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 spectra of the polymer thin films varied with irradiating white light at 100 mW cm^-2 irrespective of their molecular architectures. The absorption peaks corresponding to alkyl side chains in the 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 changes to that of fluorenone. The reaction rate constant of the formation of C=O bonds for APFO3 was 1.64 × 10^-5 s^-1, lower than those for 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 was improved by designing a donor-acceptor type molecular architecture incorporating photostable electron-deficient benzothiadiazole units with photo-unstable fluorene units.