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

TL;DR

This study addresses whether residual tensile strain alters the optical band gap $E_g$ in semicrystalline P3HT thin films and PEDOT:PSS/P3HT stacks on flexible PET substrates. Using ex-situ UV–Visible spectroscopy and a standardized Tauc analysis, the authors quantify shifts in $E_g$ under uniaxial strains from 1% to 10%, applying robust statistical tests including equivalence testing and HC3 regression. The main finding is that $"ΔE_g"$ remains effectively zero up to 7% strain, with a reproducible 4–5 meV increase at 10% strain across all sample configurations; this threshold behavior is largely independent of annealing or stack architecture. The results provide concrete strain-threshold benchmarks for optical-property stability, enabling simpler and more accurate device-level simulations and informing the design of strain-resilient flexible organic photovoltaics.

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 ($ΔE_g$) 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.

Figures (4)

• Figure 1: Two-panel overview of the measurement workflow and apparatus. (a) Schematic illustration of the experimental workflow. Samples were first characterized by UV–Vis spectroscopy in their unstretched state, then subjected to tensile strain for 30 minutes using a motorized stretcher, and subsequently transferred back to the spectrometer for a second UV–Vis measurement. (b) Photograph of the custom motorized stretcher used to apply strain. Note that the mechanical straining and optical measurements were performed sequentially (ex-situ).
• Figure 2: KDEs of the optical band gap $E_g$ under three annealing conditions, with fitted normal distributions for reference. The KDEs use a Gaussian kernel (base function) and Scott's bandwidth rule ($h = \sigma n^{-1/5},\ n=12$) Scott1992. Rug marks along the axis show the 12 measured $E_g$ values for each condition.
• Figure 3: Representative Tauc plot illustrating the method used to extract the optical band gap $E_g$. The straight line corresponds to the optimal linear fit within the selected onset region, and the dashed vertical line marks the intercept used to determine $E_g$.
• Figure 4: Mean band gap shift ($\Delta E_g$) versus strain with 95% CIs across all stacks and annealing conditions.