(Invited) Thermochromic Phase Transitions in VO2-Based Thin Films for Energy-Saving Applications
Authors: Barron S C , Gorham J M , Green M. L. 
Published: February 2020

Materials that contribute to more sustainable technologies are in high demand, and the highest impact can be made in the structures that are most ubiquitous. Widely implemented, thermochromic ‘smart’ building windows have the potential to save energy by reducing the heating and cooling loads on HVAC systems.1,2 Exploiting a crystalline phase transition in VO2, these window coatings shift from infrared transparent on cold days when solar heating is desirable to infrared reflective as the ambient temperature increases. In pure VO2, this transition occurs at 68°C, but the incorporation of 1-5% cation impurities can depress the transition temperature to Earth ambient temperatures.
In this talk we describe our results on infrared thermochromism in V1-xMxO2thin films, for M = Ta, Nb, W, Hf, and others.
To efficiently screen the vast compositional space of transitional metal impurities in VO2, we employ a combinatorial materials technique for both material synthesis and characterization. Thin film libraries of continuously varying composition are prepared by pulsed laser deposition (PLD) from two or more unary metal oxide targets, e.g. V2O5 and Ta2O5. Taking advantage of the spatial non-uniformity of deposition rates from each target, we produce library samples of composition V1-xMxO2where x can vary from <0.01 to 0.3 across a single 3” diameter wafer.
A special high-throughput spectroscopic reflectance measurement apparatus has been constructed for screening these libraries for thermochromic transitions. A thermoelectric heater/chiller with a 3” diameter plate maintains library temperature at a setpoint between 5°C and 85°C, while near infrared (NIR) reflectance measurements are made at many library locations, and hence many unique compositions.
In Figure 1, for each composition represented on the V1-xTaxO2 sample library, the NIR reflectance switches from a low reflectance state to a high reflectance state, a consequence of the crystalline phase transition that occurs. The transition is reversible with a <10°C hysteresis, and the material returns to its low reflectance state as the material is cooled back to 20°C. As the Ta impurity content increases, the transition temperature decreases.
Figure 2 shows depression of transition temperatures in VO2 due to the incorporation of Nb or Ta. Both are found to effect a temperature depression that is linear with concentration at -4 °C/atomic percent. Additionally, the higher valence state impurities, such as W, Nb, and Ta, depress the VO2transition temperature while Hf incorporation has little to no effect.
‘Smart’ window coatings enabled by the metal-insulator phase transition in lightly-substituted VO2have significant energy-saving potential in terms of decreased heating and cooling needs for commercial and residential buildings. Critically, the NIR transmission/reflection switch occurs due to ambient temperature, and so the technology is passive, requiring no external controls or power systems.
References
1. S. M. Babulanam, T. S. Eriksson, G. A. Niklasson, and C. G. Granqvist, Solar Energy Materials, 16, 347–363 (1987).
2. C. Lampert, "Chromogenic Switchable Glazing: Towards the Development of the Smart Window" Proceedings of Window Innovations Conference LBL-37766 (1995).