Recent Advances in the Development of Semiconducting DPP‐Containing Polymers for Transistor Applications

TLDR: This progress report summarizes the numerous DPP-containing polymers recently developed for field-effect transistor applications including diphenyl-DPP and dithienyl- DPP-based polymers as the most commonly reported materials and highlights fundamental structure-property relations such as the relationships between the thin film morphologies and the charge carrier mobilities observed for D PP- containing polymers.

Abstract: This progress report summarizes the numerous DPP-containing polymers recently developed for field-effect transistor applications including diphenyl-DPP and dithienyl-DPP-based polymers as the most commonly reported materials, but also difuranyl-DPP, diselenophenyl-DPP and dithienothienyl-DPP-containing polymers. We discuss the hole and electron mobilities that were reported in relation to structural properties such as alkyl substitution patterns, polymer molecular weights and solid state packing, as well as electronic properties including HOMO and LUMO energy levels. We moreover consider important aspects of ambipolar charge transport and highlight fundamental structure-property relations such as the relationships between the thin film morphologies and the charge carrier mobilities observed for DPP-containing polymers.

Figures

Figure 9. Energy-minimized structure (B3LYP/6-31G(d)) of a P69 trimer (where R = CH3) with a visualization of the LUMO and HOMO molecular orbitals at the top and bottom, respectively. Green and red represent the isosurfaces of opposite phase of the wavefunctions and demonstrate the delocalization of both the HOMO and LUMO. Reproduced with permission from ref. [59] . Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Table 1. Diphenyl-DPP Polymers

Figure 1. General structure of copolymers containing the diphenyl-DPP (3,6-diphenyl-2,5diketopyrrolo[3,4-c]pyrrole) unit.

Figure 4. General structure of copolymers containing the diaryl-DPP (3,6-diaryl-2,5diketopyrrolo[3,4-c]pyrrole) unit.

Figure 3. General structure of copolymers containing the dithienyl-DPP (3,6-dithienyl-2,5diketopyrrolo[3,4-c]pyrrole) unit.

Figure 2. Gaussian-predicted (B3LYP/6-31G* level) conformations of diphenyl-DPP (a) and dithienyl-DPP (b) in front view (top) and side view (bottom) illustrating the increased torsional backbone twist (27° dihedral angle) in diphenyl-DPP relative to dithienyl-DPP (12° dihedral angle).

Full text

1
DOI: 10.1002/adma.((please add manuscript number))
Recent Advances in the Development of Semiconducting DPP-Containing Polymers for
Transistor Applications
By Christian B. Nielsen,* Mathieu Turbiez, and Iain McCulloch
[*] Dr. C. B. Nielsen, Prof. I. McCulloch
Department of Chemistry and Centre for Plastic Electronics
Imperial College London, London SW7 2AZ (United Kingdom)
E-mail: c.nielsen@imperial.ac.uk
Dr. M. Turbiez
Organic Electronic Materials Basel
BASF Schweiz AG, Schwarzwaldallee 215, 4002 Basel (Switzerland)
Keywords: conjugated polymers, donor-acceptor copolymers, field-effect transistors,
semiconducting polymers, organic electronics
This progress report summarizes the numerous DPP-containing
polymers recently developed for field-effect transistor applications
including diphenyl-DPP and dithienyl-DPP-based polymers as the
most commonly reported materials, but also difuranyl-DPP,
diselenophenyl-DPP and dithienothienyl-DPP-containing polymers. We discuss the hole and
electron mobilities that were reported in relation to structural properties such as alkyl
substitution patterns, polymer molecular weights and solid state packing, as well as electronic
properties including HOMO and LUMO energy levels. We moreover consider important
aspects of ambipolar charge transport and highlight fundamental structure-property relations
such as the relationships between the thin film morphologies and the charge carrier mobilities
observed for DPP-containing polymers.
1. Introduction
Since the first polythiophene field-effect transistor (FET) was fabricated in 1986,
[1]
polymeric
semiconductors have made considerable progress, now reaching performances similar to
amorphous silicon. Unlike their inorganic counterpart, semiconducting polymers have the
potential for low cost synthesis and can be processed from solution on roll to roll machinery
((Picture for
Abstract 40mm
broad, 50 mm
high))

