## 3 Results and Discussion
### Morphology and Structure of the Films
At 700 \({}^{\circ}\)C, the glass of the substrate started to melt leading to a decrease of the intensity of all reflexes. The amorphous character of the WO\({}_{3}\) thin films annealed at temperatures \(T\leq 300\)\({}^{\circ}\)C was confirmed by Raman spectroscopy. In the Raman spectra (Figure S3) measured for the films prepared without and with PEG at \(T\leq 300\)\({}^{\circ}\)C a broad peak at 770 cm\({}^{-1}\) was observed caused by vibrations of W6+-O bonds [12; 45] while a relatively sharp peak at 950 cm\({}^{-1}\) reflects the stretching of W6+=O bonds characteristic for amorphous WO\({}_{3}\)[12; 45; 46]. For the WO\({}_{31}\)up600 thin film, another broad peak at 220 cm\({}^{-1}\) was noticed which might correspond to the bridging of O-W4+-O bonds and indicates the presence of W4+ ions and oxygen vacancies within the films [12; 45]. Thermogravimetry (TG) of the dried precursor, resembling films following spin-coating, was performed to monitor the decomposition of the polymers during heat treatment and to detect possible phase transformations of WO\({}_{3}\). In Figure 3a, the TG curves for the dried peroxotungstic acid precursor solutions with and without PEG are shown. Starting from room temperature up to a temperature of 100 \({}^{\circ}\)C, a small loss in mass of around 1% followed by a larger loss from 100 \({}^{\circ}\)C to 200 \({}^{\circ}\)C was observed caused by the desorption of water and organic solvents as well as the transformation of peroxotungstic acid to WO\({}_{3}\)[14; 27; 29]. For the precursors with PEG, mainly its decomposition contributed to this mass loss [27]. As expected, the drop in mass from room temperature to 250 \({}^{\circ}\)C is  
Figure 2: In situ GIXRD patterns of a WO\({}_{31}\)up600 film that was annealed up to 700 \({}^{\circ}\)C with reflexes assigned to triclinic WO\({}_{3}\) (JCPDS: 83-0947) and FTO marked with asterisks and dots, respectively. larger for the precursor containing PEG 600 since a larger amount of polymer is present. Within the three hours of treatment at 250 \({}^{\circ}\)C, the mass of the precursor without additives decayed by 2% whereas the precursors containing PEG 400 or PEG 600 reduced their mass by only 1% or 0.4%. All three precursors quickly stabilized indicating that keeping them for about 1 h at 250 \({}^{\circ}\)C is sufficient for the preparation of amorphous WO3 thin films. The subsequent small loss of mass (2%) upon further heating to 700 \({}^{\circ}\)C can be assigned to the phase transition of amorphous to crystalline WO3 [14; 27; 29], which was independently proven by XRD (Figures 2 and S2). The TG curves for the precursors containing the block copolymer PIB50-\(b\)-PEO45 or PIB50-\(b\)-PEO45 in combination with PEG 600 (Figure 3b) follow the same trend upon heating to 300 \({}^{\circ}\)C. At this temperature, however, a mass loss of about 3% was observed over 12 h, indicating continued decomposition of the polymer. Upon subsequent heating to 800 \({}^{\circ}\)C, a significant mass loss of another 3% (WO31 mp) or 7% (WO31 ump) was observed consisting of both, crystallization of WO3 [27] and completion of polymer decomposition in the case of WO31 ump. For WO31 mp, however, the mass loss of the precursor is similar to the one of the precursor without any additives confirming that residuals of the polymer were still present in the films. Since amorphous WO3 thin films were aimed at, a temperature not higher than 300 \({}^{\circ}\)C (Figures 2 and S2) had to be chosen for the heat treatment of the films and it cannot be excluded that the WO31 mp thin films might still contain some polymer fragments. Comparing the initial loss of mass for the different precursors reveal that the precursor without any additives and the one containing PIB50-\(b\)-PEO45 have lost about the same amount of mass (15-18%) up to a temperature of 250 \({}^{\circ}\)C while the precursors with PEG 600 have lost more than half of their original mass.