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The dye adsorption optimization of ZnO nanorod-based dye-sensitized solar cells
Available online at www.sciencedirect.com
ScienceDirect Solar Energy 105 (2014) 14–19 www.elsevier.com/locate/solener
The dye adsorption optimization of ZnO nanorod-based dye-sensitized solar cells Xiang Fang a, Yan Li a, Shuai Zhang a,b, Li Bai a, Ningyi Yuan a,b,⇑, Jianning Ding a,b,⇑ a
Center for Low-Dimensional Materials, Micro-Nano Devices and Systems, Jiangsu Key Laboratory for Solar Cell Materials and Technology, Changzhou University, Changzhou 213164, Jiangsu, China b Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou 213164, Jiangsu, China Received 29 September 2013; received in revised form 17 February 2014; accepted 31 March 2014
Communicated by: Associate Editor Sam-Shajin Sun
Abstract A critical factor in enhancing the conversion efficiency of ZnO nanorod array-based dye-sensitized solar cells (DSSCs) is the dye adsorption, which is determined by the dye adsorptivity of single nanorod and the density of nanorod arrays. We show that the density of the nanorod arrays increases gradually with increasing seed layer thickness, resulting in an enlarged surface area for dye adsorption. In addition, increasing the number of desorption/adsorption cycles can also lead to a continuous increase in the total amount of dye loading. As a result, we observe a significant enhancement in the photocurrent densities and conversion efficiencies of ZnO nanorod array-based DSSCs. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: ZnO nanorods; Dye-sensitized solar cells; Seed layers; Dye adsorption
1. Introduction Compared to conventional silicon solar cells, dye-sensitized solar cells (DSSCs) have been attracting increasing attention owing to their advantages such as low cost, high stability and easy fabrication (Regan and Gratzel, 1991). The photoanode of a DSSC is conventionally a TiO2 porous membrane (Hwang et al., 2012; Craig et al., 2003; Anal et al., 2013; Huang et al., 2013). However, ZnO is considered to be the most promising alternative. In recent years, various ZnO nanostructure-based photoeletrodes
⇑ Corresponding authors. Address: Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou 213164, Jiangsu, China. Tel./fax: +86 0519 86450008. E-mail addresses: [email protected] (N. Yuan), [email protected]. cn (J. Ding).
http://dx.doi.org/10.1016/j.solener.2014.03.039 0038-092X/Ó 2014 Elsevier Ltd. All rights reserved.
have been synthesized to enhance the photovoltaic performance of solar cells (Il-Doo Kim et al., 2007; Hui Li et al., 2012; Lanlan et al., 2010; Giannouli and Spiliopoulou, 2012; Zhang et al., 2009). Among these, the use of highly ordered one-dimensional (1D) nanostructure arrays is more advantageous than using porous electrode materials. 1D ZnO nanostructure arrays can increase the photon diffraction, which not only leads to enhanced light absorption, but also shortens the electron diffusion distance, thereby reducing charge recombination (Yang et al., 2013; Guo et al., 2012). Baxter and Aydil (2005) first fabricated ZnO nanorod-based DSSC in 2005 by using a hydrothermal method; they achieved an overall photoelectric conversion efficiency of 0.5%. Since then, nanostructures of 1D ZnO nanorods and its derivatives (Martinson et al., 2007; Schlur et al., 2013; Sadia Ameen et al., 2012; McCune et al., 2012; Guo et al., 2013) have been widely
X. Fang et al. / Solar Energy 105 (2014) 14–19
investigated as photoelectrodes for enhancing the DSSC performance. Nonetheless, ZnO nanorod array based DSSCs usually have relatively lower photoelectric conversion efficiencies, which may be caused by the smaller specific surface area and the reduced dye loading. To solve this problem, many researchers set out to synthesize longer nanorods (Qiu et al., 2010; Weiguang et al., 2011; Gonzalez-Valls et al., 2011). Some others have tried to prepare hierarchical nanostructures, which show slightly improved dye adsorption (Guerin et al., 2012; Huu et al., 2013). Except for enlarging the adsorption area, optimization of the nanorod/dye interface provide another promising measure. It is well understood that the dye is adsorbed onto the nanorods through the interaction between the hydroxyl groups on ZnO surface and the carboxylic groups on the peripheral ligands of the dye molecule (Gratzel, 2000). Indeed, some attempts have been made to optimize the interface before dye adsorption, such as exposure to a plasma (Wang and Lin, 2010), TiCl4 post-treatment (Sommeling et al., 2006) and modification with various materials (HCl, NH4OH, etc.) (Jang et al., 2013). Here we propose a multiple optimization of nanorod/dye interface through the desorption/adsorption processes, which is beneficial for a single monolayer of attached dye molecules and help to increase the total dye adsorption of the arrays. In this paper, we study the effect of the number of desorption/adsorption cycles on dye loading. In addition, we also studied the influence of seed layer thickness on nanorod density, with the purpose of increasing dye loading efficiency. 