Single-Walled Carbon Nanotube Thin-Film Counter Electrodes for Indium Tin Oxide-Free Plastic Dye Solar Cells Kerttu Aitola, a,z Antti Kaskela, a Janne Halme, a Virginia Ruiz, a,b Albert G. Nasibulin, a Esko I. Kauppinen, a,c and Peter D. Lund a a Department of Applied Physics, Aalto University, 00076 Aalto, Finland b CIDETEC-IK4—Centre for Electrochemical Technologies, E-20009 Donostia-San Sebastián, Spain c Technical Research Centre of Finland, Biotechnology, FI-02044 VTT, Finland The use of a thin carbon nanotube CNT counter electrode CE on plastic in a dye solar cell DSC is demonstrated as an alternative to expensive indium tin oxide and platinum materials. Optically transparent, single-walled CNT films synthesized by the aerosol CVD method and dry-printed on PET plastic substrates functioned as both the catalyst and conducting layer of the DSC CE. The best charge-transfer resistances and sheet resistances for the random network-type film were around 89  cm 2 and 60 /, respectively, making them suitable for low-intensity DSC applications. A solar cell efficiency of 2.5% was reached at an illumination of 8 mW/cm 2 . The photocurrent generation of the cells was found to decrease when a non-purified CNT-CE was used. The electrochemical removal of iron catalyst particles from the CNT films reduced the detrimental effect and stabilized the performance of the DSC. © 2010 The Electrochemical Society. DOI: 10.1149/1.3500367 All rights reserved. Manuscript submitted June 15, 2010; revised manuscript received September 1, 2010. Published October 21, 2010. Dye solar cells DSCs are a promising option for photovoltaic energy conversion. 1 One attractive feature of DSCs is that they can be produced by roll-to-roll techniques on flexible plastic and metal substrates. 2-4 However, finding cheap photo- and counter electrode CE materials which endure bending has been challenging. The commonly used flexible CE type has been a platinum nanoparticle layer deposited on polyethylene terephthalate PET or polyethylene naphthalate polymer foil coated with indium tin oxide ITO, e.g., by sputtering. However, Pt and In are expensive and rare materials used extensively in DSCs, and there is a general need to replace them with more abundant alternative materials. The expanding utilization of conductive and transparent ITO in several electro-optical applications has increased the demand and thus the price of indium from 10 cents per gram in the early 2000s to 60 cents per gram in the late 2000s. 5 The increasing demand and price combined with the device design-limiting features of ITO, such as its poor flexure resistance, 6 motivate the study of alternative transparent and conductive materials, including single-walled car- bon nanotube SWCNT networks, which could replace ITO in cer- tain applications, such as DSCs. One interesting alternative material for DSC counter electrodes is carbon, which in its different forms has been found to possess both electrical conductivity and catalytic activity toward the tri-iodide reduction reaction, and can thus replace both Pt and ITO or fluorine- doped tin oxide FTO simultaneously at the DSC counter electrode when applied as a thick screen-printed porous layer on glass substrates. 7,8 Recently, great interest has emerged in the application of carbon nanotubes CNTs for this purpose. Individual SWCNTs have sev- eral advantageous properties, such as superior electrical conductivity and mechanical strength. It is anticipated that these properties could, to some extent, be transferred to the bulk properties of SWCNT films, which render them interesting for use in diverse applications. Thin, randomly oriented SWCNT films are already flexible 9 and conductive at such thicknesses when the films are partially transpar- ent, and they have an inherently high specific surface area, which is most probably needed for good catalytic properties. There is indeed some evidence of SWCNTs being catalytically active toward the tri-iodide reduction reaction of the DSC counter electrode. 10,11 The catalytic activity of a material used as a catalyst in an elec- trochemical device is described by the exchange current density of the charge-transfer reaction, i 0 , which is inversely proportional to the charge-transfer resistance, R CE . The target value for the catalytic activity and for the R CE of a material used as a DSC CE catalyst depends on the application area. If a DSC is operated under full sunlight, where the cell may produce current values in the range of 10 mA/cm 2 , the R CE should be as low as possible, preferably less than 10  cm 2 . A resistance of that order would cause a voltage loss of the order of 0.1 V, which is still quite a small value compared to the V OC value of 0.7 V 14% of the V OC . If a DSC is intended to be used in low light intensities, for instance indoors, it produces less current, and a higher R CE value can be tolerated, since the voltage loss caused by the resistance is nonetheless small because of the low current density. Graphite and carbon black mixtures have been successfully ap- plied to the CE of DSCs on rigid and flexible substrates. 2,7,12,13 Such a material was also previously tested in our group, and the R CE s of these films were as low as 0.5–2  cm 2 at a thickness of 10–20 m. 2 However, the durability of these materials toward bend- ing was questionable: the material flaked off the substrate, especially when the thickness of the film exceeded 40 m. There are some reports where a CNT film has been used as DSC counter electrode material. 10,11,14-17 In almost all the cases, the CNT films have been on rigid FTO glass, replacing only the Pt catalyst layer of the CE, and good catalytic activities have been achieved. For instance, Trancik et al. obtained R CE s of 1.8–15.5  cm 2 for spray-coated SWCNT films on FTO glass with transmittance from 40 to 90%, respectively; however, the electrolyte that was used was prepared in an aqueous solution instead of the common organic solvents. 10 They also observed that R CE was lowered with the ozone treatment of the films, causing defects in the CNTs, and concluded that defects seem to be catalytically active sites. In another study, a nanocarbon ink was prepared from multiwalled CNTs, Nafion and ethanol, and from this a 50 m thick film was formed. 11 Here, the R CE was as low as 2.5  cm 2 . Only in one previous study SWCNT film has been incorporated in a DSC without an additional conductive layer beneath: Suzuki et al. used a plain SWCNT film of unknown thickness on a membrane filter because of the manufacturing method, attached to a plain glass substrate, as their DSC CE, and obtained a cell efficiency of 4.5%. 14 CNT films have only been used on alternative DSC sub- strates in one study, 18 where the authors drop-casted SWCNT solu- tion onto a 2 mm thick stainless steel substrate, and obtained a DSC efficiency of about 3.9%. Plain plastic substrates with a conducting and catalytic CNT layer have not been used as DSC CEs before, but SWCNT on PET electrodes has been used for polymer–fullerene bulk-heterojunction solar cells. 19,20 In most of the above-mentioned studies the nanocarbon films were several micrometers thick and applied to rigid conducting sub- z E-mail: kerttu.aitola@tkk.fi Journal of The Electrochemical Society, 157 12 B1831-B1837 2010 0013-4651/2010/15712/B1831/7/$28.00 © The Electrochemical Society B1831 Downloaded 25 Oct 2010 to 130.233.181.5. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp