Tuning Orbital Energetics in Arylene Diimide Semiconductors. Materials Design for Ambient Stability of n-Type Charge Transport

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Abstract
Structural and electronic criteria for ambient stability in n-type organic materials for organic field-effect transistors (OFETs) are investigated by systematically varying LUMO energetics and molecular substituents of arylene diimide-based materials. Six OFETs on n+-Si/SiO2 substrates exhibit OFET response parameters as follows: N,N‘-bis(n-octyl)perylene-3,4:9,10-bis(dicarboximide) (PDI-8): μ = 0.32 cm2 V-1 s-1, Vth = 55 V, Ion/Ioff = 105; N,N‘-bis(n-octyl)-1,7- and N,N‘-bis(n-octyl)-1,6-dibromoperylene-3,4:9,10-bis(dicarboximide) (PDI-8Br2): μ = 3 × 10-5 cm2 V-1 s-1, Vth = 62 V, Ion/Ioff = 103; N,N‘-bis(n-octyl)-1,6,7,12-tetrachloroperylene-3,4:9,10-bis(dicarboximide) (PDI-8Cl4): μ = 4 × 10-3 cm2 V-1 s-1, Vth = 37 V, Ion/Ioff = 104; N,N‘-bis(n-octyl)-2-cyanonaphthalene-1,4,5,8-bis(dicarboximide) (NDI-8CN): μ = 4.7 × 10-3 cm2 V-1 s-1, Vth = 28, Ion/Ioff = 105; N,N‘-bis(n-octyl)-1,7- and N,N‘-bis(n-octyl)-1,6-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2): μ = 0.13 cm2 V-1 s 1, Vth = −14 V, Ion/Ioff = 103; and N,N‘-bis(n-octyl)-2,6-dicyanonaphthalene-1,4,5,8-bis(dicarboximide) (NDI-8CN2): μ = 0.15 cm2 V-1 s-1, Vth = −37 V, Ion/Ioff = 102. Analysis of the molecular geometries and energetics in these materials reveals a correlation between electron mobility and substituent-induced arylene core distortion, while Vth and Ioff are generally affected by LUMO energetics. Our findings also indicate that resistance to ambient charge carrier trapping observed in films of N-(n-octyl)arylene diimides occurs at a molecular reduction potential more positive than ∼ −0.1 V (vs SCE). OFET threshold voltage shifts between vacuum and ambient atmosphere operation suggest that, at Ered1 < −0.1 V, the interfacial trap density increases by greater than ∼1 × 1013 cm-2, while, for semiconductors with Ered1 > −0.1 V, the trap density increase is negligible. OFETs fabricated with the present n-type materials having Ered1 > −0.1 V operate at conventional gate biases with minimal hysteresis in air. This reduction potential corresponds to an overpotential for the reaction of the charge carriers with O2 of ∼0.6 V. N,N‘-1H,1H-Perfluorobutyl derivatives of the perylene-based semiconductors were also synthesized and used to fabricate OFETs, resulting in air-stable devices for all fluorocarbon-substituted materials, despite generally having Ered1 < −0.1 V. This behavior is consistent with a fluorocarbon-based O2 barrier mechanism. OFET cycling measurements in air for dicyanated vs fluorinated materials demonstrate that energetic stabilization of the charge carriers results in greater device longevity in comparison to the OFET degradation observed in air-stable semiconductors with fluorocarbon barriers.