Symmetrically functionalized diketopyrrolopyrrole with alkylatedthiophene moiety: from synthesis to electronic devices applications

Akshaya K. Palai • Jihee Lee • Minkyung Jea •

Hanah Na • Tae Joo Shin • Soonmin Jang •

Seung-Un Park • Seungmoon Pyo

Received: 9 January 2014 / Accepted: 14 February 2014 / Published online: 11 March 2014

� Springer Science+Business Media New York 2014

Abstract A symmetrically conjugated molecular semi-

conductor derived from diketopyrrolopyrrole (DPP), i.e., 2,5-

dihexadecyl-3,6-bis(5-(3-hexylthiophen-2-yl)thiophen-2-yl)

pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione [DPP(3HT)2], has

been synthesized using the Stille coupling reaction. The

optical band gap of DPP(3HT)2 is found to be 1.65 eV with

HOMO energy level of -5.13 eV. DPP(3HT)2 is thermally

stable up to 348 �C exhibiting only 5 % weight loss. X-ray

diffraction analysis revealed the DPP(3HT)2 to be poly-

crystalline with a high degree of crystallinity. It showed

highly narrow and strong diffraction peaks with a d-spacing

of ca. 30.18 A (at 2h = 2.09�, full width at half maximum:

2h = 0.126�) indicating an end-to-end packing distance with

the long alkyl chains (along the a-axis) being slightly tilted

and partially interdigitated for effective packing. Field-effect

transistors and complementary inverters employing

DPP(3HT)2 as a p-channel semiconductor have been fabri-

cated and characterized.

Introduction

Organic field-effect transistors (OFETs) became an area of

great interest among materials researchers in recent years

because of their potential applicability in various electronic

devices, such as logic circuits, flexible displays, memory

storage and photovoltaics [1–9]. As materials for high-

performance and cost-effective organic electronic devices,

p-conjugated polymers and small organic molecules, offer

numerous advantages when compared to their inorganic

counterparts, such as low cost, lightweight, the possibility

to tune band gaps, and the production of flexible large-area

devices [10–15]. Compared to conjugated polymers, small

molecule semiconductors which possess well-defined

molecular weight and high purity can readily be prepared,

enhancing charge carrier mobility and allowing consistent

device performance [16–21].

Among such molecular semiconductors, diketopyrrolo-

pyrrole (DPP) derivatives have recently received consider-

able attention because they exhibit good photophysical

properties along with excellent chemical, thermal, and

mechanical stability [22]. In addition, their chemical struc-

ture can be engineered in a relatively straightforward manner

to tailor solubility and optoelectronic properties [23, 24].

Because of their solution processability and favorable opto-

electronic properties, they have been considered to be suited

for application as the active material in organic photovoltaics

[25–27], dye-sensitized solar cells [28], and OFETs [29, 30].

Several DPP derivatives have been reported to act as an

active material in OFETs [31]. Functionalized DPP deriv-

atives that carry a thiophene moiety, in particular, exhibit

high hole field-effect mobility owing to their tendency to

self-assemble into organized domains [32]. A reasonable

electron mobility has furthermore been achieved for cyano-

functionalized thiophene-based DPP derivatives [33, 34].

Akshaya K. Palai and Jihee Lee contributed equally to this work.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10853-014-8116-4) contains supplementarymaterial, which is available to authorized users.

A. K. Palai � J. Lee � M. Jea � H. Na � S.-U. Park � S. Pyo (&)

Department of Chemistry, Konkuk University, 120 Neungdong-ro,

Gwangjin-gu, Seoul 143-701, Republic of Korea

e-mail: pyosm@konkuk.ac.kr

T. J. Shin

Pohang Accelerator Laboratory, Pohang 790-784,

Republic of Korea

S. Jang

Department of Chemistry, Sejong University, 209 Neungdong-ro,

Gwangjin-gu, Seoul 143-747, Republic of Korea

123

J Mater Sci (2014) 49:4215–4224

DOI 10.1007/s10853-014-8116-4

Recently, balanced charge carrier mobility of holes and

electrons was observed by the Nguyen group for FETs

based on benzothiadiazole-functionalized DPP derivatives,

using Si/SiO2 as the gate dielectric [35]. It can be con-

cluded that such functionalized molecular DPP derivatives

form potential candidates for application as the active

material in high-performance OFETs. In addition, for their

versatile applications such as flexible electronic devices,

various low-cost gate dielectrics such as polymer dielec-

trics are also necessary.

