Tunneling and Origin of Large Access Resistance in Layered-Crystal Organic Transistors

Layered crystallinity of organic semiconductors is crucial to obtaining high-performance organic thin-film transistors (OTFTs), as it allows both smooth-channel-gate-insulator interface formation and efficient two-dimensional carrier transport along the interface. However, the role of vertical transport across the crystalline molecular layers in device operations has not been a crucial subject so far. Here, we show that the interlayer carrier transport causes unusual nonlinear current-voltage characteristics and enormous access resistance in extremely high-quality single-crystal OTFTs based on 2-decyl-7-phenyl[1]benzothieno[3; 2-b][1]-benzothiophene (Ph-BTBT-C-10) that involve inherent multiple semiconducting pi-conjugated layers interposed, respectively, by electrically inert alkyl-chain layers. The output characteristics present layer-number (n)-dependent nonlinearity that becomes more evident at larger n (1 <= n <= 15), demonstrating tunneling across multiple alkyl-chain layers. The n-dependent device mobility and four-probe measurements reveal that the alkyl-chain layers generate a large access resistance that suppresses the device mobility from the intrinsic value of about 20 cm(2) V-1 s(-1). Our findings clarify the reason why device characteristics are distributed in single-crystal OTFTs.