Anthocyanin-sensitized solar cells using carbon nanotube films as counter electrodes

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Nanotechnology 19 (2008) 465204 (5pp) doi:10.1088/0957-4484/19/46/465204

Anthocyanin-sensitized solar cells usingcarbon nanotube films as counterelectrodesHongwei Zhu1,2, Haifeng Zeng1, Venkatachalam Subramanian1,Charan Masarapu1, Kai-Hsuan Hung1 and Bingqing Wei1,3

1 Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USA2 Key Laboratory for Advanced Manufacturing by Material Processing Technology,Department of Mechanical Engineering, Tsinghua University, Beijing 100084,People’s Republic of China

E-mail: weib@udel.edu

Received 21 August 2008, in final form 15 September 2008Published 21 October 2008Online at stacks.iop.org/Nano/19/465204

AbstractCarbon nanotube (CNT) films have been used as counter electrodes in natural dye-sensitized(anthocyanin-sensitized) solar cells to improve the cell performance. Compared withconventional cells using natural dye electrolytes and platinum as the counter electrodes, cellswith a single-walled nanotube (SWNT) film counter electrode show comparable conversionefficiency, which is attributed to the increase in short circuit current density due to the highconductivity of the SWNT film.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Carbon nanotube (CNT) thin films have attracted considerableacademic and industrial interest due to their remarkablemechanical, optical, and electronic properties, and many otherunique characteristics [1–18]. They have had significantimpact on a variety of emerging technologies and marketsfrom light emitting diodes to energy conversion devices andother flexible electronics [10]. For example, the uniqueelectronic and structural properties of CNTs have promptedresearchers to incorporate them as integrative building blocksinto solar cells [18, 19]. In a typical dye-sensitized solarcell (DSSC), ruthenium-based complexes (e.g. N3, N719) andplatinum have been the preferred materials for the dye andthe counter electrode because of their high electrochemicalactivity [20]. However, the costs of both the ruthenium-basedcomplexes and platinum prevent the materials from havingany large-scale applications in solar cells, thereby forcing theresearch community to find alternatives. Recently, naturaldyes (e.g. anthocyanin) [21–24] and carbon [25–27] have beeninvestigated in DSSCs in order to produce low cost solar cellswith reasonable performance. An ultra-fast electron injection

3 Author to whom any correspondence should be addressed.

(with a transient absorption signal <100 fs) was identifiedfor an anthocyanin-sensitized solar cell [21]. In DSSCswith a liquid electrolyte, the conversion efficiency shouldbecome higher by using nanocarbon electrodes because of theirhigh surface areas, which will enhance the electrochemicalactivity of the electrode [25]. The recent developmentof depositing ultra-thin and homogeneous CNT films ontransparent glass electrodes can be conveniently realized withthe benefit of lowering the process cost and improving themechanical flexibility. In particular, macroscale single-walledcarbon nanotube (SWNT) thin films have been depositeddirectly on various flexible substrates from metallic foilsto polymer films by a floating chemical vapor depositiontechnique [16]. The SWNT macrofilms thus produced displaymore excellent properties than regular carbon nanostructures,implying promising applications in optoelectronics, fieldemission, supercapacitors, lithium batteries, etc. Self-assembled nanotube networks in these films will facilitate thedevice fabrication process and could provide opportunities forcreating high performance solar cell devices with nanoscalebuilding blocks. Motivated by this prospect, we utilized SWNTmacrofilms as counter electrodes in anthocyanin-sensitizedsolar cells by taking advantages of their intrinsic largesurface areas and high electron conductivity. For comparison

0957-4484/08/465204+05$30.00 © 2008 IOP Publishing Ltd Printed in the UK1

Nanotechnology 19 (2008) 465204 H Zhu et al

Table 1. Comparison of short circuit current density, open circuit voltage, fill factor, and conversion efficiency of cells sensitized with naturaldyes.

Samples R (�/square) S (m2 g−1) Jsc (mA cm−2) Voc (V) FF η (%) Ref.

Purified SWNTs 20 700 10.6 0.33 0.42 1.46Raw SWNTs 80 150 3.14 0.45 0.4 0.57MWNTs 25 500 3.42 0.46 0.39 0.62DWNTs 80 500 2.32 0.44 0.44 0.45ACFs 100 1300 2.0 0.43 0.43 0.37BP 2000 300 1400 0.44 0.41 0.44 0.08XC 72 100 240 1.0 0.45 0.4 0.18Jp carbon 120 1000 0.5 0.44 0.5 0.11Pt 10.9 0.50 0.27 1.49 [31]Pt 9.0 0.59 0.54 — [32]Pt 2.84 0.43 0.46 0.56 [33]Pt 2.72 0.41 0.63 0.7 [23]— 1.1 0.55 0.52 — [24]— 2.3 0.71 — — [34]— 3.2 0.48 — — [35]

purposes, counter electrodes made of other carbon structures,including multi-walled CNTs (MWNTs), double-walled CNTs(DWNTs), activated carbon fabrics (ACFs), and different highsurface area carbon blacks, were also investigated.

