Air-stable polythiophene-based thin film transistors processed using oxidative chemical vapor deposition: Carrier transport and channel/metallization contact interface

• Unsubstituted polythiophene (PT) films are successfully synthesized using oCVD and integrated into organic TFTs.
• Low threshold voltages (−1 V – 0.3 V) and high field effect mobility of 0.02 cm2/Vsec is achieved.
• oCVD PT-TFTs present excellent air-stability over time.
• The carrier transport mechanisms in PT (charge screening effect and ionized impurity scattering) are suggested.

Oxidative chemical-vapor-deposition (oCVD) provides a facile route to polymerize and deposit insoluble monomers in thin film form. Here, we report on oCVD polythiophene (PT)-based organic thin film transistors (OTFTs) that present both high mobility and excellent stability over time in air. The measured field effect mobility (μFE) is ∼0.02 cm2/V sec with the low threshold voltage between −1 V and 0.3 V. Additionally the PT OTFTs show no evidence of performance degradation after 3 months exposure in air. The transmission line model (TLM) enables the determination of the specific contact resistance (ρC) of oCVD PT channel/metallization interface and reveals that ρC is improved with increasing gate bias. The oCVD PT channel conductivity (σch) and carrier density (p) were evaluated from more than 100 devices using TLM measurements and the relation of σch = qpμFE. Carrier transport analysis suggests that the charge screening effect governs hole carrier mobility in the carrier density regime below approximately 1018/cm3 where an increase in carrier density leads to higher mobility. We also demonstrate photo-conductivity of oCVD PT through an increase in device on-state current and the field effect mobility when the PT OTFT is illuminated. Strategies to further enhance the performance of the materials and devices are also suggested.

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