Molecular origin of photostability for fluorene-based donor–acceptor type photovoltaic polymers

Quantitative analyses of photodegradation for three fluorene-based photovoltaic polymers, poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (APFO3), polyfluorene (PFO), and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) were conducted to understand the molecular origin of photostability for polymers. The Fourier transform infrared (FT-IR) spectra of polymer thin-films varied by irradiating white light at 100 mWcm-2 irrespective of their molecular architectures. The absorption peaks corresponding to alkyl side chains in a fluorene unit decreased, whereas those for polymers that did not comprise carbonyl groups increased. This spectral variation indicates that alkyl side chains in the fluorene unit decompose when the molecular structure of fluorene varies to that of fluorenone. The reaction rate constant of formation of C=O bond for APFO3 was 1.64×10-5 s-1, lower than PFO (7.59×10-5 s-1) and F8BT (2.64×10-5 s-1), under light irradiation at 30 ˚C. The photostability of the polymers improves by designing a donor–acceptor type molecular architecture—incorporating photostable electron deficient benzothiadiazole units with photo-unstable fluorene units.