Strain‐Induced Optical and Molecular Transformations in Poly(Ethylene Terephthalate) Films for Organic Electronic Applications

Poly(ethylene terephthalate) (PET) films are widely used in flexible electronics and optoelectronics, where mechanical durability and optical performance under strain are essential for device reliability. This study investigates the effects of cold‐drawing amorphous PET under ambient conditions on its optical and molecular characteristics, using ultraviolet–visible (UV–Vis) and Raman spectroscopy. It examines how varying strain levels, from 0% (unstretched) to 30%, affect transparency, vibrational modes, and molecular reorganization. UV–Vis absorbance measurements, performed ex‐situ after strain release, reveal persistent changes in light transmission, including up to 100% increased absorption in the UVA and visible regions, particularly following large deformations. Raman spectra show that strains above 5% cause irreversible shifts in vibrational lines and increased full width at half maximum (FWHM). These spectral changes suggest partial molecular reorientation and emerging structural order, consistent with prior reports on cold‐drawn amorphous PET. The phonon mode coupled with C‐O stretching [O‐CH 2 ] shows the strongest response to mechanical stress. This work provides insight into strain‐induced optical and structural changes in PET, informing strategies to improve the performance of PET‐based devices in strain‐sensitive applications such as organic solar cells (OSCs), organic light‐emitting diodes (OLEDs), and flexible sensors.