Effect of ZnO Buffer Layer on the Bending Durability of ZnO:Ga Films Grown on Flexible Substrates: Investigation of Surface Energy, Electrical, Optical, and Structural Properties

ZnO:Ga (GZO) transparent conducting oxide (TCO) films are deposited on flexible polyethersulfone (PES) substrates using the radio frequency sputtering technique. The bending durability of flexible TCOs is improved by inserting 100-nm-thick ZnO buffer layers. The strain of the samples with and without buffer layers arising from bending is studied using X-ray diffraction. After the insertion of 100-nm-thick ZnO buffer layers, the strain of GZO films without ZnO after outward and inward bending for 2000 cycles decreases from 2.063 ×10
^-3
and 2.203 × 10
^-3
to 1.74 × 10
^-3
and 1.966 × 10
^-3
, respectively. Such strain variation is caused by the difference in adhesive force at the surface of 100-nm-thick ZnO/PES and PES. The surface energy of PES and 100-nm-thick ZnO/PES bent outwards and inwards for 2000 cycles increased from 34.38 and 30.56 to 36.45 and 36.03 mJ/m
²
, respectively.