2. Experimental section 2.1. Synthesis of ZnO nanorod arrays Firstly, ZnO seed layers with different thickness (10 nm, 20 nm, 50 nm, 75 nm, 100 nm) were deposited on fluorinedoped SnO2 (FTO) conductive glass substrates using atomic layer deposition (ALD). Prior to the deposition of seed layers, the FTO substrates were cleaned by successive ultrasonication in acetone, ethanol and deionized water for 15 min. Then the ZnO nanorod arrays were grown using a hydrothermal method. The growth solution was a mixture of 0.025 M zinc nitrate (Shanghai Sinpeuo Fine Chemical Co., Ltd.) and 0.025 M hexamethylenetetramine (HMTA, Jiangsu Yonghua Fine Chmical Co., Ltd.). The FTO substrates with seed layers were immersed into the growth solution and sealed in a glass bottle. The reaction temperature was maintained at 90 °C for 24 h. After the growth, the samples were taken out from the solution and immediately rinsed with deionized water and ethanol to remove the residues on the surface, followed by drying in air at 90 °C for 10 min. Finally, the samples were sintered at 450 °C for 30 min to enhance the crystallization of obtained nanorod arrays and reinforce the contact between the ZnO films and the substrates.
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2.2. Fabrication of ZnO-nanorod-based DSSCs Di-tetrabutylammonium cis-bis (isothiocyanato) bis (2,20 -bipyridyl-4,40 -dicarboxylato)ruthenium (II) bistetrabutylammonium (N719) was used as a sensitizer. ZnObased photoelectrodes were prepared by immersing the ZnO nanorod array films grown on FTO glasses in the 0.3 mM ethanolic solution of N719 for 2 h. After sensitization, the photoelectrodes were rinsed with ethanol to remove excess dye and dried. The sensitization process was repeated several times (1–5 cycles) to investigate its effect on dye loading. The specific steps were as follows: (1) desorption: after initial sensitization, the samples were rinsed with deionized water, followed by drying; (2) readsorption: the dried samples were immersed in 0.3 mM N719 solution for 1 h; and (3) repeating the desorption/adsorption cycle. The dye-absorbed photoelectrodes were assembled into a typical sandwich-type cell (Seigo Ito et al., 2008). FTO substrates with Pt layers were used as counter electrodes. A 100 lm – thick Surlyn spacer separated the photoelectrode and the counter electrode. The electrolyte solution, which comprised of 0.5 M lithium iodide, 0.05 M iodine, and 0.5 M 4-tert-butylpyridine in acetonitrile, was injected between the electrodes by capillary action. In this case, the active area of the cell was typically 0.25 cm2. 2.3. Characterization The morphologies and structures of samples were characterized using field-emission scanning electron microscopy (FESEM). The UV–Vis absorption spectra were measured on a UV2450 spectrophotometer. The total amount of dye adsorbed was determined by immersing the ZnO films into a 0.1 M NaOH solution in water/ethanol (1:1) and then measuring the absorption spectrum. The photovoltaic properties of the fabricated DSSCs were measured using a Keithley model 2400 multisource meter under a simulated AM 1.5G illumination with an intensity of 100 mW cm1. 3. Results and discussion The typical top-view FESEM images of well-aligned ZnO nanorod arrays grown on seed layers of varying thicknesses are shown in Fig. 1a–e. As seen as in Fig. 1a–c, the nanorods grown on seed layers of 10–50 nm have a diameter ranging between 100 and 250 nm, with an average value of about 200 nm. When the thickness of seed layers is larger than 75 nm, the obtained nanorods become more uniform and finer in the diameter, about 100 nm (seen in Fig. 1d–e). As known, the morphology of substrates plays a crucial role in the ZnO nanorods growth process, yet the seed layers with enough thickness can screen the effect of substrates and lead to formation of homogeneous nanorods (Xiang et al., 2013). Besides, with increasing the thickness of the seed layers, the density of the obtained nanorods increased gradually. Fig. 1f–j showed the
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X. Fang et al. / Solar Energy 105 (2014) 14–19
Fig. 1. FESEM images of ZnO nanorod arrays grown on ZnO seed layers with different thickness: (a) 10 nm, (b) 20 nm, (c) 50 nm, (d) 75 nm, (e) 100 nm. (f)–(j) shows the corresponding cross-section FESEM images of the samples in (a)–(e) respectively.
