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Performance of ZnO dye-sensitized solar cells with various nanostructures as anodes
Solid State Sciences 13 (2011) 1354e1359
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Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Performance of ZnO dye-sensitized solar cells with various nanostructures as anodes Zhifeng Liu*, Yabin Li, Chengcheng Liu, Jing Ya, Wei Zhao, Lei E, Dan Zhao, Li An Department of Materials Science and Engineering, Tianjin Institute of Urban Construction, 300384 Tianjin, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2010 Received in revised form 14 March 2011 Accepted 11 April 2011 Available online 20 April 2011
Several types of ZnO nanostructured films, including nanoparticle films, ordered porous films, nanorod films, nanotube films and porous nanosheet films, were fabricated and used as photoanodes in dyesensitized solar cells (DSSCs). Their performance was described and compared in detail. DSSCs based on ordered porous films presented relatively higher conversion efficiency (0.7%) than that of DSSCs based on nanoparticle films (0.2%). DSSCs based on nanorod films showed the maximal efficiency up to 1.3%, mainly attributed to the c-axis oriented structure which could provide the faster conduction pathway for charge transport. Although nanotube films with large surface area could absorb more dyes, the efficiency of DSSCs based on nanotube films was only 1.2% and wasn’t as high as that of nanorod DSSCs, which may be caused by the charge-carrier recombination losses at the defects formed in the etching. Moreover, the porous ZnO nanosheets photoanode showed an improved efficiency by 2.5 times as compared to ZnO nanosheets samples due to increase the dye loading and light harvesting. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: ZnO Photoanode Solar cell Efficiency Nanotubes Porous nanosheets
1. Introduction Over the last few years, the dye-sensitized solar cells (DSSCs) have been attracting lots of attention since the initial report by Gratzel’s group due to their potential application for development of low-cost, large area photovoltaics [1]. DSSCs with TiO2 nanoporous films have been demonstrated with a power conversion efficiency of 11% in which a thick layer of nanoporous film provides a large surface area of anchoring the light harvesting dye molecules [1,2]. However, slow electron percolation through the interconnected nanoparticles and the charges recombination between injected electrons and electron acceptors in the electrolyte hinder the performance of TiO2 DSSCs [3,4]. In order to increase the efficiency of energy conversion, many semiconductor films, such as ZnO [5], SnO2 [6] and Nb2O5 [7], since their band gap energy are similar to TiO2, were proposed to improve charge transfer. ZnO is a IIeVI compound n-type semiconductor with unique properties such as transparency in the visible and high infrared reflectivity, acoustic characteristics, high electrochemical stability and excellently electronic properties, which could be an alternative material for solar cell. To achieve high performance of DSSCs, ZnO photoanode is required to possess high surface area as well as good
electrical, electrochemical and structural properties. However, efficiency of DSSCs based on ZnO nanoparticle films was only reached 0.4% early in 1994 by Gratzel’s group due to trap-limited diffusion for electron transport, a slow mechanism that may limit device efficiency, especially at longer wavelengths [2,4,8]. Then a version of ZnO nanorods/nanowires DSSCs had been introduced and got good score [9e11]. In addition, 2D ZnO nanostructures, such as nanosheets and nanobelts, have also been studied for DSSC applications on account of the fact that they also have a special structure, which can improve the conversion efficiency [12,13]. Although there are many reports on the preparation and application of ZnO photoanodes in DSSCs via different techniques [12e17], comparison among different ZnO photoanodes ranging from nanoparticles, nanorods, nanotubes and nanosheets is important, which can give some revelations about the effect of ZnO photoanode structure on the DSSCs cell performance. Here we report and discuss the performance of ZnO DSSCs with nanoparticle films, ordered porous films, nanorod films, nanotube films and porous nanosheets films, fabricated by different techniques. 2. Experimental 2.1. Substrates treatment
* Corresponding author. Tel.: þ86 22 23085236; fax: þ86 22 23085110. E-mail address: [email protected] (Z. Liu). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.04.005
All the ZnO photoanode films of DSSCs in our experiment were prepared on the indium tin oxide (ITO, 10 U cm-1) glass substrates,
Z. Liu et al. / Solid State Sciences 13 (2011) 1354e1359
which were ultrasonically rinsed for 15min in acetone, iso-propyl alcohol and ethano absolution, respectively.
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85 C for 1 h, which were thoroughly rinsed in distilled water in order to remove any residual salt or amino complex, and then were dried in air at room temperature.
