High-performance single-crystal-based organic field-effect transistors from π-extended porphyrin derivatives

Article abstract of DOI:10.1002/chem.201100599

Single-crystal OFET: Single-crystals of new porphyrin derivatives TTPH2 and TTPZn were obtained from slow diffusion of solvents (see graphic). The organic field-effect transistor (OFET) devices based on these single crystals show relatively high mobilities of 6.2× 10^-2 cm² V ^-1 s^-1 for TTPH2 and 0.32 cm² V^-1 s^-1 for TTPZn with a high on-off current ratio of >10⁴. Copyright

Full text of DOI:10.1002/chem.201100599

DOI: 10.1002/chem.201100599
High-Performance Single-Crystal-Based Organic Field-Effect Transistors
from p-Extended Porphyrin ACTHNUGRTENUNGDerivatives
Mai Ha Hoang,[a] Youngmee Kim,[b] Sung-Jin Kim,[b] Dong Hoon Choi,[a] and
Suk Joong Lee*[a]
Significant research interests have recently emerged for
the development of new materials for organic field-effect
transistors (OFETs), due to their great promise for use in
electronic and optoelectronic applications.[1,2] Among them,
porphyrin derivatives are of particular interest because of
their unique properties in photonic and electronics.[3] As a
large and flat conjugated tetrapyrrole macrocycle that can
be decorated with a variety of metals and substituents, por-
phyrins have been widely used in solar energy conversion,
electron transfer, and artificial photosynthesis,[4] but they
have been relatively less exploited as building blocks for the
fabrication of OFETs.[5]
ing of building blocks at the molecular level that are closely
related with the performance of OFET.[1,2] Most of high per-
formance non-porphyrin organic semiconductors are based
on highly ordered molecular packing with strong p–p inter-
actions, showing high crystallinity. However, structural stud-
ies of these organic semiconductors have also rarely been
exploited due to the difficulties of single-crystal growth even
with small conjugated molecules, such as pentacene, ru-
brene, and anthracene derivatives. Nevertheless, recent ach-
ievement on porphyrin-based crystalline OFETs has been
successfully demonstrated, but showed relatively low device
performance.[8]
In fact, most of the organic semiconductors rely on the in-
herent structural uniqueness of building blocks, such as pla-
narity and p conjugation, and their packing and interactions
with adjoining building blocks. In this regard, porphyrins
would be one of the best candidates for organic transistors
because they could impart multiple inter- and/or intra-inter-
actions, such as hydrogen bonding, p–p stacking, electrostat-
ic interactions, as well as metal–ligand coordination that
could be generated by synthetic modification of the porphy-
rin framework.[6] Therefore, well-defined crystalline nano-
and micro-size structures, such as fibers, rods, ribbons,
plates, sheets, cubes, wheels, rings, and grids, are successfully
reported.[3,7]
However, the performance of OFET devices based on
porphyrin derivatives show relatively low carrier mobility,
with a range of 10À6 to 10À1 cm2 VÀ1 sÀ1, compared with their
inorganic analogues,[5] although they have great potentials as
mentioned above. Most of the porphyrin OFET devices are
based on thin films or crystalline objects prepared by spin-
coating or vacuum-deposition processes. Therefore, a deeper
interpretation of systems has hardly succeeded due to the
lack of information about the molecular structure and pack-
Herein, we report on single crystalline wires from p-ex-
tended porphyrin derivatives TTPH2 and TTPZn, and their
unique electronic properties in OFET devices along with
single-crystal structures.
TTPH2 was readily obtained in relatively high yield
(21%) by using a typical porphyrin condensation reaction
between
and pyrrole in propionic acid. TTPZn was further obtained
by metallation of TTPH2 with Zn(OAc)2 in 78% yield
4-((5-hexylthiophene-2-yl)ethynyl)benzaldehyde
AHCTUNGTRENNUNG
(Scheme 1). Detailed synthetic procedures are described in
the Supporting Information section (Scheme S1). Cyclic vol-
tammetry analysis of TTPH2 and TTPZn reveals that
HOMO, LUMO, and bandgap energy (Eg) are found to be
À5.33, À3.40, and 1.93 eV for TTPH2 and À5.28, À3.48, and
1.80 eV for TTPZn, respectively (Table S1). A relatively low
bandgap is found in metalloporphyrin TTPZn.
Upon layering solutions of TTPH2 and TTPZn in toluene
over hexanes and allowing the resulting mixtures to stand
over 7–14 days, narrowly dispersed single-crystalline wires
were obtained in both cases, which were around 300–500 mm
in length and about 4–15 mm in width identified by scanning
electron microscopy (SEM) analysis (Figure 1).
