energy, e is the positive elementary charge, n0 is the concentration in the dark, and n is the free electron density in the photoanode film under illumination. The above n value is affected mainly by two processes. (1) Electron photogeneration achieved by the electron injection from the photoexcited dye to the semiconductor, which can be maintained at a stationary rate because the reduced electrolyte species (i.e. I ions) are able to regenerate the oxidized dye. (2) Photoinjected electron recombination by reaction with the electrolyte-oxidized species (i.e. I2 or I 3 ), which is thought to be predominant in comparison with electron recapture by the oxidized dye because the regeneration of the oxidized dye by I is significantly faster than the charge transfer from TiO2 to the oxidized dye (Ito et al., 2005; Zaban et al., 2003). Namely, the n value increases by the electron photogeneration at a rate (aabsI0, where I0 is the incident light intensity, and aabs is the dye’s absorption coefficient), and decreases by the photoinjected electron recombination at a rate U(n) that depends on the electron density in the electrode (Zaban et al., 2003). Clearly, the light absorption and the charge recombination properties have a major impact on the n value, and then on the dark current and the VOC. As for the present situation, the reduced dark current of the ZnO bilayer film-based solar cell in comparison with the nanoparticles film-based one indicates that ZnO microflowers overlayer not only decreases the charge recombination, but also enhances the light harvesting efficiency due to its light scattering effect. Both of them (the reduced charge recombination and the enhanced light harvesting efficiency) could lead to an enhanced free electron density (n) in the bilayer film, and then to the increased VOC value, which is consistent with the above observations from the EIS spectra and the OCVD curves. As can be seen from Table 1, all of photoelectrochemical parameters (such as JSC, VOC, and FF) of the ZnO nanoparticles-based solar cell are enhanced with different degrees, and therefore, the average global conversion efficiency was improved from 2.31% to 3.20% after the ZnO micro-flowers scattering layer was loaded. This result indicates that coating a ZnO micro-flowers scattering layer over the ZnO nanoparticle film to fabricate bilayer film-based solar cell is a promising way to improve the performance of the ZnO-based DSSCs. 4. Conclusions

Fig. 9. Dark current (a) and I–V (b) curves of the ZnO-based solar cells fabricated with ZnO nanoparticles film or the ZnO micro-flowers/ nanoparticles bilayer film photoanode.

ZnO micro-flower-shaped particles were prepared by a simple solution deposition method without need of seed or keeping fresh solution, and a ZnO bilayer film electrode consisting of ZnO micro-flowers film as a scattering layer over a ZnO nanoparticles film as an underlayer was suc-

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