2.2. Preparation of ZnO nanoparticle films 2.6. Preparation of porous ZnO nanosheets ZnO powders were synthesized by mixing 0.1 M zinc nitrate water solution with 0.1 M diethanolamine, adding a small quantity of cetyltrimethylammonium bromide (CTAB) n-butanol solution. The mixed solution was then allowed to age in a thermostat at 80 C for 24 h. The product was washed with deionized water, centrifuged at a high speed and finally dried. As-prepared ZnO powders were mixed with ethanol and stirred overnight, resulted in a colloidal suspension with ZnO content of 20 wt%. At last, the suspension was coated onto the ITO glass substrate by scalpel using scotch tape as frame and spacer. After drying in air, the nanoparticle films were calcined in air up to 500 C at a heating rate of 2 C/min. 2.3. Preparation of ordered ZnO porous films Ordered ZnO porous films were prepared by cathodic electrodeposition with polystyrene spheres templates as pore-forming assistant [18]. 2.4. Preparation of ZnO nanorod arrays Large-scale arrays of oriented single-crystal ZnO nanorods was successfully fabricated on electrodeposited ZnO seed layer by aqueous solution method from zinc nitrate and hexamethylenetetramine at low temperature (typically 95 C) [19]. 2.5. Preparation of ZnO nanotube arrays ZnO nanotube arrays were obtained by chemical etching asprepared nanorod arrays using 0.1 M alkaline solution (KOH) at
Porous ZnO naosheets films were prepared by electrochemical deposition at fixed water bath temperature (70 C) in an aqueous solution containing 0.5 M Zn(NO3)2 solution with pH ¼ 5.0 0.1 using polystyrene spheres (PS) template as pore-forming assistant [20]. The ZnO nanosheets were also prepared by electrochemical deposition method under the same conditions only without PS template assistant. 2.7. Assembly of ZnO dye-sensitized solar cells The above several types of ZnO films were used as photoanodes in DSSCs with 2-mm thickness, which can be controlled by technologic parameters [18e21], sensitized in a 0.05 mM ethanol solution of Ruthenium(II)cis-di(thiocyano) bis(2,20 -bypyridyl-4,4’dicarboxylic acid) (N3) dyes for at least 12 h at 60 C. The excess unanchored dyes were rinsed off using absolute ethanol and dried in air, then covered with platinum sheet as counter electrodes. The internal space of the cell was filled with liquid electrolyte (0.5 M LiI, 0.05 M I2) dissolved in acetonitrile by capillary action. 2.8. Characterization Morphology of the films was observed by PHILIPS XL-30 environment scanning electron microscopy (ESEM) and transmission electron microscopy (TEM, JEOL 100CX-II). X-ray diffraction (XRD) patterns of the films were recorded with a Rigaku D/max-2500 using Cu Ka radiation (l ¼ 0.154059 nm). Optical transmittance of photoanode films was examined by DU-8B UV/VIS double-beam
Fig. 1. Typical SEM images of different ZnO films used as photoanodes in DSSCs (a) nanoparticle films (the inset is a TEM image); (b) ordered porous films; (c) nanorod arrays; (d) nanotube arrays.
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Z. Liu et al. / Solid State Sciences 13 (2011) 1354e1359 Table .1 Parameters of ZnO dye-sensitized solar cells with different photoanode films.
Fig. 2. Comparison of ordered ZnO porous films before and after sensitized by N3 dyes.
spectrophotometer. Photocurrent of the ZnO DSSCs was measured under irradiation of a xenon lamp (80 mW cm2) with global AM1.5 condition, and photocurrentevoltage curves of the ZnO DSSCs were obtained using a potentiostat (TD3691, Tianjin Zhonghuan CO., LTD, China). It should be noted that in order to lessen the errors on the efficiency measurements, five cell samples based on every type ZnO films were used.
3. Results and discussion Fig. 1 shows the typical SEM images of ZnO nanoparticle films, ordered porous films, nanorod arrays and nanotube arrays, respectively. The nanoparticle films are uniform and composed of particles with the diameter about 10e20 nm, illuminated as the inset one in Fig. 1 (a). In Fig. 1 (b), porous ZnO films prepared by electrodeposition method on ITO glass substrates covered with PS array templates show an ordered, honeycomb-like macroporous structure. The average diameter of pores is about 350 nm. The film is so robust that no cracking or deformation occurs during the removal of PS by immersing in toluene. Well-aligned ZnO nanorod arrays in large-scale can be successfully fabricated on electrodeposited ZnO seed layer (Fig. 1 (c)), and the well-defined
Fig. 3. Photocurrent-voltage curves of DSSCs with different ZnO photoanode films.
Photo anode films
Voc(mV)
Jsc(mA cm2)
FF(%)
h(%)
Nano particle films Ordered porosu films Nanorods films Nanotubes films
300 468 442 386
0.92 1.96 3.75 4.70
58 63 62 53
0.2 0.7 1.3 1.2
crystallographic planes of the hexagonal nanorods can be clearly identified from the high-magnification SEM image (inset image in Fig. 1 (c)), indicating the nanorods grow along the [001] direction. Fig. 1 (d) illustrates the representative SEM image of ZnO nanotube arrays transformed from as-prepared ZnO nanorod arrays by etching in an alkaline solution. Interestingly, the etching is selective and only takes place in the center of nanorods, which is attributed to the two polar planes, (001) and (001), in wurtzite ZnO crystal. The two planes with high surface energy are metastable, while the nonpolar planes parallel to c-axis are the most stable planes with a lower surface energy. As a result, the etching rate of the polar (001) plane is faster than that of the nonpolar planes [22]. Moreover, ZnO as an amphoteric oxide can react with hydroxyl ions and produce soluble salts as the following:
ZnO þ 2OH % ZnO2 2 þ H2 O
(1)
Therefore, ZnO nanotubes are formed by selective etching on the polar and nonpolar planes in case there is an appropriate OH concentration in the solutions. Fig. 2 presents the ordered ZnO porous films before and after sensitized by N3 dyes. As expected, the sensitized film (Fig. 2 (b)) appears brown due to anchoring N3 dyes. Other photoanodes settled with dyes, including nanoparticle/nanorod/nanotube films, appear similar to ordered porous films, revealing that the dyes are well adsorbed on the surface of ZnO films. Fig. 3 gives the photocurrentevoltage curves of the ZnO DSSCs with nanoparticle films, ordered porous films, nanorod films and nanotube films, while their photoelectrochemical properties are listed in Table 1. During the photocurrent measurements, the cell efficiency (h) is expressed by the following equation:
h ¼ ðVoc Jsc FFÞ=Pin
(2)
FF ¼ Vopt Jopt =Voc Jsc
(3)
Fig. 4. XRD pattern of the ZnO nanorod arrays.