In this work, we describe the synthesis, structure, OFET,

and complementary inverter properties of a symmetrically

functionalized DPP derivative, 2,5-dihexadecyl-3,6-bis(5-

(3-hexylthiophen-2-yl)thiophen-2-yl)pyrrolo[3,4-c]pyrrole-

1,4(2H,5H)-dione DPP(3HT)2. The alkylthiophene unit is

introduced to achieve an extended and rigid conjugated

DPP core with an improved molecular organization, hence

ensuring enhanced p–p interactions [36]. In addition, the

alkylthiophene units (3-hexylthiophene in his study) will

contribute to the tuning of the onset oxidation potential [24,

37] so that the HOMO energy level of this small molecule

will display compatibility with the work function of gold

electrodes (source/drain), which, in turn, is a prerequisite

for potential application as the p-type material in OFETs.

The optical, electrochemical, and thermal properties of

DPP(3HT)2 are studied and its structure analyzed using

X-ray diffraction (XRD) and density functional theory

(DFT) calculations. To investigate applicability of

DPP(3HT)2 as a semiconductor, FETs and complementary

inverters based on organic/inorganic gate dielectrics are

fabricated and their characteristics are evaluated.

Experimental

Synthesis of 2,5-dihexadecyl-3,6-bis(5-(3-

hexylthiophen-2-yl)thiophen-2-yl)pyrrolo[3,4-

c]pyrrole-1,4(2H,5H)-dione [DPP(3HT)2]

Anhydrous toluene (10 mL) and anhydrous DMF (2 mL)

were added to a mixture of compound 3 (100 mg,

0.11 mmol), compound 4 (86.30 mg, 0.275 mmol), and

Pd(PPh3)4 (11 mg, 0.010 mmol) in a 50 mL two-necked

flask. The reaction mixture was heated to 120 �C and

stirred for 24 h under argon. The mixture was cooled,

added into MeOH (100 mL) and stirred for 20 min. The

resulting solid was collected by vacuum filtration and

chromatographed on silica using a 1:1 (v/v) mixture of

petroleum ether and CHCl3 as the eluent, to afford the title

compound as a deep blue solid (65 mg, 66.22 %). 1H NMR

(400 MHz, CDCl3): d 9.02 (d, 2H), 7.28 (d, 2H), 7.26 (d,

2H), 6.97 (d, 2H), 4.10 (t, 4H), 2.83 (t, 4H), 1.76 (m, 4H),

1.67 (m, 4H), 1.44–1.29 (m, 64H), 0.90 (m, 12H). 13C

NMR (100 MHz, CDCl3): d 161.31, 242.24, 141.29,

139.30, 136.30, 130.61, 129.9, 128.82, 126.88, 125.28,

108.0, 42.4, 31.94, 31.72, 30.57, 30.14, 29.71, 29.67,

29.62, 29.37, 29.32, 26.98, 22.70, 22.65, 14.13, 14.10.

HRMS (ESI): m/z calcd. for C66H100N2O2S4 [M ? H]?

1080.66, found 1080.6691.

Fabrication of OFET devices

OFETs with top-contact and bottom-gate geometry were

fabricated on Si/SiO2 and ITO/CL-PVP (cross-linked

poly(4-vinylphenol) substrates. The substrates were cleaned,

dried, and UV-ozone treated prior to use. For polymer gate

dielectric based OFET, our previous reported procedure was

employed to obtain a 450 nm thin film of CL-PVP over the

patterned ITO substrate [38]. On top of the dielectric (either

Si/SiO2 or ITO/CL-PVP), a 50 nm uniform DPP(3HT)2 film

was deposited at room temperature by thermal evaporation at

a rate of 0.2 A s-1, under high vacuum (5 9 10-6 Torr). To

complete the device, Au top-contact source and drain elec-

trodes were then deposited on top of the uniform film at a rate

of 0.3 A s-1, under the same vacuum conditions. A shadow

mask (W 9 L = 50 lm 9 1000 lm) was employed during

Au deposition to position the DPP(3HT)2 film within the

channel region.

