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): mu = 0.32 cm2 V(-1) s(-1), Vth = 55 V, I(on)/I(off) = 10(5); N,N'-bis(n-octyl)-1,7- and N,N'-bis(n-octyl)-1,6-dibromoperylene-3,4:9,10-bis(dicarboximide) (PDI-8Br2): mu = 3 x 10(-5) cm2 V(-1) s(-1), Vth = 62 V, I(on)/I(off) = 10(3); N,N'-bis(n-octyl)-1,6,7,12-tetrachloroperylene-3,4:9,10-bis(dicarboximide) (PDI-8Cl4): mu = 4 x 10(-3) cm2 V(-1) (s-1), Vth = 37 V, I(on)/I(off) = 10(4); N,N'-bis(n-octyl)-2-cyanonaphthalene-1,4,5,8-bis(dicarboximide) (NDI-8CN): mu = 4.7 x 10(-3) cm2 V(-1) s(-1), Vth = 28, I(on)/I(off) = 10(5); N,N'-bis(n-octyl)-1,7- and N,N'-bis(n-octyl)-1,6-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2): mu = 0.13 cm2 V(-1) s(-1), Vth = -14 V, I(on)/I(off) = 10(3); and N,N'-bis(n-octyl)-2,6-dicyanonaphthalene-1,4,5,8-bis(dicarboximide) (NDI-8CN2): mu = 0.15 cm2 V(-1) s(-1), Vth = -37 V, I(on)/I(off) = 10(2). 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 I(off) 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 approximately -0.1 V (vs SCE). OFET threshold voltage shifts between vacuum and ambient atmosphere operation suggest that, at E(red1) < -0.1 V, the interfacial trap density increases by greater than approximately 1 x 10(13) cm(-2), while, for semiconductors with E(red1) > -0.1 V, the trap density increase is negligible. OFETs fabricated with the present n-type materials having E(red1) > -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 approximately 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 E(red1) < -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.

Taking up the semiconducting baton: Organic field‐effect transistors fabricated with a new class of extremely electron‐deficient cyanated perylene diimides are air‐stable and exhibit n‐type mobilities as high as 0.64 cm2 V−1 s−1 (see scheme). Devices can be fabricated from vapor‐deposited and solution‐cast films as well as top‐ and bottom‐contact electrode configurations.

We report on two multi-chromophore building blocks that self-assemble in solution and on surfaces into supramolecular light-harvesting arrays. Each building block is based on perylene-3,4:9,10-bis(dicarboximide) (PDI) chromophores. In one building block, N-phenyl PDI chromophores are attached at their para positions to both nitrogens and the 3 and 6 carbons of pyromellitimide to form a cross-shaped molecule (PI-PDI(4)). In the second building block, N-phenyl PDI chromophores are attached at their para positions to both nitrogens and the 1 and 7 carbons of a fifth PDI to produce a saddle-shaped molecule (PDI(5)). These molecules self-assemble into partially ordered dimeric structures (PI-PDI(4))(2) and (PDI(5))(2) in toluene and 2-methyltetrahydrofuran solutions with the PDI molecules approximately parallel to one another primarily due to pi-pi interactions between adjacent PDI chromophores. On hydrophobic surfaces, PDI(5) grows into rod-shaped nanostructures of average length 130 nm as revealed by atomic force microscopy. Photoexcitation of these supramolecular dimers in solution gives direct evidence of strong pi-pi interactions between the excited PDI chromophore and other PDI molecules nearby based on the observed formation of an excimer-like state in <130 fs with a lifetime of about 20 ns. Multiple photoexcitations of the supramolecular dimers lead to fast singlet-singlet annihilation of the excimer-like state, which occurs with exciton hopping times of about 5 ps, which are comparable to those observed in photosynthetic light-harvesting proteins from green plants.

A series of eight perylene diimide (PDI)‐ and naphthalene diimide (NDI)‐based organic semiconductors was used to fabricate organic field‐effect transistors (OFETs) on bare SiO2 substrates, with the substrate temperature during film deposition (Td) varied from 70–130 °C. For the N,N′‐n‐octyl materials that form highly ordered films, the mobility (µ) and current on‐off ratio (Ion/Ioff) increase slightly from 70 to 90 °C, and remain relatively constant between 90 and 130 °C. Ion/Ioff and µ of dibromo‐PDI‐based OFETs decrease with increasing Td, while films of N,N′‐1H,1H‐perfluorobutyl dicyanoperylenediimide (PDI‐FCN2) exhibit dramatic Ion/Ioff and µ enhancements with increasing Td. Increased OFET mobility can be correlated with higher levels of molecular ordering and minimization of film morphology surface irregularities. Additionally, the effects of SiO2 surface modification with trimethylsilyl and octadecyltrichlorosilyl monolayers, as well as with polystyrene, are investigated for N,N′‐n‐octyl dicyanoperylenediimide (PDI‐8CN2) and PDI‐FCN2 films deposited at Td = 130 °C. The SiO2 surface treatments have modest effects on PDI‐8CN2 OFET mobilities, but modulate the mobility and morphology of PDI‐FCN2 films substantially. Most importantly, the surface treatments result in substantially increased Vth and decreased Ioff values for the dicyanoperylenediimide films relative to those grown on SiO2, resulting in Vth > 0.0 V and Ion/Ioff ratios as high as 108. Enhancements in current modulation for these high‐mobility, air‐stable, and solution‐processable n‐type semiconductors, should prove useful in noise‐margin enhancement and further improvements in organic electronics.

No abstract