An unsymmetrically π-extended porphyrin-based single-crystal field-effect transistor and its anisotropic carrier-transport behavior

Article abstract of DOI:10.1002/chem.201202894

Anisotropic charge transport: Single-crystal organic field-effect transistor devices derived from aggregates of thiophene-appended porphyrins display very high mobility (0.27 cm² V^-1 s^-1). This behavior is due to staircase stacking of the porphyrins with distances between layers of 3.17(7) A. Furthermore, the charge-transport behavior is anisotropic owing to an anisotropic molecular arrangement in the single-crystal microplates. Copyright

Full text of DOI:10.1002/chem.201202894

COMMUNICATION
DOI: 10.1002/chem.201202894
An Unsymmetrically p-Extended Porphyrin-Based Single-Crystal
Field-Effect Transistor and Its Anisotropic Carrier-Transport Behavior
Soojung Choi,[a] Seung Hyun Chae,[a] Mai Ha Hoang,[a] Kyung Hwan Kim,[a]
Jung A Huh,[a] Youngmee Kim,[b] Sung-Jin Kim,[b] Dong Hoon Choi,*[a] and
Suk Joong Lee*[a]
For the fabrication of electronic devices such as organic
field-effect transistors (OFETs), the use of well-ordered
crystalline thin films and well-defined single crystals (SCs)
are promising for the fabrication of charge-transporting
layers.[1] In this regard, the solubility of organic semiconduc-
tors has become one of the most important factors and
much attention has been focused on the development of
highly soluble materials because they should lead to scalable
and practical low-cost devices.[2] Most organic semiconduc-
tors involve planar p-conjugated structures because they
pack efficiently into well-ordered crystalline arrangements.
Considering the above, the unique structural features of por-
phyrins should make them useful building blocks for FETs.
Porphyrins are a focus of interest in the area of photonics
and electronics[3] and their properties can be altered by in-
troducing various substituents to the large flat conjugated
macrocyclic porphyrin ring.[4] Porphyrins have been used for
solar energy conversion, photoinduced charge transfer, and
artificial photosynthesis.[5] Despite their many structural ad-
vantages, porphyrin derivatives have been relatively less ex-
ploited as charge-transporting layers for OFET devices.[6]
For them to become good OFET materials, high crystallini-
ty, close p–p stacking, and structural planarity are required.
When they pack into well-ordered crystalline arrangements
in the charge-transporting layer, p-extended porphyrins are
tion; therefore, the migration of either electrons or holes
that are generated on the active layer would have a specific
directionality.
Herein, we describe p-extended porphyrin derivatives
TEPP and DTEPP and their unique electrical properties in
OFET devices; we also describe the anisotropic carrier-
transport behavior of single-crystal microplates of TEPP in
an OFET (SC-OFET) device as a function of the orienta-
tion of these microplates.
TEPP was readily obtained through a Sonogashira cou-
pling reaction between 2-ethynyl-5-hexylthiophene and
[5,15-dibromo-10,20-di(4-hexylphenyl)porphyrinato]zinc (II)
(see the Supporting Information, Scheme S1). Cyclic voltam-
metry analysis of TEPP reveals that the energy of the
HOMO and the LUMO, and the bandgap energy (Eg) are
À5.23, À3.43, and 1.80 eV respectively (see the Supporting
Information, Table S1), thus suggesting that it is a suitable
p-type semiconducting material for OFET fabrication.[8]
Upon layering solutions of TEPP in chloroform over hex-
anes and allowing the resulting mixtures to rest over 7–
14 days, narrowly dispersed single-crystal microplates are
obtained (see the Figure S2A).
The single-crystal X-ray structure of TEPP is shown in
Figure 1. The porphyrin core is nearly planar and two thio-
phene rings are almost coplanar with the porphyrin core.
The plane of the phenyl rings make an angle of 74.3(3)8
with the porphyrin plane (Figure 1A). In a packing diagram,
the molecules are stacked in a staircase fashion through the
interaction between the thiophene moieties and the ZnII ion
with a closest-layer distance of 3.17(7) ꢀ generating arrays
while maximizing van der Waals interactions (Figure 1).
