Title:Assessing Band Gap Stability of Organic Semiconductor Thin Films for Flexible Electronic Applications

Abstract:Integration of organic semiconductors into flexible electronics requires that their optoelectronic properties remain stable under mechanical deformation. Among these, the optical band gap governs exciton generation and limits photovoltaic voltage, making it a key parameter for strain-resilient design. In this work, we investigate band gap shifts in poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/P3HT thin films deposited on flexible poly(ethylene terephthalate) (PET) substrates under uniaxial tensile strain ranging from 1\% to 10\%. Samples were subjected to mechanical deformation and then characterized by ultraviolet--visible (UV--Vis) absorption spectroscopy. The optical band gaps extracted using a standardized Tauc analysis and statistically validated through equivalence testing and robust regression models. We find that up to 7\% strain, the band gap shift ($\Delta E_g$) remains effectively invariant, independent of annealing condition or stack configuration, demonstrating electronic stability. However, at 10\% strain, all groups exhibit a reproducible widening of $\sim$4--5~meV. This threshold-like behavior marks a transition from mechanical accommodation to electronic perturbation. These findings confirm that the optical band gap in semicrystalline P3HT-based thin films is robust under practical deformation, which provides clear strain thresholds to inform mechanical modeling and device-level simulation of flexible organic optoelectronic systems.