Cesium polytungstate in sputtered solar control films. II. Electronic structure and water-induced defects

The origin of low conductivity and high near-infrared absorption observed in sputtered Cs-polytungstate (CPT) thin films was studied using x-ray photoelectron spectroscopy (XPS), thermal desorption spectroscopy (TDS), and first-principles calculations. The film's resistivity was directly correlated with the intensity of XPS W⁵⁺4f peaks, and the mobility of the conduction band (CB) electrons was evaluated as 1.27 × 10⁻⁷ cm² V⁻¹ s⁻¹, both implying the dominance of W⁵⁺-trapped electrons in the CB. The presence of H₂O residue in the films was detected from XPS O 1s and TDS spectra, suggesting that the incorporation of water upon film deposition induced numerous W/Cs defects reported in Part I. The two representative types of optical profiles were reproduced by first-principles calculations with W-deficient CPT and H₂O-incorporated pyrochlore, respectively. We conclude that water-induced W/Cs defects annihilated free electrons in the films, causing loss of conductance and plasma reflection, whereas W⁵⁺-trapped electrons generated by O and W vacancies underwent polaronic excitations that generated large near-infrared absorption.