Journal of Power Sources 168 (2007) 533–536 Short communication Tandem dye-sensitized solar cell-powered electrochromic devices for the photovoltaic-powered smart window Kwang-Soon Ahn a,∗ , Sung Jong Yoo c , Moon-Sung Kang b , Ji-Won Lee b , Yung-Eun Sung c a Energy Laboratory, Corporate R&D Center, Samsung SDI Co., Gyeonggi-do 449-577, South Korea b Energy and Environment Lab., Samsung Advanced Institute of Technology (SAIT), Gyeonggi-do 446-712, South Korea c School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, South Korea Received 20 October 2006; received in revised form 15 December 2006; accepted 19 December 2006 Available online 12 March 2007 Abstract Tandem dye-sensitized solar cell (DSSC)-electrochromic (EC) devices were realized using two-faced transparent conducting oxide (TCO). To supply sufficient voltage to drive the EC devices, two series connected, semitransparent DSSCs were fabricated with 7nm-thick, dye-adsorbed TiO 2 and 4 nm-thick Pt layers. The two series connected, semitransparent DSSCs that were used had an open circuit voltage and short circuit current density of about 1.35 V and 3.96 mA cm -2 , respectively, at 1-sun. The tandem DSSC-EC devices showed an optical density difference of 1.2 at 750 nm and reasonable response times of about 60 and 45 s during the coloring and bleaching processes, respectively, indicating that the two series-connected, semitransparent DSSCs could be used as the power sources in the tandem photovoltaic-powered EC devices. © 2007 Elsevier B.V. All rights reserved. Keywords: Dye-sensitized solar cell; Electrochromic device; Photovoltaic; Optical density difference; Response time 1. Introduction The interest in the application of photovoltaic (PV) cells to low power devices such as small electronic devices and elec- trochromic (EC) devices has grown considerably, because of the striking energy savings which they allow [1–6]. PV-EC devices have two types of structures, viz. side-by-side and tandem struc- tures. Benson et al. [2] reported that both designs are attractive and can reduce the installation cost of the EC windows by reduc- ing or eliminating the electrical wiring and electrician labor costs. The PV cell is located around the perimeter and over the entire area of the EC window for the side-by-side and tan- dem structures, respectively. The bleached transmittance of the tandem Si-based PV-EC devices was less than that of the side- by-side devices [3,5,6], because the sunlight passes through both components in the tandem structures. Nevertheless, the tandem ∗ Corresponding author. Present address: National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, CO 80401, USA. Tel.: +1 303 384 6469; fax: +1 303 384 6491. E-mail addresses: kwang-soon ahn@nrel.gov, kstheory@paran.com (K.-S. Ahn). structure may have other advantages, such as the fact that the PV cells may be used as an “energy window”, as well as the power sources of the EC devices. Dye-sensitized solar cells (DSSCs) are known as one of the promising types of PV cells for use as energy windows [7–10], because they are less inexpensive and can be easily scaled up to large-area devices. The DSSC consists of a dye-sensitized TiO 2 layer, a Pt catalyst layer, and an electrolyte containing a redox couple (I - /I 3 - ) between them. When a DSSC is illuminated by sunlight, the adsorbed dye molecules on the surface of the meso- porous TiO 2 absorb the light and become excited. The absorption of light by the dye is followed by the injection of an electron from the excited state of the dye to the conduction band of the TiO 2 and its subsequent transfer to the transparent conducting oxide (TCO). Finally, the electron flows through the external circuit. The transparency of the DSSCs may be improved by develop- ing adequate dye molecules and counter catalysts. On the other hand, Si-based PV cells have an intrinsic problem. Inorganic semiconductors like Si absorb intrinsically all wavelengths that correspond to energies above their energy bandgap. Therefore, the only solution to improve their transparency is to decrease the thickness of the Si-based PV cell. Gao et al. [6] reported 0378-7753/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2006.12.114