Z. Liu et al. / Solid State Sciences 13 (2011) 1354e1359
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Fig. 5. Optical transmittance spectra of ZnO nanorod/nanotube films.
Fig. 7. Photocurrent-voltage curves of the DSSCs based on ZnO nanosheets.
where Pin is the power of incident white light, FF is fill factor, Vopt and Jopt are voltage and current for maximum powder output, and Voc and Jsc are open circuit photovoltage and short circuit photocurrent, respectively. Comparing in terms of open circuit voltage, short circuit photocurrent, fill factor and power conversion efficiencies, it can be found that morphology of ZnO photoanode films have profound impact on the performances of DSSCs. DSSCs based on ZnO nanoparticle films (nanoparticle DSSCs) composed of interconnected spherical particles show the lowest efficiency (0.2%) due to charge-carrier recombination losses at grain boundaries between nanoparticles [2]. The average values of open circuit voltage (Voc) and short circuit current (Jsc) for nanoparticle DSSCs are 300 mV and 0.92 mA cm-2, respectively. While the values are 468 mV and 1.96 mA cm-2 for DSSCs based on ZnO ordered porous film. And the DSSCs based on ZnO ordered porous film perform higher conversion efficiency (0.7%) than nanoparticle DSSCs (0.2%), mainly caused by the fewer defects in ordered porous films than in nanoparticle films. Moreover, the photo-electric conversion efficiency of DSSCs based on ZnO porous films will be improved by decreasing the pore diameter because of the increasing surface area for dye adsorption from the porous structure. The higher conversion efficiency (1.3%) for DSSCs based on ZnO nanorod films (nanorod DSSCs) can be ascribed to its special nanostructure. Fig. 4 exhibits the typical XRD pattern of ZnO nanorods arrays film. The result shows all the diffraction peaks can be indexed to the wurtzite structure. In comparison with standard powder diffraction pattern (PDF#65e3411), much stronger intensity of (002) peak to others manifests nanorods are well preferentially oriented in the direction of c-axis, which
coincides with the SEM results. Well-aligned single-crystal nanorods can provide faster electron transportation channels than grain boundaries which associate with traps or/and barriers. And larger surface area of nanorod arrays will benefit the dyes adsorbing and improve the light harvest efficiency. Furthermore, injected electrons can transport directly through oriented nanorods to conducting substrates, which greatly reduces the recombination losses. Fig. 5 gives the optical transmittance spectra of the ZnO nanorod and nanotube arrays attached by N3 dyes, both of the films show the maximum adsorption peak around 535 nm corresponding to N3 dyes. Although the stronger and wider absorption peak for nanotube films indicates that more dyes are adsorbed and then result in the increase of Jsc, DSSCs based on nanotube films (nanotube DSSCs) present an efficiency of 1.2%, slight lower than that of nanorod DSSCs. In addition, Jsc and Voc of nanotube DSSCs are 4.70 mA cm-2 and 386 mV, while the values of nanorod DSSCs are 3.75 mA cm-2 and 442 mV. As mentioned above, the higher Jsc of nanotube DSSCs is attributed to the larger surface area that anchors more dyes. More defects may be produced during the process of ZnO nanotubes by chemical etching, leading to the electrolyte direct contact with the ITO substrate and the lower Voc. Both surface area and defects simultaneously play crucial roles on the performance of nanotube DSSCs. Actually, the conversion efficiency of nanotube DSSCs isn’t as high as one of nanorod DSSCs, due to the charge-carrier recombination losses at the defects of the direct contact between electrolyte and ITO substrate, even though the nanotube arrays with large surface area can absorb more dyes. The ZnO nanosheets can be obtained in an aqueous solution containing 0.5 M Zn(NO3)2 solution with pH ¼ 5.0 0.1 when the
Fig. 6. SEM images of ZnO nanosheets at different conditions (a) potential is 0.8 V and without PS template; (b) potential is 0.8 V and with PS template; (c) potential is 0.9 V and with PS template.
