Developing non-invasive processing methodologies and understanding the materials properties of solution-processable organic semiconductors for organic electronics
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Essential to the success of organic electronics, and in particular organic thin-film transistors, is the realization of stable, high-mobility, electrically-active organic materials that can enable low-cost solution-based processing methods. The development of viable solution-processable organic semiconductors helps make this possible. Consequently, understanding the materials properties of solution-processable organic semiconductors and how the processing conditions associated with device fabrication affect device performance are key to realizing low-cost organic electronics. In this work, we focused on understanding the processing-structure-property relationships of a solutionprocessable organic semiconductor, triethylsilylethylnyl anthradithiophene (TES ADT). Specifically, we demonstrated how a solvent-vapor annealing process can induce the crystallization of TES ADT post device processing. Bottom-contact thin-film transistors with annealed TES ADT routinely exhibit an average charge-carrier mobility of 0.1 cm 2 /V-s, which is sufficient to drive backplane circuitry in flexible display applications. Additionally, we demonstrated that the manner in which source and drain electrodes are defined significantly affects the performance of the resulting TES ADT thin-film transistors. Specifically, the yield of functioning top-contact TES ADT thinfilm transistors with electrodes defined by evaporation through a shadow mask directly on the organic semiconductor is low, and of the functioning devices, the charge-carrier mobility varies significantly (0.01 – 0.1 cm2 /V-s). In comparison, top-contact TES ADT thin-film transistors with electrodes defined separately and then laminated against the organic semiconductors have high yield and high charge-carrier mobility (0.2 ± 0.06 cm 2 /V-s). This result emphasizes the importance of adapting existing or developing new thin-film transistor fabrication techniques to overcome the materials limitations of organic semiconductors. Along the same vein, we also demonstrated an elastomeric stamp-based, solventless printing process, nanotransfer printing (nTP), for the additive patterning of copper electrodes and interconnects of feature sizes 1 – 500 μm. These printed copper patterns differ from similarly printed gold patterns in that they are not electrically conductive. Leaching the elastomeric stamps in hot toluene prior to printing, however, allowed us to routinely print conductive copper features with an average resistivity of 31 μΩ-cm. Another aspect of thin-film transistor fabrication that is crucial for optimal device performance (i.e., low off currents and low leakage currents) is the patterning and isolation of the organic semiconductor between neighboring devices. We demonstrated two novel techniques for patterning TES ADT. The first technique utilizes UV light in the presence of dichloroethane vapors to simultaneously pattern and crystallize TES ADT. TES ADT thin-film transistors patterned with this technique exhibit high chargecarrier mobility (0.1 cm2 /V-s) and low off currents (10-11 A). The second patterning technique uses a PDMS stamp to selectively remove TES ADT from the non-channel regions of the thin-film transistor. This technique can be used to pattern both as-spun and crystalline TES ADT thin films. Crystalline TES ADT thin-film transistors patterned with this technique exhibit an average charge-carrier mobility of 0.2 cm2 /V-s and low off currents on the order of 10-11 A, while amorphous TES ADT thin films that are first patterned and then crystallized exhibit an average charge-carrier mobility of 0.1 cm2 /V-s and off currents on the order of 10-10 A.
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