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We present a theoretical study of degradation mechanisms for photoinducedoxidation in organic polymers in the condensed phase, usingpoly(3-hexylthiophene)(P3HT) as an example. Applying density functional theorywith a hybrid density functional and periodic boundary conditions that accountfor steric effects and permit the modeling of interchain chemical reactions, weinvestigate reaction pathways that may lead to the oxidation of thiophenebackbone as a critical step toward disrupting the polymer conjugation. Wecalculate energy barriers for reactions of the P3HT backbone with oxidizingagents including hydroxyl radical (OH$\cdot$), hydroperoxide (ROOH), andperoxyl radical (ROO$\cdot$), following a UV-driven radical reaction startingat the $\alpha$-carbon of the alkyl side chain as suggested by infrared (IR)and X-ray photoemission (XPS) spectrosocopy studies. The results stronglysuggest that an attack of OH$\cdot$ on sulfur in P3HT is unlikely to bethermodynamically favored. On the other hand, an attack of a peroxyl radical onthe side chain on the P3HT backbone may provide low barrier reaction pathwaysto photodegradation of P3HT and other polymers with side chains. The condensedphase setting is found to qualitatively affect predictions of degradationprocesses.

First Principles Study of Photo-oxidation Degradation Mechanisms in P3HT for Organic Solar Cells