2. Experimental details

2.1. Preparation of photo electrodes

Nanostructured TiO2 coatings as photo electrodes wereprepared from a commercial TiO2 powder (Degussa P25,average size 25 nm) using a screen printing technique asdescribed previously [21, 28]. The TiO2 suspension was firstcoated on a fluorine-doped tin oxide (FTO) coated conductiveglass (30 �/square, Hartford Glass Co.), which was thenheated at 450 ◦C in air for 30 min, resulting in a thin layer,about 6 μm thick. The natural dye anthocyanin was extractedfrom frozen blackberries using methanol/acetic acid/water(25:4:21) [21]. The photo electrodes were immersed in the dyesolution for at least 3 h to ensure that the dye molecules werefully absorbed.

2.2. Preparation of counter electrodes

Counter electrodes from various nanocarbon materials werecoated differently on the conductive FTO glasses in the presentwork because of different initial features. In detail, theraw SWNT macrofilm (about 1 μm thick) was depositedon an FTO glass using a floating chemical vapor depositiontechnique [16], followed by an ethanol wetting in order toenhance the film–substrate adhesion. A free-standing purifiedSWNT film was obtained by a post treatment which employeda combination of oxidation (heated at 450 ◦C in air for 1 h) andrinsing with hydrochloric acid (37% HCl) [15, 16] and thentransferred to an FTO glass to make a conformal and uniformcoating (about 500 nm thick). MWNTs [29] were first treatedin hydrochloric acid at 50 ◦C for 12 h to remove the catalystsand then stirred in a 3:1 concentrated H2SO4/HNO3 mixtureat 50 ◦C for 10 min to introduce some functional groups andimprove the conductivity. The MWNT sample was then filteredusing a polypropylene filter paper. A thin film of entangledMWNTs (about 10 μm thick) was coated on an FTO glass

after completely drying the sample. Vertically aligned DWNTfilms were prepared on an Al-coated silicon wafer by a water-assisted CVD process [30] and then transferred to an FTOglass to make a 10 μm thick film after the ethanol wetting.Three different carbon blacks were tested: (i) Black Pearl(BP) 2000; (ii) Vulcan XC 72 (Cabot Co.); and (iii) Japanesecarbon black (Pred Materials Inc.). The counter electrodes ofthese carbon blacks were fabricated by mixing 80 wt% of thecarbon black and 20 wt% of PVDF-HFP binder. A slurry ofthe above mixture was made using N-methyl-2-pyrrolidone(NMP) as a solvent and this was subsequently brush-coatedonto an FTO glass to form a 5 μm thick film. The coatedFTO glass was dried at 110 ◦C in air for 1 h for the removalof the solvent. Highly microporous ACF, which is an excellentadsorbent in gas and liquid media, was commercially obtainedfrom Challenge Carbon Technology Co.

All the cells were assembled as illustrated in figure 1, withan active area of about 1 cm2. The KI3 electrolyte consistedof 0.5 M KI and 0.05 M I2 in anhydrous ethylene glycol. Thecells were illuminated by a Newport solar simulator under AM1.5 (100 mW cm−2) irradiation. The applied potential andphoto current were measured and recorded using a Keithley2601 sourcemeter.

3. Results and discussion

Compared with the cells using other carbon counter electrodes,the cells with CNT counter electrodes in general show betterperformance in conversion efficiency (η), which can beattributed to the increase in short circuit current density (Jsc)

(summarized in table 1, and figures 2 and 3). For example, theSWNT cells show the best performance compared with othernanocarbon cells. The cell with purified SWNT film counterelectrode shows 1.46% conversion efficiency, a much betterperformance in comparison with cells using carbon blacks asthe counter electrodes. The η and Jsc values are about 1.5times higher and 3 times higher than the previously reportedvalues for anthocyanin-sensitized solar cells [21], respectively,and about 2 times higher and 4 times higher than the values forcells using other natural dyes [23, 24]. They are comparable

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Figure 1. Schematic of a DSSC cell with a carbon counter electrode and SEM morphologies of different carbon materials. The inset shows ahigh resolution TEM image of SWNT bundles.

with the highest value ever reported for a cell using a naturaldye and Pt counter electrode [31] (see table 1).

The purpose of this work is to introduce a new formof nanotube counter electrode: a robust SWNT thin filmto improve the transport of charge carriers from the light-harvesting photoelectrode. The films were directly preparedusing our unique floating CVD technique [16]. It should benoted that the SWNT thin films and other nanotube materialsused in these experiments were all in macroscopic form,namely mixtures of semiconducting and metallic tubes withrandom chirality distribution (statistically, 2/3 semiconductingand 1/3 metallic). The performance of the whole film wasevaluated instead of that of individual semiconducting ormetallic tubes.