corresponding cross-section FESEM images of nanorods shown in Fig. 1a–e. It indicate that almost all ZnO
nanorods have a uniform length of 5 lm. It means that the thickness of seed layers have little impact on the length
X. Fang et al. / Solar Energy 105 (2014) 14–19
of obtained ZnO nanorods, but can profoundly affect their diameter and density. Fig. 2 shows the optical measurements (reflectance (Fig. 2a) and transparence (Fig. 2b)) of the ZnO nanorod array films in the visible light region (400–800 nm). The minimum reflectance and transparency was obtained when the seed layer of the obtained ZnO film is 100 nm thick. This result illustrates that increasing the thickness of the seed layers can help to improve the light absorption of the obtained films. Table 1 lists the parameter values of dye-sensitized solar cells prepared using ZnO nanorod arrays grown on seed layers with various thicknesses. Under each condition, five pieces of solar cells are assembled, and the exactly parameter values are determined by the average of their test results. The uncertainties in the results are caused by minute differences in the morphology of ZnO films, especially the length of nanorods, and the assembly of solar cells (Giannouli, 2013). Fig. 3 shows the dependence of photocurrent–voltage (J–V) characteristics of a ZnO nanorod
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Table 1 Parameters of dye-sensitized solar cells prepared using ZnO nanorod arrays grown on seed layers with various thicknesses. (nm)
Jsc (mA cm2)
Voc (V)
FF
10 20 50 75 100
1.85 1.95 2.05 2.10 2.08
0.55 0.55 0.55 0.55 0.55
0.37 0.37 0.36 0.36 0.36
(±0.10) (±0.11) (±0.10) (±0.11) (±0.13)
(±0.02) (±0.01) (±0.01) (±0.02) (±0.01)
g (%) (±0.05) (±0.01) (±0.03) (±0.02) (±0.01)
0.38 0.39 0.40 0.42 0.42
(±0.02) (±0.01) (±0.04) (±0.02) (±0.03)
Fig. 3. J–V characteristics of solar cells prepared using ZnO nanorod arrays grown on seed layers with various thickness.
Fig. 2. Optical measurements of ZnO nanorod films grown on seed layers with various thickness: (a) reflectance, (b) transmittance.
