Year of Publication
Doctor of Philosophy (PhD)
Arts and Sciences
Dr. John E. Anthony
Inorganic materials have dominated electronic applications such as photovoltaic cells, thin film transistors (TFTs) and light emitting diodes (LEDs). However developments in the field of organic electronics over the past three decades have enabled the use of organic materials in these devices. While significant improvements have been made to improve their electronic properties there are several road blocks towards commercial application. One of the significant obstacles is the poor charge carrier mobility associated with organic semiconductors processed by well established printing methods. The goal of my research project is to improve the charge carrier mobility of solution cast films of acene semiconductors by partial halogenation and heteroatom substitution. Spin coated films of triisopropylsilylethynylated difluoropentacene exhibited higher hole mobility compared to TIPS pentacene due to contact induced nucleation of pentacene on perfluorobenzenethiol treated gold electrodes. The success of this project allowed me to further investigate the effect of degree of fluorination on the electronic properties of pentacene. A series of trialkylsilylethynylated tetrafluoro and octafluoropentacenes were synthesized and their performances in thin film transistors and solar cells were explored. Solar cells made from these materials using poly(3-hexylthiophene) as donor exhibited poor open circuit voltages (Voc) resulting in low power conversion efficiency (PCE). Better device performances were achieved using pentacenes having single halogen substituent.
In order to improve the charge carrier mobility in TFTs soluble trialkylsilylethynylated hexacenes were explored. However these molecules exhibited a greater tendency to photo-dimerize in solution and solid state. Partial halogenation was used as a tool to improve the solution stability of reactive hexacene. The improved solution stability of partially halogenated hexacenes allowed me to successfully extend this approach to heptacene and nonacene.
Finally a series of new trialkylsilylethynylated anthradiselenophenes were synthesized to improve molecular ordering in the solid state by increasing non-bonding Se – Se interaction. However single crystal x-ray diffraction studies revealed no such interaction between the acene chromophore resulting in poor device performance.
Purushothaman, Balaji, "PARTIALLY HALOGENATED ACENES AND HETEROACENES FOR ORGANIC ELECTRONICS" (2011). University of Kentucky Doctoral Dissertations. 124.