It has been found that the photovoltaic performance isstrongly affected by the specific surface areas of the carbonmaterials [26, 27]. It has been proven that a nanocarboncounter electrode has low charge transfer resistance basedon impedance spectroscopy measurements, owing to its highsurface area [36]. However, our result shows that carbonblacks with larger specific surface areas (see table 1) donot give higher conversion efficiency. Besides the highsurface area, we speculate that the 1D nature of SWNTssignificantly contributes to the charge transfer process owingto its improved electric conductivity. The SWNT electrodes

have exhibited much better DSSC performance than other highsurface area carbon blacks. In comparison with graphite andcarbon blacks, SWNTs have smaller work function (∼4.5 eV)and lower electron binding energy, which may benefit thecharge transfer. It is worth noting that the utilization ofCNT electrodes affects the kinetics of charge transfer at theelectrolyte–carbon interface, as carbon blacks and nanocarbonmaterials have dissimilar surfaces onto which iodide/iodineabsorbed. The contact interface between the counter electrodeand surrounding electrolyte is better in CNT materials, whichhave nanoscale conducting channels and therefore are expectedto enhance the electrochemical activity of the electrodes, asdemonstrated from the experimental results.

Among the carbon materials, the SWNT film alsoshows a high electrochemical activity in the iodide/tri-iodideredox reaction because of the one-dimensional nanofeature ofSWNTs which can provide better electron transport. In detail,the redox reaction involved in a DSSC is catalyzed by thecounter electrode. The solar energy can excite electrons in dyemolecules: photon + dye → electron + dye+. Those excitedelectrons are transferred from the dye to the TiO2, whichtransfers the electrons to the electrode, producing electricity.This leaves the dye material slightly positive (oxidized), andit needs an electron from the counter electrode (here the onecoated with different carbon materials) to make it neutral again.

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0.0

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cy (

%)

MWNT film

10 11 120 1 2 3 4 5 6 7 8 9Current density (mA/cm2)

Raw SWNT filmDWNT array

ACF

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Purified SWNT film

Figure 2. Efficiency versus current density for different carboncounter electrodes.

Figure 3. I–V characteristics of DSSC cells with different carboncounter electrodes at 100 mW cm−2 light intensity.

The iodide/iodine electrolyte will bring electrons from thecounter electrode to reduce the dye cycling between iodide(I−) and tri-iodide (I−3 ): dye+ + 3I− → dye + I−3 . The tri-iodide is restored to iodide by taking the electron from thecounter electrode: I−3 + electron → 3I−. This reaction iscatalyzed by the carbon coating, which combines sufficientconductivity and heat resistance as well as corrosion resistanceand electrocatalytic activity for tri-iodide reduction [25]. Thediameters of CNTs are in the order of 10 nm, and SWNTs havethe smallest diameters (about 1 nm). These materials exhibitvery large aspect ratios (>103) and establish percolationpathways that can provide the means for a high carrier mobilityand an efficient charge transfer.

The electrical conductivity also plays an important rolein the photovoltaic performance. The greater conductivityof the SWNT film makes it possible to retain an adequatelateral electrical conductivity while reducing the thicknessof the conductive layer. Upon removal of the impurities

Voc

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Figure 4. Short-term stability of a DSSC with a purified SWNTcounter electrode.

(iron particles and amorphous carbon) from SWNT films,the electric conductivity is enhanced (see table 1). Thepurification process also introduced a large amount of oxygen-containing functional groups which have beneficial effects onthe conductivity of SWNTs. Their strong electronegativity aselectron acceptors tends to move the Fermi level toward thevalence band and to increase the hole density in SWNTs [37].It would be preferable to align all the SWNTs in the filmsparallel to the desired direction of conduction, but suchalignment may be difficult to achieve at low cost. Even withoutsuch alignment in our SWNT films, they have already shownthe possibility to attain adequate lateral electrical conductivity.

As shown in figure 4, a cell made using a purified SWNTcounter electrode showed good stability. Assembled cellsstored at room temperature were used to evaluate the stability.During the test, the fill factor varied moderately around 0.42.There was no notable change in Voc. This stability confirmsthat SWNT films can be readily used as counter electrodes inDSSCs.

4. Conclusions

In summary, different carbon structures have been investigatedas the counter electrodes in natural dye anthocyanin-sensitizedsolar cells. These preliminary results show that SWNTsare an attractive material for solar energy applications. Theuse of SWNT films as counter electrodes to replace Pt isexpected to afford the following advantages: (1) nanoscaleconducting channels; (2) large surface area; (3) light weight;(4) high flexibility; and (5) low cost. In addition toenhancing the conversion efficiency, the incorporation of CNTscan potentially improve the mechanical and environmentalstability. Future studies are aimed at optimizing the filmstructures with controlled thickness and tube coverage forfurther improving the conversion efficiency.

Acknowledgments

This work was financially supported by National ScienceFoundation (NSF CMMI # 0753462) and University ofDelaware Research Foundation.

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