DSSC on the thickness of the seed layers. The images show an apparent increasing trend, with an obvious increase in the short-circuit current density (Jsc), little change in the open-circuit voltage (Voc), and a pronounced increase in the conversion efficiency (g) with an increase in the ZnO seed layer thickness from 10 nm to 100 nm. The cell with a 75 nm thick seed layer exhibit the maximum g of 0.42%, Jsc of 2.10 mA cm2, Voc of 0.55 V and FF (fill factor) of 0.36. When the thickness of seed layer is larger than 75 nm, the parameter values of DSSCs show little changes. The larger values of Jsc observed for ZnO nanorod electrodes with thicker seed layers, is ascribed to the denser growth of ZnO nanorods, which not only enhanced dye loading as a result of the enlarged surface area, but also enhanced the absorption of incident light resulting from the multi-scattering effect. It is interesting to find that repeating the desorption/ adsorption process is beneficial for enhancing dye loading. Besides increasing the thick of ZnO seed layers, sensitizing the ZnO nanorod array films more times can also result in enhanced dye adsorption. The dye adsorption of ZnO electrodes after the first cycle of sensitization is 5.57 108 mol cm2. Because the deionized water can give rise to hydrolysis of linkages formed between carboxylic acid groups of the dye and the surface of ZnO nanorods, the amount of adsorbed dye will initially decrease after rinsed by water. However, after the second sensitization, the total amount of dye actually gets a modest increase, being
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X. Fang et al. / Solar Energy 105 (2014) 14–19
5.91 108 mol cm2. When the desorption/adsorption process is repeated some more times, a continual increase in dye loading is achieved. As shown in Fig. 4, the amount of dye loading is 6.36 108 mol cm2 and 6.75 108 mol cm2 for 3 and 4 cycles respectively. The behavior may be interpreted as the desorption of weakly bound dye molecules on ZnO on rinsing with water. During readsorption, the molecules subsequently reorganize and trend to form a single monolayer of more strongly attached molecules (Bazzan et al., 2011). Moreover, the ZnO nanorods do not have enough hydroxyl groups on the surface. When exposed to water, the ZnO films can be covered with more hydroxyl groups, which are formed when the water molecules break up and form H+ and –OH species. The introducing of more hydroxyl groups resultantly makes the dye loading density increase. It is worth noticing that the adsorption saturates after 4 cycles of sensitization, and the total amount of dye adsorbed reduces a little after the fifth cycle. The current–voltage characteristics of the assembled photovoltaic cells, containing ZnO electrodes sensitized over varying number of cycles, are shown in Fig 5. Clearly, the short-circuit current density gradually increases with the number of desorption/adsorption cycles (1–4). As shown in Table 2, the maximum Jsc (3.31 mA cm2) obtained after four cycles is almost 1.6 times of that obtained after the first cycle. N719 anchors to the ZnO surface through the four carboxyl groups. Rinsing with water during the desorption/adsorption cycles changes the binding behavior of N719 onto ZnO, and prevents the formation of Zn2+/dye aggregates (Scholin et al., 2011). Hence a similar increasing trend is also observed for the open-circuit voltage and fill factor. The Voc increases from 0.55 V to 0.65 V and the FF increases from 0.36 to 0.40 when the sensitization cycle increases from 1 to 4. As a result, an approximately 100% increase in conversion efficiency is achieved. In accordance with the result shown in Fig. 4, the values of Jsc, FF and g decrease slightly after five cycles.
Fig. 5. The current–voltage curves of dye-sensitized solar cells based on ZnO nanorod array films after different sensitization cycles.
Table 2 Parameters of dye-sensitized solar cells based on ZnO nanorod array films after different sensitization cycles.
1 2 3 4 5
cycle cycles cycles cycles cycles
Jsc (mA cm2)
Voc (V)
FF
2.10 2.45 2.90 3.31 3.24
0.55 0.57 0.61 0.64 0.65
0.36 0.37 0.39 0.40 0.39
(±0.11) (±0.15) (±0.09) (±0.06) (±0.15)
(±0.02) (±0.01) (±0.02) (±0.01) (±0.03)
g (%) (±0.02) (±0.02) (±0.03) (±0.03) (±0.03)
0.42 0.52 0.69 0.84 0.82
(±0.02) (±0.07) (±0.05) (±0.09) (±0.06)
4. Conclusions In summary, we have developed new strategies for improving performance of ZnO nanorod-based dye-sensitized solar cells. Firstly, thicker ZnO seed layers is employed to adjust the size and density of the obtained nanorods. The resulting nanorod arrays exhibit enlarged surface area, which can accommodate slightly larger amount of dye. In addition, we also find that increasing the number of dye adsorption/desorption cycles improves the concentration of dye molecules adsorbed on the surface of the ZnO films. The combination of the above-mentioned strategies results in an increase in the photocurrent density and power conversion efficiency. The enhanced solar cell performance is due to the increase in short-circuit current density, as well as the open-circuit voltage. The main reasons for the increase in open-circuit voltage are the optimization of the bonding behavior onto ZnO nanorods and the reduction in formation of Zn2+/dye aggregates at the semiconductor surface. Acknowledgements
Fig. 4. UV–Vis absorption spectra of dye desorbed from ZnO nanorod array films after various sensitization cycles.
