Contact Resistance of Low-Voltage n-Channel Organic Thin-Film Transistors Based on Three Different Organic Semiconductors

Transistors based on organic semiconductors provide the possibility of fabricating electronic circuits and systems on flexible polymeric substrates, owing to the low process temperature of organic semiconductors. For mobile systems that can be used in wearable electronics and in diagnostic devices directly attached to the human body, an extremely low power consumption is a critical requirement. From a circuit-design perspective, the most effective approach to minimize the power consumption of digital electronic circuits is to combine p-channel and n-channel field-effect transistors in a complementary circuit design.<br/>While the performance and stability of p-channel organic thin-film transistors (TFTs) are already sufficient for certain applications, the performance and stability of n-channel organic TFTs are still comparatively poor. This is partially due to the fact that in n-channel organic transistors, charge transport takes place in the lowest unoccupied molecular orbital (LUMO), whose energy must fall within a relatively narrow region between approximately -4.0 to -4.5 eV to allow efficient charge exchange with the source/drain contacts while providing sufficient stability. Organic semiconductors that fulfill this requirement include π-conjugated systems with strong electron-withdrawing substituents either attached to or incorporated into the molecular backbone.<br/>Here, we compare the performance of low-voltage n-channel organic TFTs based on three promising small-molecule semiconductors having LUMO energies between approximately -4.0 to -4.5 eV, namely N,N’-bis(2,2,3,3,4,4,4-fluorobutyl)-(1,7 & 1,6)-dicyano-perylene-tetracarboxylic diimide (ActivInk N1100; Angew. Chem. Int. Ed. 43, 6363, 2004), 2,9-bis(heptafluoropropyl)-4,7,11,14-tetrabromo-1,3,8,10-tetraazaperopyrene (TAPP-Br₄; Adv. Funct. Mater. 23, 3866, 2013) and diphenylethyl-3,4,9,10-benzo[de]isoquinolino[1,8-gh]quinolinetetracarboxylic diimide (PhC₂-BQQDI; Sci. Adv. 6, eaaz0632, 2020). The TFTs were fabricated on flexible polyethylene naphthalate (PEN) substrates or on silicon substrates using a hybrid gate dielectric composed of a thin, plasma-grown aluminum oxide layer and an <i>n</i>-tetradecylphosphonic acid self-assembled monolayer. TFTs were fabricated either in the inverted staggered (bottom-gate, top-contact) or the inverted coplanar (bottom-gate, bottom-contact) device architecture, using gold for the source and drain contacts. In the coplanar TFTs, the surface of the source and drain contacts was functionalized with a chemisorbed monolayer of one of four different thiols, namely 4-methylbenzenethiol (MeTP), 4-(methylsulfanyl)-thiophenol (MeSTP), 4-methoxythiophenol (MeOTP) and benzyl mercaptane (BM) to improve the charge exchange between the contacts and the semiconductor. The semiconductors were deposited by thermal sublimation in vacuum. All electrical measurements were performed in ambient air<br/>For each of the three semiconductors, the TFTs fabricated in the coplanar device architecture were found to have a smaller contact resistance (measured using the transmission line method) than the staggered TFTs; this finding is consistent with results reported previously for low-voltage p-channel organic TFTs (Nature Commun. 10, 1119, 2019). Regardless of the semiconductor, the best TFT performance was obtained by functionalizing the contacts with MeSTP. In addition, PhC₂-BQQDI and N1100 were found to provide smaller contact resistance and larger effective carrier mobility than TAPP-Br₄. Flexible coplanar PhC₂-BQQDI TFTs with MeSTP-functionalized contacts have an intrinsic channel mobility up to 0.6 cm²/Vs, an on/off current ratio up to 10⁶, a subthreshold slope as small as 100 mV/decade, and a contact resistance as small as 90 Ohm-cm (a record for n-channel organic TFTs). These results are important in view of the realization of low-voltage organic complementary circuits for low-power flexible electronics applications.