Enhancement of Photostability through Side Chain Tuning in Dioxythiophene-Based Conjugated Polymers

Even as strategies for encapsulating organic electronic devices have become more sophisticated and accessible, resulting in impressive and encouraging long-term stabilities being reported, photodegradation of conjugated polymers continues to occur even under oxygen- and moisture-free conditions. To further enhance the photostability of conjugated materials, it is important to understand fundamental and system-independent photodegradation pathways and the structure-property relationships impacting photostability. As a means to address this, we measure the photostability of thin films under oxygen-free conditions of a family of dialkoxy-functionalized thiophene polymers and evaluate potential degradation pathways. This approach is enabled by varying the nature and placement of solubilizing groups to pinpoint structural elements and motifs that are particularly susceptible to irreversible photodegradation. Using a combination of UV-vis-NIR spectroscopy, X-ray photoelectron spectroscopy, size exclusion chromatography, nuclear magnetic resonance, and cyclic voltammetry, we evaluate if and how defects such as crosslinking, chain scission, and/or chemical changes affecting the conjugated backbone impact the rate of photodegradation and how they are affected by the placement, design, and branching point of the side chains. We are able to demonstrate that crosslinking through the alkyl side chains is the main degradation pathway under oxygen-free conditions, and that backbone twisting plays an important role in the rate of photodegradation. Based on these findings, we propose that moving away from tertiary carbon branched side chains toward either quaternary carbon branches or linear chains, and/or moving the branching point further from the conjugated backbone, are all promising and relatively straightforward strategies for enhancing photostability in conjugated polymers.