3.1.4. Bending Ability Test of AZO/PET Flexible Substrates

The bending ability test has been checked for flexible AZO/PET substrates under tensile strain. Generally, the sheet resistance (*R*s) value depends on the film composition, morphology, thickness and the substrate material. After continuous bending up to 1000 times (radius of 10 mm), the sheet resistance of AZO/PET increased only slightly (see Figure 4). The RMS roughness of blank PET substrate was 6.26 nm. AZO/PET RMS was measured to be around 12 nm and it is increasing to 22 nm after 1000 bending cycles. During the bending cycles, damage in the AZO/PET develops, most probably in the form of extended defects that collapse as micro-cracks (as evidenced by SEM image in Figure 1f), capable of interrupting electrical paths on subsequent bending. Bending in–out cycles open and close these micro-cracks, and with the accumulation of the bending cycles an effective breaking of a slightly increasing number of conductive paths occurred, thus influencing the measured resistance and appearing as increased sheet resistance. These results are confirmed further by the increased roughness shown in the AFM image of the same layer, performed after 1000 bending cycles (Figure 1c).

**Figure 4.** Sheet resistance dependence of the number of bending cycles for AZO layer on PET substrate.
