Solution-processed ambipolar field-effect transistor based on diketopyrrolopyrrole functionalized with benzothiadiazole

Article abstract of DOI:10.1002/adfm.201101820

Ambipolar charge transport in a solution-processed small molecule 4,7-bis{2-[2,5-bis(2-ethylhexyl)-3-(5-hexyl-2,2′:5′, 2″-terthiophene-5″-yl)-pyrrolo[3,4-c]pyrrolo-1,4-dione-6-yl] -thiophene-5-yl}-2,1,3-benzothiadiazole (BTDPP2) transistor has been investigated and shows a balanced field-effect mobility of electrons and holes of up to ～10^-2 cm² V^-1 s^-1. Using low-work-function top electrodes such as Ba, the electron injection barrier is largely reduced. The observed ambipolar transport can be enhanced over one order of magnitude compared to devices using Al or Au electrodes. The field-effect mobility increases upon thermal annealing at 150°C due to the formation of large crystalline domains, as shown by atomic force microscopy and X-ray diffraction. Organic inverter circuits based on BTDPP2 ambipolar transistors display a gain of over 25. Copyright

Full text of DOI:10.1002/adfm.201101820

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Solution-Processed Ambipolar Field-Effect Transistor
Based on Diketopyrrolopyrrole Functionalized with
Benzothiadiazole
Yuan Zhang, Chunki Kim, Jason Lin, and Thuc-Quyen Nguyen*
electric potential and the light emission
zone can be controlled by the applied gate
Ambipolar charge transport in a solution-processed small molecule 4,7-bis{2-
[2,5-bis(2-ethylhexyl)-3-(5-hexyl-2,2′:5′,2′′-terthiophene-5′′-yl)-pyrrolo[3,4-c]
pyrrolo-1,4-dione-6-yl]-thiophene-5-yl}-2,1,3-benzothiadiazole (BTDPP2)
transistor has been investigated and shows a balanced field-effect mobility of
electrons and holes of up to ∼10−2 cm2 V−1 s−1. Using low-work-function top
electrodes such as Ba, the electron injection barrier is largely reduced. The
observed ambipolar transport can be enhanced over one order of magni-
tude compared to devices using Al or Au electrodes. The field-effect mobility
increases upon thermal annealing at 150 °C due to the formation of large
crystalline domains, as shown by atomic force microscopy and X-ray diffrac-
tion. Organic inverter circuits based on BTDPP2 ambipolar transistors display
a gain of over 25.
voltage.[14]
Recently, several solution-processed
polymer-based OFETs using a regular
bottom-gate structure with hole mobili-
ties of ∼10−1 cm−2 V−1 s−1 have been
reported.[15−19] The charge-carrier mobility
can be improved by tuning the inter-
molecular interactions between the nearest
neighboring molecules,[20] which can be
achieved using fused aromatic rings such
as thienothiophene, cyclopentanedithi-
ophene, and naphthodithiophene.[21−23] It
is believed that the fused aromatic struc-
tures enhance π–π stacking and hence
induce higher molecular ordering, which
drastically improves charge transport.[16] However, conjugated
polymers suffer batch-to-batch variation in terms of molecular
weight and polydispersity that affect solar cell performance and
field-effect mobility.[24,25]
1. Introduction
Organic field-effect transistors (OFETs) have potential
applications in large-area displays, sensors, radiofrequency iden-
tification tags, and logic circuits with low-cost processability,
and high flexibility.[1−6] OFETs using a bottom gate structure
generally display hole transport only. This can be attributable
to the trapping of the electrons by the hydroxyl group from the
SiO2 gate dielectrics.[7] Treatment of SiO2 gate dielectric surfaces
with organic self-assembled monolayers leads to improved per-
formance in n-type OFETs fabricated from thermally evaporated
small molecules or solution-processed polymer semiconducting
layers.[7,8] Using a trap-free polymer gate dielectric layer, electron
transport in conducting polymers has been observed, revealing
a comparable field-effect mobility as holes.[9] However, for cer-
tain types of applications, ambipolar transport is required. For
example, complimentary metal-oxide-semiconductor (CMOS)
ambipolar transistors are preferable to increase the noise
margin for logic circuits.[10] Furthermore for light-emitting
transistors,[11−13] ambipolar transport is required so that the
Recently a family of soluble small organic molecules
containing a diketopyrrolopyrrole (DPP) core have been
synthesized.[26−33] DPP-based materials have been used in
solution-processable organic solar cells showing a power
conversion efficiency (PCE) up to 5.2%.[29] OFETs fabricated
from soluble DPP materials show hole mobility of ∼10−2 cm2
V−1 s−1.[34] Incorporating fused aromatic ring moieties, the
crystallinity of this type of materials can be well controlled by
choosing appropriate solvents or thermal annealing, leading
to desired film morphologies.[35] To the best of our knowledge,
ambipolar transport based on this class of materials has not
been observed previously.