Fabrication of the complementary inverter

For fabrication of the complementary inverter, DPP(3HT)2

(p-channel) and PTCDI-C13 (n-channel) were vacuum

deposited next to each layer on top of the ITO/CL-PVP

through a shadow mask, at a deposition rate of 0.3 A s-1 at

room temperature. The thickness of the channel materials

was controlled at 50 nm. The organic inverter was com-

pleted by thermally evaporating 50 nm gold electrodes

above the active layer using a shadow mask, forming the

source and drain electrodes (L 9 W = 50 lm 9 2000 lm

and L 9 W = 50 lm 9 1000 lm for the p- and n-channel

OFET, respectively).

Results and discussion

The chemical structure and synthesis of DPP(3HT)2 are

shown in Scheme 1. Pristine DPP derivative 1 was synthe-

sized as reported elsewhere [39]. To enhance its solution

processability, compound 1 was alkylated using n-bromo-

hexadecane (-C16H33) at the N,N0-positions of the lactam

ring of the DPP core to obtain alkylated compound 2.

Compound 2 was subsequently brominated with N-bromo-

succinimide (NBS) to obtain compound 3. Finally, the

symmetrical functionalized DPP derivative featuring

extended conjugation, i.e., DPP(3HT)2, was synthesized by

4216 J Mater Sci (2014) 49:4215–4224

123

the palladium-catalyzed Stille coupling of compound 3 with

(3-hexylthiophen-2-yl)trimethylstannane (4) (Scheme 1).

DPP(3HT)2 was purified by column chromatography and

characterized by 1H NMR, 13C NMR, and HRMS. The

corresponding spectra are shown in Fig. S1 and S2 (Sup-

porting Information). DPP(3HT)2 is found to be readily

soluble in organic solvents such as dichloromethane (DCM),

toluene, and 1,2-dichlorobenzene, and was obtained in

66.2 % yield.

The extended p-conjugated system of DPP(3HT)2 was

investigated by UV–vis spectroscopy. Absorption spectra

of DPP(3HT)2 in DCM as well as in the solid are shown in

Fig. 1a and summarized in Table 1. Both spectra show a

broad absorption band, ranging from 450 to 700 nm. The

p-conjugation in DPP(3HT)2 extends to the thiophene

units at either side of the DPP core and the spectrum hence

shows a red shift (67 nm) of the absorption maximum as

compared to that of the parental compound 2

(kmax = 547 nm) [23]. The absorption spectrum of a

DPP(3HT)2 film shows a red shift in the absorption max-

imum as well, reaching into the near-IR region when

compared to the dissolved compound, suggesting optimal

aggregation and p-stacking in the solid state, thus making

this compound a promising candidate for use in organic

electronics. Furthermore, the absorption spectrum of the

film is observed to be broadened, indicating delocalization

of electrons in the individual units.

In view of its application as an organic semiconductor,

the electrochemical properties of DPP(3HT)2 were studied

by cyclic voltammetry (CV) in acetonitrile, using 0.05 M

Bu4NPF6 as the supporting electrolyte. The corresponding

cyclic voltammogram of DPP(3HT)2 is shown in Fig. 1b.

Scheme 1 Molecular structure

and synthesis route of

DPP(3HT)2

Fig. 1 a UV–vis absorption spectra of DPP(3HT)2 and b CV of

DPP(3HT)2 recorded in 0.05 M Bu4NPF6 in acetonitrile (scan rate:

50 mV s-1), using ferrocene as the internal standard

J Mater Sci (2014) 49:4215–4224 4217

123

The insets of the figure show the enlarged CV curve (top

left) and cyclic voltommogram of the ferrocene without

material (top right). The onset oxidation of our material is

found at 0.36 V, which is clearly seen in enlarged CV

curve. The onset oxidation potential (0.36 V vs Fc/Fc?)

and the corresponding highest occupied molecular orbital

(HOMO) energy level (-5.13 eV, with respect to the

energy level of ferrocene (-4.8 eV, relative to vacuum))

are compiled in Table 1. The presence of the electron-

donating hexylthiophene groups in DPP(3HT)2 lowers the

onset oxidation potential, reflecting a higher lying HOMO

energy level as compared to that of parental compound 2

(-5.32 eV) [38]. Indeed, the observed effect on oxidation

potential of the electron-donating alkylthiophene moieties

has previously been reported for a set of similar DPP

derivatives [24]. Calculation of the lowest unoccupied

molecular orbital (LUMO) energy level from the HOMO

energy level and the optical band gap (measuring 1.65 eV,

as calculated from the UV–vis spectrum) yielded a value of

-3.48 eV. Since the HOMO of DPP(3HT)2 lies above the

work function of gold (-5.2 eV), upon application of an

electric field, efficient hole injection should take place from

the gold electrode into the organic semiconductor.

