Dendritic oligothiophenes: synthesis, characterization and application in organic photovoltaics

Abstract

The thesis describes the structural modification of 2:2",3":2"-terthiophene (3T)-based dendritic oligothiophenes by enlarging the pi-conjugated system and consequently broadening the absorption spectrum in order to improve the light harvesting property of the compounds for use in organic photovoltaics. Four main modifications have been realized: in chapter 2, the 3T-building block was elongated to 2:2",5":2",3":2",5":2"-quinquethiophene (5T) and used for further constructing dendrons and dendrimers up to the third generation. Ir-catalyzed borylation reaction with high regio-selectivity was introduced as a powerful synthetic tool to obtain the crucial synthetic intermediates. Conventional column chromatography as well as size-exclusive chromatography were used in the purification procedure. Chapter 3 describes the modification of a 5T-core with three branching sites resulting in unsymmetrical 5T-based dendrimers and a significant enhancement of solubility without attaching any solubilizing group, which simplified the synthesis and purification procedures. The third possibility was the modification of the peripheries of the 3T-based structures. In chapter 4 the peripheral extension with oligothiophenes and in chapter 5 with benzothiadiazole-moieties are discussed. Single crystal structure analysis revealed the planarity of the periphery which affects the bulk properties such as solubility, shifts of absorption in the solid states. The goal of broadening the absorption spectrum was achieved in all modifications and consequently, the photovoltaic tests in chapter 6 demonstrate that the modified dendritic oligothiophenes are very good candidates as donor material for organic bulk heterojunction solar cells when blended with fullerene derivatives. Up to 3.1 % of power conversion efficiency and high open circuit voltages have been achieved.