Stability of Ternary Interfaces in Printed Organic Thin-Film Transistors Showing Extremely Sharp Switching Operation

Inverted-coplanar or bottom-gate-bottom-contact (BGBC) type has several unique advantages in fabrication and application of organic thin-film transistors (OTFTs), because it allows apply a variety of fabrication processes for dielectric and/or electrode components without bringing unfavorable influences on the channel organic semiconductor (OSC) layers. However, it is also generally known that it becomes more difficult to operate BGBC-type OTFTs, as compared to bottom-gate-top-contact (BGTC) type OTFTs. Recently, it was demonstrated that both high mobility and ideal on/off switching are successfully realized in BGBC-type printed OTFTs by adopting highly lyophobic gate dielectric layers that use amorphous perfluoropolymer of Cytop. However, peculiar channel material dependence is still observed in the device performance. Here, we report that one of crucial requirements for realizing excellent device performance is the stability of semiconductor/metal/insulator ternary interfaces in the BGBC-type OTFTs.<br/>We manufactured semiconductor single-crystal thin films of various layer numbers and materials (phenyl/alkyl-substituted benzothieno[3,2-b]benzothiophene (Ph-BTBT-C<i>_n</i>) and phenyl/alkyl-substituted benzothieno[3,2-b]naphtho[2,3-b]thiophene (Ph-BTNT-C<i>_n</i>)) on highly lyophobic Cytop gate dielectric surfaces by extended meniscus-guided coating technique. We found that some TFTs exhibit notable time-dependent degradation, whose degradation speed depends significantly on the material, layer number, and also the encapsulation. For example, the single layer TFTs with Ph-BTBT-C<i>_n</i> exhibited mobility as about 5 cm²/Vs immediately after device fabrication, but decreased to 0.06 cm²/Vs after 50 hours. In contrast, multilayer (3 layers) TFTs exhibited excellent time stability up to about 400 hours, and maintained initial mobility of 8 cm²/Vs. Kelvin probe force microscopy (KPFM) measurements revealed that observed degradation is associated with the occurrence of contact resistance at around the source electrodes, whereas the degradation is dramatically reduced by stacking multilayer (two or more layers) OSC films. Microscopic characterizations by atomic force microscopy (AFM), and in-plane x-ray diffraction (XRD) indicated the sign of film deterioration at ternary interface, in case that contact resistance is observed. The results of this study suggest the importance of the quality of the semiconductor thin films at ternary interface and is expected to serve as a clue for improving the performance of BGBC-type OTFTs.<br/>[1] G. Kitahara <i>et al., Sci. Adv.</i> <b>6</b>, eabc8847 (2020). [2] H. Iino <i>et al., Nat. Commun</i>. <b>6</b>, 6828 (2015). [3] S. Inoue <i>et al., </i><i>Chem. Mater.</i> <b>27</b>, 3809 (2015). [4] S. Inoue <i>et al., Chem. Mater</i>. <b>30</b>, 5050–5060 (2018). [5] S. Arai<i> et al., Adv. Mater.</i> <b>30</b>, 1707256 (2018).