Here, we report a newly synthesized bis-DPP compound,
4,7-bis{2-[2,5-bis(2-ethylhexyl)-3-(5-hexyl-2,2′:5′,2′′-terthiophe
ne-5′′-yl)-pyrrolo[3,4-c]pyrrolo-1,4-dione-6-yl]-thiophene-5-yl}-
2,1,3-benzothiadiazole (BTDPP2), with two electron-accepting
units (DPP and benzothiadiazole, BT). The chemical structure
of BTDPP2 is shown in Figure 1a. Detailed synthetic proce-
dures can be found in the Experimental Section. The benzothi-
adiazole group strongly increases the electron affinity, leading
to n-channel transport characteristics. We observe balanced car-
rier mobilities up to 10−3 cm2 V−1 s−1 using a bottom-gate, top
electrode device architecture (Figure 1b) with appropriate top
contacts. Various top electrodes with work function (Φm) from
5.1 eV (Au) to 2.7 eV (Ba) were compared to examine the effect
of the injection barrier on the ambipolar transport. Using a
Dr. Y. Zhang, Dr. C. Kim, J. Lin, Prof. T.-Q. Nguyen
Center for Polymers and Organic Solids
Department of Chemistry and Biochemistry
University of California
Santa Barbara, Santa Barbara, CA 93106, USA
E-mail: quyen@chem.ucsb.edu
DOI: 10.1002/adfm.201101820
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nearly unchanged. With further increase in
annealing temperature to 200 °C, Id dramati-
cally drops below that of the device annealed
at 120 °C. Vg(min), defined by the gate voltage
for the crossover point of the minimal drain
current under electron accumulation, slightly
shifts towards a lower voltage when annealed
due to the change of the fixed charges at the
interface. In contrast, the OFET operated
in the hole-accumulation regime shows a
random variation of Vg(min). Mobilities were
calculated using the standard FET equation
under saturation regime (Equation 1),
√


2
2L
d
Id
µsat
=
(1)
WCi dVg
where μsat stands for the field-effect mobility at
saturation regime, Ci for the area capacitance
of the gate dielectrics, and L/W for channel
width to length ratio, the saturation mobility
μsat of BTDPP2 is calculated and plotted as a
function of annealing T (Figure 2b).
The μsat of the hole and electron are both
in the range of ∼10−4 to 10−3 cm2 V−1 s−1.
