pubs.acs.org/cm Published on Web 10/28/2009 r 2009 American Chemical Society 5508 Chem. Mater. 2009, 21, 5508–5518 DOI:10.1021/cm902531d Interplay between Energetic and Kinetic Factors on the Ambient Stability of n-Channel Organic Transistors Based on Perylene Diimide Derivatives Joon Hak Oh, † Ya-Sen Sun, † R€ udiger Schmidt, § Michael F. Toney, ‡ Dennis Nordlund, ‡ Martin K€ onemann, ) Frank W€ urthner, § and Zhenan Bao* ,† † Department of Chemical Engineering, Stanford University, 381 North South Mall, Stanford, California 94305, ‡ Stanford Synchrotron Radiation Lightsource, Stanford Linear Accelerator Center, 2575 Sand Hill Road, M/S 69, Menlo Park, California 94025, § Universit € at W€ urzburg, Institut f € ur Organische Chemie and R€ ontgen Research Center for Complex Material Systems, Am Hubland, D-97074 W€ urzburg, Germany, and ) BASF SE, GVP/C, D-67056 Ludwigshafen, Germany Received August 17, 2009. Revised Manuscript Received October 8, 2009 The effects of the interplay between energetic and kinetic factors on the air stability of n-channel organic thin-film transistors (OTFTs) were studied using two perylene diimide (PDI) compounds with distinctly different lowest unoccupied molecular orbital (LUMO) levels. On the basis of the empirical energy level windows, one compound (N,N 0 -bis(2,2,3,3,4,4,5,5,5-nonafluoropentyl)- 3,4:9,10-tetracarboxylic acid diimide (PDI-F): -3.84 eV) is at the onset region for air stability, whereas the other (N,N 0 -bis(cyclohexyl)-1,7-dicyano-perylene-3,4:9,10-tetracarboxylic acid diimide (PDI-CN 2 ): -4.33 eV) is in the air-stable region. Charge-transport behaviors under an inert atmosphere and in air were investigated as a function of active layer thickness. Charge transport in air was greatly affected by the active layer thickness for both compounds, an effect that has been overlooked so far. The ambient stability of the air-unstable PDI-F TFTs increased significantly for thicknesses over ∼10 monolayers (ML). Surprisingly, the previously considered “air-stable” PDI- CN 2 TFTs were not stable in air if the active layer thickness was less than ∼4 ML. The molecular packing and orientation of the PDI thin films were investigated using grazing incidence X-ray diffraction (GIXD) and near-edge X-ray absorption fine structure (NEXAFS). We found that the minimum thickness required for air stability is closely related to the LUMO level, film morphology, and film growth mode. Introduction Understanding and controlling charge transport in organic thin-film transistors (OTFTs) has been an active research area during the past decade. This has proven to critically depend on the packing of the organic molecules, degree of order in the solid state, energetic barriers at electrode contacts, as well as on the semiconductor/di- electric interfacial properties. 1,2 Despite their intrinsic potential as ambipolar semiconductors with both p-type (hole transporting) and n-type (electron transporting) conductions, 3 the majority of organic semiconductors practically exhibit unipolar transport, typically p-type transport, because the energetically high-lying LUMO in most organic semiconductors hinders the efficient injection of electrons from the contact to the LUMO. Moreover, electron charge carriers are vulnerable to trap- ping by ambient oxidants, such as O 2 ,H 2 O, or O 3 . 3-6 Therefore, one major challenge in OTFT research is to develop high performance n-channel organic semicon- ductors; these are essential for complementary circuits offering low power consumption, high operating speed, and device stability against operational parameter varia- tions and external noise. 7,8 The strategies to stabilize field-induced electron charge carriers in OTFTs can be classified into three categories: (i) semiconductor design to lower the LUMO level, (ii) electrode contact modification, and (iii) interface engi- neering between the dielectric and semiconductor. The first strategy, based on molecular orbital energetics, is to make π-conjugated cores of organic semiconductors electron-deficient (electron-accepting) by substitution with strong electron-withdrawing and/or hydrophobic *Corresponding author. E-mail: zbao@stanford.edu. (1) Bao, Z.; Locklin, J. Organic Field-Effect Transistors; Taylor: Boca Raton, FL, 2007; Vol. 1. (2) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Bredas, J.-L. Chem. Rev. 2007, 107, 926–952. (3) Chua, L. L.; Zaumseil, J.; Chang, J. F.; Ou, E. C. W.; Ho, P. K. H.; Sirringhaus, H.; Friend, R. H. Nature 2005, 434, 194–199. (4) Weitz, R. T.; Amsharov, K.; Zschieschang, U.; Villas, E. B.; Goswami, D. K.; Burghard, M.; Dosch, H.; Jansen, M.; Kern, K.; Klauk, H. J. Am. Chem. Soc. 2008, 130, 4637–4645. (5) Jones, B. A.; Facchetti, A.; Wasielewski, M. R.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 15259–15278. (6) Chabinyc, M. L.; Street, R. A.; Northrup, J. E. Appl. Phys. Lett. 2007, 90, 123508. (7) Klauk, H.; Zschieschang, U.; Pflaum, J.; Halik, M. Nature 2007, 445, 745–748. (8) Crone, B.; Dodabalapur, A.; Lin, Y. Y.; Filas, R. W.; Bao, Z.; LaDuca, A.; Sarpeshkar, R.; Katz, H. E.; Li, W. Nature 2000, 403, 521–523.