ICMCTF 2022 Session C3-2-ThA: Thin Films for Energy Storage and Conversion II

The ALD/MLD (atomic/molecular layer deposition) technique allows the combination of inorganic and organic layers into any arbitrary frequency pattern. We have exploited ALD/MLD for (i) textile-integrated thermoelectrics, (ii) photo-switchable high-coercivity magnets, (iii) artificial SEI layers for Li-ion batteries, and (iv) active components for Li-organic microbattery. For thermoelectrics, we pioneeredZnO:organic superlattice structures, in which monomolecular organic layers alternate with nm-scale thermoelectric ZnO layers, to drastically suppress the thermal conductivity without comprising the electrical conductivity; when deposited on textiles, these films coat the textile fibers conformally so that the entire textile becomes an active part of the thermoelectric device.^1,2To realize flexible and photo-switchable magnets, we have combined nanoscale layers of the rarest trivalent iron oxide polymorph ε-Fe₂O₃ exhibiting giant coercive field values with azobenzene layers undergoing reversible trans-cis-trans isomerization reactions upon successive UV and visible light irradiations.^3,4 To mimic the composition of the naturally forming SEI layers in Li-ion batteries, we developed a three-precursor ALD/MLD process, Li-HMDS+ethylene glycol+CO₂, for the targeted lithium ethyl carbonate films.⁵ Finally, for the Li-organic microbattery application, our new active-material arsenal comprises various intriguing intercalated-type layered Li-organic materialsthat experience minimal changes in crystal structure upon the electrochemical Li⁺-ion intercalation.⁶

1. R. Ghiyasi, M. Milich, J. Tomko, P.E. Hopkins & M. Karppinen, Appl. Phys. Lett. 118, 211903 (2021).
2. G. Marin, R. Funahashi & M. Karppinen, Adv. Eng. Mater. 22, 2000535 (2020).
3. A. Khayyami, A. Philip & M. Karppinen, Angew. Chem.58, 13400 (2019).
4. A. Philip, J.-P. Niemelä, G.C. Tewari, B. Putz, T.E.J. Edwards, M. Itoh, I. Utke & M. Karppinen,ACS Appl. Mater. Interfaces12, 21912 (2020).
5. J. Heiska, M. Madadi & M. Karppinen, Nanoscale Adv.2, 2441 (2020).
6. J. Multia, J. Heiska, A. Khayyami & M. Karppinen, ACS Appl. Mater. Interfaces12, 41557 (2020).

The design of a transparent thermoelectric material is a promising technology for touch–screen displays and solar cell applications, rendering a more sustainable powering of the device. In order to enhance the thermoelectric performance, the material must have a high Seebeck coefficient, high electrical conductivity but low thermal conductivity [1]. Modifying the atomic structures of TiO₂ by deliberately introducing defects can enhance its properties to a great extent, while a cationic doping of TiO₂ has been documented to improve its electrical conductivity [2]. This work reports the production and characterization of optically transparent Nb-doped TiO₂ thin films with enhanced thermoelectric properties deposited on glass and Si by reactive d.c. magnetron sputtering in high vacuum. The purpose of these films is to harvest thermal energy from the environment and convert it to electrical energy. Several process parameters, such as reactive and working gas flow rate, deposition temperature, target current density and post-annealing conditions, directly affect the morphology and crystalline structure of the thin films. The optimization of these parameters results in thin films with thickness of 120-300 nm, maximum average optical transmittance in the visible range of 73 %, n-type electrical resistivity of 0.05 W·cm, thermal conductivity below 1.7 W·m^-1·K^-1 and a maximum absolute Seebeck coefficient of 223 mV·K^-1. The resulting maximum thermoelectric power factor is 60 mW∙K^-2∙m^-1 and the maximum thermoelectric figure of merit is 0.014. Hence, modifying the optical, electric, thermal and thermoelectric properties of the thin films enables their suitability for applications as transparent electrodes in photovoltaic systems and touch displays, amongst other devices.

References

[1]R. Venkatasubramanian, E. Siivola, T. Colpitts, B.O. Quinn, Thin-film thermoelectric devices with high room-temperature figures of merit, Nature. 413 (2001) 597–602.

[2]C.J. Tavares, M. V. Castro, E.S. Marins, A.P. Samantilleke, S. Ferdov, L. Rebouta, M. Benelmekki, M.F. Cerqueira, P. Alpuim, E. Xuriguera, J.P. Rivière, D. Eyidi, M.F. Beaufort, A. Mendes, Effect of hot-filament annealing in a hydrogen atmosphere on the electrical and structural properties of Nb-doped TiO2 sputtered thin films, Thin Solid Films. 520 (2012) 2514–2519. doi:10.1016/j.tsf.2011.10.031.