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

Abstract

Layered crystallinity of organic semiconductors is crucial to obtain 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-crystalline OTFTs based on Ph-BTBT-C 10 that involve inherent multiple semiconducting π-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 tunnelling 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 /Vs. Our findings clarify the reason why device characteristics are distributed in single-crystalline OTFTs.

Crystallized Monolayer Semiconductor for Ohmic Contact Resistance, High Intrinsic Gain, and High Current Density

“…[8] In the devices with staggered structure, the contact resistance has two major components: i) the resistance at the M/OSC interface (referred as R int ), and ii) the resistance across the OSC layer (namely, the access resistance, R a ). [9] Unlike conventional amorphous organic thin films, crystalline organic semiconductor with regular lattice structure and molecularly flat surfaces is an excellent candidate to develop closely packed interfaces with the metal electrodes. [10,11] Various approaches have been studied to improve the R int at M/OSC interfaces, including tuning the work function of metal electrodes by using oxidation, [12] forming self-assembled monolayers or inserting thin layers with large dipole moments, [13][14][15][16] and employing thin layers for Fermi-level depinning.…”

Semiconductive Single Molecular Bilayers Realized Using Geometrical Frustration

“…It eventually allows to realize efficient 2D carrier transport and thus high‐performance organic thin‐film transistors (TFTs) . However, these materials usually form thin flake‐like crystals or crystalline thin films, composed of multiply‐stacked molecular bilayer units . No method has been reported so far for producing the corresponding SMBs.…”

“…Typical mobility is estimated as 2.6 cm 2 V −1 s −1 in the linear regime and 6.0 cm 2 V −1 s −1 in the saturation regime. Notably, these values are slightly lower than those of single‐crystalline Ph‐BTBT‐C 10 TFTs with channels composed of a larger number of bilayer units . We consider that the result could be ascribed to the limited thickness of the channel layer in the above SMB‐based TFTs, which should suppress the carrier transport through the channels.…”

“…The SMB formation technique as presented all the above is based on a unique characteristic that a class of asymmetric rod‐like molecules of (πCore)‐C n 's exhibits very high layered crystallinity due to the formation of bilayer‐type LHB packing. It is also reported that the class of packing is useful for fabricating high‐performance organic TFTs . Basically, the LHB packing is composed of two types of intermolecular contacts, i.e., T‐shaped (core‐to‐edge) and slipped parallel (core‐to‐core) contacts between the neighboring planar πCores that are arranged in glide and translational symmetry, respectively (see Figure d).…”

Effect of access resistance on the experimentally measured temperature–carrier mobility dependence in highly-crystalline DNTT-based transistors

“…19 Note that this feature is hard to observe even in some layeredstructure organic single crystals. 20 In the following discussion on charge transport, 20 nm-thick samples were selected because their access resistance was much lower compared with that in 50 nmthick samples.…”