5 Polymer Solar Cells: Development of π-Conjugated Polymers …
105
naphthobischalcogenadiazoles (NXz) such as naphtho1,2-c:5,6-c
]bis[1, 2, 5]oxadiazole (NOz) [33, 34], naphtho1,2-c:5,6-c
]bis[1, 2, 5]thiadiazole (NTz) [35, 36], and
naphtho1,2-c:5,6-c
]bis[1, 2, 5]selenadizaole (NSz) [33] (Fig. 5.14) are emerging
acceptor units that can be used for semiconducting polymers. The advantages of NXz
over BXz are the extended π-electron system that enhances the intermolecular interactions and thus the ordering of the polymers, and the enhanced electron deficiency
that deepens the HOMO and LUMO energy levels. Further, with its centrosymmetric
structure, NXz is beneficial for designing more regularly structured polymers, which
would contribute to the polymer ordering, relative to BXz with an axisymmetric
structure. In particular, NOz and NTz are found to be promising building units for
π-conjugated polymers.
5.3.2 Quaterthiophene–Naphthobisthiadiazole Polymer
(PNTz4T): Comparison with Benzothiadiazole Analog
NTz is the most widely studied unit among the family of NXz [37, 38]. While the
group of Cao and Huang has reported a series of benzodithiophene–NTz polymers
and their application to solar cell [35], we have been studying a series of quaterthiophene–NTz polymers, such as PNTz4T (Fig. 5.15a) [36]. In these polymers, the
2-decyltetradecyl (DT) long branched alkyl group was used as the substituent to
ensure sufficient solubility in solvents, which was attached to the thiophene rings
neighboring the NTz moiety at β-positions heading the unsubstituted bithiophene
moiety to avoid steric hindrance between NTz and alkylthiophene. The absorption
of PNTz4T covered whole visible range with the absorption maximum (λ max ) of
725 nm and the absorption edge (λ edge ) reaching 805 nm, which were red-shifted for
approximately 50 nm relative to those of PBTz4T (Fig. 5.15b). As a consequence, the
E g was determined to be 1.54 eV, which was 0.1 eV smaller than that of PBTz4T. Both
the HOMO and LUMO energy levels were deeper for PNTz4T than PBTz4T. These
results are most likely due to the strong electron-poor nature of NTz as compared to
BTz.
The hole mobility, evaluated by using OFET devices, for PNTz4T was as high
as 0.56 cm
2 V
−1 s
−1 and approximately one order of magnitude higher than that
for PBTz4T (~0.05 cm
2 V
−1 s
−1 ). In solar cells with a conventional structure
(ITO/PEDOT:PSS/(polymer/PC 61 BM)/LiF/Al), whereas the PBTz4T cell showed
a PCE of 2.6% (J SC = 5.6 mA cm
−2 , V OC = 0.74 V, FF = 0.63), the PNTz4T cell
showed an approximately twofold higher PCE of 6.3% (J SC = 12.0 mA cm
−2 , V OC
= 0.76 V, FF = 0.69) (Fig. 5.15c).
The higher photovoltaic performance of PNTz4T relative to PBTz4T was understood by investigating the thin-film structure on the basis of 2D GIXD measurements.
Figure 5.16a–d shows the 2D GIXD patterns of PNTz4T and PBTz4T in the neat and
blend film. In the PNTz4T neat film, diffractions assignable to the lamellar structure, up to the fifth order, and a diffraction assignable to the π–π stacking structure,
105
naphthobischalcogenadiazoles (NXz) such as naphtho1,2-c:5,6-c
]bis[1, 2, 5]oxadiazole (NOz) [33, 34], naphtho1,2-c:5,6-c
]bis[1, 2, 5]thiadiazole (NTz) [35, 36], and
naphtho1,2-c:5,6-c
]bis[1, 2, 5]selenadizaole (NSz) [33] (Fig. 5.14) are emerging
acceptor units that can be used for semiconducting polymers. The advantages of NXz
over BXz are the extended π-electron system that enhances the intermolecular interactions and thus the ordering of the polymers, and the enhanced electron deficiency
that deepens the HOMO and LUMO energy levels. Further, with its centrosymmetric
structure, NXz is beneficial for designing more regularly structured polymers, which
would contribute to the polymer ordering, relative to BXz with an axisymmetric
structure. In particular, NOz and NTz are found to be promising building units for
π-conjugated polymers.
5.3.2 Quaterthiophene–Naphthobisthiadiazole Polymer
(PNTz4T): Comparison with Benzothiadiazole Analog
NTz is the most widely studied unit among the family of NXz [37, 38]. While the
group of Cao and Huang has reported a series of benzodithiophene–NTz polymers
and their application to solar cell [35], we have been studying a series of quaterthiophene–NTz polymers, such as PNTz4T (Fig. 5.15a) [36]. In these polymers, the
2-decyltetradecyl (DT) long branched alkyl group was used as the substituent to
ensure sufficient solubility in solvents, which was attached to the thiophene rings
neighboring the NTz moiety at β-positions heading the unsubstituted bithiophene
moiety to avoid steric hindrance between NTz and alkylthiophene. The absorption
of PNTz4T covered whole visible range with the absorption maximum (λ max ) of
725 nm and the absorption edge (λ edge ) reaching 805 nm, which were red-shifted for
approximately 50 nm relative to those of PBTz4T (Fig. 5.15b). As a consequence, the
E g was determined to be 1.54 eV, which was 0.1 eV smaller than that of PBTz4T. Both
the HOMO and LUMO energy levels were deeper for PNTz4T than PBTz4T. These
results are most likely due to the strong electron-poor nature of NTz as compared to
BTz.
The hole mobility, evaluated by using OFET devices, for PNTz4T was as high
as 0.56 cm
2 V
−1 s
−1 and approximately one order of magnitude higher than that
for PBTz4T (~0.05 cm
2 V
−1 s
−1 ). In solar cells with a conventional structure
(ITO/PEDOT:PSS/(polymer/PC 61 BM)/LiF/Al), whereas the PBTz4T cell showed
a PCE of 2.6% (J SC = 5.6 mA cm
−2 , V OC = 0.74 V, FF = 0.63), the PNTz4T cell
showed an approximately twofold higher PCE of 6.3% (J SC = 12.0 mA cm
−2 , V OC
= 0.76 V, FF = 0.69) (Fig. 5.15c).
The higher photovoltaic performance of PNTz4T relative to PBTz4T was understood by investigating the thin-film structure on the basis of 2D GIXD measurements.
Figure 5.16a–d shows the 2D GIXD patterns of PNTz4T and PBTz4T in the neat and
blend film. In the PNTz4T neat film, diffractions assignable to the lamellar structure, up to the fifth order, and a diffraction assignable to the π–π stacking structure,