（公財）日本板硝子材料工学助成会，40（2022）

環境調和熱電ハーベスタ実現のための高分子ナノ粒子導入された酸化物薄膜の開発
芝浦工業大学

工学部

メレパオロ

Development of Oxide Thin Films Embedded with Polymer Nanoparticles for Realization ofEnvironmentally Friendly Thermoelectric Harvesters
Paolo Mele
College of Engineering, Shibaura Institute of Technology（MOD）
により作製しSnO 2 ナノ粒子を添加した Al ドープ ZnO の薄膜を有機金属堆積法た。室温での最高の熱電性能は 600℃の Ar+ 1 %H 2 流下で 6 時間のポストアニールを行ったところ、室温で最高のゼーベック係数 S =- 73 µV/K、力率 PF = 7 . 10 µW/mK 2 であった。
We fabricated by metal-organic deposition technique (MOD) thin films of Al-doped ZnO
added with SnO2 nanoparticles. The best thermoelectric performance at room temperature
is: Seebeck coef ficient S =-73μV/K and power factor PF = 7.10 µW/mK2, after a postannealing of 6h in flowing Ar+1%H2 at 600 ˚C.
1. Introduction
A huge amount ( 66 %) of the total energy obtained from natural fossil resources is lost
every year as exhaust heat [ 1 ]. A highly promising method for energy recovery from suchheat sources is the utilization of thermoelectric (TE) materials that can convert various typesof waste heat flows directly into electricity without moving parts.
The major obstacle to the wide diffusion of TE materials is the conversion efficiency,measured with the scale of figure of merit ZT = (σS 2 ) × T/(κel +κph) (eq. 1 ), where σ is theelectrical conductivity, S is the Seebeck coef ficient (S =ΔV/ΔT, being V the electricalvoltage), κ is the total thermal conductivity (κ = κel +κph, where κel and κph are the electronic
and phononic contribution, respectively) and T is the operation temperature. To
commercialize a module based on thermoelectric materials, ZT ≧ 1 (efficiency ≧ 10 %) isrequired. The parameters σ, S, κ are not independently tunable: it is not possible to increase
σand reduce κ at the same time since κel = LTσ (eq. 2 ) with L = 2 . 44 × 10 - 8 W Ω K- 2 (Lorentznumber). On the other hand, κph = 1 / 3 CvΛ (eq. 3 ) with C = heat capacity, v = speed ofphonons, Λ= phonons mean free path.
Purpose of this research is to improve the conversion ef ficiency of thermoelectricmaterials by enhancement of ZT through depression of κ. To this purpose, we aim to reduceΛ inserting artificial nanoparticles as phonon scatterers inside thermoelectric materials.
According to equations ( 1 ), ( 2 ) and ( 3 ), to reach the goals of κ = 1 ~ 3 W m- 1 K- 1 and ZT > 1at T > 600 K, it is needed to insert in the film nanodefects which size is 20 nm or less and− 268 −NSG Found. Mat. Sci. Eng. Rep.

