In Situ Real-Time Measurements for Ambipolar Channel Formation Processes in Organic Double-Layer Field-Effect Transistors of CuPc and F16CuPc

Molecular donor–acceptor heterostructures have been utilized to obtain ambipolar characteristics in organic field-effect transistors (OFETs). In the present work, we examined the surface morphologies of the double-layer thin films of the donor–acceptor pair of copper phthalocyanine (CuPc) and its fluorinated analogue (F 16 CuPc). We then performed in situ real-time measurements on the formation processes of ambipolar transport channels in the double-layer CuPc/F 16 CuPc and F 16 CuPc/CuPc FETs, with gradual growths of the top layers on the bottom layers under high vacuum conditions. When CuPc was deposited on the F 16 CuPc bottom layer, the n-type mobility was immediately enhanced because of an electron carrier injection to the F 16 CuPc layer. In the case of CuPc top-layer growth, ambipolar properties were clearly seen when the top layer was thicker than 2.4 ML. Finally, the CuPc/F 16 CuPc FET exhibited a well-balanced ambipolar transport with p- and n-type mobilities of 2.6 × 10 –2 and 1.4 × 10 –2 cm 2 /V s, respectively. In contrast, when F 16 CuPc was deposited on the CuPc bottom layer, the p-type transport of CuPc was suppressed because of the formation of trap states at the interface. Further deposition of F 16 CuPc resulted in the recovery of p-type transport in CuPc and produced an ambipolar transport with p- and n-type mobilities of 3.2 × 10 –3 and 3.7 × 10 –3 cm 2 /V s, respectively, when F 16 CuPc was thicker than 3.2 ML. This suggested that smoother interfaces between CuPc and F 16 CuPc would produce higher mobilities in the heterostructure OFETs.