Anneke Georg, Freiburg Materials Research Centre, Freiburg, Germany Andreas Georg, Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany Ursa Opara Krasovec, Faculty of Electrical Engineering, University of Ljubljana, Ljubljana, Slovenia

Photoelectrochromic windows represent a special kind of switching windows. The energy for colouring is provided by sunlight, so that a voltage supply is not required. The transmittance can be decreased on illumination and can be increased again in the dark. In contrast to photochromic devices, the system is externally switchable under illumination.

Our photoelectrochromic window consists of several components: a dye-covered nanoporous TiO2 layer, which is situated on a nanoporous electrochromic layer, such as WO3, two glass substrates coated with a transparent conductive oxide, of which one is coated with Pt, an iodide/tri-iodide redox couple and Li+ ions in a solid ion conductor. All the layers can be kept quite thin, so that they are transparent. The pores of the TiO2 and WO3 layers are filled with the electrolyte.

This configuration is a particularly advantageous combination of the dye solar cell and an electrochromic element. The colouring time is independent of the area, the transmittance can be varied also in the illuminated state, and the system can also be switched by an auxiliary external voltage. Initial samples with solid electrolyte change their visible transmittance from 62 % to 1.6 %, their solar transmittance from 41 % to

0. 8 %. The time for colouring and bleaching is about 10 minutes.

Introduction

Photoelectrochromic systems combine electrochromic layers [1, 2] and dye solar cells [3, 4]. Electrochromic layers change their transmittance reversibly when electrons and cations are injected. In photoelectrochromic systems, the dye solar cell provides the energy for the coloration of the electrochromic layer. Thus, the transmittance of the photoelectrochromic device can be decreased under illumination and can be increased again when illuminated or in the dark. An external voltage supply is not required. Applications of these devices include, for example, switchable sunroofs in cars or smart windows in buildings.

We developed the photoelectrochromic configuration illustrated in fig. 1, which is a particularly advantageous device. It consists of several components (fig.1): a dye-covered nanoporous TiO2 layer, a porous electrochromic layer, such as WO3, two glass substrates coated with a transparent conductive oxide (TCO), of which one is coated with Pt, an iodide/triiodide redox couple and Li+ ions in an organic solvent. Both the TiO2 and the Pt layers can be kept quite thin, so that they are transparent. The pores of the TiO2 and WO3 layers are filled with the electrolyte.

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During illumination (upper part of fig.1), a dye molecule absorbs a photon of the incident light. Then an electron is rapidly injected from the excited state of the dye into the conduction band of the TiO2 and diffuses to the WO3. Ionised dye molecules are reduced by I" in the electrolyte according to the reaction: 3I" ^ I3" + 2e". Li+ ions intercalate into the WO3 and keep the charges balanced. Because of the injection of electrons, the WO3 changes its colour from transparent to blue.

If electrons are allowed to flow via an external circuit from the WO3 via a TCO layer to the Pt electrode (lower part of fig.1, external switch closed), then the Pt catalyses the reverse reaction, i. e. the reduction of I3" to I". Li+ leaves the WO3, and the WO3 is bleached fast. This process occurs also during illumination. If the external switch is open, electrons can leave the WO3 only by loss reactions. This process is very slow.

With a liquid electrolyte, the device’s visible (solar) transmittance under 1000W/m2 of illumination changes from 51% to 5% (35% to 1.5%) with switching times of about 3 minutes. Using a solid electrolyte, a visible transmittance change from 62% to 1.6% and a solar transmittance change from 41% to 0.8% are achieved with switching times of about 10 min. The colouring time is independent of the area.

An alternative photoelectrochromic configuration was first published in [5]. The colouring and the bleaching are competing processes, because the bleaching is possible only via loss reactions. Therefore, either fast colouring and bleaching with a small transmittance change [5] or a large transmittance change with slow bleaching is achievable [6], or an external voltage is used for bleaching [7]. In our new device, the materials can be optimised for colouring and bleaching independently, so it simultaneously allows fast colouring and bleaching, and high contrast [8].

In [8] we introduced this new device and discussed the differences to the alternative photoelectrochromic system and the advantages of our new system.

Experiments with different layer configurations of photoelectrochromic devices were reported in [9]. From these experiments we concluded that the loss reactions of electrons from the TiO2 can be neglected compared to the loss reactions of electrons from the WO3.

We investigated both liquid electrolytes [8,9] and solid electrolytes [10]. Liquid electrolytes allow a faster switching, but need good sealing to be stable on the long term, whereas

solid electrolytes, especially polymer electrolytes, show slower switching properties but are more suitable for most window applications.