The single-crystal X-ray structures of TTPH2 and TTPZn
are shown in Figure 2. The porphyrin core in TTPH2 is
puckered with angles between the N1/N2/N3/N4 plane and
each pyrrol ring of 10.88 (N1/C2/C3/C4/C5), 7.28 (N2/C7/C8/
C9/C10), 11.68 (N3/C12/C13/C14/C15), and 7.68 (N4/C17/
C18/C19/C20) as shown in Figure 2a and b and Figure S1 in
the Supporting Information. Interestingly, the porphyrin
core in TTPZn is also puckered with angles between the
N1/N2/N3/N4 plane and each pyrrol ring of 6.8, 13.3, 9.1,
and 10.88 (Figure 2 f and g and Figure S3 in the Supporting
Information). Four phenyl rings are tilted from the N1/N2/
[a] Dr. M. H. Hoang, Prof. D. H. Choi, Prof. S. J. Lee
Department of Chemistry, Research Institute for Natural Sciences
Korea University, 5 Anam-dong, Sungbuk-gu, Seoul 136-701 (Korea)
Fax : (+82)2-924-3141
E-mail: slee1@korea.ac.kr
[b] Dr. Y. Kim, Prof. S.-J. Kim
Department of Chemistry and NanoScience
Ewha Womanꢀs University, Seoul 136-701 (Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100599.
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COMMUNICATION
and 27.7, 30.4, 34.4, and 49.98 for TTPZn. Great similarities
are found in the crystal structures and crystal packing in
both systems. In the packing diagram representations, the
porphyrins adopt multi-layered arrangements (Figure 2c
and h) generating an array, which maximizes their van der
Waals interactions (Figures S2 and S4 in the Supporting In-
formation). The second layer is stacked in a slightly slipped
manner with respect to the first layer, and each layer is
piled up in a zig-zag fashion (ababa type) in both TTPH2
and TTPZn crystals (Figure 2e and j). However, the distan-
ces between the layers are quite different and are found to
be 3.81 ꢂ for TTPH2 and 3.65 ꢂ for TTPZn.
To investigate the electrical characteristics of individual
single crystals of TTPH2 and TTPZn, OFETs (bottom-gate,
top-contact) were fabricated on a SiO2/Si substrate with N-
doped polycrystalline silicon as a gate electrode and an n-
octyltrichlorosilane (OTS)-treated SiO2 surface layer as a di-
electric gate insulator followed by deposition of gold elec-
trodes using shadow masks. The channel lengths are 110 mm
for TTPH2 and 103 mm for TTPZn. The devices were dried
in vacuum at room temperature for 24 h before testing them
at room temperature and in air.
The resulting single-crystal OFET devices show typical p-
channel FET characteristics as shown in Figure 3. The
output characteristics show very good saturation behavior
and clear saturation currents that are quadratically related
to the gate bias, even using Au-based source/drain electro-
des with a relatively large electron injection barrier.[9]
The carrier mobility (m) was calculated by using the satu-
ration region transistor equation, IDS =(W/2L)mC0CAHTUNGTRENNNUG
(VGÀVt)2,
in which IDS is the source-drain current, VG the gate voltage,
C0 the capacitance per unit area of the dielectric layer, and
Vt the threshold voltage.[10] Experimental results indicate
that the devices fabricated from single crystal of TTPH2
and TTPZn display excellent OFET performances. The car-
rier mobilities are 6.2ꢃ10À2 cm2 VÀ1 sÀ1 for TTPH2 and
0.32 cm2 VÀ1 sÀ1for TTPZn with a high on–off current ratio
of >104 (Figure 3). To the best of our knowledge, these
values are among the best for porphyrin-based OFET devi-
ces.[5] Since the mobility is highly dependent on the stacking
distance between building blocks in the solid state, the
single-crystal device of TTPZn, which has the shorter pack-
ing distance between layers, shows much better device per-
formance than that of TTPH2 evidenced by single-crystal
X-ray structure analysis. The smaller bandgap (Eg) of
TTPZn (Eg =1.80 eV) compared with TTPH2 (Eg =1.93 eV)
further supports that the Zn metallation could make the
porphyrin system behave as a better FET device.
Scheme 1. Porphyrin building blocks.