The thermal properties of DPP(3HT)2 were evaluated

using thermogravimetric analysis (TGA) at a heating rate of

10 �C min-1, under nitrogen atmosphere (Fig. S3, Supporting

Table 1 Optical, electrochemical, and thermal properties of DPP(3HT)2

DPP derivatives kmax (nm) Egc (eV) Eox,onset

d (V) HOMOe (eV) LUMOf (eV) Tdg (�C) Td

h (�C)

Solutiona Filmb

DPP(3HT)2 349, 409, 580, 614 364, 554, 666(s) 1.65 0.36 -5.13 -3.48 348 380

a UV–vis spectra of polymers recorded in CH2Cl2 solutionb UV–vis spectra of films deposited inside the quartz cuvvettec Optical band gapd Measured in acetonitrole/Bu4NPF6 versus Ag/Ag?

e Estimated from Eonset according to EHOMO = -4.8 - e (Eonset - E1/2 Fc) eV where E1/2 Fc = 0.03 Vf LUMO levels were calculated from the HOMO values and the optical band gapg Degradation temperature (Td) at 5 % weight lossh Td at 10 % weight loss

Fig. 2 Electron density of a HOMO and b LUMO of DPP(3HT)2 as calculated by DFT at the B3LPY/6-31G(d,p) level

4218 J Mater Sci (2014) 49:4215–4224

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Information). Thermal decomposition is observed at temper-

atures of 348 and 380 �C, exhibiting 5 and 10 % weight loss,

respectively (see Table 1). These results indicate that the

material did not degrade during thin-film formation by vacuum

deposition and suggest that it was stable over the temperature

range generated during operation of the device.

To characterize the geometry and electronic structure of

the DPP(3HT)2 molecule arising from donor–acceptor (D–

A) interactions, DFT calculation of the respective elec-

tronic states was performed at the B3LPY/6-31G(d,p)

level. Before DFT calculations, a series of structures was

obtained from simulated annealing-based conformational

analysis using SPARTAN10 [40]. The resulting best five

structures were subjected to semi-empirical AM1 calcula-

tions and followed by HF-SCF with a minimum STO-3G

basis set. Finally, two stable conformers, displaying energy

differences of over 10 kcal mol-1 with the other structures,

were used as the initial inputs for DFT calculation at the

B3LPY/6-31G(d,p) level. We used a polarizable continuum

model with DCM to represent the solvation effect. The

optimal geometry of DPP(3HT)2 is shown in Fig. S4. The

dihedral angles between the DPP core and the adjacent

thiophene moiety were 0.88/0.89�, while that of the two

thiophene rings measured 10.14/12.71�, respectively. The

low dihedral angles suggest the DPP(3HT)2 structure to be

planar. Similar dihedral angles have been reported for a

series of DPP derivatives [41]. The extended conjugated

molecular length (aromatic part) plays a crucial role in

charge delocalization and measures ca. 18.63 A, while the

molecular length along the long axis (i.e., that of the alkyl

group on the lactam moiety) is 44.65 A. The molecular

orbital diagrams of the DPP(3HT)2 molecule show the

HOMO (Fig. 2a) and the LUMO (Fig. 2b) to exhibit sim-

ilar electron densities and to be localized along the con-

jugated backbone. Such localization of the HOMO and

LUMO orbital on D–A units has been reported elsewhere

for other DPP derivatives [42].

The structure of DPP(3HT)2 was further investigated by

XRD (Fig. 3). The XRD patterns suggest a high degree of

crystallinity and the existence of polycrystalline domains, in

analogy to other organic molecular materials [43–45]. The

DPP(3HT)2 powder shows considerably narrow (full width at

half maximum: 2h = 0.126�) and strong 1st order diffraction

peaks at 2h = 2.09�, with a d-spacing of ca. 30.18 A and

multi-order diffraction peaks at 4.19�, 6.26�, and 10.51�, while

the 4th order peak is absent (Fig. 3a, red arrows and asterisk).