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Z. Liu et al. / Solid State Sciences 13 (2011) 1354e1359
Table 2 Parameters of DSSCs with different ZnO nanosheets photoanode films. Photoanode film
Va(mV)
Jsc(mA cm2)
FF(%)
h(%)
ZnO nanosheets ZnO nanosheets(B) ZnO nanosheets(W)
307 348 373
2.1 3.3 4.2
55 54 53
0.4 0.8 1.0
potential is 0.8 V or 0.9 V [20]. The morphologies of the ZnO nanosheets obtained were examined by SEM analysis. Fig. 1 shows the typical SEM images of ZnO nanosheets with different morphologies. These ZnO nanosheets standing on the ITO substrate exhibit the regular hexagon end planes with about 4-5 um in diameter and 100 nm in thickness. Moreover, it can be seen from Fig. 6 (a) that the surface of ZnO nanosheets electrodeposited under above conditions without PS template assistant is smooth, and there is a mass of gaps existing among the crystals. However, ZnO nanosheets (as shown in Fig. 6 (b) and (c)) with porous structure can be synthesized under the same conditions using PS template as pore-forming assistant. During the preparation of porous ZnO nanosheets, the ITO substrate covered with PS was firstly soaked in Zn(NO3)2 aqueous solution at 70 C for 30 min, which could weaken the fastness of PS particles in some extent before the electrochemical deposition reaction [17]. Then, at the action of electric field, the chemical reaction taking place at the working electrode (the cathode in this case) is as the following: ZnðNO3 Þ2 þ2e /ZnOY þ NO 3 þ NO2
(4)
The growth of ZnO at the spaces of PS further weakened the fastness of PS particles and the weak PS, which was caused both by the soaking in electrolyte and the hustle of ZnO growth, might drift toward the solution due to the action of electric field. It was known that Zn precursor in solution had positive charge and PS had negative charge, so there was an electrostatic attraction between the Zn precursor and PS. As a result, PS were allured and embedded by ZnO crystals during the electrodeposition. Porous ZnO nanosheets films could be obtained after the samples were soaked in toluene to dissolve the PS. It should be noted that the electrodeposition potential has an important role on the morphology of porous ZnO nanosheets. In Fig. 6 (b), PS can be only embedded in the bottom of ZnO nanosheets (porous ZnO nanosheets(B)) at 0.8 V because the drift speed of PS is slower than the growth of ZnO crystals. However, the drift speed of PS increases with the relative increasing of potential. So PS can be embedded in whole ZnO nanosheets (porous ZnO nanosheets(W))
at 0.9 V and uniform porous structure can be obtained after soaking the sample in toluene to dissolve the PS (as shown in Fig. 6 (c)). Fig. 7 gives the photocurrentevoltage curves of the DSSCs based on the ZnO nanosheets using N3 as sensitizer, and the photoelectrochemical properties of these DSSCs listed in Table 2 The cell performances of the DSSCs based on porous ZnO nanosheets(B) and porous ZnO nanosheets(W) are higher than ZnO nanosheets based DSSCs and are expected to be high. The average values of open circuit voltage (Voc) and short circuit current (Jsc) for porous ZnO nanosheets(B) based DSSCs are 348 mV and 3.3 mA cm-2, respectively, While the values are 307 mV and 2.1 mA cm-2 for DSSCs based on ZnO nanosheets. Moreover, the porous ZnO nanosheets(B) based DSSCs exhibit an improved efficiency by 100% corresponding to that of ZnO nanosheets. And the porous ZnO nanosheets(W) based DSSC reached a total efficiency of 1.0%, which is 2.5 times as high as that of the nanosheets based DSSC (0.4%). All of these results indicate that, porous ZnO nanosheets can enhance cell performance compared to the nanosheets without porous structure. As we all know, except the effect of recombination of the photo-excited electrons on the cell properties, one reason for the low conversion efficiency of traditional DSSCs is the dye loading on semiconductor. Another reason may be explained by the solar light utilization. The ZnO nanosheets photoanode with porous structure can overcome the two limitations above. The dye loading and solar light harvesting of ZnO nanosheets based DSSCs can be schematically illustrated in Fig. 8, where the porous structure may be formed by using PS template as pore-forming assistant during the growth of ZnO crystals; the dye is very easy to load on the porous ZnO nanosheets photoanodic film due to the enlargement of surface area, which significantly increases the cell performances. From Fig. 7 and Table 2, compared to the ZnO nanosheets, the porous ZnO nanosheets, both porous ZnO nanosheets(B) and porous ZnO nanosheets(W), show an improved cell performances. Especially, there is an overall increase of cell performance for porous ZnO nanosheets(W) based DSSCs due to the uniform pore distributing in the whole nanosheets. It should be noted that the enhanced solar light harvesting behavior is also associated with the sufficient dye loading. Besides, the nanosheets with porous structure are very efficient on increasing the light harvesting, which significantly improve the current and is beneficial for the final conversion efficiency. Moreover, as shown in Fig. 8 (c), some solar light can further reflect after through the pore in the nanosheets, resulting in maximum light harvesting in porous nanosheets photoanode.
Fig. 8. The schematic diagram of the dye loading and light harvesting for ZnO nanosheets based DSSC.
Z. Liu et al. / Solid State Sciences 13 (2011) 1354e1359
4. Conclusions Several types of ZnO nanostructured films, including nanoparticle films, ordered porous films, nanorod films, nanotube films and nanosheet films, were fabricated and used as photoanodes in dye-sensitized solar cells. Their performance was described and compared in detail. DSSCs based on ordered porous films presented relatively higher conversion efficiency (0.7%) than that of DSSCs based on nanoparticle films (0.2%), due to the charge-carrier recombination losses at grain boundaries for nanoparticle films. DSSCs based on nanorod films showed the maximal efficiency up to 1.3%, mainly attributed to the c-axis oriented structure which could provide the faster conduction pathway for charge transport. Although nanotube films with large surface area could absorb more dyes, the efficiency of DSSCs based on nanotube films was only 1.2% and wasn’t as high as that of nanorod DSSCs, which may be caused by the charge-carrier recombination losses at the defects formed in the etching. In addition, the porous ZnO nanosheets photoanode showed remarkable influence on the final performance of the DSSCs. IeV characteristic measurement indicates an improved efficiency by 2.5 times as compared to ZnO nanosheets samples due to increase the dye loading and light harvesting.
Acknowledgements The authors gratefully acknowledge financial support from the Key Project of Chinese Ministry of Education (No. 208008), China
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Postdoctoral Science Foundation Funded Project (No. 20080440674) and China Postdoctoral Science Special Foundation (No. 201003294).
