Organic field-effect transistors based on disubstituted perylene diimides: Effect of alkyl chains on the device performance

Article abstract of DOI:10.1016/j.mencom.2014.09.020

The preparation and systematic investigation of n-type semiconductor materials based on disubstituted perylene diimides (PDIs) possessing alkyl chains with variable length (from C₄ to C₁₂) are reported. It was shown that electrical characteristics of field-effect transistors can be tuned by changing the length of the alkyl chains in the PDIs used as semiconductor materials.

Full text of DOI:10.1016/j.mencom.2014.09.020

Mendeleev
Communications
Mendeleev Commun., 2014, 24, 306–307
Organic field-effect transistors based on disubstituted perylene diimides:
effect of alkyl chains on the device performance
Alexander V. Mumyatov, Lidiya I. Leshanskaya, Denis V. Anokhin, Nadezhda N. Dremova and Pavel A. Troshin*
Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka,
Moscow Region, Russian Federation. Fax: +7 496 522 3507; e-mail: troshin2003@inbox.ru
DOI: 10.1016/j.mencom.2014.09.020
The preparation and systematic investigation of n-type semiconductor materials based on disubstituted perylene diimides (PDIs)
possessing alkyl chains with variable length (from C4 to C12) are reported. It was shown that electrical characteristics of field-effect
transistors can be tuned by changing the length of the alkyl chains in the PDIs used as semiconductor materials.
R
Perylene-3,4,9,10-tetracarboxydiimides, which are also known
as perylene diimides (PDIs), were studied as industrial colorants
and pigments.1 These pigments have excellent chemical, thermal,
photo and weather stability, and they can be applied as high-
grade industrial paints.2 In addition, perylene diimides exhibit
high electron affinity and good charge transport properties. Thus,
they can be used as n-type semiconductors in organic field-effect
transistors (OFETs),3,4 organic solar cells,5 photoswitches,6 laser
dye applications7 etc.
O
O
O
N
O
O
Zn(OAc)2·H2O
+ 2H2O
+ 2RNH2
Quinoline,
reflux
O
O
O
N
N,N'-Diphenyl-3,4,9,10-perylenetetracarboxylic diimide was
among the first perylene diimides investigated in OFETs, which
showed a field-effect electron mobility of 1.5×10–5 cm2 V–1 s–1.8
The charge carrier mobility measured in similar devices based on
unsubstituted perylene diimide was almost one order of magnitude
lower (3.5×10–6 cm2 V–1 s–1).9 Perylene diimides bearing alkyl
substituents exhibited higher charge carrier mobility: 5.6×10–4,
0.1, 5.0×10–2, 0.6, 0.52, 2.1, 0.4 and 1.2 cm2 V–1 s–1 for dimethyl-
PDI,10 dipentyl-PDI,11 diheptyl-PDI,12 dioctyl-PDI,11 didodecyl-
PDI,11 ditridecyl-PDI,13 dibutadecyl-PDI,14 and dioctadecyl-PDI,14
respectively. Unfortunately, these values were obtained in OFETs
possessing different geometry, comprising different dielectric
layers and fabricated using different film deposition techniques.
Therefore, it is impossible to compare these data and reveal the
most efficient PDI derivative with an optimum alkyl chain length
suitable for the fabrication of high-performance OFETs.
Here, we report the preparation and characterization of disub-
stituted perylene diimides in order to reveal the effect of alkyl
substituents on the performance of these semiconductor materials
in OFETs.
The synthesis was carried out according to a known procedure
based on the condensation of perylene-3,4,9,10-tetracarboxylic
dianhydride with an appropriate aliphatic amine in a high-boiling
solvent (quinoline) with the addition of a catalytic amount of zinc
acetate (Scheme 1).15 The PDIs obtained in 80–90% isolated
yields were purified by multiple gradient vacuum sublimation.
Commercial N,N'-dimethyl-3,4,9,10-perylenetetracarboxylic
diimide 1 (Aldrich) was also purified by sublimation.
Organic field-effect transistors comprising PDIs as semicon-
ductor materials were fabricated using the following procedure:
The 40×60 mm glass slides were cleaned by sonication in a base
piranha solution (a mixture of hydrogen peroxide and ammonia).
Then, aluminum gate electrodes (~100 nm) were deposited on
the glass slides by thermal vacuum evaporation. Thin and compact
dielectric layers of aluminum oxide were formed on the surface of
the gate electrodes by the electrochemical oxidation of aluminum
in a potentiostatic regime at 10 V according to a procedure reported
O
O
R
2–9
2 R = C4H9
3 R = C5H11
4 R = C6H13
5 R = C7H15
6 R = C8H17
7 R = 2-ethylhexyl
8 R = C10H21
9 R = C12H25
Scheme 1
previously.16 The passivation organic layer was deposited on the
surface of the dielectric oxide by spin-coating a solution of benzo-
