5 Polymer Solar Cells: Development of π-Conjugated Polymers …
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far for polymer/fullerene cells. Further, an NOz-based polymer, PNOz4T, was found
to be able to significantly reduce the photon energy loss of the cell close to that
for inorganic solar cells. Importantly, PNOz4T has allowed efficient photoinduced
charge separation even with a very small driving force energy of nearly 0 eV. As a
result, PNOz4T exhibited the maximum PCE of 8.9%, which was the highest value
for polymer/fullerene systems with small E loss such as lower than 0.6 eV.
5.4 Conclusion and Outlook
This chapter summarized our recent studies on the development of π-conjugated
polymers based on thiazolothiazole and naphthobischalcogenadiazoles. The
electron-poor nature of the building units resulted in donor–acceptor (D–A) backbones when incorporated into a polythiophene backbone. With the large π-electron
system in addition to the D–A motif, the polymers formed highly ordered crystalline structures with close π–π stacking, leading to high charge carrier mobilities,
which greatly benefits the photovoltaic performance. We discovered that the backbone orientation can be controlled by the careful design of the side chain composition.
Further, we found that there is distribution of the orientation motifs through the film
thickness. Thus, the match between the orientation including its distribution and the
cell stack is crucial for maximizing the efficiency. The use of such polymers having
high crystallinity and favorable orientation afforded high-efficiency polymer solar
cells with thick active layers. In parallel, we also discovered that the photon energy
loss, which is one of the most critical issues in organic solar cells, can be reduced as
small as that for inorganic solar cells by molecular design, as evidenced by PNOz4T.
We believe that these findings will be important guidelines for the development of
new π-conjugated polymers and for the further improvement of the polymer solar
cells.
A crucial issue would be that the reduction of the photon energy loss is always
accompanied by a loss of driving force energy for charge separation, giving rise to
a lower photocurrent (J SC ). Although PNOz4T showed relatively high photocurrent
with the EQE more than 60% even under a small photon energy loss, such tradeoff is still remained unsolved. Importantly, however, recent studies in this area have
shown that the use of non-fullerene n-type materials [58–60] can realize small photon
energy loss as small as ~0.5 eV and high photocurrent with EQEs reaching 80% at the
same time, leading to significantly high efficiencies of ~18% [61]. Further, studies
for understanding the mechanism underneath such unconventional phenomenon in
PNOz4T as well as those non-fullerene materials would lead to new strategy in
designing novel high-performance materials and thereby higher efficiencies.
117
far for polymer/fullerene cells. Further, an NOz-based polymer, PNOz4T, was found
to be able to significantly reduce the photon energy loss of the cell close to that
for inorganic solar cells. Importantly, PNOz4T has allowed efficient photoinduced
charge separation even with a very small driving force energy of nearly 0 eV. As a
result, PNOz4T exhibited the maximum PCE of 8.9%, which was the highest value
for polymer/fullerene systems with small E loss such as lower than 0.6 eV.
5.4 Conclusion and Outlook
This chapter summarized our recent studies on the development of π-conjugated
polymers based on thiazolothiazole and naphthobischalcogenadiazoles. The
electron-poor nature of the building units resulted in donor–acceptor (D–A) backbones when incorporated into a polythiophene backbone. With the large π-electron
system in addition to the D–A motif, the polymers formed highly ordered crystalline structures with close π–π stacking, leading to high charge carrier mobilities,
which greatly benefits the photovoltaic performance. We discovered that the backbone orientation can be controlled by the careful design of the side chain composition.
Further, we found that there is distribution of the orientation motifs through the film
thickness. Thus, the match between the orientation including its distribution and the
cell stack is crucial for maximizing the efficiency. The use of such polymers having
high crystallinity and favorable orientation afforded high-efficiency polymer solar
cells with thick active layers. In parallel, we also discovered that the photon energy
loss, which is one of the most critical issues in organic solar cells, can be reduced as
small as that for inorganic solar cells by molecular design, as evidenced by PNOz4T.
We believe that these findings will be important guidelines for the development of
new π-conjugated polymers and for the further improvement of the polymer solar
cells.
A crucial issue would be that the reduction of the photon energy loss is always
accompanied by a loss of driving force energy for charge separation, giving rise to
a lower photocurrent (J SC ). Although PNOz4T showed relatively high photocurrent
with the EQE more than 60% even under a small photon energy loss, such tradeoff is still remained unsolved. Importantly, however, recent studies in this area have
shown that the use of non-fullerene n-type materials [58–60] can realize small photon
energy loss as small as ~0.5 eV and high photocurrent with EQEs reaching 80% at the
same time, leading to significantly high efficiencies of ~18% [61]. Further, studies
for understanding the mechanism underneath such unconventional phenomenon in
PNOz4T as well as those non-fullerene materials would lead to new strategy in
designing novel high-performance materials and thereby higher efficiencies.