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Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Performance of ZnO dye-sensitized solar cells with various nanostructures as anodes Zhifeng Liu*, Yabin Li, Chengcheng Liu, Jing Ya, Wei Zhao, Lei E, Dan Zhao, Li An Department of Materials Science and Engineering, Tianjin Institute of Urban Construction, 300384 Tianjin, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2010 Received in revised form 14 March 2011 Accepted 11 April 2011 Available online 20 April 2011
Several types of ZnO nanostructured films, including nanoparticle films, ordered porous films, nanorod films, nanotube films and porous nanosheet films, were fabricated and used as photoanodes in dyesensitized solar cells (DSSCs). Their performance was described and compared in detail. DSSCs based on ordered porous films presented relatively higher conversion efficiency (0.7%) than that of DSSCs based on nanoparticle films (0.2%). DSSCs based on nanorod films showed the maximal efficiency up to 1.3%, mainly attributed to the c-axis oriented structure which could provide the faster conduction pathway for charge transport. Although nanotube films with large surface area could absorb more dyes, the efficiency of DSSCs based on nanotube films was only 1.2% and wasn’t as high as that of nanorod DSSCs, which may be caused by the charge-carrier recombination losses at the defects formed in the etching. Moreover, the porous ZnO nanosheets photoanode showed an improved efficiency by 2.5 times as compared to ZnO nanosheets samples due to increase the dye loading and light harvesting. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: ZnO Photoanode Solar cell Efficiency Nanotubes Porous nanosheets
1. Introduction Over the last few years, the dye-sensitized solar cells (DSSCs) have been attracting lots of attention since the initial report by Gratzel’s group due to their potential application for development of low-cost, large area photovoltaics [1]. DSSCs with TiO2 nanoporous films have been demonstrated with a power conversion efficiency of 11% in which a thick layer of nanoporous film provides a large surface area of anchoring the light harvesting dye molecules [1,2]. However, slow electron percolation through the interconnected nanoparticles and the charges recombination between injected electrons and electron acceptors in the electrolyte hinder the performance of TiO2 DSSCs [3,4]. In order to increase the efficiency of energy conversion, many semiconductor films, such as ZnO [5], SnO2 [6] and Nb2O5 [7], since their band gap energy are similar to TiO2, were proposed to improve charge transfer. ZnO is a IIeVI compound n-type semiconductor with unique properties such as transparency in the visible and high infrared reflectivity, acoustic characteristics, high electrochemical stability and excellently electronic properties, which could be an alternative material for solar cell. To achieve high performance of DSSCs, ZnO photoanode is required to possess high surface area as well as good
electrical, electrochemical and structural properties. However, efficiency of DSSCs based on ZnO nanoparticle films was only reached 0.4% early in 1994 by Gratzel’s group due to trap-limited diffusion for electron transport, a slow mechanism that may limit device efficiency, especially at longer wavelengths [2,4,8]. Then a version of ZnO nanorods/nanowires DSSCs had been introduced and got good score [9e11]. In addition, 2D ZnO nanostructures, such as nanosheets and nanobelts, have also been studied for DSSC applications on account of the fact that they also have a special structure, which can improve the conversion efficiency [12,13]. Although there are many reports on the preparation and application of ZnO photoanodes in DSSCs via different techniques [12e17], comparison among different ZnO photoanodes ranging from nanoparticles, nanorods, nanotubes and nanosheets is important, which can give some revelations about the effect of ZnO photoanode structure on the DSSCs cell performance. Here we report and discuss the performance of ZnO DSSCs with nanoparticle films, ordered porous films, nanorod films, nanotube films and porous nanosheets films, fabricated by different techniques. 2. Experimental 2.1. Substrates treatment
* Corresponding author. Tel.: þ86 22 23085236; fax: þ86 22 23085110. E-mail address: [email protected] (Z. Liu). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.04.005
All the ZnO photoanode films of DSSCs in our experiment were prepared on the indium tin oxide (ITO, 10 U cm-1) glass substrates,
Z. Liu et al. / Solid State Sciences 13 (2011) 1354e1359
which were ultrasonically rinsed for 15min in acetone, iso-propyl alcohol and ethano absolution, respectively.
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85 C for 1 h, which were thoroughly rinsed in distilled water in order to remove any residual salt or amino complex, and then were dried in air at room temperature.