These layers are built up in a zigzag fashion in the crystal
(Figure 1D).
highly beneficial because their structural planarity max
ACHTUNGTRENNUNG
ACHTUNGERTNiNUNG -
mizes p–p interactions, thus increasing carrier mobility.[7]
The use of unsymmetrically p-extended porphyrins may
lead to strong p–p interactions, which would promote the
formation of either well-defined crystalline films or single-
crystals owing to J-type aggregation along a certain direc-
[a] S. Choi, S. H. Chae, Dr. M. H. Hoang, Dr. K. H. Kim, J. A. Huh,
Prof. D. H. Choi, Prof. S. J. Lee
The absorption spectra reveal a slight red shift of 22 nm
upon TEPP film formation, thus indicating effective inter-
molecular interactions:[9] whereas the absorption spectrum
of a solution of TEPP exhibits Sorꢁt bands at 458 nm and
Q bands at around 650 nm, a film of TEPP on quartz glass
plates show a broadened and slightly red-shifted Sorꢁt band
at 480 nm and Q bands at around 675 nm. The PL spectrum
of the film shows an emission band at 725 nm, whereas that
of the solution shows a band at band at 671 nm (see the
Supporting Information, Figure S1A). Therefore, the high
degree of intermolecular interaction between the porphyrins
Department of Chemistry
Research Institute for Natural Sciences, Korea University
5 Anam-dong, Sungbuk-gu, Seoul 136-701 (Korea)
Fax : (+82)2-925-4284
E-mail: slee1@korea.ac.kr
dhchoi8803@korea.ac.kr
[b] Dr. Y. Kim, Prof. S.-J. Kim
Department of Chemistry and NanoScience
Ewha Womans University, Seoul 136-701 (Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202894.
Chem. Eur. J. 2013, 19, 2247 – 2251
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2247

Figure 1. Single-crystal X-ray structure and packing diagram representations of TEPP. (A) The structure with thermal ellipsoids at 50% probability;
(B) packing diagram looking down through a axis; (C) co-facial packing diagram with adjacent porphyrins showing clear J-aggregations with a layer dis-
tance of 3.17 ꢀ; (D) packing diagram with omission of hexyl groups, hydrogen atoms, and the disordered atoms for clarity.
is shown by film formation (Figure S1A), thus suggesting
that TEPP is tightly packed in the film, a characteristic that
leads to the expectation of enhanced device performance.
To investigate the electrical characteristics of individual
single crystal of TEPP, OFETs (bottom-gate and top-contact
transistors) were fabricated on SiO2/Si substrate with
N-doped polycrystalline silicon as the gate electrode and n-
octyltrichlorosilane (OTS)-treated SiO2 surface layer as the
dielectric gate insulator; gold electrodes were then deposit-
ed using shadow masks.[10] The film devices were prepared
by spin coating chloroform solutions of the porphyrins and
single-crystal devices were prepared by the slow diffusion of
chloroform solutions over hexanes. The devices were dried
under vacuum at room temperature for 24 hours before test-
ing them at room temperature in air. The output characteris-
tics showed very good saturation behavior and clear satura-
tion currents (Figure 2A and 2C). The carrier-mobility (m)
values were obtained by analyzing more than 20 different
devices and were calculated by using the saturation-region
0.02 cm2 VÀ1 sÀ1, together with high on/off current ratio (Ion/
off =1.0ꢃ105) and low threshold voltage (Vth ꢀÀ10 V). The
single-crystal OFETs exhibited a field-effect mobility of
0.27 cm2 VÀ1 sÀ1, together with high on/off current ratio (2.2ꢃ
103) and threshold voltages (Vth ꢀÀ2 V, Figure 2B). Re-
markably, the field-effect mobility of the TEPP single-crys-
tal devices was about 14 times higher than that of the film-
state devices. This relatively high field-effect mobility of
single-crystal devices is presumably due to the denser mo-
lecular arrangement[12] characterized by the close-packing
distance between porphyrin layers (3.17(7) ꢀ) and the effec-
tive J-aggregations shown by analysis of the single-crystal X-
ray structure (Figure 1). Differential scanning calorimetric
(DSC) measurements provided further evidence of the
densely packed aggregation of the TEPP film. DSC analysis
showed distinct crystalline–isotropic transition temperatures
of 1168C and 1118C (see the Supporting Information, Fig-
ure S6 and Table S3). The relatively high melting tempera-
ture is due to the tight packing of the porphyrin building
blocks.[13]
transistor equation, IDS =(W/2L)mC0CATHUNGTRENNNUG
(VGÀVth)2, where IDS is
the source-drain current, VG the gate voltage, C0 the capaci-
tance per unit area of the dielectric layer, and Vth the thresh-
old voltage.[11] According to the transfer characteristics (see
the Supporting Information, Figure S8B), the film devices
made of TEPP provided field-effect mobility of
Encouraged by results obtained from the analysis of
TEPP, the electrical properties of OFET devices derived
from a porphyrin with bithiophene units (DTEPP), which
has more extended p-conjugation, were investigated with
the anticipation that such a porphyrin would lead to aggre-
2248
www.chemeurj.org
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 2247 – 2251

Single-Crystal Field-Effect Transistor
COMMUNICATION
Figure 2. Electrical characterization of single-crystal OFET. Output (A) and transfer (B) curves of OFET devices made from TEPP; output (C) and
transfer (D) curves of OFET devices made from DTEPP (insets: optical-microscope images of devices, VDS =À120 V).
