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. 2017 Mar 1;10(3):245.
doi: 10.3390/ma10030245.

Mechanical Properties of ZTO, ITO, and a-Si:H Multilayer Films for Flexible Thin Film Solar Cells

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Free PMC article

Mechanical Properties of ZTO, ITO, and a-Si:H Multilayer Films for Flexible Thin Film Solar Cells

Claudia Hengst et al. Materials (Basel). .
Free PMC article

Abstract

The behavior of bi- and trilayer coating systems for flexible a-Si:H based solar cells consisting of a barrier, an electrode, and an absorption layer is studied under mechanical load. First, the film morphology, stress, Young's modulus, and crack onset strain (COS) were analyzed for single film coatings of various thickness on polyethylene terephthalate (PET) substrates. In order to demonstrate the role of the microstructure of a single film on the mechanical behavior of the whole multilayer coating, two sets of InSnOx (indium tin oxide, ITO) conductive coatings were prepared. Whereas a characteristic grain-subgrain structure was observed in ITO-1 films, grain growth was suppressed in ITO-2 films. ITO-1 bilayer coatings showed two-step failure under tensile load with cracks propagating along the ITO-1/a-Si:H-interface, whereas channeling cracks in comparable bi- and trilayers based on amorphous ITO-2 run through all constituent layers. A two-step failure is preferable from an application point of view, as it may lead to only a degradation of the performance instead of the ultimate failure of the device. Hence, the results demonstrate the importance of a fine-tuning of film microstructure not only for excellent electrical properties, but also for a high mechanical performance of flexible devices (e.g., a-Si:H based solar cells) during fabrication in a roll-to-roll process or under service.

Keywords: ITO; flexible substrates; mechanical properties; multilayers; silicon-based solar cells.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
SEM images of zinc tin oxide (ZTO), indium tin oxide (ITO), and a-Si:H films for film thicknesses of 200 nm and 400 nm.
Figure 2
Figure 2
(a) X-ray diffraction patterns of representative ITO-1 and ITO-2 coatings. The peaks are assigned to the In1.9Sn0.05O2.95 phase by filled squares and to the Sn(Sn2In4)O12 phase by filled triangles; (b) (222)-diffraction peak of ITO-1 films with varying thickness. For a better comparison of the full width at half maximum (FWHM), the peaks are normalized to the maximum intensity of the (222) peak. For the 200 nm film, the shoulder of the peak belongs to the Sn(Sn2In4)O12 phase.
Figure 3
Figure 3
Focused ion beam (FIB) cuts of (a) ≈200 nm and (b) ≈2020 nm ITO-1 films. For better visualization, the contrast was enhanced in (a) and grain boundaries are indicated by dotted lines in the left image section. PET: polyethylene terephthalate.
Figure 4
Figure 4
(a) Characteristic stress–strain curves for ITO-1 films of varying thickness after subtracting the substrate contribution. For comparison, the initial strain due to prestraining was shifted to zero. Stresses beyond the linear region at small strains do not correspond to the applied stress in the coating due to film rupture; (b) Young’s moduli that were extracted from the initial slope of stress–strain curves. Error bars indicate the standard deviation for averaged values in the case of several measurements.
Figure 5
Figure 5
Crack onset strain (COS) for single films of a-Si:H, ITO-1, ITO-2, and ZTO with varying film thickness on 25 µm PET substrates. The dashed line indicates the proportionality of the critical strain and 1/tf for a-Si:H coatings. The inset shows an example of channeling cracks in a ca. 400 nm ITO-1 film. Arrows indicate the direction of the applied tensile load.
Figure 6
Figure 6
Force–strain curves of two bilayer systems containing ITO-1 (dotted line) and ITO-2 (solid line) and of a PET (HB3) reference substrate (grey solid line). Color bars indicated the range of measured COS values for single films of I: 400 nm a-Si:H, II: 200 nm ITO-2, and III: 200 nm ITO-1. To allow for a better comparison, lines with the slope of the reference curve are superimposed to the bilayer curves.
Figure 7
Figure 7
Cross-sections of bilayers of a-Si:H and (a) ITO-1 or (b) ITO-2 films on PET after tensile loading perpendicular to the crack path up to a maximum strain value as indicated in the images. The right image in (a) shows a detail of the layer stacks with enhanced contrast for a better visualization of grains; (c) A crack across a ZTO/ITO/a-Si:H trilayer after maximum applied strain as indicated in the images.

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References

    1. Van den Donker M.N., Gordijn A., Stiebig H., Finger F., Rech B., Stannowski B., Bartl R., Hamers E.A.G., Schlatmann R., Jongerden G.J. Flexible amorphous and microcrystalline silicon tandem solar modules in the temporary superstrate concept. Sol. Energy Mater. Sol. Cells. 2007;91:572–580. doi: 10.1016/j.solmat.2006.11.012. - DOI
    1. Söderström T., Haug F.J., Terrazzoni-Daudrix V., Ballif C. Optimization of amorphous silicon thin film solar cells for flexible photovoltaics. J. Appl. Phys. 2008;103:114509. doi: 10.1063/1.2938839. - DOI
    1. Myong S.Y., Jeon L.S. N-type amorphous silicon-based bilayers for cost-effective thin-film silicon photovoltaic devices. Curr. Appl. Phys. 2014;14:151–155. doi: 10.1016/j.cap.2013.10.021. - DOI
    1. Ablayev G.M., Abramov A.S., Nyapshaev I.A., Vygranenko Y.K., Yang R., Sazonov A.Y., Shvarts M.Z., Terukov E.I. Flexible photovoltaic modules based on amorphous hydrogenated silicon. Semiconductors. 2015;49:679–682. doi: 10.1134/S1063782615050024. - DOI
    1. Wilken K., Paetzold U.W., Meier M., Prager N., Fahland M., Finger F., Smirnov V. Nanoimprint texturing of transparent flexible substrates for improved light management in thin-film solar cells. Phys. Status Solidi Rapid Res. Lett. 2015;9:215–219. doi: 10.1002/pssr.201510040. - DOI
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