2.2. Preparation of ZnO nanoparticle films 2.6. Preparation of porous ZnO nanosheets ZnO powders were synthesized by mixing 0.1 M zinc nitrate water solution with 0.1 M diethanolamine, adding a small quantity of cetyltrimethylammonium bromide (CTAB) n-butanol solution. The mixed solution was then allowed to age in a thermostat at 80 C for 24 h. The product was washed with deionized water, centrifuged at a high speed and finally dried. As-prepared ZnO powders were mixed with ethanol and stirred overnight, resulted in a colloidal suspension with ZnO content of 20 wt%. At last, the suspension was coated onto the ITO glass substrate by scalpel using scotch tape as frame and spacer. After drying in air, the nanoparticle films were calcined in air up to 500 C at a heating rate of 2 C/min. 2.3. Preparation of ordered ZnO porous films Ordered ZnO porous films were prepared by cathodic electrodeposition with polystyrene spheres templates as pore-forming assistant [18]. 2.4. Preparation of ZnO nanorod arrays Large-scale arrays of oriented single-crystal ZnO nanorods was successfully fabricated on electrodeposited ZnO seed layer by aqueous solution method from zinc nitrate and hexamethylenetetramine at low temperature (typically 95 C) [19]. 2.5. Preparation of ZnO nanotube arrays ZnO nanotube arrays were obtained by chemical etching asprepared nanorod arrays using 0.1 M alkaline solution (KOH) at
Porous ZnO naosheets films were prepared by electrochemical deposition at fixed water bath temperature (70 C) in an aqueous solution containing 0.5 M Zn(NO3)2 solution with pH ¼ 5.0 0.1 using polystyrene spheres (PS) template as pore-forming assistant [20]. The ZnO nanosheets were also prepared by electrochemical deposition method under the same conditions only without PS template assistant. 2.7. Assembly of ZnO dye-sensitized solar cells The above several types of ZnO films were used as photoanodes in DSSCs with 2-mm thickness, which can be controlled by technologic parameters [18e21], sensitized in a 0.05 mM ethanol solution of Ruthenium(II)cis-di(thiocyano) bis(2,20 -bypyridyl-4,4’dicarboxylic acid) (N3) dyes for at least 12 h at 60 C. The excess unanchored dyes were rinsed off using absolute ethanol and dried in air, then covered with platinum sheet as counter electrodes. The internal space of the cell was filled with liquid electrolyte (0.5 M LiI, 0.05 M I2) dissolved in acetonitrile by capillary action. 2.8. Characterization Morphology of the films was observed by PHILIPS XL-30 environment scanning electron microscopy (ESEM) and transmission electron microscopy (TEM, JEOL 100CX-II). X-ray diffraction (XRD) patterns of the films were recorded with a Rigaku D/max-2500 using Cu Ka radiation (l ¼ 0.154059 nm). Optical transmittance of photoanode films was examined by DU-8B UV/VIS double-beam
Fig. 1. Typical SEM images of different ZnO films used as photoanodes in DSSCs (a) nanoparticle films (the inset is a TEM image); (b) ordered porous films; (c) nanorod arrays; (d) nanotube arrays.
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Z. Liu et al. / Solid State Sciences 13 (2011) 1354e1359 Table .1 Parameters of ZnO dye-sensitized solar cells with different photoanode films.
Fig. 2. Comparison of ordered ZnO porous films before and after sensitized by N3 dyes.
spectrophotometer. Photocurrent of the ZnO DSSCs was measured under irradiation of a xenon lamp (80 mW cm2) with global AM1.5 condition, and photocurrentevoltage curves of the ZnO DSSCs were obtained using a potentiostat (TD3691, Tianjin Zhonghuan CO., LTD, China). It should be noted that in order to lessen the errors on the efficiency measurements, five cell samples based on every type ZnO films were used.
3. Results and discussion Fig. 1 shows the typical SEM images of ZnO nanoparticle films, ordered porous films, nanorod arrays and nanotube arrays, respectively. The nanoparticle films are uniform and composed of particles with the diameter about 10e20 nm, illuminated as the inset one in Fig. 1 (a). In Fig. 1 (b), porous ZnO films prepared by electrodeposition method on ITO glass substrates covered with PS array templates show an ordered, honeycomb-like macroporous structure. The average diameter of pores is about 350 nm. The film is so robust that no cracking or deformation occurs during the removal of PS by immersing in toluene. Well-aligned ZnO nanorod arrays in large-scale can be successfully fabricated on electrodeposited ZnO seed layer (Fig. 1 (c)), and the well-defined
Fig. 3. Photocurrent-voltage curves of DSSCs with different ZnO photoanode films.
Photo anode films
Voc(mV)
Jsc(mA cm2)
FF(%)
h(%)
Nano particle films Ordered porosu films Nanorods films Nanotubes films
300 468 442 386
0.92 1.96 3.75 4.70
58 63 62 53
0.2 0.7 1.3 1.2
crystallographic planes of the hexagonal nanorods can be clearly identified from the high-magnification SEM image (inset image in Fig. 1 (c)), indicating the nanorods grow along the [001] direction. Fig. 1 (d) illustrates the representative SEM image of ZnO nanotube arrays transformed from as-prepared ZnO nanorod arrays by etching in an alkaline solution. Interestingly, the etching is selective and only takes place in the center of nanorods, which is attributed to the two polar planes, (001) and (001), in wurtzite ZnO crystal. The two planes with high surface energy are metastable, while the nonpolar planes parallel to c-axis are the most stable planes with a lower surface energy. As a result, the etching rate of the polar (001) plane is faster than that of the nonpolar planes [22]. Moreover, ZnO as an amphoteric oxide can react with hydroxyl ions and produce soluble salts as the following:
ZnO þ 2OH % ZnO2 2 þ H2 O
(1)
Therefore, ZnO nanotubes are formed by selective etching on the polar and nonpolar planes in case there is an appropriate OH concentration in the solutions. Fig. 2 presents the ordered ZnO porous films before and after sensitized by N3 dyes. As expected, the sensitized film (Fig. 2 (b)) appears brown due to anchoring N3 dyes. Other photoanodes settled with dyes, including nanoparticle/nanorod/nanotube films, appear similar to ordered porous films, revealing that the dyes are well adsorbed on the surface of ZnO films. Fig. 3 gives the photocurrentevoltage curves of the ZnO DSSCs with nanoparticle films, ordered porous films, nanorod films and nanotube films, while their photoelectrochemical properties are listed in Table 1. During the photocurrent measurements, the cell efficiency (h) is expressed by the following equation:
h ¼ ðVoc Jsc FFÞ=Pin
(2)
FF ¼ Vopt Jopt =Voc Jsc
(3)
Fig. 4. XRD pattern of the ZnO nanorod arrays.
