Publication Date:
2015-05-09
Description:
Thin films of doped VO 2 were deposited, analyzed, and optimized with regard to their solar energy transmittance ( T sol ) and visible/luminous light transmittance ( T lum ) which are important parameters in the context of smart window applications in buildings. The doping with alkaline earth metals (AEM) like Mg, Ca, Sr, or Ba increased both T sol and T lum due to a bandgap widening and an associated absorption edge blue-shift. Thereby, the brown-yellowish color impression of pure VO 2 thin films, which is one major hindrance limiting the usage of VO 2 as thermochromic window coating, was overcome. Transparent thin films with excellent switching behavior were prepared by sputtering. Highly doped V 1− x Me x O 2 ( Me = Ca, Sr, Ba) kept its excellent thermochromic switching behavior up to x ( Me ) = Me /( Me + V) = 10 at. % doping level, while the optical bandgap energy was increased from 1.64 eV for undoped VO 2 to 2.38 eV for x (Mg) = 7.7 at. %, 1.85 eV for x (Ca) = 7.4 at. %, 1.84 eV for x (Sr) = 6.4 at. % and 1.70 eV for x (Ba) = 6.8 at. %, as well as the absorption edge is blue shifted by increasing AEM contents. Also, the critical temperature ϑ c , at which the semiconductor-to-metal transition (SMT) occurs, was decreased by AEM doping, which amounted to about −0.5 K/at. % for all AEM on average. The critical temperature was determined by transmittance-temperature hysteresis measurements. Furthermore, T sol and T lum were calculated and were found to be significantly enhanced by AEM doping. T lum increased from 32.0% in undoped VO 2 to 43.4% in VO 2 doped with 6.4 at. % Sr. Similar improvements were found for other AEM. The modulation of the solar energy transmittance ΔT sol , which is the difference of the T sol values in the low and high temperature phase, was almost constant or even slightly increased when the doping level was increased up to about 10 at. % Ca, Sr, or Ba.
Print ISSN:
0021-8979
Electronic ISSN:
1089-7550
Topics:
Physics
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