The Influence of Thiophene Moieties on the Molecular Orientation and Interface Properties of Low Band Gap Polymers

The properties of low band gap (LBG) polymers in donor-acceptor based bulk heterojunction (BHJ) solar cells are strongly influenced by their morphology and ability for self-organization in thin-films. For two related polymer pairs the influence of the introduction of additional (hexyl-) thiophene moieties on both the electronic structure and the ability for self-organization in thin films is studied using Ultraviolet Photoelectron spectroscopy (UPS) and Near-Edge X-Ray Absorption Fine Structure spectroscopy (NEXAFS). NEXAFS in fluorescence detection mode at the sulfur K edge was used to investigate molecular orientation. Exemplary NEXAFS spectra are compared to DFT calculations. We find that the introduction of additional (hexyl-) thiophene moieties in the polymer structure does not affect significantly basic electronic parameters and the energy level alignment at interfaces, but has a strong impact on their self-organization properties.[1] This will affect the behavior of such polymers in devices. We thank the group of Prof. Scherf (University of Wuppertal, Germany) for the synthesis of LBG polymers.

[1] M. Ivanovic et al., Solar Energy Materials and Solar Cells, 157 (2016) 286-294