## 3 Results and Discussion
### Photothermal Conversion Performance  
According to the definition of absorptance (\(\alpha\)), transmittance (\(\tau\)), and reflectance (\(\rho\)), it is known that \(\alpha+\tau+\rho=1\). Because the infrared reflected layer is opaque, consequently, \(\tau\approx 0\), and the absorptance \(\alpha=1-\rho\). The photothermal conversion property (\(\alpha\), absorptance, and \(\varepsilon\), emissivity) was observed using an ultraviolet-visible-near-infrared spectrophotometer and emissometer. Figure 11 shows the effect of Al content on reflectivity of TiMoAlON film, which confirms the above analysis that the ratio of oxide and nitride has an effect on solar spectrum absorptivity.  
TiMoAlON film has low reflectivity in the spectrum range of 300-900 nm, then it increases sharply in the near-infrared spectrum range, as seen in Figure 11. According to [PERSON]'s law of thermal radiation, high reflectivity in the infrared region demonstrates a relatively low heat radiation when it serves at high temperature. Absorptivity in the visible spectrum increases with Al content, but when its content is over 10 at.%, absorptivity is similar in the range from visible to infrared. The difference is just in the ultraviolet  
Figure 11: Effect of Al content on reflectivity of TiMoAlON selective absorbing films.  
Figure 10: Cross-sectional TEM image of TiMoAlON film with Mo content of 3 at%.  
spectrum. When Al content is increased to 12 at.%, absorptivity decreases by 0.05 in the ultraviolet spectrum range. Al doping in TiN matrix can change its bond structure (e.g., TiN to (Ti,Al)N and (Ti,Al)(ON)) [14; 15], so absorptivity is varied accordingly to different matrix materials, but the superfluous Al atom does not take part in the reactive sputtering process. Considering the higher reflectivity in the UV spectrum region, TiMoAlON film with Al content of 10 at.% can achieve the highest absorptivity.  
To serve in non-vacuum environment at high temperatures, the as-deposited films were baked in atmosphere at 500 \({}^{\circ}\)C for 8 h to evaluate their thermal stability. Figure 12 shows reflectivity variation with Mo content after thermal oxidation in atmosphere at 500 \({}^{\circ}\)C for 8 h. Under the experimental conditions, 10 at.% Al is the optimum doping amount. Absorptivity has little change after thermal oxidation, which is consistent with [PERSON]'s conclusion about thermal stability of TiAlN/TiAlON/Si\({}_{3}\)N\({}_{4}\) tandem absorbers prepared using direct current magnetron sputtering [16; 17; 18]. It has high absorptivity, \(\alpha\) = 0.94, and low emissivity, \(\varepsilon\) = 0.02 at room temperature and \(\varepsilon\) = 0.10 at 500 \({}^{\circ}\)C. The as-deposited films doped with Mo do not improve absorptivity at the early stage because of strong intrinsic absorption, but surface plasma resonance absorption caused by the embedded ultra-fine Mo in oxide ceramic matrix will take effect after thermal oxidation, which offsets the intrinsic absorption reduction.  
When doped with 2 at.% Mo, film has the highest absorptivity, absorptivity of which in the visible-light range almost reached 0.97. When Mo content is below 2 at.%, optical absorption becomes weaker with lower Mo atomic percent. When Mo content was increased to 3 at.%, Mo particles aggregated and formed a multilayer structure, which remarkably decreased Mo specific area. After thermal oxidation, absorptivity in the visible spectrum becomes lower.