Charge Transport, Trapping, and Interfacial Effects in High Electron Affinity Arylene Diimide Semiconductor Thin Films

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Small molecule organic materials based on perylene- and naphthalene- diimide are synthesized and characterized for use as semiconductor thin films in organic field-effect transistors. The orbital energetics of the materials are investigated via electrochemical, spectroscopic, and computational techniques. The thin film solids are characterized by x-ray diffraction techniques and surface microscopy. Finally, the electrical properties of the thin films are tested with organic field-effect transistor (OFET) measurements to characterize electron mobility, threshold voltage, and current on-off ratios. This study begins with development of synthetic methodology to functionalize arylene diimide cores with highly electron withdrawing groups to manipulate LUMO energetics and thus electron carrier stability. Dicyanated arylene diimides yield materials with reduction potentials ~ 0 V vs. S.C.E., with minimal alterations to the band gap of the material. Thin films of these semiconductors yield OFETs with high electron mobility (> 0.1 cm^2V^-1s^-1), low threshold voltages, and environmental stability of the charge carriers. Furthermore, thin films of these materials can be deposited from solution due to the increase solubility upon functionalization, and these materials are found to be compatible with bottom-contact device configurations. After the synthetic methodology and initial electrical characterization were completed, a systematic investigation of carrier stability and trapping due to air in thin films of core-substituted arylene diimides reveals that materials with an Ered1 < ~ -0.1 V vs. S.C.E. are susceptible to trapping by air, and the trap density in the OFET increases by > 10^13 cm^-2. Thin films of materials with Ered1 > -0.1 vs. S.C.E. show air-stable OFET operation with minimal hysteresis over extended periods of time. Additionally, steric-induced distortions of the arylene diimide core are found to modulate the transistor electron mobility in core-substituted arylene diimide thin films Consequently, a study of gate dielectric surface treatments reveals that interfacial surface chemistry controls the charge carrier density in the thin films, thus modulating the off-current in the arylene diimide OFET. Functionalization of the gate dielectric with self-assembled monolayers or polymer films decreases the initial carrier concentration compared to on bare silicon dioxide for materials with Ered1 > -0.1 V vs. S.C.E.. The decreased off-current leads to Ion/Ioff as high as 10^8 for air-stable dicyano arylene diimide semiconductor thin films. Finally, the materials properties of the dicyanated derivatives are also investigated for fabrication of unique device structures. Dicyano naphthalene diimide was utilized to fabricate the first flexible, transparent, air-stable, n-channel organic transistors. Dicyano perylene diimide films were used to fabricate air-stable organic-CMOS circuitry for clock generation and memory from both vapor- and solution-deposited thin films. These circuits operate at unprecedented speeds for organic-based complementary circuits.

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  • 08/07/2018
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