A new approach for design of organic electrochromic devices with inter-digitated electrode structure $ Galit Bar a , Galit Strum a , Raz Gvishi a,Ã , Nina Larina b , Vladimir Lokshin b , Vladimir Khodorkovsky b , Larissa Grinis c , Arie Zaban c , Irina Kiryuschev d a Electro-optics Division, Soreq NRC, Yavne 81800, Israel b Interdisciplinary Center of Nanoscience CINaM (CNRS UPR 3118),13288 Marseille Cedex 9, France c Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel d Irus Optomechanics LTD, Israel article info Article history: Received 25 September 2008 Received in revised form 21 July 2009 Accepted 15 August 2009 Available online 16 September 2009 Keywords: Electrochromic device Inter-digitated electrode Viologen Electrophoretic deposition Porous titania abstract We present a new approach for design of organic electrochromic devices (ECD) with inter-digitated electrode (IDE) structure and three-electrode dynamic operation. The advantages of the IDE design include the ability to produce fast and homogenous color change over large areas. In addition, it enables fabrication of multi-color devices. Our method involves photolithographic etching of ITO followed by electrophoretic deposition (EPD) and mechanical compression of porous titania to produce finely patterned electrodes with high surface area. The titania layer is chemically modified by new stable and reversible electrochromic viologen derivatives involving phenylphosphonic acid anchoring moiety. The new device demonstrates reversible and strong color change from colorless to deep blue and yellow. & 2009 Elsevier B.V. All rights reserved. 1. Introduction Recent development of new display technology is attracting considerable attention and significant amount of funding. More- over, many research groups concentrate on fabrication of flexible displays. Electrochromic devices (ECDs) are expected to enter this niche and fulfill the requirements from such devices [1–8]. High contrast ratio and short response time are among their properties and they are also available in various colors [9–11]. In addition, ECD power consumption is low compared to conventional displays, which makes them attractive for light weight applica- tions such as cellular phones and electronic paper. The flexible ECDs are expected to be light and in addition will be less fragile compared to rigid devices. All together, ECDs and flexible ECDs in particular are very attractive for a variety of applications [12–16]. Previous reports on organic electrochromic devices described a multilayer structure with the electrochromic material either bonded to the substrate [9,10,15–18] or dissolved in the sand- wiched solution [19]. Our technique aims to improve the existing technologies and it seems to be promising in manufacturing flexible ECDs. The concept of our method is fabrication of organic ECD in an inter- digitated structure and three-electrode dynamic operation. In IDE the distance between anode and cathode is minimized and thus the voltage drop problem is reduced and the electric field is homogenously distributed over all the electrode area. Therefore, the advantages of the inter-digitated design include the ability to produce homogenous color changes over large areas, fast response time and possibility to fabricate multi-electrode or multi-color devices [20]. To prepare the inter-digitated pattern in fine structure and high quality on glass or PET we used lithographic etching. On top of the ITO we deposit a nanoporous layer of titania to increase the surface area of the working electrode (WE) by several orders of magnitude. This affords higher electrochromic material concentration on the electrode surface and, therefore, improves the contrast ratio and color intensity of the device. The conventional method to prepare porous titania electrodes is based on a viscous titania paste that is spread on the substrate. This method is not suitable for preparation of patterned electro- des with a fine structure (electrode finger widths of 1–2 mm). For this reason we used EPD to make the nanoporous TiO 2 layer [21,22]. EPD is suitable for deposition of porous materials on conductive electrodes with widths that are fractions of a millimeter. Further, the EPD technique enables selective ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.08.013 $ Paper presented at the 8th International Meeting on Electrochromism (IME-8), Seoul, Korea, August 24–28, 2008. Ã Corresponding author. Tel.: +972 8 9434325; fax: +972 8 9434823. E-mail address: rgvishi@soreq.gov.il (R. Gvishi). Solar Energy Materials & Solar Cells 93 (2009) 2118–2124