Except for the devices annealed at 150 °C, the
hole FET mobility of 4 × 10−4 cm2 V−1 s−1 is
almost the same for the as-cast and annealed
Figure 1. a) Chemical structure of BTDPP2. b) FET device structure.
films and it is lower than that of the electron
mobility by a factor of two. The FET mobility
low-work-function Ba contact (Φm ∼ 2.7 eV), the drain–source
currents of holes and electrons are both enhanced by more
than one order of magnitude compared to OFETs with Al and
Au contacts. Both the hole and electron mobilities increase
from 10−4 to 10−3 cm2 V−1 s−1 with increased the annealing tem-
perature up to 150 °C. Atomic force microscopy (AFM) shows
a transition of the BTDPP2 film morphology from a fiber-
like (as-cast) to crystalline-like features (above 200 °C). X-ray
diffraction (XRD) measurements on BTDPP2 films prove the
formation of crystalline and close intermolecular packing upon
thermal annealing. Organic complementary inverters using
BTDPP ambipolar OFETs show effective inversion of the input
signals operating in both the first and third quadrants. Gains
exceeding 25 have been achieved as a result of balanced trans-
port properties.
is almost constant when T ≤ 120 °C, suggesting that the film
morphology does not change when annealed at such tempera-
tures. The sharp increase of μsat up to 2.1 × 10−3 cm2 V−1 s−1
when annealing temperature increases from 120 °C to 150 °C
may indicate an improved film morphology and/or molecular
packing. Under different annealing temperatures, all BTDPP2
transistors demonstrate balanced hole and electron carrier
mobilities.
To further understand the transport results observed
above, AFM was employed to probe change in surface mor-
phology as a functional of thermal annealing temperature.
Topographic images of the as-cast and annealed films are
shown in Figure 3 with a scan size of 2 μm × 2 μm. The as-
cast film exhibits continuous fiber-like features, similar to
what has been observed on other DPP derivatives.[34] Upon
thermal annealing, the fiber-like structures gradually evolve
into a particle-like structures showing sharper boundaries.
Compared to the recently reported morphology of 2,5-di-n-
hexyl-3,6-bis(5′′-n-hexyl[2,2′;5′,2′′]terthiophen-5-yl)pyrrolo[3,-
4-c]pyrrole-1,4-dione (DHT6DPPC6) films,[34] which displays
large crystalline domains upon annealing at 150 °C, large
crystalline domains of BTDPP2 films were only observed at
higher annealing temperatures (240 °C). The average size of
these domains is around 0.2 μm × 1 μm. Figure 3f shows the
surface roughness as a function of annealing temperature. The
roughness of the as-cast film decreases from 3.3 nm to 0.5 nm
when annealed at 150 °C and inversely increases to 2.5 nm
when T = 240 °C where large crystalline domains are formed.
The highest mobility value is observed from films with small
2. Results and Discussion
We first study the effect of thermal annealing on BTDPP2
film morphology and on the charge transport in OFET devices
utilizing gold top contact. BTDPP2 films were annealed for
30 min at different temperatures inside a nitrogen glovebox.
Figure 2a shows the saturation drain current with Vd = ±60 V
as a function of gate bias upon various annealing temperatures
(T). We observe the lowest drain current (Id) from the as-cast
BTDPP2 films. The current increases with increasing annealing
temperature up to 150 °C. This enhancement holds both for
the hole and electron currents, while the leakage currents are
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higher annealing temperatures. This result helps explain why
thermal annealing at 150 °C leads to highest hole and elec-
tron mobilities.
Next, we explore the use of low-work-function metal
electrodes on the OFET performance. One of the advantages
of applying bottom-gate, top-electrode device geometry is the
great flexibility to utilize different metal contacts by which
the injection barriers can be tuned. The OFET transfer curves
using Au (Φm = 5.0 eV), Al (Φm = 4.3 eV), Ca (Φm = 3.0 eV), and
Ba (Φm = 2.7 eV) contacts are compared in Figure 5. Figure 5a
shows the electron current Id at low drain bias Vd. Clearly Id
increases when the work function is lowered. This is attributed
to the reduced electron injection barrier of Ba and Ca, leading
to an Ohmic contact with the LUMO of BTDPP2 (3.7 eV from
cyclic voltammetry). Surprisingly, the transfer characteristics of
Ba OFETs display a more ambipolar feature compared to those
with Au and Al contacts. To further understand this obser-
vation, the electron and hole currents in the saturation regime
are compared in Figures 5b and 5c with, Vd = ±60 V. Under the
same carrier concentration induced by the gate dielectrics, the
lowered electron injection barrier with Ba or Ca contacts results
in higher drain currents in the first quadrant. Interestingly, the
hole current in the third quadrant using Ba electrode is also
much higher, leading to symmetric transfer characteristics.
