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, humidity level as well as temperature are monitored using infrared and UV/vis absorption 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 toward 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 regioregular to regiorandom significantly accelerates the degradation, probably due to the higher triplet yield of the regiorandom polymer.

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The influence of oxygen and light on the energy level alignment of poly(3-hexylthiophene) thin films (∼100 nm) is investigated quantitatively using X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) in order to probe energetic shifts and the element distribution in the film. Upon simultaneous exposure to oxygen and light a reversible shift of the occupied states of about 0.3 eV is observed together with the presence of reversibly and irreversibly bound oxygen species. Whereas the reversibly bound species correlates with the energy shifts, the latter does not alter the electronic structure. The observations are discussed in terms of light-induced p-doping of the semiconducting material by oxygen which is completely reversible upon vacuum annealing of the polymer film above the glass transition temperature of 150 °C.

A versatile and efficient tip-enhanced spectroscopic imaging technique based on a parabolic mirror (PM) assisted near-field optical microscope is demonstrated. The replacement of the conventional objective lens with a parabolic mirror allows the non-restricted investigation of sample materials regarding their opacity. In addition, an improved signal collection efficiency and effective excitation of the longitudinal plasmonic oscillation in the tip apex are obtained. The capabilities of PM-assisted tip-enhanced Raman (TER) and photoluminescence (PL) imaging in distinguishing the individual domains made of different chemical components in poly (3-hexythiophene)/[6, 6]-penyl-C 61 butyric acid methyl ester (P3HT/PCBM) solar cell blend film and in the investigation of the plasmonic properties of geometrically well-defined Au cones are demonstrated.

The photodegradation kinetics of regioregular poly-3-hexylthiophene (P3HT) thin films (∼150 nm) upon illumination with both visible light of 525 nm and UV light of 365 nm has been studied using UV/vis and FTIR spectroscopy in order to probe both the disruption of the π-conjugated system and the chemical product evolution. Reaction effectiveness and product evolution turn out to depend strongly on the wavelength of the incident light. Under UV light, the π-conjugated system and the hexyl side chain are degraded almost simultaneously, involving Norrish-type reactions that lead to complete disintegration of the alkyl chains. Under visible light only the π-conjugated system is destroyed, whereas the aliphatic side chains are hardly affected. While the reaction under UV light is consistent with a radical reaction starting at the α-carbon of the alkyl side chain, the reaction under visible illumination is more likely to involve a photosensitized species that primarily attacks the π-conjugated system of the polymer.