Comparisons of the Thermal Stability of Poly(3,4-Ethylenedioxythiophene) (PEDOT) and ITO on Flexible Substrates

Organic materials have been widely used in photovoltaic solar applications because of their flexibility and low cost. Poly(3,4-ethylenedioxythiophene) (PEDOT) has been known to have good mechanical stability under stretching and bending and also has good electrical and optical properties. Unlike inorganic materials, PEDOT can withstand high strains. In flexible solar cells, modules are usually exposed to heating-cooling cycles during day and night. When the structure of a thin film on a flexible substrate is exposed to high temperatures, the film and the substrate will expand at different rates; this will produce a residual strain caused by the thermal mismatch between the two layers. In contrast, indium-tin oxide (ITO) is brittle, and its conductivity decreases significantly as the ITO films are exposed to stretching or bending strains, especially in flexible/foldable solar cell applications. Cracks in ITO appear at very low strains that might cause failure in the conductive layer because of the combination of a very thin film of brittle ceramic material applied to a polymer substrate. In this work, a comparison of the thermal stability was conducted between PEDOT and ITO on flexible polyethylene terephthalate (PET) substrate. The performances of the two candidates were assessed during thermal bending fatigue and accelerated thermal cycling experiments. This work was designed to study the behavior of ITO and PEDOT under thermal cycling and thermal bending fatigue.