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I. Osaka
wavelength range of 400–600 nm and E g of approximately 1.9 eV. The absorption
bands were very much blue-shifted compared to those of NTz-based polymers, which
probably originated in the weak electron deficiency of the TZNT unit. The inverted
OPV cells that used TZNT-based polymers had relatively high V OC s of up to 0.92 V,
which resulted from the relatively deep HOMO energy levels. PTZNT2 gave the
highest PCE of 7.11% among these polymers.
15.3.4 NPz-Based Polymers
Our group has been involved in the synthesis of a series of NPz-based polymers. We
first reported an NPz-based polymer (PNPz4T; Fig. 15.10b) in which tetramethyl NPz
was combined with the quaterthiophene unit, similar to PNTz4T (Mikie et al. 2017).
PNPz4T had an E g of 1.76 eV, which was wider than that of PNTz4T. The HOMO
and LUMO energy levels of PNPz4T were –5.05 eV and –3.02 eV, respectively, both
of which were shallower than those of PNTz4T. The results suggest that NPz has
weak electron deficiency relative to NTz. The photovoltaic cell that used PNPz4T
(ITO/ZnO/PNPz4T:PC 61 BM/MoO x /Ag) showed a maximum PCE of 2.82% with
J SC of 7.9 mA cm
–2 , V OC of 0.59 V, and FF of 0.60.
One major advantage of NPz is that the substituents can be introduced at 2-, 3-, 8-,
and 9-positions. We introduced one ester group to each of those positions, which was
copolymerized with oligothiophene donor units (PeNPz2T, PeNPz3T, and PeNPz4T)
(Fig. 15.10c) (Mikie and Osaka 2019). Owing to the electron-withdrawing property of
the ester group, these polymers had deep HOMO and LUMO energy levels. Whereas
the HOMO energy levels varied in the range of –5.1 to –5.4 eV with the oligothiophene unit, the LUMO energy levels were consistently approximately –3.5 eV.
Further, it was interesting to note that the absorption range was red-shifted by more
than 150 nm relative to that of PNPz4T with methyl or dodecyl substituents. E g was
around 1.50 eV. The polymers formed relatively highly crystalline structures with
the face-on orientation, which were also favorable for OPVs, in both neat and blend
films. In contrast, PeNPz2T with the bithiophene donor unit showed weak crystallinity, likely ascribed to the steric hindrance between the substituents. Although
the OPV cells that used the polymers (ITO/ZnO/polymer:PC 61 BM/MoO x /Ag) had
moderate PCEs that were as high as 6.3%, the narrow E g of the polymers was indeed
advantageous.
Peng and co-workers reported on BDT-NPz polymers PBDT-NQxT and PBDTSNQxT (Fig. 15.10d), in which the octylthienyl group was introduced at each of the 2-,
3-, 8-, and 9-positions of the NPz unit (Yu et al. 2017). PBDT–NQxT and PBDTS–
NQxT had HOMO energy levels of –5.24 eV and –5.31 eV, and E g s of 1.80 eV and
1.81 eV, respectively. OPV cells were fabricated with the combination of PC 71 BM
and a non-fullerene compound (ITIC) as the n-type material. The PC 71 BM-based
cell showed moderate PCEs of up to 7%, which were likely limited by the short
photoresponse range below 700 nm. On the other hand, the ITIC-based cell had a
photoresponse range of up to 800 nm, thereby resulting in the highest PCE of 11.5%.
I. Osaka
wavelength range of 400–600 nm and E g of approximately 1.9 eV. The absorption
bands were very much blue-shifted compared to those of NTz-based polymers, which
probably originated in the weak electron deficiency of the TZNT unit. The inverted
OPV cells that used TZNT-based polymers had relatively high V OC s of up to 0.92 V,
which resulted from the relatively deep HOMO energy levels. PTZNT2 gave the
highest PCE of 7.11% among these polymers.
15.3.4 NPz-Based Polymers
Our group has been involved in the synthesis of a series of NPz-based polymers. We
first reported an NPz-based polymer (PNPz4T; Fig. 15.10b) in which tetramethyl NPz
was combined with the quaterthiophene unit, similar to PNTz4T (Mikie et al. 2017).
PNPz4T had an E g of 1.76 eV, which was wider than that of PNTz4T. The HOMO
and LUMO energy levels of PNPz4T were –5.05 eV and –3.02 eV, respectively, both
of which were shallower than those of PNTz4T. The results suggest that NPz has
weak electron deficiency relative to NTz. The photovoltaic cell that used PNPz4T
(ITO/ZnO/PNPz4T:PC 61 BM/MoO x /Ag) showed a maximum PCE of 2.82% with
J SC of 7.9 mA cm
–2 , V OC of 0.59 V, and FF of 0.60.
One major advantage of NPz is that the substituents can be introduced at 2-, 3-, 8-,
and 9-positions. We introduced one ester group to each of those positions, which was
copolymerized with oligothiophene donor units (PeNPz2T, PeNPz3T, and PeNPz4T)
(Fig. 15.10c) (Mikie and Osaka 2019). Owing to the electron-withdrawing property of
the ester group, these polymers had deep HOMO and LUMO energy levels. Whereas
the HOMO energy levels varied in the range of –5.1 to –5.4 eV with the oligothiophene unit, the LUMO energy levels were consistently approximately –3.5 eV.
Further, it was interesting to note that the absorption range was red-shifted by more
than 150 nm relative to that of PNPz4T with methyl or dodecyl substituents. E g was
around 1.50 eV. The polymers formed relatively highly crystalline structures with
the face-on orientation, which were also favorable for OPVs, in both neat and blend
films. In contrast, PeNPz2T with the bithiophene donor unit showed weak crystallinity, likely ascribed to the steric hindrance between the substituents. Although
the OPV cells that used the polymers (ITO/ZnO/polymer:PC 61 BM/MoO x /Ag) had
moderate PCEs that were as high as 6.3%, the narrow E g of the polymers was indeed
advantageous.
Peng and co-workers reported on BDT-NPz polymers PBDT-NQxT and PBDTSNQxT (Fig. 15.10d), in which the octylthienyl group was introduced at each of the 2-,
3-, 8-, and 9-positions of the NPz unit (Yu et al. 2017). PBDT–NQxT and PBDTS–
NQxT had HOMO energy levels of –5.24 eV and –5.31 eV, and E g s of 1.80 eV and
1.81 eV, respectively. OPV cells were fabricated with the combination of PC 71 BM
and a non-fullerene compound (ITIC) as the n-type material. The PC 71 BM-based
cell showed moderate PCEs of up to 7%, which were likely limited by the short
photoresponse range below 700 nm. On the other hand, the ITIC-based cell had a
photoresponse range of up to 800 nm, thereby resulting in the highest PCE of 11.5%.