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The application of new polymer materials for organic solar cells requires a detailed understanding of the orientation and the electronic properties of the system because these have a dramatic effect on the device performance and efficiency. The films were prepared by doctor blade casting, a simple preparation route from solution. We study the orientation of the low band gap polymers poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) and poly-[2,6-(4,4bis(2-ethyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-5,8-(2,3-dioctylquinoxaline)] (PCPDTQ) as well as of the state of the art polymer poly(3-hexyl-thiophene) (P3HT) using near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Clear angular dependence in the NEXAFS spectra is observed for P3HT and PCPDTBT pointing to well-orientated films.

The photo-degradation of the low band-gap polymer Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) during irradiation under white light (AM 1.5 conditions) has been studied in pristine polymer films as well as in blend films with [6,6]-Phenyl C61 butyric acid methyl ester (PC[60]BM). In order to gain insight into the degradation process, FTIR spectroscopy has been used to follow the evolution of different subunits of the polymer and to probe the chemical product formation. In contrast to other polymers, not the alkyl side chains but the π-conjugated system is preferentially oxidized during the first stages of degradation. Furthermore it has been shown that the subunits of the polymer backbone are differently affected by the degradation. Blending the polymer with PC[60]BM leads to a significantly larger impact on the stability of the cyclopentadiene group compared to the benzene ring of the benzothiadiazole group.

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