Under a large drain bias, the strong electric field leads to a
pronounced Schottky barrier lowering and hence facilitates the
carrier injection both for the electrons and holes. In contrast
to unipolar OFETs, the electric potential in ambipolar OFETs
reaches a minimum at some point far from either the drain or
source electrodes as the gate bias approaches that applied to
the drain. Thus, the drain current at saturation regime actually
comprised of both types of charge carriers, similar to a p–n
junction. Using low-work-function contacts, the enhancement
of the electron injection upon high drain bias helps attract
more holes injected from the counterpart electrode. This is
the main reason that the drain current of the Ba devices under
both the hole and electron accumulations is large compared to
the devices using Al or Au electrodes. The carrier mobility with
different top electrodes is summarized in Table 1. BTDPP2
demonstrates balanced saturation FET mobilities ranging from
10−4 to roughly 1 × 10−2 cm2 V−1 s−1. The difference between
the hole and electron mobility is low and insensitive to the elec-
trode used. Utilizing a low-work-function Ba contact facilitates
electron injection into BTDPP2 increasing not only the elec-
tron, but hole mobility when operating the OFETs in the satu-
ration regime. The hole and electron mobilities are enhanced
by one order of magnitude compared to those using Au or Al
electrodes.
Figure 2. a) Saturation transfer characteristics of BTDPP2 FETs upon
different annealing temperature using Au top contact. b) BTDPP2 carrier
mobility as a function of annealing temperature.
domains and low surface roughness (150 °C). Large domain
formation at 240 °C lead to discontinuous grain boundaries,
which are detrimental to charge transport, and hence the car-
rier mobility decreases from 2.1 × 10−3 to 1.0 × 10−4 cm2 V−1 s−1
for holes and from 2.1 × 10−3 to 3.5 × 10−4 cm2 V−1 s−1 for
electrons.
From AFM images, one can only probe the surface mor-
phology. To probe change in the internal morphology as a
function of annealing temperature, we performed XRD with
the normal-to-surface mode and extracted the intermolecular
spacing (d-spacing) of BTDPP2 films. Figure 4a shows the
XRD spectra of as-cast and thermal annealed BTDPP2 films.
There is no XRD peak observed for the as-cast film. The
2θ XRD peak appears at 6.2 degrees for the BTDPP2 film
annealed at 120 °C, which shifts to 6.33 degrees for films
annealed at 150 °C and further to 4.63 degrees when annealed
at 200 °C, and 240 °C, respectively, implying that the intermo-
lecular spacing becomes closer. The intensity of the XRD sig-
nals increases with increasing annealing temperature, indica-
tive of the enhanced film crystallinity. For the films annealed
at 120 °C and 150 °C, the XRD peak intensities are quite weak,
indicating a disorder structure consistent with AFM images
shown in Figure 3a–c. The d-spacing was calculated using
Bragg’s law, λ = 2d sinθ, where λ is the X-ray wavelength,
and plotted in Figure 4b for various annealing conditions.
When annealed at 150 °C, the d-spacing abruptly decreases
from 16.5 Å to 13.95 Å and remains nearly unchanged at
In inorganic FETs, the threshold voltage Vth refers to the
onset of strong inversion. Below Vth, since the conduction
channel is totally depleted, there should ideally be no current
flowing from the drain to the source.[36] In contrast, organic
FETs work in accumulation and the current in an inver-
sion regime is hardly attainable.[37] Thus, defining the Vth in
organic FETs is difficult as already addressed by Horowitz.[38]
Although classical MOSFET equations are good approxima-
tions for describing the Vth in OFETs, these equations neglect
the dependence of the carrier mobility by the carrier concen-
tration and electric fields.[39] To examine the Vth of BTDPP2
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by impedance analysis agrees well with the
results extracted directly from the transfer
characteristics (Figure 5) under saturation
regime using the following equation:
Vd
= Vg(min) − V
(2)
th
2
where Vg(min) is the gate voltage for inflec-
tion point of Id. The relatively low Vth and
equivalent mobility both demonstrate well-
balanced, ambipolar transport in BTDPP2
transistors.