This work was supported by the National High Technology Research and Development Program 863 of China (2011AA050511), Jiangsu “333” Project, the National Natural Science Foundation of China
X. Fang et al. / Solar Energy 105 (2014) 14–19
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ScienceDirect Solar Energy 105 (2014) 14–19 www.elsevier.com/locate/solener
The dye adsorption optimization of ZnO nanorod-based dye-sensitized solar cells Xiang Fang a, Yan Li a, Shuai Zhang a,b, Li Bai a, Ningyi Yuan a,b,⇑, Jianning Ding a,b,⇑ a
Center for Low-Dimensional Materials, Micro-Nano Devices and Systems, Jiangsu Key Laboratory for Solar Cell Materials and Technology, Changzhou University, Changzhou 213164, Jiangsu, China b Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou 213164, Jiangsu, China Received 29 September 2013; received in revised form 17 February 2014; accepted 31 March 2014
Communicated by: Associate Editor Sam-Shajin Sun
Abstract A critical factor in enhancing the conversion efficiency of ZnO nanorod array-based dye-sensitized solar cells (DSSCs) is the dye adsorption, which is determined by the dye adsorptivity of single nanorod and the density of nanorod arrays. We show that the density of the nanorod arrays increases gradually with increasing seed layer thickness, resulting in an enlarged surface area for dye adsorption. In addition, increasing the number of desorption/adsorption cycles can also lead to a continuous increase in the total amount of dye loading. As a result, we observe a significant enhancement in the photocurrent densities and conversion efficiencies of ZnO nanorod array-based DSSCs. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: ZnO nanorods; Dye-sensitized solar cells; Seed layers; Dye adsorption
1. Introduction Compared to conventional silicon solar cells, dye-sensitized solar cells (DSSCs) have been attracting increasing attention owing to their advantages such as low cost, high stability and easy fabrication (Regan and Gratzel, 1991). The photoanode of a DSSC is conventionally a TiO2 porous membrane (Hwang et al., 2012; Craig et al., 2003; Anal et al., 2013; Huang et al., 2013). However, ZnO is considered to be the most promising alternative. In recent years, various ZnO nanostructure-based photoeletrodes
⇑ Corresponding authors. Address: Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou 213164, Jiangsu, China. Tel./fax: +86 0519 86450008. E-mail addresses: [email protected] (N. Yuan), [email protected]. cn (J. Ding).
http://dx.doi.org/10.1016/j.solener.2014.03.039 0038-092X/Ó 2014 Elsevier Ltd. All rights reserved.
have been synthesized to enhance the photovoltaic performance of solar cells (Il-Doo Kim et al., 2007; Hui Li et al., 2012; Lanlan et al., 2010; Giannouli and Spiliopoulou, 2012; Zhang et al., 2009). Among these, the use of highly ordered one-dimensional (1D) nanostructure arrays is more advantageous than using porous electrode materials. 1D ZnO nanostructure arrays can increase the photon diffraction, which not only leads to enhanced light absorption, but also shortens the electron diffusion distance, thereby reducing charge recombination (Yang et al., 2013; Guo et al., 2012). Baxter and Aydil (2005) first fabricated ZnO nanorod-based DSSC in 2005 by using a hydrothermal method; they achieved an overall photoelectric conversion efficiency of 0.5%. Since then, nanostructures of 1D ZnO nanorods and its derivatives (Martinson et al., 2007; Schlur et al., 2013; Sadia Ameen et al., 2012; McCune et al., 2012; Guo et al., 2013) have been widely
X. Fang et al. / Solar Energy 105 (2014) 14–19
investigated as photoelectrodes for enhancing the DSSC performance. Nonetheless, ZnO nanorod array based DSSCs usually have relatively lower photoelectric conversion efficiencies, which may be caused by the smaller specific surface area and the reduced dye loading. To solve this problem, many researchers set out to synthesize longer nanorods (Qiu et al., 2010; Weiguang et al., 2011; Gonzalez-Valls et al., 2011). Some others have tried to prepare hierarchical nanostructures, which show slightly improved dye adsorption (Guerin et al., 2012; Huu et al., 2013). Except for enlarging the adsorption area, optimization of the nanorod/dye interface provide another promising measure. It is well understood that the dye is adsorbed onto the nanorods through the interaction between the hydroxyl groups on ZnO surface and the carboxylic groups on the peripheral ligands of the dye molecule (Gratzel, 2000). Indeed, some attempts have been made to optimize the interface before dye adsorption, such as exposure to a plasma (Wang and Lin, 2010), TiCl4 post-treatment (Sommeling et al., 2006) and modification with various materials (HCl, NH4OH, etc.) (Jang et al., 2013). Here we propose a multiple optimization of nanorod/dye interface through the desorption/adsorption processes, which is beneficial for a single monolayer of attached dye molecules and help to increase the total dye adsorption of the arrays. In this paper, we study the effect of the number of desorption/adsorption cycles on dye loading. In addition, we also studied the influence of seed layer thickness on nanorod density, with the purpose of increasing dye loading efficiency. 