2
for high throughput, low cost production resulting in light weight, flexible and unbreakable
devices. The semiconducting properties are directly related to their molecular ordering,
molecular weight, growth mode, and purity.
[2]
In this context, diketopyrrolopyrrole (DPP)
based polymers are currently displaying some of the highest mobilities due to the remarkable
aggregating properties of the DPP moities.
[3]
The potential of DPP containing polymers as
semiconductor materials for organic field-effect transistor (OFET) and organic photovoltaic
(OPV) application was discovered in 2005 by Mathieu Turbiez.
[4]
Since the first
demonstrations of ambipolar mobilities of 0.1cm
2
/Vs,
[5, 6]
work has escalated on this very
promising chemical platform to reach record p-type mobilities of 8.2 cm
2
/Vs and n-type
mobilities of 1.56cm
2
/Vs.
[7, 8]
In this progress report, we will summarize the broad scope of
publications reporting on various DPP containing polymers and highlight the relationship
between the thin film morphology and the mobilities observed.
2. Diphenyl-DPPs
The first diphenyl-DPP-based polymers (Figure 1) were reported as early as 1993 by Yu and
co-workers for photorefractive applications,
[9]
but very few studies of the semiconducting
properties of diphenyl-DPP polymers have since been published (Table 1). The vinylene-
copolymer P1 showed a rather low hole mobility mainly ascribed to the formation of an
amorphous solid state structure.
[10]
P2, on the other hand, with a solubilized phenylene-
vinylene motif and linear alkyl chains on the DPP unit rather than branched chains as in the
case of P1, showed a significantly higher hole mobility of 5.4 10
-4
cm
2
/Vs.
[11]
After annealing
at 150°C, X-ray diffraction (XRD) of P2 thin films showed peaks corroborating both lamellar
order and - stacking, which correlates well with the large increase in charge carrier
mobility when compared to P1. To date, the best OFET properties from a diphenyl-DPP-
based copolymer have been reported by Li and co-workers, who copolymerized the diphenyl-
DPP unit with bithiophene to afford P3.
[12]
Initial SCLC measurements revealed a hole

3
mobility of 2.1 10
-4
cm
2
/Vs and an electron mobility of 4.7 10
-5
cm
2
/Vs and after fabrication
of OFET devices, a hole mobility of 0.04 cm
2
/Vs was achieved. Interestingly, only slightly
inferior results were obtained with a lower molecular weight batch of P3. Several other
thiophene-containing units have also been incorporated into diphenyl-DPP polymers as
evident from Table 1 (P4-P6).
[13, 14]
In a comparison of two fused bithiophene systems,
namely the cyclopentadithiophene (CPDT, P4) and the dithienopyrrole (DTP, P5), Chen and
co-workers found the latter copolymer to perform better in a FET device with a hole mobility
of 2.2 10
-3
cm
2
/Vs.
[14]
The lower alkyl density of the DTP-unit as compared to the CPDT-unit
with two branched alkyl substituents is most likely responsible for the improved local order
(observed as fibrils by atomic force microscopy (AFM) analysis) and hence the larger charge
carrier mobility in thin films of P5.
Whereas numerous diphenyl-DPP-based donor-acceptor type copolymers have been
applied with considerable success in organic photovoltaic (OPV) devices,
[15]
a remarkable
lack of similarly high-performing transistor materials is evident from the data presented in
Table 1. One explanation for this is illustrated in Figure 2. Steric hindrance between the
oxygen atoms of the lactams and the phenyl -hydrogen atoms prevents a coplanar
conformation. Quantum-chemical calculations predict a hydrogen-oxygen distance of 2.33 Å
(the sum of the Van der Waal radii is 2.61 Å) and a dihedral angle of 27° between the phenyl
and the DPP-unit as a consequence of this electronic repulsion. There is likely to also be an
energetic penalty for planarization of the link between the opposite end of the phenyl group
and the comonomer of choice (Ar in Figure 1). These backbone twists are expected to prevent
strong intermolecular - interactions and hence a tight and ordered packing, which is often
associated with good charge transport properties. The prediction of a low degree of backbone
coplanarity and a high probability of disorder on a macroscopic scale for diphenyl-DPP