gates with stronger intermolecular p–p interactions and thus
devices with better performance. DTEPP was similarly syn-
thesized using a Sonogashira coupling reaction between 5’-
bromo-4-hexyl-5-methyl-2,2’-bithiophene[14] and [5,15-bis-
5.61ꢃ10À3 cm2 VÀ1 sÀ1 (Vth ꢀÀ13 V; see the Supporting Infor-
mation, Figure S8C and S8D). Unexpectedly, the field-
effect mobilities of DTEPP-based devices were about four
times lower than TEPP-based devices. The lower field-effect
mobility of DTEPP is presumably due to its lack of planari-
ty.[15] Although extension of p-conjugation was expected by
replacing the thiophene rings with bithiophene rings, the
planes of the rings of the latter are presumably tilted slightly
because of steric hindrance between the two adjacent hydro-
gen atoms at the 3 and 3’-positions,[15] thus hampering effec-
tive conjugation. Therefore, DTEPP has lower molecular
order and a less dense arrangement of molecular layers.
DSC measurements provided further evidence to support
the less dense molecular packing and poorer aggregation
properties of DTEPP; distinct crystalline–isotropic transi-
tions were not detected (see the Supporting Information,
Figure S6 and Table S3).
Remarkably, when a device was fabricated such that the
longitudinal axis of the single-crystal of TEPP was placed
90 degrees with respect to the source-to-drain direction, nei-
ther measurable outputs nor gate effects were observed, be-
havior that was characteristic of an insulator. Furthermore,
when a device in which the longitudinal axis of the single-
crystal of TEPP was placed 45 degrees with respect to the
source-to-drain direction, the resulting field-effect mobility
was only approximately 25% of value obtained from the
device in which the axis was 0 degrees with respect to the
source-to-drain direction (Figure 3). As has been clearly
demonstrated by single-crystal analysis, stacked J-aggregat-
ed arrays are effectively lined up along the longitudinal di-
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
energy of the HOMO and the LUMO, and the bandgap
energy (Eg) are À5.12, À3.37, and 1.75 eV, respectively (see
the Supporting Information, Table S1). Homogeneous
single-crystal needles were obtained after layering solutions
of porphyrins in chloroform over hexanes for 7–14 days (see
the Supporting Information, Figure S2B) but they were not
suitable enough to carry out single-crystal X-ray structural
analysis. The absorption spectra reveal a slight red shift of
about 21 nm upon DTEPP film formation, thus indicating
effective intermolecular interactions: the absorption spec-
trum of a solution of DTEPP exhibits Sorꢁt bands at 466 nm
and Q bands at around 670 nm, a film of DTEPP on quartz
glass plates show a broadened and slightly red-shifted Sorꢁt
band at 487 nm and Q bands at around 710 nm. The PL
spectrum of the solution sample shows a band at 692 nm,
whereas that of the film shows two bands at 752 and 831 nm
(see the Supporting Information, Figure S1B). Therefore,
significant J-aggregations are observed upon film formation.
