|Ph.D Student||Shritz Rozalia|
|Subject||Synthesis and Characterization of Solution Processable|
Small Molecules and Polymers Bearing Labile
|Department||Department of Chemistry||Supervisor||Professor Yoav Eichen|
|Full Thesis text|
With the discovery of the conductivity of oxidized polyacetylene about four decades ago, the field of organic electronics was born and over the time since then it has continued to expand. The main examples of current-day device types are Field Effect Transistors (FETs), solar cells and light emitting diodes. Among the advantages of organic semiconductor technologies are the ability to design and modify the structure of organic semiconducting compound and its properties and possibility to fabricate printable and flexible electronic devices.
The conductivity of organic semiconductors is derived from the presence of p-conjugated backbones. However, these structural elements in the molecule also render it insoluble in common organic solvents, and therefore, solubilizing side chains are added. In the finalized device the organic semiconductors are set in the solid state, in which these solubilizing groups normally detract from the desired electronic properties of the material by interrupting the preferential packing orientation of the conjugated backbone. To overcome this drawback caused by the solubilizing groups, it would be advantageous to enable the removal of these groups following the deposition of the organic semiconductor within the device. Thus, the research presented herein focuses on the design of p-conjugated polymers and small molecules for organic electronics, whose uniqueness is that they bear solubilizing groups which are thermally-cleavable from the main backbone.
In this work the synthesis and characterization of a series of six naphthalene diimide-based small molecules bearing t-BOC as thermo-cleavable solubilizing groups is described. Three representative molecules of this series were fully characterized in terms of their electronic properties and their band gaps were calculated. Film morphology and “washability” of these materials were additionally examined. It was found, that small molecules bearing ≥18% of t-BOC groups did not present appropriate morphology for OFET devices. Therefore, six naphthalene diimide -based polymers bearing different concentrations (0% to ~8.3%) of t-BOC groups were synthesized and characterized. Additionally, OFET devices were fabricated and investigated before and after annealing. The morphology that provided the best OFET device performance was found at the ratio 30%:70% for t-BOC groups: bulky not thermo-cleavable groups (2-(n-decyl) tetradecyl imide), with m= 0.01 cm2/Vs. Two competitive processes were found to occur while increasing t-BOC concentration in the examined polymers: improvement in film morphology, up to the optimum t-BOC concentration (6.26%), and the rise in the concentration of interface traps associated with the cleavage of t-BOC groups. After t-BOC thermolysis, the resulting imide moieties are capable of forming hydrogen bonds, leading to enhanced crystallinity and consequently to better electronic properties of the OFETs. Therefore, thermally induced reorganization is necessary after the deposition step, which, in turn, proceeds easier, when there is an optimum number of bulky solubilizing and “free” imide groups. These findings clarify the necessity to take into account the optimum ratio of cleavable- versus un-cleavable solubilizing side groups when designing materials for organic electronics.