N-Heteroacenes in Organic Field-Effect Transistors

Linear acenes are a widely studied class of materials in the field of Organic Electronics. Their aromatic system and the strong interaction of the π-electrons of neighbouring molecules in the solid state allow an efficient charge transport in these materials. The defined molecular structure of these small molecules and the possibility to tune their optical and electronic properties as well as the solid state packing through careful chemical design and synthesis have resulted in numerous applications of acenes in organic transistors and optoelectronic devices. This work focuses on the application of N-Heteroacenes and non-conjugated pentacene-based polymers as semiconductors in solution-processed organic field-effect transistors. These devices are used to evaluate the charge transport properties of the materials and derive structure-function relationships for the various compounds. To draw structure-function relationships from the studies described in this thesis, a myriad of characterisation techniques was employed to obtain an insight into the optical, electronic, electrical and morphological properties of each material. The effects of order, energetics and processing of the semiconductor on the transistor performance are all investigated. While non-conjugated pentacene-based polymers offer an ease of processability, their amorphous nature inhibits efficient hole transport, resulting in a relatively poor transistor performance. The fashion in which the pentacene systems are connected to the polymer backbone changes their flexibility and therefore affecting the injection behaviour and charge transport properties. The nitrogen substitution in N-Heteroacenes results in an energetic stabilisation of the frontier molecular orbitals, allowing for an enhanced electron injection into these materials. For the symmetrical tetraazapentacene and two halogenated phenazine derivatives relatively high electron mobilities were achieved demonstrating their potential for future application as n-type semiconductors in organic field-effect transistors. The use of N-heteroacenes is not limited to electron transport only. Their N,N’-dihydro forms are electron rich compounds that exhibit good hole transport. This is demonstrated for differently substituted tetraazapentacenes as well as for a N,N’-dihydro diazahexacene and -heptacene. For these materials it is shown that the processing conditions not only affect the macroscopic transistor performance, but also influence the formation of polymorphs in thin films. The solid state packing of functionalised acenes is typically determined by their solubilising side chains. A norbornadienyl substitution at the side chain of the well-known 6,13-bis(triisopropylsiliylethynyl)pentacene and its tetraaza derivative was shown to result in an enhancement of the charge transport properties for the p-type derivatives and deterioration of the performance of the n-type transistors. These observations are related to changes in the charge transfer integrals, the film microstructure and the solid state packing. In conclusion, this work contributes to the development of guidelines for the design and synthesis of next generation N-heteroacenes to be applied in the future in state of the art organic electronic devices

Solvent-antisolvent interactions in metal halide perovskites

The fabrication of metal halide perovskite films using the solvent-engineering method is increasingly common. In this method, the crystallisation of the perovskite layer is triggered by the application of an antisolvent during the spin-coating of a perovskite precursor solution. Herein, we introduce the current state of understanding of the processes involved in the crystallisation of perovskite layers formed by solvent engineering, focusing in particular on the role of antisolvent properties and solvent-antisolvent interactions. By considering the impact of the Hansen solubility parameters, we propose guidelines for selecting the appropriate antisolvent and outline open questions and future research directions for the fabrication of perovskite films by this method

Stability of Hydroxo/Oxo/Fluoro Zirconates vs. Hafniates - A DFT Study

We performed density functional theory (DFT) calculations on binary and ternary oxo/fluoro crystals of the geochemical twin pair zirconium and hafnium to evaluate and compare their stabilities. This is the first DFT study on bulk ZrF4 or HfF4, as well as on a hypothetical ZrOF2 or HfOF2 bulk crystal. For α-MO2, β-MF4 and MOF2, we have found significantly higher cohesive energies for the respective hafnium species. This suggests a considerable gap in affinity toward fluorine and oxygen between the twin pair in the solid state. In agreement with experimental findings, this gap is slightly more pronounced for fluorine. This study is also the first to evaluate the theoretical, endothermic mono-hydroxylation of the respective fluorides or oxyfluorides to model the difference in affinity toward fluoride versus hydroxide. For these, we could also find a slight energetic preference for the hafnium compound

Cation exchange synthesis of AgBiS2 quantum dots for highly efficient solar cells

Silver bismuth sulfide (AgBiS2) nanocrystals have emerged as a promising eco-friendly, low-cost solar cell absorber material. Their direct synthesis often relies on the hot-injection method, requiring the application of high temperatures and vacuum for prolonged times. Here, we demonstrate an alternative synthetic approach via a cation exchange reaction. In the first-step, bis(stearoyl)sulfide is used as an air-stable sulfur precursor for the synthesis of small, monodisperse Ag2S nanocrystals at room-temperature. In a second step, bismuth cations are incorporated into the nanocrystal lattice to form ternary AgBiS2 nanocrystals, without altering their size and shape. When implemented into photovoltaic devices, AgBiS2 nanocrystals obtained by cation exchange reach power conversion efficiencies of up to 7.35%, demonstrating the efficacy of the new synthetic approach for the formation of high-quality, ternary semiconducting nanocrystals