A R T I C L E S
Weitz et al.
amples are n-channel FETs based on pentacene11 and poly(9,9-
dioctylfluorene) (ref 1) that employ low-workfunction metal
contacts and a dielectric interface with a reduced density of
active electron traps. Generally, all of these transistors operate
properly only in a vacuum or in an inert atmosphere, apparently
because certain species present in ambient air, such as oxygen,
easily diffuse into the semiconductor and destabilize or trap the
negative charge carriers in the channel. (This does not mean
that the organic semiconductor is destroyed by oxygen exposure;
devices typically recover upon returning to an inert ambient.)
A third strategy that has been developed and has led to the
demonstration of organic n-channel FETs that can be operated
in air is the functionalization of conjugated compounds with
fluorine-rich substituents. This was first reported by Bao and
co-workers for copper phthalocyanine (CuPc).12 FETs based on
CuPc only show p-channel operation, whereas FETs based on
the perfluorinated counterpart, F16CuPc, operate as n-channel
FETs in air with a mobility of 0.03 cm2/Vs and excellent long-
term stability.13,14
ever, although these materials have demonstrated n-channel FET
operation, not all of them operate in air. It appears that fluorine
functionalization generally helps in obtaining n-channel FETs
but that it promotes air stability only for certain types of
materials (such as CH2C6H4CF3-functionalized naphthalene
carboxylic diimide FETs that show an n-channel mobility of
0.12 cm2/Vs both in vacuum and in air15) but not for others
(such as FETs based on perfluorinated pentacene28 or fluoro-
alkyl-oligothiophenes25). The exact mechanism for this remains
unclear, especially because in some cases fluorine functional-
ization appears to actually reduce the mobility. For example,
FETs based on CH2(CF2)6CF3-substituted naphthalene carboxy-
lic diimide have shown n-channel mobilities that are three times
smaller than the n-channel mobilities of FETs based on the
(CH2)7CH3-substituted derivatives (∼0.05 cm2/Vs vs 0.16 cm2/
Vs, both measured in vacuum).15
Recently, Jones et al. synthesized two perylene carboxylic
diimide derivatives with a cyano-functionalized perylene core
and obtained n-channel FETs with excellent performance in
air.29 One of the compounds was end-functionalized with
cyclohexane groups and showed a mobility of 0.1 cm2/Vs,
whereas the other was end-functionalized with fluoroalkyl
substituents and exhibited a mobility of 0.6 cm2/Vs. Apparently,
functionalizing only the perylene core with strongly electron-
This raises the question about the mechanism by which the
fluorine-containing substituents promote air stability. Fluorine
functionalization to create organic semiconductors for n-channel
FETs works for other material systems as well, for example,
for naphthalene carboxylic diimides,15-17 perylene carboxylic
diimides,18,19 oligothiophenes,20-26 and for pentacene.27,28 How-
withdrawing groups (e.g., cyano groups29 or chlorine groups30,31
)
is sufficient to induce air stability, whereas the additional
functionalization of the diimide groups with electron-withdraw-
ing substituents promotes a further increase of the mobility in
air. In the case of fluoroalkyl-substituted naphthalene and
perylene carboxylic diimide derivatives, the observation of air
stability has been attributed to the close packing of the
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