2. Experimental section 2.1. Synthesis of ZnO nanorod arrays Firstly, ZnO seed layers with different thickness (10 nm, 20 nm, 50 nm, 75 nm, 100 nm) were deposited on fluorinedoped SnO2 (FTO) conductive glass substrates using atomic layer deposition (ALD). Prior to the deposition of seed layers, the FTO substrates were cleaned by successive ultrasonication in acetone, ethanol and deionized water for 15 min. Then the ZnO nanorod arrays were grown using a hydrothermal method. The growth solution was a mixture of 0.025 M zinc nitrate (Shanghai Sinpeuo Fine Chemical Co., Ltd.) and 0.025 M hexamethylenetetramine (HMTA, Jiangsu Yonghua Fine Chmical Co., Ltd.). The FTO substrates with seed layers were immersed into the growth solution and sealed in a glass bottle. The reaction temperature was maintained at 90 °C for 24 h. After the growth, the samples were taken out from the solution and immediately rinsed with deionized water and ethanol to remove the residues on the surface, followed by drying in air at 90 °C for 10 min. Finally, the samples were sintered at 450 °C for 30 min to enhance the crystallization of obtained nanorod arrays and reinforce the contact between the ZnO films and the substrates.
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2.2. Fabrication of ZnO-nanorod-based DSSCs Di-tetrabutylammonium cis-bis (isothiocyanato) bis (2,20 -bipyridyl-4,40 -dicarboxylato)ruthenium (II) bistetrabutylammonium (N719) was used as a sensitizer. ZnObased photoelectrodes were prepared by immersing the ZnO nanorod array films grown on FTO glasses in the 0.3 mM ethanolic solution of N719 for 2 h. After sensitization, the photoelectrodes were rinsed with ethanol to remove excess dye and dried. The sensitization process was repeated several times (1–5 cycles) to investigate its effect on dye loading. The specific steps were as follows: (1) desorption: after initial sensitization, the samples were rinsed with deionized water, followed by drying; (2) readsorption: the dried samples were immersed in 0.3 mM N719 solution for 1 h; and (3) repeating the desorption/adsorption cycle. The dye-absorbed photoelectrodes were assembled into a typical sandwich-type cell (Seigo Ito et al., 2008). FTO substrates with Pt layers were used as counter electrodes. A 100 lm – thick Surlyn spacer separated the photoelectrode and the counter electrode. The electrolyte solution, which comprised of 0.5 M lithium iodide, 0.05 M iodine, and 0.5 M 4-tert-butylpyridine in acetonitrile, was injected between the electrodes by capillary action. In this case, the active area of the cell was typically 0.25 cm2. 2.3. Characterization The morphologies and structures of samples were characterized using field-emission scanning electron microscopy (FESEM). The UV–Vis absorption spectra were measured on a UV2450 spectrophotometer. The total amount of dye adsorbed was determined by immersing the ZnO films into a 0.1 M NaOH solution in water/ethanol (1:1) and then measuring the absorption spectrum. The photovoltaic properties of the fabricated DSSCs were measured using a Keithley model 2400 multisource meter under a simulated AM 1.5G illumination with an intensity of 100 mW cm1. 3. Results and discussion The typical top-view FESEM images of well-aligned ZnO nanorod arrays grown on seed layers of varying thicknesses are shown in Fig. 1a–e. As seen as in Fig. 1a–c, the nanorods grown on seed layers of 10–50 nm have a diameter ranging between 100 and 250 nm, with an average value of about 200 nm. When the thickness of seed layers is larger than 75 nm, the obtained nanorods become more uniform and finer in the diameter, about 100 nm (seen in Fig. 1d–e). As known, the morphology of substrates plays a crucial role in the ZnO nanorods growth process, yet the seed layers with enough thickness can screen the effect of substrates and lead to formation of homogeneous nanorods (Xiang et al., 2013). Besides, with increasing the thickness of the seed layers, the density of the obtained nanorods increased gradually. Fig. 1f–j showed the
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X. Fang et al. / Solar Energy 105 (2014) 14–19
Fig. 1. FESEM images of ZnO nanorod arrays grown on ZnO seed layers with different thickness: (a) 10 nm, (b) 20 nm, (c) 50 nm, (d) 75 nm, (e) 100 nm. (f)–(j) shows the corresponding cross-section FESEM images of the samples in (a)–(e) respectively.
