Page 326 - Organic Electronics in Sensors and Biotechnology

Organic Electronics in Memories and Sensing Applications 303

fused benzene rings. Pentacene has been widely studied as a p-type
semiconductor for OFETs. The highest field-effect mobility obtained
2
15
is 3 cm /(V . s) for pentacene thin films on polymer dielectrics and
16
6 cm /(V . s) on chemically modified SiO /Si substrates.
2
2
Substituted acenes based on silylethynyl, 17–19 such as 6,13-bis
(triisopropylsilylethynyl) (TIPS) pentacene and triethylsilylethynyl
anthradiophene (TES-ADT), not only are highly soluble but also
exhibit high crystallinity with mobility as high as 1 cm /(V . s). TES-
2
ADT as-deposited film is known to be amorphous and subsequent to
19
annealing-induced controlled crystallization of the thin films. This
shows enormous progress in the direction of solution-processed
organic semiconductors for OFETs.
n-Type Semiconductors
As far as n-type semiconductors are concerned, there are still only a
few which show high charge mobilities as compared to p-type semi-
conductors. Among them naphthalene/perylene derivatives, copper
perfluorophthalocyanine, and fluoroalkyl-substituted oligothiophene
are promising (see Fig. 8.2). Among all n-type oligomers such as N, N-
dioctyl-3,4,9,10-perylenetetracarboxylic diimide have shown the high-
est mobilities 0.6 to 1.7 cm /(V . s). Quinode oligomers such as terthio-
2
phene-based quinodimethane stabilized by dicyanomethylene groups
2
at each end show large electron mobilities up to 0.2 cm /(V . s) with
high on/off ratios. However, none of these materials have been applied
so far in devices with the exception of devices for the measurement of
charge carrier mobilities. Among the fullerenes, C is well known to
60
have high electron mobilities up to 6 cm /(V . s) depending on the crys-
2
tallinity of the film. Based on these high mobility C thin-film devices,
60
seven-stage ring oscillators were demonstrated with an operating fre-
quency as high as 50 kHz. These devices have been operated only
20
under inert conditions. Small-molecule organic semiconductors can
also be rendered solution-processable by attachment of flexible side
chains. Among the fullerene derivatives, phenyl C -butyric acid
61
2
methyl ester (PCBM) have mobilities as high as 0.2 cm /(V . s) when the
21
film is solution-cast on polymeric dielectrics.
Design of molecules for electron transport with increased elec-
tron affinity has been achieved through substitution with electron-
egative elements to allow efficient electron injection into the lowest
occupied molecular orbital (LUMO) level and to increase hydropho-
bicity. Fluorinated compounds are more air-stable, for example,
F CuPc, perfluoropentacene. Interestingly there are plenty of n-type
16
organic semiconductors with unpublished mobilities (see Fig. 8.3).
Among polymeric n-type semiconductors, ladder-type polymers
such as BBL have shown electron mobilities of 0.1 cm /(V . s). A poly-
2
mer synthesized by Stille coupling of N, N'-dialkyl-1,7-dibromo-
3,4,9,10-perylene diimide with a distannyl derivative of dithienothi-
ophene has shown mobilities of 1.3 × 10 cm /(V . s) (see Fig. 8.2).
−2
2