Photo-oxidation Kinetics of Poly-3-hexylthiophene Thin Films

Abstract

Poly-3-hexylthiophene (P3HT) is a widely employed, organic semiconductor used in the field of organic solar cells. However there is still a lack of understanding about the basic degradation mechanisms which leads to the decomposition of the material limiting the lifetime. Quantitative information about competing reaction pathways under ambient conditions is hardly available. In the present work the influence of environmental factors on the degradation process of P3HT film has been investigated quantitatively. The decay kinetics of the polymer absorption during variation of intensity and spectral distribution of the incident light, oxygen concentration, ozone concentration, humidity level as well as temperature are monitored using infrared, UV/VIS and photoelectron spectroscopy. Additionally the oxygen diffusion into the polymer film has been investigated using fluorescence spectroscopy under the same experimental conditions. The degradation process is light initiated with a strong increase of the effectiveness towards the ultraviolet region of the spectrum. The observed photo oxidation is not oxygen diffusion limited although an activation energy of 26 kJmol-1 was observed for both degradation and oxygen diffusion. The observed kinetics, especially its dependence on wavelength of the incident light, point to a radical- based degradation process in the solid state rather than a singlet oxygen based mechanism as it is observed in the liquid phase. Furthermore the presence of humidity strongly affects the degradation process although water itself does not decompose the polymer. Changing the structure of the polymer from regio-regular to regio-random significantly accelerates the degradation, probably due to the higher triplet yield of the regio-random polymer. Ozone degradation is negligible at ambient ozone levels.

Poly-3-hexylthiophene (P3HT) is a widely employed, organic semiconductor used in the field of organic solar cells. However there is still a lack of understanding about the basic degradation mechanisms which leads to the decomposition of the material limiting the lifetime. Quantitative information about competing reaction pathways under ambient conditions is hardly available. In the present work the influence of environmental factors on the degradation process of P3HT film has been investigated quantitatively. The decay kinetics of the polymer absorption during variation of intensity and spectral distribution of the incident light, oxygen concentration, ozone concentration, humidity level as well as temperature are monitored using infrared, UV/VIS and photoelectron spectroscopy. Additionally the oxygen diffusion into the polymer film has been investigated using fluorescence spectroscopy under the same experimental conditions. The degradation process is light initiated with a strong increase of the effectiveness towards the ultraviolet region of the spectrum. The observed photo oxidation is not oxygen diffusion limited although an activation energy of 26 kJmol-1 was observed for both degradation and oxygen diffusion. The observed kinetics, especially its dependence on wavelength of the incident light, point to a radical- based degradation process in the solid state rather than a singlet oxygen based mechanism as it is observed in the liquid phase. Furthermore the presence of humidity strongly affects the degradation process although water itself does not decompose the polymer. Changing the structure of the polymer from regio-regular to regio-random significantly accelerates the degradation, probably due to the higher triplet yield of the regio-random polymer. Ozone degradation is negligible at ambient ozone levels.