corresponding cross-section FESEM images of nanorods shown in Fig. 1a–e. It indicate that almost all ZnO
nanorods have a uniform length of 5 lm. It means that the thickness of seed layers have little impact on the length
X. Fang et al. / Solar Energy 105 (2014) 14–19
of obtained ZnO nanorods, but can profoundly affect their diameter and density. Fig. 2 shows the optical measurements (reflectance (Fig. 2a) and transparence (Fig. 2b)) of the ZnO nanorod array films in the visible light region (400–800 nm). The minimum reflectance and transparency was obtained when the seed layer of the obtained ZnO film is 100 nm thick. This result illustrates that increasing the thickness of the seed layers can help to improve the light absorption of the obtained films. Table 1 lists the parameter values of dye-sensitized solar cells prepared using ZnO nanorod arrays grown on seed layers with various thicknesses. Under each condition, five pieces of solar cells are assembled, and the exactly parameter values are determined by the average of their test results. The uncertainties in the results are caused by minute differences in the morphology of ZnO films, especially the length of nanorods, and the assembly of solar cells (Giannouli, 2013). Fig. 3 shows the dependence of photocurrent–voltage (J–V) characteristics of a ZnO nanorod
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Table 1 Parameters of dye-sensitized solar cells prepared using ZnO nanorod arrays grown on seed layers with various thicknesses. (nm)
Jsc (mA cm2)
Voc (V)
FF
10 20 50 75 100
1.85 1.95 2.05 2.10 2.08
0.55 0.55 0.55 0.55 0.55
0.37 0.37 0.36 0.36 0.36
(±0.10) (±0.11) (±0.10) (±0.11) (±0.13)
(±0.02) (±0.01) (±0.01) (±0.02) (±0.01)
g (%) (±0.05) (±0.01) (±0.03) (±0.02) (±0.01)
0.38 0.39 0.40 0.42 0.42
(±0.02) (±0.01) (±0.04) (±0.02) (±0.03)
Fig. 3. J–V characteristics of solar cells prepared using ZnO nanorod arrays grown on seed layers with various thickness.
Fig. 2. Optical measurements of ZnO nanorod films grown on seed layers with various thickness: (a) reflectance, (b) transmittance.
