ICMCTF 2021 Virtual Conference Session C3: Thin Films for Energy Applications: Solar, Thermal, and Photochemical

The design of a transparent conducting oxide (TCO) material with thermoelectric properties is a promising technology for touch–screen displays and solar cell applications. In this work, TiO₂ doped with Nb thin films were deposited by d.c. magnetron sputtering. Several process parameters were adjusted, such as reactive gas (oxygen) partial pressure and deposition time and temperature, which affect the morphology and crystalline structure of the thin films. Hence, by modifying the optical, electric, thermal and thermoelectric properties of the produced TiO₂:Nb thin films, enables their suitability for thermal energy harvesters in devices in order to render them more sustainable. For optimized deposition conditions, TiO₂:Nb thin films with an optical transmittance up to 85 %, a relatively low electrical resistivity (>10 Ω∙cm), low thermal conductivity (<2 W·m^-1·K^-1), and a high absolute Seebeck coefficient (>200 µV∙K^-1) corresponding to a power factor of 125 µW∙K ∙m and ZT figure of merit close to 0.1 were attained, as seen in Figure 1. Both anatase and rutile crystalline phases were discerned in the X-ray diffractograms. Scanning electron microscopy observations provided evidence of a dense microstructure and a smooth film surface with an average thickness of 120 nm. From Figure 2, X-ray photoelectron spectroscopy experiments confirms that Nb⁵⁺ ions substitute Ti⁴⁺ in the TiO₂ lattice, providing a charge unbalance to the matrix. Furthermore, due to larger ionic radii, Nb⁵⁺ scatter phonons more efficiently and reduce the thermal conductivity, which is essential for enhancing the thermoelectric property.

The solar thermal flat panel insulated with high vacuum can have excellent efficiency performances in the mid temperature range (150-300°C) if equipped with an optimized selective solar absorber. We present 3 multilayer coatings optimized to work at 100, 200 and 300°C. Optimization has been obtained by maximizing the efficiency at the designed operating temperature by a genetic algorithm. The coatings (based on Cr and Cr2O3) have been deposited by DC reactive magnetron sputtering starting from a Cr target on glass and on 3 industrial copper substrates (OFE, OF, ETP). The single layers have been measured by standard characterization techniques (ellissometry, integrating sphere, AFM, X-Rays Diffraction…), whereas the produced multilayers have been also investigated by using a proprietary system [1] able to measure the absorber efficiency as function of temperature up to the stagnation temperature and above[2]. Influence of several deposition parameters on the multilayer performance will be presented. To enhance the absorptivity the multilayers have been covered with antireflective coatings based on SiO2 and/or SiNx.

[1] R.Russo et al. Optic Express 26 (2018) A480

[2] C. D’alessandro et al. submitted to this conference

Organic inorganic perovskites have attracted a great attention as next generation solar cells due to their excellent properties such as high molar extinction coefficient, low temperature processability, ambipolar nature, large diffusion length and small exciton energies. The compact or blocking layers have been studied extensively in dye sensitized solar cells, however, there are only a few reports on the effect of these layers on perovskite solar cells (PSCs). Herein, we employed a thin compact layer of spin coated TiO₂ (c-TiO₂) (<50 nm) in order to reduce series resistance and improve transmittance. Thethickness of c-TiO₂ (7-35 nm) was optimized by changing the precursor concentration as well as spinning speed. The prepared c-TiO₂ as well as mesoporous layer of TiO₂ (m-TiO₂) were thoroughly characterized using UV-Vis spectroscopy, Raman, Atomic Force Microscopy, Cyclic voltammetry and electrochemical techniques.Further, PSCs were fabricated under ambient conditions with high humidity (~RH 80%) using CuSCN as hole transporting layer (HTL). The power conversion efficiency of the optimized device found to be improved by 50 % on increasing the thickness of ETL from 7 nm to 15 nm.