Influence of the Side Chain Structure on the Electronic Structure and Self-Organization Properties of Low Band Gap Polymers

The properties of low band gap polymers in devices such as solar cells are strongly influenced by their morphology and ability of self-organization in thin films and interface properties. We study the influence of alkyl and alkoxy side chain position for four conjugated, alternating oligothiophene-benzothiadiazole copolymers on the molecular orientation in thin films and electronic interface properties using photoemission, X-ray absorption spectroscopy (XAS) at the sulfur K edge, and polarization modulation-infrared reflection–absorption spectroscopy (PMIRRAS). The interface charge transfer (ICT) model is used to explain interface properties of the polymers on substrates with different work functions. We find that the position of the side chains has a significant influence on the orientation and thus on self-organization properties of the polymers in thin films, whereas the electronic structure is less affected. The preferred molecular orientation is further affected by annealing, leading to a higher degree of ordering. Results from complementary methods with different surface sensitivities (XAS in total electron yield and fluorescence mode and PMIRRAS) are discussed.