For comparison, we have also investigated the performan-
ces of the corresponding film devices prepared by spin coat-
ing the solutions of TTPH2 and TTPZn in chloroform. In-
terestingly, the device performances drop down dramatically
and the observed carrier mobilities (m) are in the range of
about 2.3ꢃ10À4–3.5ꢃ10À4 cm2 VÀ1 sÀ1 (Figure S7 and S8 in
the Supporting Information). These dramatic drops in mobi-
lity are presumably due to a decrease of their molecular
order upon haphazard aggregations of porphyrins during the
Figure 1. SEM (a and b) and optical microscopy (c and d) images of
single-crystalline wires obtained from TTPH2 (a and c) and TTPZn (b
and d).
N3/N4 porphyrin core plane by 52.7, 79.3, 53.7, and 80.88
for TTPH2, and by 47.3, 77.7, 48.1, and 80.48 for TTPZn.
Four thiophene rings are also tilted with respect to the por-
phyrin core plane by 20.4, 25.7, 32.8, and 43.38 for TTPH2,
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S. J. Lee et al.
shows two bands at 658 and
720 nm, whereas two bands at
664 and 724 nm are observed
for the solution sample of
TTPH2 (Figure S6a in the Sup-
porting Information). There-
fore, insignificant configuration-
al changes are observed upon
film formation. However, in
case of TTPZn, the extinction
spectrum of the film shows a
broadened and slightly redshift-
ed Sorꢄt band at 432 nm and
redshifted multiple
Q bands
around 557 nm. The PL spec-
trum of the film from TTPZn
shows one broad band at
646 nm showing the typical be-
havior of a closely aggregated
system, whereas two bands at
602 and 647 nm are observed
for the solution sample of
TTPZn (Figure S6b in the Sup-
porting Information). This ob-
servation suggests that TTPZn
is becoming much more tightly
packed upon film formation
and therefore better crystal
device performance is expected.
Differential scanning calori-
metric (DSC) measurements
further assist the densely
packed aggregation of the
TTPZn film. They exhibit dis-
tinct crystalline–isotropic transi-
tion temperatures of 263 and
3218C for TTPH2 and TTPZn
films, respectively (Figure S9 in
the Supporting Information). A
relatively high melting tempera-
ture is observed for the TTPZn
film that is strongly related to
the high density of tightly
packed metalloporphyrins.
Figure 2. Single-crystal X-ray structures and packing diagram representations of TTPH2 (a–e) and TTPZn (f–
i). Hydrogen atoms are omitted for clarity.
film formation. However, the UV/Vis absorption and photo-
luminescence (PL) spectra in solution and film forms of
TTPH2 and TTPZn show that TTPZn generates densely
packed aggregates in the solid state (Figure S6 in the Sup-
porting Information). The absorption spectra reveal slight
redshifts of around 5–8 nm upon the film formation in both
TTPH2 and TTPZn that indicate ineffective intermolecular
interactions. For example, the solution sample of TTPH2 ex-
hibits Sorꢄt bands at 426 nm with multiple Q bands around
474 nm, the extinction spectrum of the film from TTPH2 on
quartz glass plates shows a broadened and slightly redshifted
Sorꢄt band at 433 nm and redshifted multiple Q bands
around 522 nm. The PL spectrum of the film from TTPH2
However, carrier mobilities for both film systems of
TTPH2 and TTPZn show similar values that are much
lower compared with their crystal analogues. Although
TTPZn is providing heavily packed aggregation and low
bandgap, lack of molecular order in the film is responsible
for low mobility, which is similar to that of the TTPH2 film.
In summary, single crystals of new porphyrin derivatives
TTPH2 and TTPZn have been obtained from slow diffusion
of solvents. The OFET devices based on these single crystals
display excellent characteristics with high mobilities of 6.2ꢃ
10À2 cm2 VÀ1 sÀ1 for TTPH2 and 0.32 cm2 VÀ1 sÀ1 for TTPZn
with a high on–off current ratio of >104. Due to low crystal-
linity, their film device performances drop dramatically.
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High-Performance Organic Field-Effect Transistors
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Figure 3. Electrical characterization of single-crystal OFETs. Output (a) and transfer (b) curves of OFET device made of TTPH2 (Inset: SEM image of
device, VDS =À80 V), and output (c) and transfer (d) curves of OFET device made of TTPZn (Inset: SEM image of device, VDS =À110 V).
These new porphyrin derivatives have demonstrated that
porphyrin building blocks have strong potential for use in
electronic and optoelectronic applications.
Keywords: field-effect transistors
· molecular devices ·
porphyrins · semiconductors · thin films
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Received: February 23, 2011
Published online: May 26, 2011
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