density 1024 ~ 1025 m- 3 .
To realize this goal, we used at first stable, ear th-abundant, inexpensive, andenvironmentally friendly oxide thin films of n-type Al-doped ZnO (AZO) added with SnO 2
nanosized inclusions. The introduction of artificial nanodefects (nanoparticles, nanopores,nanolayers) in thermoelectric oxide thin films has been tried using several vacuum
deposition techniques (atomic layer deposition: ALD [ 2 ], pulsed laser deposition: PLD [ 3 ]), but
not using room pressure chemical route like metal-organic deposition (MOD).
In this work, we focused on the systematic addition of SnO 2 nanoparticles to AZO films via
MOD and we preliminary characterized the obtained thin films at room temperature.
2. Experimental details
The fabrication of n-type AZO thin films was carried on by means of metal-organic
deposition (MOD) system consisting on a spin coater (Kyowariken K- 35951 )and a firing
furnace (AS-ONE TMF- 700 N) equipped with Mass flow controller (Horiba SEC-E 40 MK 3 ).
The AZO solution for MOD was prepared via chemical reaction between stoichiometricamounts of Zn(CH 3 COO) 2 · 2 H 2 O (Cica 18008 - 00 ) and AlCl 3 · 6 H 2 O (Cica 01155 - 00 ) usingas solvent a mixture of 1 : 3 = EtOH : 2 -propanol (Cica 14032 - 79 and 20322 - 09 , respectively)
with HOCH 2 CH 2 NH 2 (MEA, Aldrich E9508) as chelating agent. The reaction was conducted
at 60 ˚C in a beaker under continuous stirring for 1 h.
MOD method consisted of 3 steps: (i) dropping of few mL of oxide organic solution on asilica glass substrate (preliminary cleaned to assure good adhesion of solution) then spun athigh rate: this high-speed stirring determines the formation of amorphous thin layer; (ii)firing the sample (substrate + layer) in a tube furnace at proper temperature in air; (iii)
annealing the sample under controlled gas flow, leading to the crystallization of a pure thinfilm layer. The three steps were repeated several times to obtain a multilayered thin film ofthe proper thickness.
All the parameters to prepare pure AZO films were varied to find the optimal conditions asfollows: concentration of AZO solution = 0 . 15 M; Al content = 3 %; amount of MEA = 5 drops;
No. of layers = 5 ; spinning rate = 3000 RPM; spinning time = 30 s; pre-annealing T = 350 ˚C;annealing T = 600 ˚C; annealing atmosphere: Ar+ 1 % H 2 ; gas flow: 200 sccm. The search forproper annealing time (considering 1 , 3 , 6 , 9 h) is described in the results section.
The effect of SnO 2 nanoparticles was evaluated after addition of increasing number ofdrops ( 1 , 3 , 6 and 10 ) of SnO 2 solution (Aldrich 901079 - 10 ML) to the AZO solution before
per forming the spinning, firing, and annealing steps. According to the vendor ’s
specifications, the SnO 2 concentration is 2 . 5 wt% in butanol and the nanoparticles’size is inthe order of 7 ~ 10 nm.
The crystallinity of the pure and SnO 2 -added films was evaluated by X-ray diffractometer
Smart Lab 3 (Rigaku). Surface of the films was observed by SEM-EDS (JEOL/JSM- 7100 F).
The thickness of a typical AZO film was evaluated by profilometer Filmetrics Profilm 3 D(KLA). The room-temperature Seebeck coefficient of AZO films was evaluated by the probe
PTM- 3 (Joule Yacht) and the room-temperature resistance was evaluated by a tester.
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（公財）日本板硝子材料工学助成会，40（2022）

3. Experimental results
3.1 Pure AZO films
The effect of annealing time on AZO films prepared with the optimal conditions describedin experimental section was studied.

Fig.

1 (left) XRD patterns for AZO films annealed at 600 ˚C at different times underAr+H 2 gas flow;(right) magnification of the ( 002 ) peakFigure 1 shows the XRD pattern of AZO films by MOD technique with different annealingtime. The observed plane is consistent with ZnO structure matching with PDF card no:
01 - 082 - 9744 . The hexagonal phase structure is attained for the prepared AZO thin films
with different annealing time. Cr ystallite size D was calculated using the conventional
relation D = 0 . 9 λ/βcosθ and lattice strain by the relation ε =β cotθ/ 4 , where β, θ and λrepresent the FWHM, Bragg’s angle and X-ray CuKα wavelength ( 1 . 5406 Å), respectively.
The crystallite size has been increased with increasing annealing time as demonstrated by
the decrease of FWHM value. Consequently, the calculated strain values were decreased
gradually. Here, there is no peak shift occurs with respect to the annealing temperature.
Moreover, the peak intensity is consistently increased as a function of annealing
temperature. On the other side, the lattice constant and cell volume has been increased andthe higher values were obtained for 6 h annealing time. All the crystalline parameters andrelated results are summarized in Table 1 .
The morphology of films is reported in figure 2. The film annealed 1h shows a meandering
structure, which disappears by increasing the annealing time. The films are porous, andhexagonal grains which size is about 100 nm start appearing with 6 h of annealing time
The values of room temperature ρ, S and power factor PF = S 2 /ρ are shown in Table 2
using as thickness the value of 200 nm for all films. The ρ value has been decreased from0 . 176 to 0 . 074 Ω cm from the sample annealed 1 h to the sample annealed 6 h in Ar+H 2 .
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Table 1 – Crystalline parameters of pure AZO films annealed at 600 ˚C in Ar+H 2

This is due to the improvement in film`s cr ystallinity. The magnitude of the S values
considerably decreases with respect to increase of doping concentration. The magnitude ofthe S values slightly reduced as a function of annealing time. The calculated PF has been
increased from 3 . 83 to 7 . 10 μW/m.K 2 from the sample annealed 1 h to the sample annealed
6 h in Ar+H 2 . The best annealing time has been found as 6 h.

