Nanotechnology for solar energy conversion:

smart materials for the building envelope

Due to their fascinating optical and electronical properties, nanometer-scaled structures play an important role in solar energy conversion. In our research group, we develop novel micro- and nanostructured smart materials for active and passive solar energy applications being integrated into the building envelope. We focus on thin films, because these allow covering a large surface area with a small quantity of raw material – and we focus on their electronic and optical properties, because these closely interrelated properties are most important for the utilisation of solar energy.

Nanocomposite coatings consist typically of dielectric, semiconductor or metal nanocrystals being embedded in a dielectric matrix. Effective medium theories such as the Bruggeman and the Ping Sheng theory are used to model the dielectric function of nanocomposite materials. The method of finite differences in the time domain (FDTD) allows nowadays to simulate the interaction of light with complex structures on the micrometric and nanometric scale. Micro- and/or nanostructuring materials can result in very interesting features such as tunable band-gaps by quantum confinement, adjustable complex dielectric function, enhanced mechanical stability, superhydrophobicity, frequency-selective surfaces FSS [BOU2017], etc. The semiconductor-to-metal transition observed in transition metal oxides with strong electron correlations can be exploited for smart switching coatings [KRA2017].

Our unique and specifically designed experimental infrastructure allows us the plasma-deposition of novel nanocomposite thin films and the subsequent characterization of their electronic properties by photoelectron spectroscopies (XPS & UPS). Recently we have added surface characterization with nanometric lateral resolution by scanning tunnelling microscopy (STM) and spectroscopy (STS). Using these methods, we determine features such as the alignment of valence and conduction band edges at an interface of a heterostructure, the difference in workfunction leading to a built-in electrostatic field, band bending at surfaces and interfaces as well as local variations of the energy gap in polycrystalline films. Because the above-mentioned characterization techniques are very surface sensitive, it is a crucial advantage that the deposited thin films can be analysed in-line without breaking the vacuum.  We synthesise new materials, create novel micro- and nanostructures, study the occurring physical phenomena, and tailor the systems to the targeted solar energy applications.   

The scope of envisaged solar energy applications is large. Recent work focuses especially on smart materials, such as thermochromic selective solar absorber coatings for overheating protection of solar thermal systems [KRA2022a], and electrochromic coatings for switchable windows [BOU2021], [FLE2022], [FLE2023]. Further applications include microstructured glazing with strong seasonal dependence of the solar heat gains [GON2018], photoluminescent quantum dot solar concentrators for photovoltaic energy conversion, antireflection coatings on solar collector glazing, colored coatings with high solar transmittance for novel glazing of photovoltaic facades [JOL2017], selective solar absorber coatings for thermal solar collectors and thermoelectric power generation [KRA2022b], fuel cells [NI2023], microelectronics [NIK2022], as well as novel insulating glazing with high transmittance for the microwaves of mobile communication [FLE2020].


Selected references

[BOU2021]             Bouvard O., Burnier L., Lagier M., Schüler A., Strong coloration of nanoporous tungsten oxides by in-vacuo lithiation for all-solid-state electrochromic devices, Thin Solid Films, 2021, 730, 138700.

[BOU2017]             Bouvard, O., Lanini, M., Burnier, L., Witte, R., Cuttat, B., Salvadè, A., Schüler, A., Structured transparent low emissivity coatings with high microwave transmission, (2017) Applied Physics A: Materials Science and Processing, 123 (1), art. no. 66.

[BUR2023]              Burnier L., Jamaly N., Delaporte H., Durussel S., Fleury J., Schüler, A., Energy saving glazing with high MIMO performance, manuscript in preparation

[FLE2023]               Fleury, J., Burnier, L., Lagier, M., Shukla, S., Manwani K., Panda, E., Schüler, A., Electrochromic device with hierarchical metal mesh electrodes: Transmittance switching in the full spectral range of solar radiation, Solar Energy Materials and Solar Cells 257 (2023) 112345

[FLE2022]               Fleury, J., Burnier, L., Schüler, A., Electronic properties and ion migration of in vacuo lithiated nanoporous WO3:Mo thin films, Journal of Applied Physics, 2022, 131(1), 015301.

[FLE2020]               Fleury, J., Lanini, M., Pose, C., Burnier, L., Salvadè, A., Zimmermann, E., Genoud, C., Schüler, A., Wide band-pass FSS with reduced periodicity for energy efficient windows at higher frequencies, Applied Physics A: Materials Science and Processing, 2020, 126(6), 417.

[FLO2022]               Florio, P., Tendon, X., Fleury, J., Costantini C. , Schueler A., Scartezzini J.-L., Performance Assessment of a nZEB Carbon Neutral Living/Office Space and Its Integration into a District Energy-Hub, Energies 2022, 15(3), 793

[GON2018]             Gong, J., Delaunay, A., Kostro, A., Schüler A., Development of a novel mechanical micro-engraving method for the high-aspect-ratio microstructures of an advanced window system, (2018) Microelectronic Engineering, 191, pp. 48-53.

[JOL2017]               Jolissaint, N., Hanbali, R., Hadorn, J.-C., Schüler, A., Colored solar façades for buildings, (2017) Energy Procedia, 122, pp. 175-180. Describes our novel PV technology integrated to building façades.

[KRA2022a]            Krammer, A., Matilainen, A., Pischow, K., Schüler, A., VO2:Ge based thermochromic solar absorber coatings, Solar Energy Materials and Solar Cells, 2022, 240, 111680.

[KRA2022b]            Krammer, A., Lagier, M., Schüler, A., In-line electronic and structural characterization of reactively sputtered Cu-Co-Mn black spinel oxides, Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films, 2021, 39(5), 053411.

 [KRA2017]             Krammer, A., Magrez, A., Vitale, W.A., Mocny, P., Jeanneret, P., Guibert, E., Whitlow, H.J., Ionescu, A.M., Schüler, A., Elevated transition temperature in Ge doped VO2 thin films, (2017) Journal of Applied Physics, 122 (4), art. no. 045304. 

[LAG2021]              Lagier, M., Bertinotti, A., Bouvard, O., Burnier, L., Schüler, A., Optical properties of in vacuo lithiated nanoporous WO3:Mo thin films as determined by spectroscopic ellipsometry, Optical Materials, 2021, 117, 111091.

[NI2023]                 Ni, W., Meibom, J.L., Hassan, N.U., Chang, M., Chiuan, Y.-C., Krammer, A., Sun, S., Zheng, Y., Bai, L., Ma, W., Lee, S., Jin, S., Luterbacher, J.S., Schüler, A., Chen, H.M., Mustain, W.E., Hu, X., Synergistic interactions between PtRu catalyst and nitrogen-doped carbon support boost hydrogen oxidation, Nature Catalysis (2023).

[NIK2022]               Nikoo, M.S., Reza Soleimanzadeh, R., Krammer, A., Marega, G.M., Yunkyu Park, Y., Son, J., Schueler, A., Andras Kis, A., Moll, P.J.W., Matioli, E., Electrical control of glass-like dynamics in vanadium dioxide for data storage and processing, Nature Elecronics, 2022, Vol. 5, 96–603