Poly(diketopyrrolopyrrole-terthiophene) for Ambipolar Logic and
Johan C. Bijleveld,†Arjan P. Zoombelt,†Simon G. J. Mathijssen,†,‡Martijn M. Wienk,†
Mathieu Turbiez,§Dago M. de Leeuw,‡and Rene ´ A. J. Janssen*,†
Molecular Materials and Nanosystems, EindhoVen UniVersity of Technology, P.O. Box 513,
5600 MB EindhoVen, The Netherlands, Philips Research Laboratories, High Tech Campus 4, 5656 AE EindhoVen,
The Netherlands, and BASF, Klybeckstrasse 141, 4002 Basel, Switzerland
Received September 4, 2009; E-mail: firstname.lastname@example.org
Polymers based on diketopyrrolopyrrole (DPP) have been known
for a relatively long time1but are only recently emerging as
promising candidates for use in optoelectronic applications, par-
ticularly in field-effect transistors (FETs)2and organic photovoltaic
cells (OPCs).3,4OPC efficiencies up to 4.45% have been reached
with DPP polymers and oligomers.3e,4cHere we present a new DPP
based polymer, PDPP3T (Figure 1), that features high, almost
balanced charge carrier mobilities for both electrons and holes in
combination with an extended optical absorption toward the IR
region. These favorable characteristics allow PDPP3T to be used
for the construction of a FET-based inverter and to further enhance
the power conversion efficiency of DPP-based materials for OPCs.
PDPP3T was designed to have an unsubstituted terthiophene unit
between each pair of DPP units along the chain.3dIn comparison
with other DPP polymers that generally have more extended and
complex conjugated segments and carry solubilizing chains,3
terthiophene induces additional planarity, which enhances packing
and charge carrier mobility. By using extended side chains [i.e.,
2-hexyldecyl (HD)] on DPP, we compensated for the expected loss
in solubility. This allowed us to obtain PDPP3T in high molecular
weight, which is a crucial factor for its photovoltaic performance
PDPP3T was synthesized by Suzuki polymerization from the
dibromo-DPP monomer2aand 2,5-thiophenebis(boronic ester) (Figure
1). The use of Pd2(dba)3/PPh3as the catalyst yielded PDPP3T with
Mn) 54 000 g/mol [polydispersity index (PDI) ) 3.15]. Of all the
organic solvents tested, PDPP3T readily dissolves (>1 mg/mL) only
in chloroform. A much lower Mnof 10 000 g/mol (PDI ) 2.4) was
obtained when Pd(PPh3)4was used as the catalyst. Details of synthesis
and characterization are given in the Supporting Information (SI).
The onset of optical absorption at 1.36 eV in o-dichlorobenzene
(ODCB) solution and at 1.30 eV in a thin film (see the SI) classifies
PDPP3T as a small-band-gap polymer. The HOMO and LUMO levels
were found at +0.07 and -1.49 V vs Fc/Fc+by cyclic voltammetry
in ODCB solution (i.e., -5.17 and -3.61 eV vs vacuum) (see the
FETs with PDPP3T (Mn) 54 000 g/mol) as the semiconductor
were fabricated on a heavily doped silicon wafer covered by a 200
nm thick layer of thermally grown silicon dioxide, which acted as
common gate electrode and gate dielectric, respectively. Gold
electrodes were defined by standard photolithography. The polymer
was applied by spin-coating and subsequently dried in vacuum at
110 °C for 72 h to remove traces of solvent. Charge transport was
studied as a function of bias at room temperature. Typical ambipolar
transfer characteristics are presented in Figure 2a,b for negative
and positive drain biases. The PDPP3T transistors exhibited nearly
balanced hole and electron mobilities of 0.04 and 0.01 cm2V-1
s-1, respectively. These values are among the highest reported for
single-component ambipolar transistors.2aSurprisingly, these mo-
bilities were found to be almost independent of molecular weight:
the Mn) 10 000 g/mol version of PDPP3T gave µh) 0.05 cm2
V-1s-1and µe) 0.008 cm2V-1s-1(see the SI).
Two identical ambipolar transistors were combined into inverters,
with the common gate as the input voltage (Figure 2c inset).5Figure
2c shows the output voltage (VOUT) as a function of the input voltage
(VIN) at constant supply bias (VDD). The steepness of the inverter
curve indicated a gain of ∼30, which is comparable to that of state-
of-the-art CMOS-like inverters6and much higher than usually
obtained for unipolar logic.7
†Eindhoven University of Technology.
Pd2(dba)3, PPh3, K3PO4, Aliquat 336, toluene/water, 115 °C, 72 h.