A possible molecular packing diagram of the three molecular

axes (a, b, and c) is shown in Fig. 3b. Since the molecular

length of DPP(3HT)2 along the direction of the longer alkyl

chains bonded to the DPP unit is ca. 44.65 A, a d-spacing of

30.18 A is attributed to the end-to-end packing distance along

the long alkyl chains (a-axis), suggesting that they are slightly

tilted and partially interdigitated for effective packing

(Fig. 3b). The XRD pattern, furthermore, shows a weak dif-

fraction at 2h = 5.92� with a d-spacing of ca. 10.68 A and a

higher order peak at 2h = 11.84� (as indicated by the blue

arrows, Fig. 3a), which can be attributed to the p-conjugated

molecular axis (b-axis). Based on the above two series of

diffraction peaks, the peaks at 2h = 7.22�, 8.60�, and 10.24�are assigned to (110), (210), and (310) reflections, respec-

tively. The diffraction peaks above 2h = 12� are assigned to

p–p stacking of the conjugated backbones (c-axis) and further

higher long-range ordering. Similar XRD patterns were

reported for another symmetrical DPP derivative [46].

To study the applicability and semiconducting proper-

ties of DPP(3HT)2, the compound was employed as the

active layer in OFETs constructed from SiO2 or CL-PVP

gate dielectrics. Figure 4 shows a schematic representation

of top-contact, bottom-gate OFETs containing a 50 nm

DPP(3HT)2 film deposited on Si/SiO2 (Fig. 4a) or indium

Fig. 3 a Powder XRD patterns of DPP(3HT)2 in wide span

(2h = 1–20�) and b schematic representation of the possible molecular

packing of DPP(3HT)2 along the three molecular axes (a, b, and c) (Color

figure online)

J Mater Sci (2014) 49:4215–4224 4219

123

tin oxide (ITO)/CL-PVP (Fig. 4b) gate dielectrics. A

50-nm-thick gold electrode was used as source and drain

for both devices. Tapping-mode AFM images over

5 lm 9 5 lm of the DPP(3HT)2 thin film on SiO2 or CL-

PVP, thermally vacuum deposited, are shown in Fig. 4c, d,

respectively, indicating the DPP(3HT)2 film to be com-

posed of grain-like crystals and no significant difference in

the shape of the crystal domain is found.

The output characteristics (i.e., drain current, IDS, versus

drain voltage, VDS) of the DPP(3HT)2 OFET on the Si/

SiO2 dielectric are depicted in Fig. 5a. The output char-

acteristics as a function of gate bias show dominant hole

transport behavior. Such p-type behavior can be ascribed to

the close match between the HOMO level of the material

and work function of the gold electrode, with the injection

barrier between DPP(3HT)2 and gold *0.1 eV. An

increasing gate bias (VGS) generates more charge carriers,

which accumulate at the interface of semiconductor and

gate dielectric in the channel region, resulting in an

increased drain current. The transfer curves of the OFET

are depicted in Fig. 5b, allowing calculation of IDS vs VGS.

The field effect mobility in the saturation regime was

subsequently determined by the following equation:

IDS ¼ WCi=2Lð Þl VGS � Vthð Þ2; ð1Þ

where Ci is the capacitance per unit area of the dielectric

layer and Vth is the threshold voltage. Furthermore, L and

W are the channel length and width, respectively, of the

device. The threshold voltage of the device was determined

by plotting the square root of IDS against VGS and extrapo-

lating to IDS = 0. The inverse subthreshold slope (ss), being

a measure of switching speed of the device, was calculated

by ss = [dlog(IDS)/dVGS]-1. The hole mobility (l), thresh-

old voltage, on-to-off current ratio (Ion/Ioff), and ss of the

Fig. 4 Schematic

representation of top-contact

bottom-gate OFETs based on

a Si/SiO2 and b ITO/CL-PVP

gate dielectrics, using

DPP(3HT)2 as the

semiconducting channel. c AFM

height image (5 lm 9 5 lm) of

DPP(3HT)2 films on Si/SiO2

and d glass/ITO/CL-PVP

substrates

Fig. 5 a Typical output and b transfer characteristics of the top-contact,

bottom-gate device preparedusinga 50-nm-thick thin film of DPP(3HT)2

on Si/SiO2 gate dielectrics (W 9 L = 1000 lm 9 L = 50 lm)