Surprisingly, when devices derived from DTEPP were
tested for their performance, a single crystal of DTEPP
showed low field-effect mobility of 0.066 cm2 VÀ1 sÀ1 with
low threshold voltages (Vth ꢀÀ8 V) (Figure 2C and 2D),
whereas the film device showed a slightly high mobility of
Chem. Eur. J. 2013, 19, 2247 – 2251
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2249

S. J. Lee, D. H. Choi et al.
transport behavior was unam-
biguously confirmed in TEPP-
based single-crystal OFETs.
Experimental Section
Crystal
data
for
TEPP:
C68H24N4O0.25S2Zn, Mr =1030.40, Mon-
oclinic (C2/c), a=32.200(6), b=
10.045(2),
c=21.663(4) ꢀ,
b=
116.98(3)8,
V=6244(2) ꢀ3,
Z=4,
(mMoKa)=0.500 mmÀ1
,
16944 reflec-
tions measured, 6085 unique (Rint
=
0.0775) which were used in all calcula-
tions, final R=0.0873 (wR=0.2275)
with reflections having intensities
greater than 2s, GOF (F2)=0.999.
CCDC 888055 contains the supple-
mentary crystallographic data for
TEPP. These data can be obtained
free of charge from the Cambridge
Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
The authors acknowledge the financial
support from the Korea Research
Foundation Program (2012R1A2A1-
Figure 3. Anisotropic carrier-transport behavior of single-crystal OFET devices made from TEPP. Optical-mi-
croscope images of single-crystal devices under polarized light (left column); schematic representations of mo-
lecular packing in crystals (middle column); transfer curves (right column). The crystals are positioned such
that the longitudinal axis of the crystal is 08 (A), 458 (B), and 908 (C) with respect to the source-to-drain direc-
tion.
AHCTUNGTRENNUNG 01008797 and NRF20120002285) and
Priority Research Centers Program
(NRF20120005860) through the Na-
tional Research Foundation of Korea
(NRF) funded by the Ministry of Edu-
cation, Science and Technology
(MEST).
rection of a single crystal, an alignment that favors effective
Keywords: anisotropic behavior · carrier transport · field-
effect transistors · porphyrins · semiconductors
carrier transport; however, no efficient p-orbital overlap is
observed in the latitudinal direction of the crystal, thus im-
peding carrier mobility. Therefore, we have shown that
single-crystal devices derived from the unsymmetrically p-
extended porphyrin, TEPP, exhibit anisotropic carrier-trans-
port behavior.
[1] a) P. Gao, D. Beckmann, H. N. Tsao, X. Feng, V. Enkelmann, M.
Baumgarten, W. Pisula, K. Mꢄllen, Adv. Mater. 2009, 21, 213; b) Y.
Zhou, T. Lei, L. Wang, J. Pei, Y. Cao, J. Wang, Adv. Mater. 2010, 22,
1484.
[2] a) F. Silvestri, A. Marrocchi, M. Seri, C. Kim, T. J. Marks, A. Fac-
chetti, A. Taticchi, J. Am. Chem. Soc. 2010, 132, 6108; b) Z. Wei, W.
Hong, H. Geng, C. Wang, Y. Liu, R. Li, W. Xu, Z. Shuai, W. Hu, Q.
Wang, D. Zhu, Adv. Mater. 2010, 22, 2458.
[3] a) A. J. Jiang, D. K. P. Ng, Acc. Chem. Res. 2009, 42, 79; b) C. M.
Che, H. F. Xiang, S. S. Y. Chui, Z. X. Xu, V. A. L. Roy, J. J. Yan,
W. F. Fu, P. T. Lai, I. D. Williams, Chem. Asian J. 2008, 3, 1092.
[4] a) L. Zang, Y. Che, J. S. Moore, Acc. Chem. Res. 2008, 41, 1596;
b) S. Laschat, A. Baro, N. Steinke, F. Giesselmann, C. Hꢅgele, G.
Scalia, R. Judele, E. Kapatsina, S. Sauer, A. Schreivogel, M. Tosoni,
Angew. Chem. 2007, 119, 4916; Angew. Chem. Int. Ed. 2007, 46,
4832.