Synthesis and structure of PDPP3T. Reaction conditions:
Figure 2. Ambipolar transfer characteristics for (a) negative and (b) positive
drain biases, varying from (20 to (80 V in steps of 20 V. (c) Inverter
characteristics of an inverter based on two identical FETs. The channel
length and width were 10 and 2500 µm, respectively.
Published on Web 11/03/2009
10.1021/ja907506r CCC: $40.75 2009 American Chemical Society
16616 9 J. AM. CHEM. SOC. 2009, 131, 16616–16617
Photovoltaic cells containing PDPP3T (Mn) 54 000 g/mol) and Download full-text
[6,6]phenyl-C61-butyric acid methyl ester (PCBM) were fab-
ricated by spin-coating on a PEDOT:PSS/ITO transparent front
electrode. The LiF/Al back electrode was evaporated in vacuum.
Cells with an active layer spin-coated from chloroform had a
relatively low performance because of formation of larger PCBM
clusters.3aControl over the morphology is crucial for bulk-
heterojunction solar cells, and several strategies to accomplish a
more favorable morphology, such as thermal treatment and the use
processing additives, have been developed.8Adding a small amount
(∼25 mg/mL) of diiodooctane (DIO) to the mixture before spin-
coating significantly improved the efficiency of the cells, mainly
as a result of an increase in photocurrent. The best cells were
obtained for PDPP3T:PCBM in a 1:2 weight ratio and gave an
open-circuit voltage (Voc) of 0.68 V, a fill factor (FF) of 0.67, and
a short-circuit current density (Jsc) of 8.3 mA/cm2under simulated
AM1.5 (100 mW/cm2) conditions,9resulting in an estimated power
conversion efficiency (η) of 3.8% (Figure 3a). The monochromatic
external quantum efficiency (EQE) showed a very sharp onset at
the optical band gap and rose to ∼33% in the 750-850 nm region
To increase the photocurrent in the visible region, we changed
the electron acceptor to PCBM, which possesses an increased
absorption coefficient in that part of the spectrum.10Because
PCBM is less soluble in chloroform than PCBM, an
increase of the DIO concentration to ∼100 mg/mL was required
to reach optimum device performance. The optimized cells had
PDPP3T:PCBM in a 1:2 weight ratio and provided Voc) 0.65
V, Jsc) 11.8 mA/cm2, and FF ) 0.60, resulting in an η value of
4.7%. The need for a larger amount of DIO with PCBM
strengthens the view of Lee et al.11that the role of the processing
additive is to keep the fullerene in solution longer, giving the
polymer more time to aggregate. This reduces the formation of large
PCBM crystals and realizes a superior morphology of the active
layer. The EQE of the cell with PCBM was similar to that of
the PCBM cell in the long-wavelength region but much higher
in the visible region, accounting for the higher short-circuit current.
In contrast to the constant FET mobility, a dramatic reduction
in OPC performance was observed when the lower-Mn (10 000
g/mol) version of PDPP3T was used in combination with PCBM
or PCBM (Figure 3b,d). Under identical processing conditions,
the power conversion efficiency did not exceed 1.3 or 2.7%,
respectively. The differences were caused by a reduction in
photocurrent (Figure 3b,d vs 3a,c) and exemplify the importance
of molecular weight as a crucial parameter in the formation of
efficient bulk heterojunctions. In this respect, it is interesting to
note that the use of DIO did not have an effect on µhor µefor pure
PDPP3T (see the SI).
With Voc) 0.65-0.68 V and an optical band gap (Eg) of 1.30
eV, the PDPP3T:PCBM combination approaches the minimum
offset of eVoc) Eg- 0.6 eV that we recently put forward as being
a threshold for bulk-heterojunction solar cells.12The fact that we
approached the minimum offset required for electron transfer to
occur could be the reason that the EQE was less than that found in
the more efficient (6%) cells published lately.13
In conclusion, PDPP3T exhibits ambipolar transport in FETs with
nearly balanced electron and hole mobilities in the 10-2cm2V-1
s-1range, making it an interesting candidate for CMOS-like circuits.
At a high molecular weight, PDPP3T reaches η ) 4.7% in
photovoltaic cells when combined with PCBM and has a
photoresponse up to 900 nm.
Acknowledgment. This research was supported by a TOP Grant
from NWO-CW and is part of the Joint Solar Programme,
cofinanced by FOM, CW, and the Foundation Shell Research.
Supporting Information Available: Synthetic procedures and
spectroscopic and electrochemical data. This material is available free
of charge via the Internet at http://pubs.acs.org.
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Figure 3. (a, b) J-V curves and (c, d) EQEs for PDPP3T:PCBM bulk-
heterojunction solar cells with (a, c) high and (b, d) low Mn.
J. AM. CHEM. SOC. 9 VOL. 131, NO. 46, 2009