Investigating post-deposition semiconductor dewetting and fabrication of ultra-thin organic semiconductor thin-film transistors (Conference Presentation)

Post-deposition semiconductor dewetting is the transformation of a (nearly) closed organic-semiconductor monolayer into separated individual islands of multilayer height [1]. We have recently observed this phenomenon in both ultra-thin (1-3 nm) and thin (25-40 nm) films of the small-molecule semiconductor dinaphthothienothiophene (DNTT) [2,3]. Since the gate-field-induced carrier channel is located in close vicinity to the semiconductor-dielectric interface, the accelerated pace of dewetting of ultra-thin semiconductor films is relevant to the performance and stability of organic thin-film transistors (TFTs). We have therefore fabricated bottom-gate, bottom-contact TFTs based on 2 nm and 25 nm-thick DNTT films. Compared to the relatively stable charge-carrier mobility of 1.1 cm2/Vs for the 25-nm-DNTT TFT, the 2-nm-DNTT TFTs show a sharp decrease from 0.2 cm2/Vs to 0.011 cm2/Vs over 72 hours after fabrication. To stabilize the TFT performance, we have explored strategies to prevent ultra-thin DNTT films from dewetting, including substrate cooling and semiconductor encapsulation, and fabricated stable DNTT TFTs with monolayer semiconductor thickness. Encapsulation with vacuum-deposited polytetrafluoroethene (PTFE) or titanyl phthalocyanine (TiOPc) leads to a relative decrease in mobility by only 12% and 44%, compared to 99.6% for TFTs without encapsulation over 28 hours after device fabrication. [1]T. Breuer et al., ACS Appl. Mater. Interfaces, 9, 8384, (2017). [2]K. Takimiya et al., Sci. Technol. Adv. Mater., 8, 273, (2007). [3]U. Zschieschang et al., Organic Eletronics, 12, 1370, (2011).