4
copolymers is in good agreement with the observed charge carrier mobilities and the lack of
reports on crystallinity for this class of materials.
3. Dithienyl-DPPs
As a logical consequence of the anticipated backbone twist when placing phenyl groups
adjacent to the DPP-unit, thiophene units have subsequently been introduced instead to afford
the dithienyl-DPP motif. As can be seen in Figure 2, the smaller thiophene unit with only one
-hydrogen atom can orient in a nearly coplanar fashion with favorable intramolecular
sulfur-oxygen interactions (S-O distance of 3.03 Å predicted, sum of Van der Waal radii is
3.32 Å) and dihedral angles of 12° between the DPP unit and its adjacent thiophene units.
The prospect of dithienyl-DPP copolymers (general structure depicted in Figure 3) in OFET
applications is reflected in the large number of polymers of this type included in Table 2.
Zoombelt and co-workers synthesized the homopolymer of a solubilized diphenyl-
DPP unit (P7) and found it to exhibit ambipolar FET properties, although the reported values
around 10
-4
cm
2
/Vs for both the hole and electron mobilities were quite low.
[16]
More
recently, Li and co-workers have also reported on P7 and found significantly improved
charge carrier mobilities on the order of 10
-2
cm
2
/Vs.
[17]
Jenekhe’s group who reported on P1
also made the corresponding dithienyl-DPP copolymer (P8) and found it to perform much
better than P1, with a hole mobility of 0.17 cm
2
/Vs and an electron mobility of 1.9 10
-2
cm
2
/Vs in a field-effect transistor.
[10]
XRD confirmed P8 to be crystalline with a - stacking
distance of 3.92 Å, which correlates well with the drastic improvement over the amorphous
diphenyl-DPP analogue (P1) and the relationship to coplanarity discussed in relation to
Figure 2. P9, having a 1,4-phenylene unit rather than the vinylene of P8, showed similar
properties with a good electron mobility of 2 10
-2
cm
2
/Vs and a slightly reduced hole mobility
of 4 10
-2
cm
2
/Vs compared to P8.
[18]
Another direct comparison of the diphenyl- and
dithienyl-DPP systems was provided by Zhang and co-workers, who found P10 to behave

5
inferior to the diphenyl-DPP analogue (P2).
[11]
Both sets of materials appear crystalline and
the observed difference in this case more likely stems from the much lower molecular
weights obtained for the P10 polymers (Table 2). In 2011, Sonar and co-workers published
their work on P11 having a naphthalene unit as the other comonomer and found this polymer
to display very good FET properties with maximum hole mobilities approaching 1 cm
2
/Vs.
[19,
20]
Instrumental in achieving these results was a thorough optimization process including
substrate surface treatments and thermal annealing of the polymer thin film at 140°C, which -
by XRD and AFM - was found to significantly improve the crystallinity of the polymer. A
series of fluorene analogous structures (fluorene (P12), carbazole (P13), and germafluorene
(P14-P15)) have been investigated for copolymerization with the dithienyl-DPP repeat
unit.
[16, 21, 22]
In FET devices, P12 displays ambipolarity, but both the electron and the hole
mobilities are quite poor (10
-4
- 10
-6
cm
2
/Vs). The carbazole- and germafluorene-containing
polymers P13 and P14 show much improved hole mobilities on the order of 10
-2
cm
2
/Vs,
whereas the germafluorene-based material with longer alkyl chains (P15) is somewhere in
between with a hole mobility of 8 10
-3
cm
2
/Vs . Elongation of the solubilizing alkyl
substituents from butyl (P14) to octyl (P15) affects the intermolecular packing drastically,
with an increase in the - stacking distance from 3.8 Å to 4.7 Å, which in this case is the
main factor responsible for the more than 5-fold decrease in charge carrier mobility.
[22]
Bearing in mind the excellent charge transport properties and the highly ordered
packing of many thiophene-based polymers,
[23]
the incorporation of thiophene-containing
comonomers was an obvious step in the continued development of DPP polymers for high-
performing transistor devices. The simplest copolymer, P16, with unsubstituted thiophene
has been studied in great detail.
[24-26]
In 2009, Bijleveld and co-workers reported on P16 with
a C6C10 (2-hexyl-1-decyl) branched alkyl chain on the DPP unit and they found a maximum
hole mobility of 5 10
-2
cm
2
/Vs and an electron mobility of 1 10
-2
cm
2
/Vs. Interestingly, they

Frequently asked questions

Q1. What have the authors contributed in "Recent advances in the development of semiconducting dpp-containing polymers for transistor applications" ?