DSSC on the thickness of the seed layers. The images show an apparent increasing trend, with an obvious increase in the short-circuit current density (Jsc), little change in the open-circuit voltage (Voc), and a pronounced increase in the conversion efficiency (g) with an increase in the ZnO seed layer thickness from 10 nm to 100 nm. The cell with a 75 nm thick seed layer exhibit the maximum g of 0.42%, Jsc of 2.10 mA cm2, Voc of 0.55 V and FF (fill factor) of 0.36. When the thickness of seed layer is larger than 75 nm, the parameter values of DSSCs show little changes. The larger values of Jsc observed for ZnO nanorod electrodes with thicker seed layers, is ascribed to the denser growth of ZnO nanorods, which not only enhanced dye loading as a result of the enlarged surface area, but also enhanced the absorption of incident light resulting from the multi-scattering effect. It is interesting to find that repeating the desorption/ adsorption process is beneficial for enhancing dye loading. Besides increasing the thick of ZnO seed layers, sensitizing the ZnO nanorod array films more times can also result in enhanced dye adsorption. The dye adsorption of ZnO electrodes after the first cycle of sensitization is 5.57 108 mol cm2. Because the deionized water can give rise to hydrolysis of linkages formed between carboxylic acid groups of the dye and the surface of ZnO nanorods, the amount of adsorbed dye will initially decrease after rinsed by water. However, after the second sensitization, the total amount of dye actually gets a modest increase, being
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X. Fang et al. / Solar Energy 105 (2014) 14–19
5.91 108 mol cm2. When the desorption/adsorption process is repeated some more times, a continual increase in dye loading is achieved. As shown in Fig. 4, the amount of dye loading is 6.36 108 mol cm2 and 6.75 108 mol cm2 for 3 and 4 cycles respectively. The behavior may be interpreted as the desorption of weakly bound dye molecules on ZnO on rinsing with water. During readsorption, the molecules subsequently reorganize and trend to form a single monolayer of more strongly attached molecules (Bazzan et al., 2011). Moreover, the ZnO nanorods do not have enough hydroxyl groups on the surface. When exposed to water, the ZnO films can be covered with more hydroxyl groups, which are formed when the water molecules break up and form H+ and –OH species. The introducing of more hydroxyl groups resultantly makes the dye loading density increase. It is worth noticing that the adsorption saturates after 4 cycles of sensitization, and the total amount of dye adsorbed reduces a little after the fifth cycle. The current–voltage characteristics of the assembled photovoltaic cells, containing ZnO electrodes sensitized over varying number of cycles, are shown in Fig 5. Clearly, the short-circuit current density gradually increases with the number of desorption/adsorption cycles (1–4). As shown in Table 2, the maximum Jsc (3.31 mA cm2) obtained after four cycles is almost 1.6 times of that obtained after the first cycle. N719 anchors to the ZnO surface through the four carboxyl groups. Rinsing with water during the desorption/adsorption cycles changes the binding behavior of N719 onto ZnO, and prevents the formation of Zn2+/dye aggregates (Scholin et al., 2011). Hence a similar increasing trend is also observed for the open-circuit voltage and fill factor. The Voc increases from 0.55 V to 0.65 V and the FF increases from 0.36 to 0.40 when the sensitization cycle increases from 1 to 4. As a result, an approximately 100% increase in conversion efficiency is achieved. In accordance with the result shown in Fig. 4, the values of Jsc, FF and g decrease slightly after five cycles.
Fig. 5. The current–voltage curves of dye-sensitized solar cells based on ZnO nanorod array films after different sensitization cycles.
Table 2 Parameters of dye-sensitized solar cells based on ZnO nanorod array films after different sensitization cycles.
1 2 3 4 5
cycle cycles cycles cycles cycles
Jsc (mA cm2)
Voc (V)
FF
2.10 2.45 2.90 3.31 3.24
0.55 0.57 0.61 0.64 0.65
0.36 0.37 0.39 0.40 0.39
(±0.11) (±0.15) (±0.09) (±0.06) (±0.15)
(±0.02) (±0.01) (±0.02) (±0.01) (±0.03)
g (%) (±0.02) (±0.02) (±0.03) (±0.03) (±0.03)
0.42 0.52 0.69 0.84 0.82
(±0.02) (±0.07) (±0.05) (±0.09) (±0.06)
4. Conclusions In summary, we have developed new strategies for improving performance of ZnO nanorod-based dye-sensitized solar cells. Firstly, thicker ZnO seed layers is employed to adjust the size and density of the obtained nanorods. The resulting nanorod arrays exhibit enlarged surface area, which can accommodate slightly larger amount of dye. In addition, we also find that increasing the number of dye adsorption/desorption cycles improves the concentration of dye molecules adsorbed on the surface of the ZnO films. The combination of the above-mentioned strategies results in an increase in the photocurrent density and power conversion efficiency. The enhanced solar cell performance is due to the increase in short-circuit current density, as well as the open-circuit voltage. The main reasons for the increase in open-circuit voltage are the optimization of the bonding behavior onto ZnO nanorods and the reduction in formation of Zn2+/dye aggregates at the semiconductor surface. Acknowledgements
Fig. 4. UV–Vis absorption spectra of dye desorbed from ZnO nanorod array films after various sensitization cycles.
This work was supported by the National High Technology Research and Development Program 863 of China (2011AA050511), Jiangsu “333” Project, the National Natural Science Foundation of China
X. Fang et al. / Solar Energy 105 (2014) 14–19
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