Tin oxide-organic superlattices for flexible thermoelectric applications by atomic/molecular layer deposition

Flexible thermoelectric (TE) heat-to-electricity conversion devices would be highly beneficial for wearable applications and also for any application in which the heat source is of complex shape.[1] A TE device is in principle simple, but it needs both n- and p-type semiconductor legs to function.
An efficient TE material should have simultaneously high electrical conductivity and low thermal conductivity, which is a challenge for conventional materials. The second challenge is that the currently employed inorganic TE materials are composed of heavy/rare/poisonous elements (Bi, Te, etc.) and are thus not environmentally sustainable. Simple metal oxides like ZnO are relevant material candidates, but their thermal conductivity is too high.
The combined atomic and molecular layer deposition (ALD/MLD) technique allows us to mix inorganic and organic precursor pulses in a sequential manner, thus enabling precise layer-engineered superlattice (SL) structures. In our previous works, we have deposited ZnO:organic SLs where monomolecular organic layers are embedded within nanoscale layers of the n-type ZnO semiconductor to block the phonon conduction at the resultant metal oxide/organic interfaces without affecting the electrical conductivity.[2] Organic layers also improve the flexibility of the film.
Here we present similar efforts for the yet-missing p-type semiconductor counterpart SnO. We use an amidinate-based tin(II) precursor which reacts well with water as the co-reactant under ALD conditions for the deposition of the SnO layers.[3] Thin films deposited at 220 °C results in phase pure SnO. Saturation for tin precursor happens at 12 s and for water at 2 s. The thickness of the films is measured by X-ray reflectivity (XRR) and the composition analyzed by Fourier-transform infrared spectroscopy (FTIR). For the organic component, we investigate various possibilities (hydroquinone, terephthalic acid, etc.); tentatively, terephthalic acid is found a promising organic component, as it has more oxidative character than e.g. hydroquinone.
References
[1] G. Marin, R. Funahashi and M. Karppinen, Textile-integrated ZnO-based thermoelectric device using atomic layer deposition, Advanced Engineering Materials 22, 2000535 (2020).

[2] R. Ghiyasi, M. Milich, J. Tomko, P.E. Hopkins and M. Karppinen, Organic-component dependent thermal conductivity reduction in ALD/MLD grown ZnO:organic superlattice thin films, Applied Physics Letters 118, 211903 (2021).

[3] N. Huster, R. Ghiyasi, D. Zanders, D. Rogalla, M. Karppinen & A. Devi, SnO deposition via water based ALD employing tin(II) formamidinate: precursor characterization and process development, Dalton Transact 51, 14970 (2022).