Nielsen et al. this paper discussed over 80 different polymers containing the DPP moiety and more than half of these exhibited an electron and/or hole mobility greater than 0.01cm 2 /Vs. 

Q2. What are the useful tools in elucidating the structural ordering of DPP-?

X-Ray techniques such as X-ray diffraction (XRD), grazing incidence X-ray diffraction (GIXD) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy areextremely useful tools in elucidating the structural ordering for -conjugated materials. 

Q3. What is the general trend related to the solid state morphology?

Some general trends related to the solid state morphology, which are commonlyobserved for these DPP polymers, are that an increased degree of crystallinity typically isassociated with an increase in charge carrier mobility and that a shortened - stackingdistance typically is associated with an increase in charge carrier mobility as well. 

Q4. What is the reason for the poorer performance of P70 relative to P71?

Bronstein and co-workers speculate that the short repeat unit of P70 brings the bulky alkyl-substituted DPP units too close to each other and thus hinders backbone coplanarity and interchain packing, which could explain the poorer FET performance of P70 relative to P71. 

Q5. What is the effect of the addition of alkyl chains on the DPP unit?

The presence of long branched alkyl chains on the DPP unit appears to have limitedimpact on the electrical performance, whereas the addition of alkyl chains on other locations along the backbone often induces detrimental steric hindrance. 

Q6. What is the reason for the improved local order of the DTP-unit?

The lower alkyl density of the DTP-unit as compared to the CPDT-unit with two branched alkyl substituents is most likely responsible for the improved local order (observed as fibrils by atomic force microscopy (AFM) analysis) and hence the larger charge carrier mobility in thin films of P5. 

Q7. What is the description of the ambipolar properties of P69?

P69 is the second example in the diselenophenyl-DPP series where the anticipated improvement in electron transport from a more stable and delocalized LUMO is confirmed experimentally; the best electron mobilityof P69 is nearly double that of P53, while electron mobilities for the two polymers are comparable. 

Q8. What is the polarizability of the selenophene unit?

This is indicative of an attractive Se-O interaction as also seen for S-O in the case of dithienyl-DPP; the coplanar conformation can moreover be accommodated by the higher polarizability of the selenophene unit, which is also seen by a slight distortion of the selenophene geometry. 

Q9. What was the performance for a DTT-containing polymer?

Prior to the reporting of P42, the best FET performance for a DTT-containing polymer was found for P45, which hada high molecular weight similar to that of P42 and a hole mobility of 3.3 10 -2 cm 2 /Vs. [49] 

Q10. What is the reason for the improvement in hole mobility observed for P16?

As firmly established for P3HT, [27] increased molecular weights are often associated with improved charge carrier mobilities and this could very well be the explanation for the improvement in hole mobility observed for P16 by Zhang and co-workers. 

Q11. What temperature did Zhang and co-workers achieve their highest hole mobility?

Both Ha’s and Zhang’s results were obtained after thermal annealing at 150°C, while Li and co-workers achieved their highest mobility with a more moderate 100°C annealing step. 

Q12. What is the prediction of a low degree of backbone coplanarity?

The prediction of a low degree of backbone coplanarity and a high probability of disorder on a macroscopic scale for diphenyl-DPPcopolymers is in good agreement with the observed charge carrier mobilities and the lack of reports on crystallinity for this class of materials. 

Q13. What is the reason for the lack of high-performing transistor materials?

Whereas numerous diphenyl-DPP-based donor-acceptor type copolymers have beenapplied with considerable success in organic photovoltaic (OPV) devices, [15] a remarkable lack of similarly high-performing transistor materials is evident from the data presented in Table 1. 

Q14. What is the potential of DPP containing polymers?

The potential of DPP containing polymers as semiconductor materials for organic field-effect transistor (OFET) and organic photovoltaic (OPV) application was discovered in 2005 by Mathieu Turbiez. [4] 

Q15. What temperature did P54 exhibit the improvement in FET performance?

Both materials showed optimum performance after 160-200°C annealing, while especially P55 suffered a drastic decrease in FET performance upon higher temperature annealing.