Fig.

2 Morphology of AZO films annealed at 600 °C at different timesunder Ar+H 2 gas flow

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（公財）日本板硝子材料工学助成会，40（2022）

Table 2 – Room temperature transport and thermoelectric properties of pure AZO films annealed at600 ˚C in Ar+H 2

3.2 SnO2-added AZO films
Figure 3 (left) illustrates theXRD patterns for pure AZO andSnO 2 added AZO thin films
prepared by MOD technique. Theobser ved planes are consistent
with hexagonal phase structure ofZnO matching with PDF card no:
01 - 071 - 3830 . The hexagonalphase structure is attained for allFig.

3 (left) XRD patter ns for SnO 2 - added AZO films
annealed at 600 °C for 6 h under Ar+H 2 gas flow;(right) magnification of the ( 002 ) peak

the prepared films. Moreover,there is no observed impurity or
secondar y peaks like SnO 2 orother Sn-r elated compounds.
Figure 3 (right) shows the enlarged XRD pattern of the predominant peak ( 002 ) for all theprepared films. The figure indicates that there is an identified peak shift occurs with respect
to SnO 2 addition. This may be attributed to atomic substitution of Sn in AZO lattice but
should be confirmed by further analyses.
Table 3 shows that the full width at half maxima (FWHM) value initially increased andthen decrease when increasing SnO 2 solution amount into AZO solution. The variation ofcrystallite size (D) follows an inverse trend. The calculated strain values were also changed
consistently with FWHM. The lattice constants (a and c) and cell volume values for all theprepared films are also reported.

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Table 3 – Crystalline parameters of SnO 2 - added AZO films annealed at 600 °C for 6 h inAr+H 2

The SEM images (not reported here) does not show any significant change of morphology
respect to the undoped AZO film deposited in same conditions.
Table 4 – Room temperature transport and thermoelectric properties of SnO 2 - added AZO films
annealed at 600 °C for 6 h in Ar+H 2

The values of room temperature ρ, S and power factor PF are shown in Table 4 using as
thickness the value of 200 nm for all films. The ρ slightly decreases and S increases withamount of SnO 2 solution up to 6 drops. 10 drops appear to be an excessive amount of dopant.
The Seebeck coefficient values shows a negative sign as conﬁrming the n-type conductivityof all samples. The obtained PF value is enhanced with the addition of SnO 2 but has been
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（公財）日本板硝子材料工学助成会，40（2022）

slightly decreased respect to the case of pure AZO film, being the best performance 7 . 06
μW/m.K 2 when 6 drops SnO 2 were added.
4. Conclusion and perspective
Pure and SnO 2 -added AZO films were prepared by MOD. The addition of SnO 2
nanoparticles affected the crystal structure of the films, but was not dramatically changing
their thermoelectric properties, at least at room temperature.
Further experiments are required in two directions: long X-rays acquisitions followed by
Rietveld analysis, and cross-sectional TEM are needed to understand if the nanoparticles ofSnO 2 have been incorporated into the films or if Sn has substituted Zn in the AZO cell;measurement of transport and thermoelectric properties at increasing temperature are due
to understand if the role of SnO 2 becomes beneficial at higher temperatures.
5. RemarkRespect to the original plan, polymer nanoparticles were not used systematically because
in a preliminar y experiment their solution segregated. In substitution, the SnO 2nanoparticles solution was mixed properly, so it was used as replacement.
6. Bibliography[ 1 ] BP stat. rev. of World energy, 2020
[ 2 ] T. Tynell, P. Mele et al., J. Mater. Chem. A 2 ( 2014 ) 12150[ 3 ] P. Mele et al, J. Nanosci. Nanotechl. 17 ( 2017 ) 16166− 274 −