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I. Osaka
are significantly reduced from that of the PNTz4T/PCBM system (~0.85 eV) and
are even smaller than 0.6 eV that has been referenced as the empirical limit for
polymer solar cells [41].
Figure 5.24c depicts the plots of eV OC against E g for the PNTz4T and PNOz4T
systems in comparison with various solar cell systems such as inorganic, perovskite,
dye-sensitized (DSSC), small molecule-based organic, and polymer solar cells, in
which the lines of E loss estimated by the Shockley–Queisser theory [56] and the
empirically estimated limit (0.6 eV) are drawn. Figure 5.24d shows the plots of PCE
against E loss for the PNTz4T and PNOz4T systems along with other organic systems
plotted in Fig. 5.24c [51]. The red squares of a–e indicate the polymer systems
with E loss ≤ 0.6 eV. It is clear that the E loss of the PNOz4T system are among the
lowest values reported so far for polymers solar cells, and even approach the values
for inorganic or perovskite solar cells, 0.4–0.5 eV. More importantly, PCEs of the
PNOz4T system are in fact the highest values among the organic systems having
E loss ≤ 0.6 eV.
We note here that, even though the EQE values for the PNOz4T cells were limited
to ca. 60%, these values are quite high for the polymer/PCBM systems with such a
small E L . For instance, although the system of DPP-2Tz polymers/PC 71 BM was
reported to have a small E L of 0.09–0.21 eV and provide a high V OC of more
than 0.9 V, it only showed low EQEs of 5–40% [57]. Therefore, to the best of our
knowledge, in demonstrating high PCEs of ~9% with a high V OC of ~1 V, small E loss
and a small energy offset at the same time, PNOz4T is regarded as quite a unique
narrow bandgap polymer. In addition, the study of charge generation dynamics in
comparison with the PNTz4T system revealed that the relatively low EQE of the
PNOz4T system originates in the relatively large domain size of the blend films and
thus limited exciton diffusion, and not originates in the small E L [51]. Thus, it is
expected that further modification of the polymer structure and/or optimization of
the morphology of the blend film can still improve EQE and thus J SC .
5.3.4 Summary
This section has highlighted the π-conjugated polymers based on naphthobisthiadiazole (NTz) and naphthobisoxadiazole (NOz). The advantages of these building units
over their counterpart units, benzothiadiazole (BTz) and benzooxadiazole (BOz), are
their stronger electron-poor nature, extended π-electron system with rigid structure,
and structural symmetry, all of which are important for developing high-performance
polymers. In fact, polymers based on NTz showed deeper HOMO and LUMO energy
levels, narrower bandgap, better ordering structures, and better device performances
in both OFETs and OPVs compared to the polymer based on BTz. In particular, a
quaterthiophene-NTz polymer, PNTz4T, exhibited efficiencies reaching 10% when
blended with PC 71 BM in the solar cell. The introduction of the fluorine atoms successfully reduced the photon energy loss, resulting in the enhanced V OC , and thereby the
improved efficiencies that are close to 11%, which are among the highest reported so
I. Osaka
are significantly reduced from that of the PNTz4T/PCBM system (~0.85 eV) and
are even smaller than 0.6 eV that has been referenced as the empirical limit for
polymer solar cells [41].
Figure 5.24c depicts the plots of eV OC against E g for the PNTz4T and PNOz4T
systems in comparison with various solar cell systems such as inorganic, perovskite,
dye-sensitized (DSSC), small molecule-based organic, and polymer solar cells, in
which the lines of E loss estimated by the Shockley–Queisser theory [56] and the
empirically estimated limit (0.6 eV) are drawn. Figure 5.24d shows the plots of PCE
against E loss for the PNTz4T and PNOz4T systems along with other organic systems
plotted in Fig. 5.24c [51]. The red squares of a–e indicate the polymer systems
with E loss ≤ 0.6 eV. It is clear that the E loss of the PNOz4T system are among the
lowest values reported so far for polymers solar cells, and even approach the values
for inorganic or perovskite solar cells, 0.4–0.5 eV. More importantly, PCEs of the
PNOz4T system are in fact the highest values among the organic systems having
E loss ≤ 0.6 eV.
We note here that, even though the EQE values for the PNOz4T cells were limited
to ca. 60%, these values are quite high for the polymer/PCBM systems with such a
small E L . For instance, although the system of DPP-2Tz polymers/PC 71 BM was
reported to have a small E L of 0.09–0.21 eV and provide a high V OC of more
than 0.9 V, it only showed low EQEs of 5–40% [57]. Therefore, to the best of our
knowledge, in demonstrating high PCEs of ~9% with a high V OC of ~1 V, small E loss
and a small energy offset at the same time, PNOz4T is regarded as quite a unique
narrow bandgap polymer. In addition, the study of charge generation dynamics in
comparison with the PNTz4T system revealed that the relatively low EQE of the
PNOz4T system originates in the relatively large domain size of the blend films and
thus limited exciton diffusion, and not originates in the small E L [51]. Thus, it is
expected that further modification of the polymer structure and/or optimization of
the morphology of the blend film can still improve EQE and thus J SC .
5.3.4 Summary
This section has highlighted the π-conjugated polymers based on naphthobisthiadiazole (NTz) and naphthobisoxadiazole (NOz). The advantages of these building units
over their counterpart units, benzothiadiazole (BTz) and benzooxadiazole (BOz), are
their stronger electron-poor nature, extended π-electron system with rigid structure,
and structural symmetry, all of which are important for developing high-performance
polymers. In fact, polymers based on NTz showed deeper HOMO and LUMO energy
levels, narrower bandgap, better ordering structures, and better device performances
in both OFETs and OPVs compared to the polymer based on BTz. In particular, a
quaterthiophene-NTz polymer, PNTz4T, exhibited efficiencies reaching 10% when
blended with PC 71 BM in the solar cell. The introduction of the fluorine atoms successfully reduced the photon energy loss, resulting in the enhanced V OC , and thereby the
improved efficiencies that are close to 11%, which are among the highest reported so