Photodegradation of Polythiophene-Based Polymers: Excited State Properties and Radical Intermediates

Polythiophene-based polymers are an important class of organic semiconductors that serve as the building blocks for polymer-based hybrid solar cells. These polymers are susceptible to oxidative photodegradation in air. A model water-soluble polymer, the sodium salt of poly[2-(3-thienyl)ethoxy-4-butylsulfonate], was employed to investigate the role of excited-state and singlet-state properties in the photodegradation of the polymer. The singlet excited state produces characteristic absorption at 770 nm with a lifetime of 22 ps. The triplet state exhibits a broad absorption in the 650−800 nm region and has a lifetime of 18.7 μs in deaerated water. The excited triplets are readily quenched by oxygen with a rate constant of 1.9 × 10<sup>9</sup> M<sup>−1</sup> s<sup>−1</sup> via both electron and energy transfer pathways. Pulse radiolysis experiments have been conducted to verify the identity of the cation radical and hydroxyl adduct of the polymer. It is evident from the photochemical experiments that the high photochemical reactivity of triplets with oxygen is responsible for the photodegradation.