Advanced Thin Film Photo-Thermal Materials and Applications

Photo-thermal material can get thermal energy from light and be applied in many fields, such as solar thermoelectric generators (STEGs) [1], solar thermophotovoltaic (STPV) [2], concentrated solar power (CSP) [3] and solar water heaters [3]. It is vital for solar thermal applications. Architecturally integrated solar thermal technologies such as solar water heaters and solar thermoelectric generators rely on spectrally selective solar absorbers. These solar absorbers need to have high solar absorptivity (α) and low thermal emissivity (ε) simultaneously to obtain high solar energy utilizing efficiency. In this chapter, TiN_xO_y based absorbers have been fabricated by using reactive magnetron sputtering. The best solar absorptivity is 97.6 %, [4] which is the highest result to the authors’ knowledge. However, high solar energy utilization efficiency always means dark blue or even black appearance and blocks architectural integrated solar applications. Or colorful appearance generated by conventional approaches will lower the solar absorptivity and increase the thermal emissivity dramatically. In order to solve the problem, colored absorbers with a TiN_xO_y absorbing layer and a TiO₂/Si₃N₄/SiO₂ dielectric stack are elaborately designed and fabricated by controlling a reflective peak in the visible range. Both the theoretical and experimental results show that the color can be tuned in a wide color gamut, while keeping solar absorptivity higher than 95 % and thermal emissivity lower than 5 %.[5] Furthermore, a patterned colorful solar absorber has been presented via changing the thickness of dielectric SiO₂, TiO₂ or Si₃N₄ layer for the first time. It will make colorful buildings converting solar light into energy [6]. A colored solar absorber array with 16 color units is demonstrated experimentally by using combinatorial deposition technique with solar absorptivity and thermal emissivity of all units higher than 92 % and lower than 5.5%, respectively. The photo-thermal absorbers can be deposited onto most types of substrates, even flexible substrates. Such high performance absorbers can also be fabricated onto thermoelectric generators to the conversion of solar energy into electricity. The demonstrated results show that the open circuit voltage of thermoelectric generators can be dramatically increased from 171 mV to 523 mV (3.1 times) when deposited the colored absorbers. Additionally, a broadband perfect absorber with ultra-low escaping photons has been developed based on TiN embedded AlN (TiN@AlN) nanocomposite [7]. A large area TiN@AlN/TiO₂/SiO₂ absorber of 200 mm × 200 mm with only about 200 nm thick, has been fabricated on glass by reactive sputtering at room temperature as example. Extremely low average escaping photons of 0.4 % has been achieved in the whole visible range of 380 ~ 780 nm. The above results show that photo-thermal absorbing coatings can be extensively applied in many fields including solar thermal and solar thermoelectric.