Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat Iridium oxide catalyst for hybrid electrochromic device based on tetramethylthiourea (TMTU) redox electrolyte Shankar Bogati, Rabin Basnet, Andreas Georg ⁎ Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstraße 2, 79110 Freiburg, Germany ABSTRACT This work reports a study on sputter coated iridium oxide film as a counter electrode (CE) for the electrochemical catalysis of tetramethylthiourea/tetra- methylformaminium disulfide (TMTU/TMFDS 2+ ) redox reaction in electrochromic (EC) devices. The iridium oxide (IrO x ) with the thickness of about 5 nm coated onto transparent conductive oxide, e.g., fluorine-doped tin oxide (F:SnO 2 ), has shown excellent catalytic properties and excellent electrochromic cycling stability in EC device. The electrochemical impedance spectroscopy (EIS) results revealed that the charge transfer resistance (R ct ) is mainly influenced by the oxygen flow rate during the sputtering process, and by the layer thickness. The IrO X films coated with low oxygen flow rate has shown a lower charge transfer resistance compared to the fully oxidized iridium oxide film. However, sub-stoichiometric layers are less transparent, and therefore less appropriate for the application. In this regard, fully oxidized and highly transparent (T visible = 86%) layer formed by the sputtering process at or above 50 sccm (standard cubic centimeters per minute) flow of oxygen having the thickness of 5 nm is used in EC devices. These layers have the R ct of 25 Ω cm 2 at 1 V bias voltage for the redox electrolytes. The cyclic voltammetry technique has shown a typical quasi-reversible nature of redox electrolyte at the same IrO x coated electrode. An electrochromic test for 550 cycles demonstrates that 5 nm of IrO x is sufficient for the stabile EC window with TMTU/TMFDS 2+ electrolyte. 1. Introduction An electrochromic phenomenon of a material can be described as a reversible change in the optical properties of a material due to the in- jection or the extraction of electrons together with compensating po- sitive charges [1]. Smart windows for buildings and vehicles have a high energy saving potential due to their control over light transmit- tance through them [2]. The hybrid type of electrochromic devices can be one of the options for the windows with large area [2]. In this type, one redox system is immobilized as a thin film on a transparent elec- trode and the other one is in solution [3]. The exchange of charges in between two redox pairs is driven by an external voltage or corre- sponding current as shown in Fig. 1. Eqs. (1) and (2) describe the two redox reactions taking place during coloration (forward) and bleaching (backward). The reduced and oxidized forms of the redox couple are abbreviated as R and O, respectively. The redox couple counterbalances the first redox reaction during double insertion or extraction of Li + ions and electrons into or from tungsten oxide (WO 3 ), respectively. + + ⇌ + − xM (cation) xe WO (transparent) M WO (blue) 3 x 3 (1) ⇌ − + R e – O (2) Commonly, WO 3 film is used an electrochromic layer. The selection of the redox electrolyte mainly depends upon its optical and electro- chemical properties such as redox potential and charge transfer resistance (R ct ) at the counter and working electrodes. Moreover, the redox electrolyte should also be non-corrosive and environmentally compatible. The utilization of iodide/triiodide ( − − I /I 3 ) as redox elec- trolyte in EC device was presented by Georg et al. [2], where a thin film of Pt (platinum) was used as counter electrode (CE). The Pt is also commonly used CE in dye-sensitized solar cells [4,5] or photoelec- trochromic devices [6–14]. Furthermore, other redox couple such as tetramethylthiourea/tetramethylformaminium disulfide (TMTU/ TMFDS 2+ ), 2,2,6,6-tetramethyl-1-piperidinyl-oxy (TEMPO)/TEMPO + and potassium ferrocyanide/potassium ferricyanide (K 4 [Fe(CN) 6 ]/ K 3 [Fe(CN) 6 ]) have been already studied [15] to be used in EC devices. Among them, TMTU/TMFDS 2+ was found to be the most promising redox couple because of its color neutrality, appropriate redox potential and excellent charge transfer resistance at sputtered WO 3 , Pt and transparent conductive oxide TCO, especially at F:SnO 2 . One of the critical issues of EC devices with redox electrolyte is the loss current, which has to be maintained by the external power supply to keep the device in colored state [16]. It represents the electron transfer in be- tween WO 3 and redox electrolyte. It can be correlated with the charge transfer resistance or the exchange current densities on the WO 3 /redox electrolyte. A rough estimation of the maximum tolerable loss current is in the range about 10 μA/cm 2 [16]. Higher loss currents lead to an inhomogeneous coloration on a large area, where the corresponding power consumption is less critical. This issue has been addressed adding https://doi.org/10.1016/j.solmat.2018.09.026 Received 10 April 2018; Received in revised form 21 September 2018; Accepted 23 September 2018 ⁎ Corresponding author. E-mail address: andreas.georg@ise.fraunhofer.de (A. Georg). Solar Energy Materials and Solar Cells 189 (2019) 206–213 Available online 16 October 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved. T