Synthesis and characterization of amorphous Cyclopentadithiophene- and Cyclopentadiene-based organic hole transport materials

Abstract

The aim of this work was the design and synthesis of novel hole transport materials with a stable amorphous glassy state. This goal was achieved with two different strategies. In Chapter 1 the spiro-concept i.e., the linkage of two different π-systems with different functions over one common sp3-hybridized atom was used. One π-system was represented by a triarylamine-flanked cyclopentadithiophene unit. The use of this unit allowed to tune the optoelectronic properties of the molecules towards their planned application purpose. The second π-system is electronically disconnected from the first one and can therefore independently be tuned and optimized. Three new spiro-cycloentadithiophene middle building blocks, containing a spiro-linked acridine, thioxanthene, or xanthene have been synthesized and described. Three spiro central building blocks were substituted with triarylamine units in the free thiophene α-positions by palladium-catalyzed Suzuki-type cross-coupling reactions to obtain five different spiro-cyclopentadithiophene based hole transport materials. Three additional hole transport materials were obtained by coupling three different triarylamine substituents to non-spiro diphenylcyclopentadithiophene building blocks. These non-spiro hole transport materials were used as reference to evaluate the influence of the spiro-linkage. All hole transport materials were electrochemically characterized to validate their applicability in perovskite solar cells. The success of the spiro-concept in the synthesis of the hole transport materials was verified through thermal analysis by differential scanning calorimetry. All materials showed high glass transition temperatures above 135 °C providing evidence for a stable amorphous state. In Chapter 2 the concept of overcrowding was used to obtain molecules with spherical structures. This forces a 3-dimensional structure and should also lead to a material with a stable amorphous state. The small cyclopentadiene moiety was chosen as a central building block. Substitution of the small cyclopentadiene derivative by Suzuki-type cross-coupling with four sterically demanding triarylamine substituents led to the desired overcrowding. Two different triarylamine substituents were used. The central dimethoxy cyclopentadiene was transformed into a cyclopentadienone to open the molecule up to further functionalization. This possibility was used to introduce a dicyanovinylene group into the molecule by a Knoevenagel condensation. The influence of the different cyclopentadiene derivatives on the optoelectronic properties was investigated with optical spectroscopy and electrochemistry. It was discovered that the central building block has a large influence on the redox behavior and on the optical spectrum. The concept of overcrowding to achieve hole transport materials with a stable amorphous state was here again verified by differential scanning calorimetry. All six hole transport materials showed glass transition temperatures of 120 °C and higher, confirming the design philosophy. In Chapter 3 a selection of the synthesized hole transport materials was employed in perovskite solar cells. The materials were investigated for their performance in the solar cells but also for their thermal stability. From the solar cell results it could be concluded that methoxy groups on the triarylamines increase the performance of the device. All of the methoxy capped hole transport materials surpassed the performance of their phenyl capped counterparts. One of the spiro-acridine and one of the diphenyl-cyclopentadiene hole transport materials both achieved very good results of 21% power conversion efficiencies in perovskite solar cells. The best result was obtained for the methoxyphenyl-capped cyclopentadienone acetal. The material could beat the spiro-MeOTAD reference device and could achieve an outstanding PCE of 23%.