4220 J Mater Sci (2014) 49:4215–4224

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OFET are estimated at ca. 3.18 9 10-3 cm2 V-1 s-1,

-28.70 V, 1.07 9 103, and 7.90 V dec-1, respectively

(Fig. 5b). The average hole mobility of 10 OFET devices

was 1.65 9 10-3 cm2 V-1 s-1 with an on-to-off current

ratio of 3.16 9 102. The performance parameters are sum-

marized in Table 2.

The suitability of DPP(3HT)2 thin films to act as the

active components in electronic devices with organic

dielectric was further tested through fabrication of top-

contact, bottom-gate OFETs that employed a 450 nm

polymer gate dielectric (CL-PVP) deposited on patterned

ITO-coated glass substrates. Capacitance of the gate

dielectric was measured using metal–insulator–metal

devices and found to be 58 pF mm-2 at 1 kHz. The output

(IDS–VDS) and transfer (IDS–VGS) characteristics are shown

in Fig. 6a, b, respectively, while the performance charac-

teristics of the CL-PVP OFET are summarized in Table 2.

Mobility, threshold voltage, on-to-off current ratio, and ss

of the ITO/CL-PVP OFET were estimated to be ca.

2.01 9 10-3 cm2 V-1 s-1, -32.9 V, 9.05 9 102, and 1.2

Vdec-1, respectively (Fig. 6b). Combined data of 10 ITO/

CL-PVP OFETs indicate a consistent performance with an

average hole mobility of 1.60 9 10-3 cm2 V-1 s-1 and an

on-to-off current ratio of 3.15 9 102 (Table 2). This

demonstrates the potential of this particular DPP derivative

for application as the active material in all-organic flexible

FETs, the more so when it is considered that application in

polymeric gate dielectrics typically decreases mobility by

three orders of magnitude for molecular semiconductor

such as 7,7,8,8-tetracyanoquinodimethane [47], and per-

formance of OFETs based on ITO/CL-PVP and Si/SiO2

gate dielectrics is comparable.

Complementary inverter devices were readily prepared by

combining p- and n-channel FETs. Specifically, we prepared

an inverter consisting of 50 nm thin films of N,N0-ditridecyl-

3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) [48,

49] and DPP(3HT)2 as the n- and p-channel semiconductor,

respectively. CL-PVP was used as a gate dielectric, while

gold acted as the top-contact electrode. A schematic repre-

sentation of the complementary inverter is shown in Fig. 7a,

along with the chemical structure of PTCDI-C13. The logic

circuit diagram of the complementary inverter is depicted in

the inset of Fig. 7b. Because current levels of p- and n-

channel OFETs need to be matched for fabrication of an

efficient complementary inverter [50]. L and W of the two

OFETs were optimized (within the limits of our device fab-

rication system). Thus, L 9 W for the n-channel OFET

measured 50 lm 9 1000 lm, whereas the dimensions of the

p-channel OFET measured 50 lm 9 2000 lm. The

DPP(3HT)2 OFET was connected to the supply voltage

(VDD), while the PTCDI-C13 OFET was connected to the

ground, with the two transistors sharing an input (VIN) and an

output (VOUT) terminal. Figure 7b, c shows the voltage

transfer characteristics (VIN–VOUT) and the corresponding

gain of the complementary inverter from 0 to 60 V at

VDD = 60 V. As can be verified from Fig. 7b, the comple-

mentary inverter shows an almost ideal transfer curve for

forward (i.e., from 0 to 60 V) and reverse (i.e., from 60 to 0

V) scans, indicating negligible hysteresis. Furthermore, the

Table 2 Performance of the FET devices prepared using DPP(3HT)2 thin film

Semiconductor Dielectric Mobility (cm2 V-1 s-1) Ion/Ioff ratio Vth (V) S.S (V/dec) No. of devices

