DOI: 10.1002/cphc.201402570 Dye-Sensitized Solar Cell Counter Electrodes Based on Carbon Nanotubes Seunghwa Hwang, [a] Munkhbayar Batmunkh, [b] Md J. Nine, [b] Hanshik Chung, [a] and Hyomin Jeong* [a] 1. Introduction The current energy crisis is one of the most serious worldwide problems, owing to rapidly growing energy consumption that leads to increases in the fuel price. Power generation through burning fossil fuels also brings significant damage to the envi- ronment. [1] Over the past few decades, various renewable energy sources from nature have been developed and used to supply our energy demands. [2–5] Producing energy from the sun may be the most ideal source, owing to its huge electricity production. The energy supply from the sun to the earth is considerable; it is estimated to be around 3  10 24 J year À1 , [6] which is almost a thousand times higher than what we cur- rently consume. Photovoltaic (PV) devices (solar cells) have been developed and categorized under the following three generations: 1) sili- con-based (Si) solar cells, [7] 2) thin semiconductor films based solar cells, [8] and 3) organic solar cells [9] and dye-sensitized solar cells (DSSCs). [10] The first generation Si-based solar cells are only the PV devices that are commercially available, because of their high conversion efficiency. However, the manufacturing processes that are used to produce Si-based solar cells are complex and the cost of the electricity produced from these cells is inherently expensive. DSSCs, one of the third-genera- tion PV cells, were designed to overcome the problems of con- ventional devices by combining high conversion efficiency with low manufacturing costs. Since their discovery by Grätzel in 1991, [11a] DSSCs have been attracting a great deal of attention because they are environ- mentally friendly, cost effective, and easy to fabricate. [11b] A conventional DSSC is composed of a film of dye -adsorbed semiconducting TiO 2 nanoparticles on a transparent conduct- ing oxide (TCO) (working electrode), an electrolyte containing the iodide/triiodide (I À /I 3 À ) redox couple, and a platinum (Pt)- based film on the TCO. [12, 13] Figure 1 schematically illustrates the working principle of DSSCs. A detailed explanation of the operational principle can be found elsewhere. [14, 15] In DSSCs, the counter electrode serves a critical role in trans- ferring the collected electrons from the external circuit and in- jecting them into the electrolyte, and thus I 3 À is reduced to I À . In a typical counter electrode, Pt is used as a catalyst because of its high electrochemical activity. [15] However, Pt is known as a rare and expensive metal, which inflates the overall cost of Dye-sensitized solar cells (DSSCs) have received significant at- tention from the scientific community since their discovery in 1991. However, the high cost and scarcity of platinum has mo- tivated researchers to seek other suitable materials for the counter electrode of DSSCs. Owing to their exceptional proper- ties such as high conductivity, good electrochemical activity, and low cost, carbon nanotubes (CNTs) have been considered as promising alternatives to expensive platinum (Pt) in the counter electrode of DSSCs. Herein, we provide a Minireview of the CNTs use in the counter electrode of DSSCs. A brief overview of Pt-based counter electrodes is also discussed. Par- ticular attention is given to the recent advances of counter electrodes with CNT-based composite structures. Figure 1. Operational principles of a typical DSSC. TCO: transparent conduct- ing oxide, CB: conduction band, V oc : open-circuit voltage. [a] S. Hwang, + Prof. H. Chung, Prof. H. Jeong Department of Energy and Mechanical Engineering Gyeongsang National University, Cheondaegukchi-Gil 38 Tongyeong, Gyeongnam 650-160 (Republic of Korea) E-mail : hmjeong@gnu.ac.kr [b] M. Batmunkh, + M. J. Nine School of Chemical Engineering The University of Adelaide Adelaide, South Australia 5005 (Australia) [ + ] These authors contributed equally  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 0000, 00, 1 – 14 &1& These are not the final page numbers! ÞÞ CHEMPHYSCHEM MINIREVIEWS