the neighboring thiophene units led to a slight curvature of the polymer backbone.
This modification increased solubility (in dichlorobenzene) and avoided very strong
aggregation, which would impede ordered film formation. The hole mobility of
such an optimized conjugated polymer was >0.1 cm
2 V
À1 s
À1 without any special
annealing or processing conditions. The linear isomer (P5 in Fig. 11, with R
0
¼ H,
R ¼ n-C 12 H 25 ) also packed very tightly, but here the π–π-interactions were too
strong and led to precipitation during processing and thus low uniformity of the
thin films. The resulting field-effect mobilities were ten times lower than those of
the curved P4. Pan et al. showed that addition of alkyl side chains to the central,
linear benzo[1,2-b:4,5-b
0 ]dithiophene unit (P5, with R
0
¼ R ¼ n-C 6 H 13 or
R
0
¼ R ¼ n-C 10 H 21 ) enabled good solubility in 1,2-dichlorobenzene and gave
highly ordered films with mobilities of 0.25 and 0.4 cm
2 V
À1 s
À1 , respectively,
and high on/off ratios of 10
6 , combined with good air-stability [109, 116, 117].
The cyclopentadithiophenes (CPDT) are another class of promising fused
thiophene-based materials. In CPDTs, the bridging carbon atom with the alkyl
substituents is sp
3 -hybridized and does not participate in the delocalization along
the polymer backbone. Films of CPDT homopolymers showed a disordered, amorphous structure and only low (10
À4 cm
2 V
À1 s
À1 ) mobilities [118]. In contrast,
copolymers of CPDT with the electron-withdrawing group 2,1,3-benzothiadiazole
(BT) (CPDT-BT; P6 in Fig. 11, with R ¼ n-C 16 H 33 ) exhibited much higher mobilities [119], which could be increased up to 3.3 cm
2 V
À1 s
À1 by optimized
processing [120, 121].
The high mobilities of the CPDT-BT copolymers renewed interest in thiophene
copolymers with electron-deficient comonomers. Fan et al. synthesized a copolymer of the highly electron-withdrawing benzobisthiadiazole (BBT) and dialkylated
quaterthiophene (P7 in Fig. 11, with R ¼ 2-octyldodecyl). This polymer packed
with a very short π–π stacking distance of 3.5 Å and exhibited an average hole
mobility of 1.8 cm
2 V
À1 s
À1 after annealing at 280
C [122]. Due to its donor–
acceptor structure, the bandgap of this polymer is quite small (0.7 eV) and the
lowest unoccupied molecular orbital (LUMO) is low enough to enable electron
injection and thus ambipolar charge transport.
All these examples show that, although P3HT is no longer in the race as a highmobility semiconducting polymer, the structural motifs of thiophenes and fused
thiophenes are still important as building blocks for the next generation of polymer
semiconductors. Even the current record holders with hole mobilities of up to
10 cm
2 V
À1 s
À1 that are based on the acceptor moiety N-alkyl diketopyrrolo-pyrrole
(DPP) are copolymers with several thiophene units (e.g., P-29-DPPDBTE
107
) or
dithienylthieno[3,2-b]thiophene (DPP-DTT
106
; P8 in Fig. 11). The high mobilities
of these polymers are not based on high crystallinity but on a morphology with small
length-scale, short contacts interconnected by polymer backbones that allow
one-dimensional transport. These short contacts are enhanced by dipolar interactions
between neighboring backbones, as for example in DPP copolymers, in which the
polar pyrrole units drive aggregation through noncovalent interactions [123].
130
J. Zaumseil
This modification increased solubility (in dichlorobenzene) and avoided very strong
aggregation, which would impede ordered film formation. The hole mobility of
such an optimized conjugated polymer was >0.1 cm
2 V
À1 s
À1 without any special
annealing or processing conditions. The linear isomer (P5 in Fig. 11, with R
0
¼ H,
R ¼ n-C 12 H 25 ) also packed very tightly, but here the π–π-interactions were too
strong and led to precipitation during processing and thus low uniformity of the
thin films. The resulting field-effect mobilities were ten times lower than those of
the curved P4. Pan et al. showed that addition of alkyl side chains to the central,
linear benzo[1,2-b:4,5-b
0 ]dithiophene unit (P5, with R
0
¼ R ¼ n-C 6 H 13 or
R
0
¼ R ¼ n-C 10 H 21 ) enabled good solubility in 1,2-dichlorobenzene and gave
highly ordered films with mobilities of 0.25 and 0.4 cm
2 V
À1 s
À1 , respectively,
and high on/off ratios of 10
6 , combined with good air-stability [109, 116, 117].
The cyclopentadithiophenes (CPDT) are another class of promising fused
thiophene-based materials. In CPDTs, the bridging carbon atom with the alkyl
substituents is sp
3 -hybridized and does not participate in the delocalization along
the polymer backbone. Films of CPDT homopolymers showed a disordered, amorphous structure and only low (10
À4 cm
2 V
À1 s
À1 ) mobilities [118]. In contrast,
copolymers of CPDT with the electron-withdrawing group 2,1,3-benzothiadiazole
(BT) (CPDT-BT; P6 in Fig. 11, with R ¼ n-C 16 H 33 ) exhibited much higher mobilities [119], which could be increased up to 3.3 cm
2 V
À1 s
À1 by optimized
processing [120, 121].
The high mobilities of the CPDT-BT copolymers renewed interest in thiophene
copolymers with electron-deficient comonomers. Fan et al. synthesized a copolymer of the highly electron-withdrawing benzobisthiadiazole (BBT) and dialkylated
quaterthiophene (P7 in Fig. 11, with R ¼ 2-octyldodecyl). This polymer packed
with a very short π–π stacking distance of 3.5 Å and exhibited an average hole
mobility of 1.8 cm
2 V
À1 s
À1 after annealing at 280
C [122]. Due to its donor–
acceptor structure, the bandgap of this polymer is quite small (0.7 eV) and the
lowest unoccupied molecular orbital (LUMO) is low enough to enable electron
injection and thus ambipolar charge transport.
All these examples show that, although P3HT is no longer in the race as a highmobility semiconducting polymer, the structural motifs of thiophenes and fused
thiophenes are still important as building blocks for the next generation of polymer
semiconductors. Even the current record holders with hole mobilities of up to
10 cm
2 V
À1 s
À1 that are based on the acceptor moiety N-alkyl diketopyrrolo-pyrrole
(DPP) are copolymers with several thiophene units (e.g., P-29-DPPDBTE
107
) or
dithienylthieno[3,2-b]thiophene (DPP-DTT
106
; P8 in Fig. 11). The high mobilities
of these polymers are not based on high crystallinity but on a morphology with small
length-scale, short contacts interconnected by polymer backbones that allow
one-dimensional transport. These short contacts are enhanced by dipolar interactions
between neighboring backbones, as for example in DPP copolymers, in which the
polar pyrrole units drive aggregation through noncovalent interactions [123].
130
J. Zaumseil