DPP(3HT)2 Si/SiO2 0.0032 (0.0016) 1.1 9 103 (102) -28.7 (-40.2) 7.9 (10.0) 10

DPP(3HT)2 ITO/CL-PVP 0.0020 (0.0016) 9.1 9 102 (102) -32.9 (-13.5) 1.2 (12.8) 10

Values in parentheses are average values from 10 devices

Fig. 6 a Typical output and b transfer characteristics of the top-

contact, bottom-gate device prepared using a 50-nm-thick thin film of

DPP(3HT)2 on CL-PVP gate dielectrics (thickness: 450 nm,

W 9 L = 1000 lm 9 50 lm)

J Mater Sci (2014) 49:4215–4224 4221

123

swing range of VOUT is almost identical to the supply voltage,

confirming zero static power consumption in the digital logic

circuit. The switching voltage, VM (defined as that voltage at

which VIN = VOUT), of the complementary inverter was

29.67 V, being almost equal to VDD/2 = 30 V. These results

clearly demonstrate the two OFETs to operate properly after

integration, suggesting that performance parameters such as

charge carrier transport and turn-on voltage of the p-channel

DPP(3HT)2 FET are well balanced with those of the

n-channel PTCDI-C13 FET. The inverter gain (i.e., dVOUT/

dVIN), defined as the speed at which VIN can be switched, is

shown in Fig. 7c, with a maximum gain of 16.9 achieved at

VIN = 29.7 V. The noise margins for the high- and low-logic

levels (i.e., NMH and NML) are calculated to be 23.51 and

25.20, respectively, for the forward bias, and 23.14 and

27.97, respectively, for the reverse bias. The gain of 16.9 and

the observed ideal switching voltage indicate logic devices

based on DPP(3HT)2 could be used in more complex logic

circuits. The transient behavior of this DPP small molecule

based organic inverter has also been studied by applying

input signal at 1 Hz frequency with VDD = 60 V, and the

results are depicted in Fig. 7d. As shown in Fig. 7d, VIN is a

square wave swapped between 0 and 60 V, and VOUT exhibits

good inversion signature. These combined static and

dynamic results further suggest practical utility of the present

inverter in complex logic circuits. Although there are many

reports in the literature where inverters have been demon-

strated by only using DPP-polymers [51–53], here we dem-

onstrate small molecular organic semiconductor based

complementary metal-oxide semiconductor (CMOS)-like

organic inverter having DPP derivative as p-type and PTCDI

derivative as n-type material on polymer gate dielectric.

Fig. 7 a 3-D representation of the organic complementary inverter

based on the ITO/CL-PVP gate dielectric, produced by integration of

the DPP(3HT)2 p-channel and PTCDI-C13 n-channel OFETs

(W 9 L = 50 lm 9 1000 lm and W 9 L = 50 lm 9 2000 lm for

n- and p-channel OFETs, respectively). b Typical voltage transfer

curve (VIN–VOUT). The inset shows the simplified circuit diagram of

the inverter. c gain of the organic complementary inverter at

VDD = 60 V. d transient response characteristics of CMOS-like

inverter measured under 0–60 V peak-to-peak square input signal at

1 Hz frequency with VDD = 60 V

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123

Conclusion

We have demonstrated the design of a p-conjugated sym-

metrically functionalized DPP molecule, DPP(3HT)2, with a

low-energy gap. This new DPP derivative was suited for

application as a p-type semiconducting material when used (in

combination with a charge-injecting gold electrode) on inor-

ganic as well as organic gate dielectrics. Under ambient con-

ditions, average hole mobilities of 1.65 9 10-3 cm2 V-1 s-1

and 1.60 9 10-3 cm2 V-1 s-1 were obtained on Si/SiO2 and

ITO/CL-PVP gate dielectrics, respectively. The utility of

DPP(3HT)2 was, furthermore, studied by constructing all

organic complementary inverters in combination with n-type

PTCDI-C13 on CL-PVP gate dielectric, demonstrating suc-

cessful device operation (both static as well as dynamic

behavior).

Supporting information

Materials, instruments used, 1H NMR, 13C NMR and

HRMS spectra, TGA thermogram and optimal geometry of

DPP(3HT)2 are available. This material is available free of

charge via the Internet.

Acknowledgements This work was supported by the National

Research Foundation (NRF) funded by Korea government (MEST)

(R11-2008-052-03,003, 2012R1A2A2A01045694). A.K.P. acknowl-

edges financial support from the 2012 KU Brain Pool Program of

Konkuk University.

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