96
I. Osaka
the present polymers, showed a texture for the edge-on orientation. Therefore, the
length difference between R
1 and R
2 , in which the length of the trunk part is taken into
account in case of branched side chains, seems to play an important role to determine
the backbone orientation. When the difference is larger the polymers form an edge-on
orientation: note that in the case of the linear–branched system, the linear side chain
must be longer. When the difference is smaller, on the other hand, the polymers tend
to form a well-ordered face-on orientation. Overall, it is very interesting that such
small difference in the side chain length and thus in the intermolecular interaction
can impact the backbone orientation.
Interestingly, however, in the polymer/PC 61 BM blend films, all the polymers
displayed the texture corresponding to face-on orientation. In other words, while the
polymers that form face-on orientation in the polymer neat films, “face-on-polymers,”
preserved their orientation, the polymers that form edge-on or bimodal orientation in
the polymer-only films, “edge-on-polymers” or “bimodal-polymers,” changed their
orientation into face-on in the blend films (Fig. 5.8). This change could be due to
the π–π interaction between the polymer and PC 61 BM, and/or the weakened π–π
stacking of the polymer in the presence of PC 61 BM.
5.2.3 Impact of Side Chain Topology in Solar Cell
Performance
Solar cells with the conventional structure (ITO/PEDOT:PSS/(polymer/PC 61 BM)/
Ca/Al) were fabricated by spin coating the chloroform solution of the
polymer/PC 61 BM blend. Figures 5.9a and b show the current density (J)–voltage
(V ) curves and the EQE spectra of the solar cells with the active layer thickness
of roughly 200 nm. The optimal polymer to PC 61 BM ratio was 1:2 for most of the
polymers, and 1:1 for PTzBT-BOOD, 1:3 for-BOBO, and 1:4 for-EHHD. The solar
cell parameters are summarized in Table 5.1. Interestingly, a notable difference in
V OC was found between the polymers with the R
1 , R
2
= linear, branched groups
(linear–branched) and R
1 , R
2
= branched, branched groups (all-branched). The cells
with the linear–branched system gave V OC of 0.81–0.82 V, whereas the cells with
all-branched system gave 0.88–0.90 V, despite the fact that the HOMO energy level
of the polymers is almost the same. The difference in V OC on the side chain composition is seemingly independent of the orientation. The V OC difference is possibly due
to the different intermolecular interaction between the polymer and PC 61 BM [25].
The weak π–π stacking crystallinity of the all-branched system compared to the
linear–branched system in the presence of PC 61 BM as described above, which can
also be counted as weaker polymer–PC 61 BM interactions, might reduce the charge
recombination, in turn leading to the high V OC .
I. Osaka
the present polymers, showed a texture for the edge-on orientation. Therefore, the
length difference between R
1 and R
2 , in which the length of the trunk part is taken into
account in case of branched side chains, seems to play an important role to determine
the backbone orientation. When the difference is larger the polymers form an edge-on
orientation: note that in the case of the linear–branched system, the linear side chain
must be longer. When the difference is smaller, on the other hand, the polymers tend
to form a well-ordered face-on orientation. Overall, it is very interesting that such
small difference in the side chain length and thus in the intermolecular interaction
can impact the backbone orientation.
Interestingly, however, in the polymer/PC 61 BM blend films, all the polymers
displayed the texture corresponding to face-on orientation. In other words, while the
polymers that form face-on orientation in the polymer neat films, “face-on-polymers,”
preserved their orientation, the polymers that form edge-on or bimodal orientation in
the polymer-only films, “edge-on-polymers” or “bimodal-polymers,” changed their
orientation into face-on in the blend films (Fig. 5.8). This change could be due to
the π–π interaction between the polymer and PC 61 BM, and/or the weakened π–π
stacking of the polymer in the presence of PC 61 BM.
5.2.3 Impact of Side Chain Topology in Solar Cell
Performance
Solar cells with the conventional structure (ITO/PEDOT:PSS/(polymer/PC 61 BM)/
Ca/Al) were fabricated by spin coating the chloroform solution of the
polymer/PC 61 BM blend. Figures 5.9a and b show the current density (J)–voltage
(V ) curves and the EQE spectra of the solar cells with the active layer thickness
of roughly 200 nm. The optimal polymer to PC 61 BM ratio was 1:2 for most of the
polymers, and 1:1 for PTzBT-BOOD, 1:3 for-BOBO, and 1:4 for-EHHD. The solar
cell parameters are summarized in Table 5.1. Interestingly, a notable difference in
V OC was found between the polymers with the R
1 , R
2
= linear, branched groups
(linear–branched) and R
1 , R
2
= branched, branched groups (all-branched). The cells
with the linear–branched system gave V OC of 0.81–0.82 V, whereas the cells with
all-branched system gave 0.88–0.90 V, despite the fact that the HOMO energy level
of the polymers is almost the same. The difference in V OC on the side chain composition is seemingly independent of the orientation. The V OC difference is possibly due
to the different intermolecular interaction between the polymer and PC 61 BM [25].
The weak π–π stacking crystallinity of the all-branched system compared to the
linear–branched system in the presence of PC 61 BM as described above, which can
also be counted as weaker polymer–PC 61 BM interactions, might reduce the charge
recombination, in turn leading to the high V OC .