Unveiling the Complexity of the Degradation Mechanism of Semiconducting Organic Polymers: Visible-Light-Induced Oxidation of P3HT Films on ZnO/ITO under Atmospheric Conditions

The oxidation of poly(3-hexylthiophene) (P3HT) films deposited on ZnO/indium tin oxide (ITO) under blue light irradiation in either dry or humid atmospheres was studied using X-ray photoelectron spectroscopy in combination with UV-vis absorption spectroscopy. From results up to 12 h of reaction, ring-opening was hardly found, and it is suggested that the water molecules chemisorbed competitively against O₂ (i.e., the major oxidizing agent), thereby decreasing the oxidation of P3HT. Beyond 12 h, thiophene ring-opening took place at the topmost surface layer of P3HT, and the humidity facilitated the ring-opening of P3HT. Regarding the oxidation of the entire P3HT thin film, the humidity did not have a large influence on the oxidation behavior of P3HT. Here, the degree of oxidation of P3HT abruptly increased when the reaction time exceeded 12 h. This suggests that the rate of oxidation of the entire P3HT film is determined by the slow diffusion of the activated oxygen species into the deeper layers of the P3HT films. We also demonstrate that the photoinduced degradation of P3HT can be retarded by turning off light between irradiation, which may be due to the reversible desorption of activated oxygen species under dark conditions.