Electronic structure and self-organization properties of low band gap polymers: The effect of the introduction of additional thiophene moieties

An approach to improve the efficiency of donor-acceptor based bulk heterojunction (BHJ) organic solar cells is the use of low band gap (LBG) polymers as donor materials. The properties of these materials in BHJ solar cells are strongly influenced by their morphology and ability for self-organization in thin-films. We studied for two related polymer pairs the influence of the introduction of additional (hexyl-) thiophene moieties on both the electronic structure and ability for self-organization in thin films using Ultraviolet photoelectron spectroscopy (UPS) and Near-Edge X-Ray Absorption Fine Structure spectroscopy (NEXAFS). In addition, the influence of mixing with PCBM and post-processing annealing on the molecular orientation is investigated. We find that the introduction of additional (hexyl-) thiophene moieties in the polymer structure does not affect significantly the electronic structure of the polymers in thin films, but has a strong impact on their self-organization properties and consequently affects their behavior in devices.