cyclobutene-silicon resin (BCB) (concentration of ~4 mg ml–
)
1
and the subsequent annealing at 250°C overnight in an argon
atmosphere. Organic semiconductor material (PDI film, ~100 nm)
was deposited by thermal evaporation in a vacuum of ~10–6 mbar
at a rate of 2–3 Å s–1. Finally, silver source and drain electrodes
(80 nm) were deposited on the top of the organic semiconductor
by thermal evaporation. Figure 1 shows the geometry of the
fabricated device.
The current–voltage characteristics of the OFET devices were
analyzed using a Keithley 2612A source measurement unit.
Figure 2 shows the representative transfer and output charac-
teristics of an OFET based on N,N'-didodecyl-3,4,9,10-perylene-
tetracarboxylic diimide 9. Thus, the transistor works at a low
voltage (6 V), and the current on/off ratio reaches 103–104.
The charge carrier mobilities of organic field-effect transistors
were calculated from transfer characteristics in a saturation
regime according to a standard method17 (Table 1). The electrical
characteristics of OFETs depend on the length of alkyl chains
attached to the PDI core. An analysis of these data allowed us to
Source
Semiconductor (PDI)
Drain
Substrate
Dielectric (Al2O3 + BCB)
Gate
Figure 1 Top contact device structure of a field-effect transistor.
© 2014 Mendeleev Communications. All rights reserved.
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Mendeleev Commun., 2014, 24, 306–307
Table 1 Electrical characteristics of OFETs (VDS = 6 V).
10–6
10–7
10–8
10–9
10–10
10–11
(a)
IDS
PDI
VGS/V
VTh/V
Ion/Ioff
me/10–3 cm2 V–1 s–1
1
2
3
4
5
6
7
8
9
–1.0–6.0
–1.5–6.0
–1.5–6.0
–1.5–6.0
–1.2–6.0
–1.0–6.0
–1.2–6.0
–1.0–6.0
–1.0–6.0
–0.7
–0.1
0.6
1.8
1.2
1.5
1.8
0.5
1.6
960
2700
764
10000
11000
4800
466
0.56
6.0
1.8
6.7
19.0
17.0
0.47
32.0
73.2
IGS
–2 –1
0
1
2
3
4
5
6
7
25700
50000
VGS/V
(b)
6
5
4
3
2
1
VGS = 6 V
VGS = 5 V
VGS = 4 V
VGS = 3 V
VGS = 2 V
VGS = 1 V
VGS = 0 V
solid thin films via molecular p–p stacking and alkyl–alkyl chain
interactions and electronic contacts between neighboring mole-
cules. A strong difference in the organization of C5-PDI (3) and
C10-PDI (8) thin films was found using scanning electron micro-
s
copy (SEM). Figure 4 shows that C5-PDI forms rod-like aggregates
with an average length of ~100 nm. On the contrary, C10-PDI
forms lamellas of approximately the same size. It is very likely
that lamellar aggregates possess better charge transport properties
compared to the rods due to unknown reasons (perhaps due to
better interconnections), which result in their superior performance
in OFETs. The observed different morphology of C5-PDI and
C10-PDI is most probably related to their different crystal packing.
In conclusion, we investigated eight different perylene diimides
as semiconductor materials for OFETs. It was shown that increase
in the length of the alkyl chains attached to the PDI core from C1 to
C12 improves both the charge carrier mobility and the on-off current
ratio of the devices. The revealed correlation between the molecular
structures of PDIs and their performances in OFETs might be
useful in the design of advanced materials for organic electronics.
0
–1
0
1
2
3
4
5
6
7
VDS/V
Figure 2 (a) Transfer and (b) output characteristics of an OFET based on
N,N’-didodecyl-3,4,9,10-perylenetetracarboxylic diimide 9.
find correlations between the molecular structure of the semicon-
ductor materials and their performance in OFETs. Figure 3(a)
shows that the charge carrier mobility increases on going from
compounds with short chains (C1–C5) to PDIs bearing long alkyl
substituents (C10–C12). It is also notable that PDI 7 bearing
branched 2-ethylhexyl side chains showed inferior performance
compared to the isomeric PDI 6 with linear n-octyl chains. It is
most likely that the branched side chains induce a stronger disorder
in the PDI films, which results in hindered charge transport and
poor semiconductor performance.
This work was supported by the Russian Ministry of Educa-
tion and Science (contract no. 11.G34.31.0055) and the Russian
Foundation for Basic Research (grant no. 12-03-31698).
The experimental data indicate that the length of the alkyl
substituents has a strong effect on the electrical performance of
PDI derivatives in OFETs. Charge transport in organic semi-
conductors strongly depends on the packing of molecules in thin
films and on the strength of intermolecular electronic interactions.
It is likely that alkyl chains affect both the ordering of PDIs in
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Received: 23rd December 2013; Com. 13/4275
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