Interface Engineering of Organic Thin Film Transistors with Self-assembled Organophosphonic Acids

Organic thin film transistors (OTFTs) are interface devices with their performance highly dependent on the interface between organic semiconductors and gate dielectrics no matter whether the organic semiconductors are processed by vacuum deposition or solution-based methods. Detailed in this thesis are studies of interface engineering for OTFTs with self-assembled organophosphonic acids, which play important roles in tuning the properties of the dielectric surface for high-performance OTFTs.;The poor crystallinity of rubrene in conventional vacuum deposited films is a well-known obstacle limiting practical applications of rubrene in thin film transistors. As described in Chapter 2, a template layer of diazapentacene (DAP) is introduced to induce crystallization of rubrene in thin film transistors. This study demonstrates that DAP is a suitable template molecule with negligible contribution to the conduction channel leading to polycrystalline thin films of rubrene with field effect mobility as high as 0.68 cm2 V --1 s--1. This induced-crystallization strategy highly depends on a unique octadecylphosphonic acid (ODPA) bilayer-step surface, which plays important roles in controlling the growth of both DAP and rubrene.;In solution-processed OTFTs, one key factor that affects the nucleation and growth of semiconductor molecules during solution-based processing is the wetting behavior of the semiconductor solution on the dielectric surface. Reported in Chapter 3 is a new strategy for preparing solution-processed OTFTs based on enhancing the surface energy of self-assembled monolayers (SAMs) by inserting polar oxygen atoms into the long alkyl chain of phosphonic acids. SAMs of these phosphonic acids on a high-k metal oxide layer of AlOy /TiOx lead to solution-processed n-channel OTFTs with high field effect mobility of up to 2.5 cm2 V--1 s--1 and low operational voltage.;Chapter 4 puts forth a new design of SAMs for interface engineering of high-performance OTFTs. This design combines a long alkyl chain and a cyclohexyl terminal group resulting in an unprecedented phosphonic acid. The SAM-modified AlOy/TiOx is applied as a general dielectric, which enables OTFTs with high field effect mobility of up to 5.7 cm2 V--1 s--1 for holes and 5.5 cm 2 V--1 s--1 for electrons, good air stability with low operating voltage, and general applicability to solution-processed and vacuum-deposited n-type and p-type organic semiconductors. These excellent properties are related to the highly-ordered, close-packed, and widely-wettable nature of the SAMs.