## 1 Introduction
The improvement in electrochromic parameters such as the coloration efficiency by nanoscal porosity is supposed to be attributable to enhanced diffusion of Li\({}^{+}\), which is a relevant issue in regard to the desired fast switching times [2]. Hence, further previous studies addressed the impact of different nanoscaled porosity on the electrochromic performance. Block copolymers such as polystyrene-_block_-polyethylene oxide (PS-_b_-PEO) [31] or Pluronic P123 [32] that are forming micelles allow the formation of mesoporous films with 4-30 nm large pores providing enhanced electrochromic characteristics such as short switching time, high coloration efficiency and large transmittance modulation [31; 32]. Note that in diverse previous studies films with spherical mesopores were investigated which inherently suffer from restricted connection between the spherical pores, impeding Li\({}^{+}\) diffusion [33; 34]. In view of such experience, a combination of block copolymers and long-chained additives might be promising to prepare WO\({}_{3}\) films with both, spherical mesopores as well as interconnected micropores. Such strategy already was successful in the preparation of titanium oxide thin films using polyisobutylene-_block_-polyethylene oxide (PIB\({}_{50}\)-_b_-PEO\({}_{45}\)) as block copolymer and 1-hexadecyl-3-methylimidazolium chloride (C\({}_{16}\)mimCl) as surfactant-like ionic liquid in different ratios to tune the porosity and permeability of titanium oxide thin films by creating a film structure with mesopores (caused by PIB\({}_{50}\)-_b_-PEO\({}_{45}\)) connected by worm-like pores (caused by C\({}_{16}\)mimCl) [35]. In the present work, thin films of WO\({}_{3}\) were prepared via spin-coating with similar polymers as structure-directing agents to influence the internal film structure and porosity and to improve the electrochromic switching characteristics. The precursor solution based on peroxotungstic acid showing a high stability can be easily and inexpensively prepared by dissolution of tungsten in hydrogen peroxide with no need of anhydrous conditions [8; 14; 36]. It is desirable to use PEG of low molecular weight to create a network of interconnected micropores in the films since such PEG can be easily removed after annealing for a shorter time compared to PEG of high molecular weight [27; 30]. Therefore, a peroxotungstic acid-based precursor solution was used with addition of PEG 400, PEG 600 (for a formation of micropores [37]) or the diblock copolymer PIB\({}_{50}\)-_b_-PEO\({}_{45}\) that had been successfully used for the preparation of mesoporous titanium oxide [35; 38]. To obtain WO\({}_{3}\) thin films with interconnected mesopores, a mixture of PEG 600 and PIB\({}_{50}\)-_b_-PEO\({}_{45}\) was established. The structural, optical, electrochemical and spectroelectrochemical properties of the WO\({}_{3}\) thin films using these approaches were characterized and compared to compact films prepared without additives. The addition of PEG 400 or PEG 600 into the precursor solution created a network of interconnected micropores within the films. An enhancement of the electrochromic switching properties compared to compact WO\({}_{3}\) films was observed. The largest transmittance modulations, highest coloration efficiencies and fastest diffusion of ions along with highly reversible and stable switching processes could be obtained for WO\({}_{3}\) films prepared with PIB\({}_{50}\)-_b_-PEO\({}_{45}\) combined with PEG 600 as additives, leading to films with interconnected mesopores. The great potential of solution-based processing for WO\({}_{3}\) suitable for the application in smart windows is thereby demonstrated.