Near infrared photothermoelectric effect in transparent AZO/ITO/Ag/ITO thin films

A new concept of oxide-metal-oxide structures that combine photothermoelectric effect with high reflectance (~ 80%) at wavelengths in the infrared (> 1100 nm) and high transmittance in the visible range is reported here. This was observed in optimized ITO/Ag/ITO structure, 20 nm of Silver (Ag) and 40 nm of Indium Tin Oxide (ITO), deposited on Aluminum doped Zinc Oxide (AZO) thin film. These layers show high energy saving efficiency by keeping the temperature constant inside a glazed compartment under solar radiation, but additionally they also show a photothermoelectric effect. Under uniform heating of the sample a thermoelectric effect is observed (S = 40 mV/K), but when irradiated, a potential proportional to the intensity of the radiation is also observed. Therefore, in addition to thermal control in windows, these low emission coatings can be applied as transparent photothermoelectric devices.


Supplementary information
Optical spectra of ITO/Ag/ITO structure Figure S1 shows the influence of Ag thickness on the transmittance, reflectance, and absorption of glass/ITO/Ag/ITO. The 20 nm thick Ag thin film shows a better compromise between high transparency in the visible and high reflection in the infrared range. Figure S1 -Transmittance, reflectance, and absorption of IAI coatings with different Ag layer thickness (10, 20 and 30 nm).

Elemental analysis of IAI multilayers
Helium ions are less efficient at exciting X-rays from materials, due to their lower excitation cross-sections and lower penetration in the matter: the excitation of elements in the substrate is much weaker, providing a better separation of the X-rays emitted by the elements in the film. Figure S2 shows that the contributions of In and Sn are clearly visible in the 40 nm thick ITO film and 20 nm IAI structure deposited onto glass, while that of Ag is detected only in the IAI structure. Spectral analysis with the GUPIX software further shows that the corresponding In-Sn concentration ratios are similar in both films. The Ca and Si contributions (the latter being well apart at 1.74 keV, and thus not shown in Figure S2) belong to the glass substrates; no other elements were detected. In addition, the substrate was further investigated through 2 MeV proton beams but only substrate glass elements were detected. Therefore, one can conclude that these samples have no contaminants at detectable levels.   Figure S3 A were collected under normal incidence of the ion beam, while spectra in Figure S3  in such case the beam ions effectively probe a larger areal density. Furthermore, part B of Figure S3 clearly shows that the RBS spectra change markedly upon large tilts, more so for the thicker gl-IA20I sample: the spectral contribution from the ITO elements broadens and its low energy flank spreads significantly. Although the fit model still describes adequately the main features of the experimental dataspectral intensities and energy profile widthsoverall agreement requires that an energy dependent spread is allowed for to get a better fit. Such spread, increasing as energy decreases is consistent with rough interfaces, comprising irregularities (holes, voids, islands, etc.) that introduce extra scattering and energy spread of the beam particles.

Morphology of silver layer and IAI multilayer
Additional information about the influence of Ag thickness on the surface morphology of glass/ITO/Ag/ITO top layer and Ag layer can be observed in the SEM images of Figure S4 Figure S4.

Optical spectra of AZO/ITO/Ag/ITO structure
The influence of Ag thickness on the transmittance, reflectance and absorption of glass/AZO/ITO/Ag/ITO samples is shown in Figure S5. The most important evidence is a drastic decrease in transmittance for Ag thickness around 30 nm corresponding to a reflector behaviour of this structure. For the 20 nm Ag thickness, the one used in the thermoelectric studies, the structure has a low reflectance in the visible region of the spectra which increases for infrared region, and the transmittance has the opposite behaviour (high in the visible range and low in the infrared region). The absorption is 20% in the visible range and 10% in the infrared range.

X-ray Diffraction (XRD)
The XRD diffractograms of monolayer and multi-layered films deposited on glass substrate are shown in Figure   S6. The XRD diffractograms show that the ITO thin film is amorphous, which would be expected since ITO for very thin layers deposited on glass without annealing has an amorphous structure 3,4 . The Ag layer shows a main peak located at 2 = 38.02° corresponds to the (111) planes, characteristic of polycrystalline silver even for nanometer layers or grains 1,5 . According to the Joint Committee on Powder Diffraction Standards (JCPDS) card No. 036-1451, AZO thin film has a hexagonal wurtzite structure with P63mc space group with the three major diffraction peaks corresponding to the (100), (002) and (101) planes 6 . Regarding AZO and IAI coated AZO, the diffractogram shows only the AZO peaks and the characteristic silver peak is no longer visible. This is because the silver layer is very thin compared to the two ITO and AZO layers. Although a single layer of ITO is amorphous, the IAI structure shows two main peaks, one clearly identified as the (111) silver peak and a second at 2 = 35.17º attributed to (400) ITO crystalline film planes 7 . At first glance it could also be attributed to Ag2O, related to some oxidation of the Ag layer before or during the ITO deposition. However, these generally lie at 2 2 = 38º -38,6º or 2 = 37° 8,9 .

Optical band gap
The band gap was determined for the ITO and AZO individual layers and for the multilayer IAI structures on glass (with and without AZO layer). The graphs of Figure S7 shows the plot of (h) 2

Optical properties of the materials used to filter the light: Black/white papers and Kapton film
White and black papers as well as Kapton film were used to reflect, absorb or filter the sun radiation on the samples of Figure 7. The reflectance spectra of white paper, the absorption spectra of black paper and the absorption band of Kapton film were measured and shown in the Figure S9. These confirm a high reflectance of white paper, and a high absorption for the black paper, both in the entire spectral region, and high absorption in the UV region for the Kapton film (for higher wavelengths, i. e. after 400 nm, the absorption is almost null).

Kelvin probe force microscopy (KPFM)
Kelvin probe force microscopy allows obtaining surface topography, morphology, roughness, surface potential and phase without contacting the sample 11 . The obtained maps for the individual samples and multilayers IAI and AZO/IAI are shown in Figure S10. KPFM measures a contact potential difference (CPD) between the sample surface and the tip, that is, the difference between the work-function of the material and the probe 12