Lower electron injection barriers observed
using Ba electrodes should result in low
contact resistance (RC). RC in an OFET
describes the potential drop across the con-
tacts, which causes a voltage loss for an
effective voltage on transport channel from
the fully applied drain–source bias.[40] In a
staggered, bottom-gate configuration, the
charge accumulation regime is formed
opposite to the top drain–source contacts
and it is found that RC shows a weaker
dependence on the injection barrier than
in the coplanar geometry.[41] Next, we study
the contact resistance and channel length
dependence of the BTDPP2 charge transport
at a low drain-source bias of Vd = 11 V (linear
regime). The OFET transfer curves using a
Ba/Al top contact are shown in Figure 6a
using various channel lengths. The applied
drain bias of 11 V is much lower than the
gate bias to ensure that the majority of car-
riers in the conduction channel are electrons
and the OFETs operate in a unipolar mode.
At high RC, one can observe a strong sup-
pression and even sign reversal of the drain
current at the linear regime. From Figure 6a,
with Vd = 11 V, the drain current clearly
increases as the gate bias exceeds 40 V. It
Figure 3. 2 μm × 2 μm AFM topographic images of BTDPP2 films prepared at different condi-
tions: a) as-cast, annealed at b) 120 °C, c) 150 °C, d) 200 °C, and e) 240 °C. f) RMS film rough-
ness versus annealing temperature.
OFETs, impedance measurements were performed on the
metal-insulator-semiconductor (MIS) diode prepared by the
same active layers. For this measurement, charge carriers in
BTDPP2 films were accumulated or depleted by sweeping
the gate voltage (Vg) leading to a different bulk capacitance,
C. Under a certain Vg, C is minimized when the charge car-
riers are entirely depleted. This given Vg is a fingerprint of
flat-band condition and can be considered the Vth. Figure 5d
shows the C of the BTDPP2 MIS diode as a function of gate
bias. Using Ba and Ca top contact, C strongly decreases with
the gate bias from 10 V to 6 V, giving rise to a minimal value
of 0.73 nF for C when Vg = Vth = 5 V. Conversely, using Au
contact the capacitance exhibits less pronounced changes and
the value of C at large positive bias lags behind that of devices
using Ba and Ca contacts. This can be explained by the dif-
ficulty of electron injection from Au, as compared to that
from low-work-function metals. As a result the charge accu-
mulation is weaker. Importantly, the Vth of ∼5 V determined
also scales with the channel length ranging from 40 to 100 μm.
This is a the signature of an Ohmic contact for electron injection
and thus the RC for the electron injection is small. Linear elec-
tron current using Al or Au top electrode is hardly attained
with large channel lengths. When applying a large enough
drain voltage (60 V), the devices using Al contacts show elec-
tron transport with a large channel length (100 μm) while
the overall magnitude is much lower than the Ba devices, by
roughly one order of magnitude (Id = 10−7 A when Vg = 60 V,
see Figure 1S, Supporting Information). From Figure 6a, the
hole current under linear regime using Ba contacts is not
measurable for different channel lengths. This is because the
hole injection barrier under linear regime is still quite high due
to the low horizontal electric field. Hence, the holes can not be
easily injected over a relatively large energetic barrier consid-
ering the difference between the Ba work function (2.7 eV) and
the highest occupied molecular orbital (HOMO) of BTDPP2
(∼5.0 eV from cyclic voltammetry). Figures 6b and 6c show the
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