Z. Liu et al. / Solid State Sciences 13 (2011) 1354e1359
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Fig. 5. Optical transmittance spectra of ZnO nanorod/nanotube films.
Fig. 7. Photocurrent-voltage curves of the DSSCs based on ZnO nanosheets.
where Pin is the power of incident white light, FF is fill factor, Vopt and Jopt are voltage and current for maximum powder output, and Voc and Jsc are open circuit photovoltage and short circuit photocurrent, respectively. Comparing in terms of open circuit voltage, short circuit photocurrent, fill factor and power conversion efficiencies, it can be found that morphology of ZnO photoanode films have profound impact on the performances of DSSCs. DSSCs based on ZnO nanoparticle films (nanoparticle DSSCs) composed of interconnected spherical particles show the lowest efficiency (0.2%) due to charge-carrier recombination losses at grain boundaries between nanoparticles [2]. The average values of open circuit voltage (Voc) and short circuit current (Jsc) for nanoparticle DSSCs are 300 mV and 0.92 mA cm-2, respectively. While the values are 468 mV and 1.96 mA cm-2 for DSSCs based on ZnO ordered porous film. And the DSSCs based on ZnO ordered porous film perform higher conversion efficiency (0.7%) than nanoparticle DSSCs (0.2%), mainly caused by the fewer defects in ordered porous films than in nanoparticle films. Moreover, the photo-electric conversion efficiency of DSSCs based on ZnO porous films will be improved by decreasing the pore diameter because of the increasing surface area for dye adsorption from the porous structure. The higher conversion efficiency (1.3%) for DSSCs based on ZnO nanorod films (nanorod DSSCs) can be ascribed to its special nanostructure. Fig. 4 exhibits the typical XRD pattern of ZnO nanorods arrays film. The result shows all the diffraction peaks can be indexed to the wurtzite structure. In comparison with standard powder diffraction pattern (PDF#65e3411), much stronger intensity of (002) peak to others manifests nanorods are well preferentially oriented in the direction of c-axis, which
coincides with the SEM results. Well-aligned single-crystal nanorods can provide faster electron transportation channels than grain boundaries which associate with traps or/and barriers. And larger surface area of nanorod arrays will benefit the dyes adsorbing and improve the light harvest efficiency. Furthermore, injected electrons can transport directly through oriented nanorods to conducting substrates, which greatly reduces the recombination losses. Fig. 5 gives the optical transmittance spectra of the ZnO nanorod and nanotube arrays attached by N3 dyes, both of the films show the maximum adsorption peak around 535 nm corresponding to N3 dyes. Although the stronger and wider absorption peak for nanotube films indicates that more dyes are adsorbed and then result in the increase of Jsc, DSSCs based on nanotube films (nanotube DSSCs) present an efficiency of 1.2%, slight lower than that of nanorod DSSCs. In addition, Jsc and Voc of nanotube DSSCs are 4.70 mA cm-2 and 386 mV, while the values of nanorod DSSCs are 3.75 mA cm-2 and 442 mV. As mentioned above, the higher Jsc of nanotube DSSCs is attributed to the larger surface area that anchors more dyes. More defects may be produced during the process of ZnO nanotubes by chemical etching, leading to the electrolyte direct contact with the ITO substrate and the lower Voc. Both surface area and defects simultaneously play crucial roles on the performance of nanotube DSSCs. Actually, the conversion efficiency of nanotube DSSCs isn’t as high as one of nanorod DSSCs, due to the charge-carrier recombination losses at the defects of the direct contact between electrolyte and ITO substrate, even though the nanotube arrays with large surface area can absorb more dyes. The ZnO nanosheets can be obtained in an aqueous solution containing 0.5 M Zn(NO3)2 solution with pH ¼ 5.0 0.1 when the
Fig. 6. SEM images of ZnO nanosheets at different conditions (a) potential is 0.8 V and without PS template; (b) potential is 0.8 V and with PS template; (c) potential is 0.9 V and with PS template.
