Tailoring the glassy phase in polymer semiconductors tunes their optical properties

Active layers in organic electronic devices often contain extensive glassy (non-crystalline) regions whose roles remain largely uncharted compared to their crystalline counterparts. Here, we demonstrate that the thermodynamic state of these glassy phases constitutes a powerful lever for tuning the optoelectronic properties of polymer semiconductors. Using poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) as a model system, we show that controlled vitrification kinetics and formation from distinct liquid precursors—including isotropic and liquid-crystalline mesophases—yield glassy PFO films with markedly different microstructures and photoluminescence characteristics. These results uncover direct correlations between glass formation pathways and functional electronic behavior, establishing the glassy state as an active design parameter rather than a passive structural byproduct. Our findings thus introduce a paradigm shift in the processing-structure-property relationships of polymer semiconductors and open new avenues for glass-phase engineering toward enhanced efficiency and stability in next-generation optoelectronic devices. Active layers in organic electronic devices often contain glassy regions whose role remains largely uncharted. Here the authors demonstrate that the thermodynamic state of these glassy phases can be used for tuning the optoelectronic properties of polymer semiconductors.