Mahya Ghorab

• This thesis investigates how uniaxial tensile strain affects the optical and molecular properties of polymer thin films used in flexible organic electronics. It focuses on poly(ethylene terephthalate) (PET) as a flexible substrate and poly(3-hexylthiophene-2,5-diyl) (P3HT) as a semiconducting polymer. The materials are examined as single layers and in PET/P3HT and PET/PEDOT:PSS/P3HT stacks at room temperature. Strain is applied reproducibly using a custom-built stretching setup. Ultraviolet–visible spectroscopy is used to track changes in optical response and band gap, while Raman spectroscopy reveals chain reorientation, conformational changes, and defect formation during and after deformation. For PET, tensile strain causes progressive optical degradation, including increased absorption across the ultraviolet and visible regions. Above ≈5% strain, the changes become irreversible and are accompanied by evidence of permanent structural modification. This identifies two regimes: elastic behavior at low strain and irreversible deformation at higher strain. For P3HT, the optical band gap remains stable up to 7% strain, regardless of annealing temperature or stack configuration. At 10% strain, a reproducible increase of about 4–5 meV appears, indicating onset of measurable electronic perturbation. Raman results show that P3HT strain response depends strongly on thermal history. Unannealed films mainly deform through reversible chain alignment. Films annealed at 50 °C show a mixed response with greater microstructural heterogeneity, while films annealed at 75 °C exhibit more constrained deformation and stronger defect accumulation. A PEDOT:PSS interlayer improves strain transfer, reduces strain localization, and enhances mechanical tolerance, especially in highly ordered films. Overall, the work defines experimentally grounded strain limits for flexible devices and clarifies how processing history and stack architecture govern mechanical and optoelectronic behavior.