Flexible n-Channel Organic Transistors with Low Contact Resistance

Three promising small-molecule organic semiconductors, namely, 2,9-bis(heptafluoropropyl)-4,7,11,14-tetrabromo-1,3,8,10-tetraazaperopyrene (TAPP-Br₄), N,N'-bis(2,2,3,3,4,4,4-fluorobutyl)-(1,7 and 1,6)-dicyano-perylene-tetracarboxylic diimide (ActivInk N1100), and diphenylethyl-3,4,9,10-benzo[de]isoquinolino[1,8-gh]quinolinetetracarboxylic diimide (PhC₂-BQQDI), are employed for the fabrication of n-channel thin-film transistors (TFTs). The TFTs are fabricated on rigid silicon and flexible polymeric substrates, in both the inverted staggered (bottom-gate, top-contact) and the inverted coplanar (bottom-gate, bottom-contact) device architecture. In the coplanar TFTs, the source and drain contacts are functionalized with one of four different thiols to minimize contact resistance. The best performance is obtained for PhC₂-BQQDI TFTs with bottom contacts functionalized using 4-(methylsulfanyl)-thiophenol (MeSTP). For these TFTs, we measure contact resistances of 130 Ω cm for TFTs fabricated on silicon substrates and 210 Ω cm for TFTs on flexible polymeric substrates; these are the smallest contact resistances reported to date for n-channel organic TFTs, despite the fact that the measurements are carried out in ambient air. The observation that functionalizing the contacts with MeSTP leads to the smallest contact resistance is consistent with the fact that MeSTP provides the lowest effective work function. The TFTs fabricated on flexible polymeric substrates showed excellent bending stability.