[5] a) C. M. Drain, A. Varotto, I. Radivojevic, Chem. Rev. 2009, 109,
1630; b) S. M. Vlaming, R. Augulis, M. C. A. Stuart, J. Knoester,
P. H. M. van Loosdrecht, J. Phys. Chem. B 2009, 113, 2273.
[6] a) P. B. Shea, C. Chen, J. Kanicki, L. R. Pattison, P. Petroff, H.
Yamada, N. Ono, Appl. Phys. Lett. 2007, 90, 233107; b) A. S. Dhoot,
S. Aramaki, D. Moses, A. Heeger, Adv. Mater. 2007, 19, 2914;
In summary, single crystals of new porphyrin derivatives
TEPP and DTEPP were prepared using the slow-diffusion
method. OFET devices based on single crystals of TEPP dis-
play high mobility (0.27 cm2 VÀ1 sÀ1) and an on/off current
ratio of greater than 103; however, single-crystal OFET de-
vices derived from DTEPP display lower mobilities
0.066 cm2 VÀ1 sÀ1. Although the replacement of thiophene
rings with bithiophene rings in going from TEPP to DTEPP
was expected to lead to extension of p-conjugation, the two
adjacent thiophene rings of the bithiophene moieties are not
coplanar, thus hampering efficient p-conjugation. The
TEPP-based single-crystal devices show significantly higher
mobility owing to the more compact molecular arrangement
with the closest p–p packing distance being 3.17(7) ꢀ be-
tween porphyrins and effective J-aggregation, as shown by
X-ray single-crystal structure analysis. Anisotropic carrier-
2250
www.chemeurj.org
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 2247 – 2251

Single-Crystal Field-Effect Transistor
COMMUNICATION
c) P. B. Shea, J. Kanicki, L. R. Pattison, P. Petroff, M. Kawano, H.
Yamada, N. Ono, J. Appl. Phys. 2006, 100, 034502.
[11] J. A. Merlo, C. R. Newman, C. P. Gerlach, T. W. Kelley, D. V.
Muyres, S. E. Fritz, M. F. Toney, C. D. Frisbie, J. Am. Chem. Soc.
2005, 127, 3997.
[12] M. D. Curtis, J. Cao, J. W. Kampf, J. Am. Chem. Soc. 2004, 126,
4318.
[13] Y. S. Kim, S. Y. Bae, K. H. Kim, T. W. Lee, J. A. Hur, M. H. Hoang,
M. J. Cho, S.-J. Kim, Y. Kim, M. Kim, K. Lee, S. J. Lee, D. H. Choi,
Chem. Commun. 2011, 8907.
[14] E. H. Cho, S. H. Chae, S. J. Lee, J. Joo, Synth. Met. 2012, 162, 813.
[15] a) M.-H. Yoon, A. Facchetti, C. E. Stern, T. J. Marks, J. Am. Chem.
Soc. 2006, 128, 5792; b) T. L. Scott, M. O. Wolf, B. O. Patrick, Can. J.
Chem. 2007, 85, 383.
[7] a) M. H. Hoang, Y. Kim, M. Kim, K. H. Kim, T. W. Lee, D. N.
Nguyen, S.-J. Kim, K. Lee, S. J. Lee, D. H. Choi, Adv. Mater. 2012,
24, 5363; b) M. H. Hoang, D. H. Choi, S. J. Lee, Synth. Met. 2012,
162, 419; c) M. H. Hoang, S. J. Kim, Y. Kim, D. H. Choi, S. J. Lee,
Chem. Eur. J. 2011, 17, 7772.
[8] M. L. Tang, J. H. Oh, A. D. Reichardt, Z. Bao, J. Am. Chem. Soc.
2009, 131, 3733.
[9] Y. H. Kim, D. H. Jeong, D. Kim, S. C. Jeoung, H. S. Cho, S. K. Kim,
N. Aratani, A. Osuka, J. Am. Chem. Soc. 2001, 123, 76.
[10] K. H. Kim, S. Y. Bae, Y. S. Kim, J. A. Hur, M. H. Hoang, T. W. Lee,
M. J. Cho, Y. Kim, J.-I. Jin, S.-J. Kim, K. Lee, S. J. Lee, D. H. Choi,
Adv. Mater. 2011, 23, 3095.
Received: August 13, 2012
Published online: December 27, 2012
Chem. Eur. J. 2013, 19, 2247 – 2251
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2251