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Table 2 Parameters of DSSCs with different ZnO nanosheets photoanode films. Photoanode film
Va(mV)
Jsc(mA cm2)
FF(%)
h(%)
ZnO nanosheets ZnO nanosheets(B) ZnO nanosheets(W)
307 348 373
2.1 3.3 4.2
55 54 53
0.4 0.8 1.0
potential is 0.8 V or 0.9 V [20]. The morphologies of the ZnO nanosheets obtained were examined by SEM analysis. Fig. 1 shows the typical SEM images of ZnO nanosheets with different morphologies. These ZnO nanosheets standing on the ITO substrate exhibit the regular hexagon end planes with about 4-5 um in diameter and 100 nm in thickness. Moreover, it can be seen from Fig. 6 (a) that the surface of ZnO nanosheets electrodeposited under above conditions without PS template assistant is smooth, and there is a mass of gaps existing among the crystals. However, ZnO nanosheets (as shown in Fig. 6 (b) and (c)) with porous structure can be synthesized under the same conditions using PS template as pore-forming assistant. During the preparation of porous ZnO nanosheets, the ITO substrate covered with PS was firstly soaked in Zn(NO3)2 aqueous solution at 70 C for 30 min, which could weaken the fastness of PS particles in some extent before the electrochemical deposition reaction [17]. Then, at the action of electric field, the chemical reaction taking place at the working electrode (the cathode in this case) is as the following: ZnðNO3 Þ2 þ2e /ZnOY þ NO 3 þ NO2
(4)
The growth of ZnO at the spaces of PS further weakened the fastness of PS particles and the weak PS, which was caused both by the soaking in electrolyte and the hustle of ZnO growth, might drift toward the solution due to the action of electric field. It was known that Zn precursor in solution had positive charge and PS had negative charge, so there was an electrostatic attraction between the Zn precursor and PS. As a result, PS were allured and embedded by ZnO crystals during the electrodeposition. Porous ZnO nanosheets films could be obtained after the samples were soaked in toluene to dissolve the PS. It should be noted that the electrodeposition potential has an important role on the morphology of porous ZnO nanosheets. In Fig. 6 (b), PS can be only embedded in the bottom of ZnO nanosheets (porous ZnO nanosheets(B)) at 0.8 V because the drift speed of PS is slower than the growth of ZnO crystals. However, the drift speed of PS increases with the relative increasing of potential. So PS can be embedded in whole ZnO nanosheets (porous ZnO nanosheets(W))
at 0.9 V and uniform porous structure can be obtained after soaking the sample in toluene to dissolve the PS (as shown in Fig. 6 (c)). Fig. 7 gives the photocurrentevoltage curves of the DSSCs based on the ZnO nanosheets using N3 as sensitizer, and the photoelectrochemical properties of these DSSCs listed in Table 2 The cell performances of the DSSCs based on porous ZnO nanosheets(B) and porous ZnO nanosheets(W) are higher than ZnO nanosheets based DSSCs and are expected to be high. The average values of open circuit voltage (Voc) and short circuit current (Jsc) for porous ZnO nanosheets(B) based DSSCs are 348 mV and 3.3 mA cm-2, respectively, While the values are 307 mV and 2.1 mA cm-2 for DSSCs based on ZnO nanosheets. Moreover, the porous ZnO nanosheets(B) based DSSCs exhibit an improved efficiency by 100% corresponding to that of ZnO nanosheets. And the porous ZnO nanosheets(W) based DSSC reached a total efficiency of 1.0%, which is 2.5 times as high as that of the nanosheets based DSSC (0.4%). All of these results indicate that, porous ZnO nanosheets can enhance cell performance compared to the nanosheets without porous structure. As we all know, except the effect of recombination of the photo-excited electrons on the cell properties, one reason for the low conversion efficiency of traditional DSSCs is the dye loading on semiconductor. Another reason may be explained by the solar light utilization. The ZnO nanosheets photoanode with porous structure can overcome the two limitations above. The dye loading and solar light harvesting of ZnO nanosheets based DSSCs can be schematically illustrated in Fig. 8, where the porous structure may be formed by using PS template as pore-forming assistant during the growth of ZnO crystals; the dye is very easy to load on the porous ZnO nanosheets photoanodic film due to the enlargement of surface area, which significantly increases the cell performances. From Fig. 7 and Table 2, compared to the ZnO nanosheets, the porous ZnO nanosheets, both porous ZnO nanosheets(B) and porous ZnO nanosheets(W), show an improved cell performances. Especially, there is an overall increase of cell performance for porous ZnO nanosheets(W) based DSSCs due to the uniform pore distributing in the whole nanosheets. It should be noted that the enhanced solar light harvesting behavior is also associated with the sufficient dye loading. Besides, the nanosheets with porous structure are very efficient on increasing the light harvesting, which significantly improve the current and is beneficial for the final conversion efficiency. Moreover, as shown in Fig. 8 (c), some solar light can further reflect after through the pore in the nanosheets, resulting in maximum light harvesting in porous nanosheets photoanode.
Fig. 8. The schematic diagram of the dye loading and light harvesting for ZnO nanosheets based DSSC.
Z. Liu et al. / Solid State Sciences 13 (2011) 1354e1359
4. Conclusions Several types of ZnO nanostructured films, including nanoparticle films, ordered porous films, nanorod films, nanotube films and nanosheet films, were fabricated and used as photoanodes in dye-sensitized solar cells. Their performance was described and compared in detail. DSSCs based on ordered porous films presented relatively higher conversion efficiency (0.7%) than that of DSSCs based on nanoparticle films (0.2%), due to the charge-carrier recombination losses at grain boundaries for nanoparticle films. DSSCs based on nanorod films showed the maximal efficiency up to 1.3%, mainly attributed to the c-axis oriented structure which could provide the faster conduction pathway for charge transport. Although nanotube films with large surface area could absorb more dyes, the efficiency of DSSCs based on nanotube films was only 1.2% and wasn’t as high as that of nanorod DSSCs, which may be caused by the charge-carrier recombination losses at the defects formed in the etching. In addition, the porous ZnO nanosheets photoanode showed remarkable influence on the final performance of the DSSCs. IeV characteristic measurement indicates an improved efficiency by 2.5 times as compared to ZnO nanosheets samples due to increase the dye loading and light harvesting.
Acknowledgements The authors gratefully acknowledge financial support from the Key Project of Chinese Ministry of Education (No. 208008), China
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Postdoctoral Science Foundation Funded Project (No. 20080440674) and China Postdoctoral Science Special Foundation (No. 201003294).
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