8 Open-Circuit Voltage in Organic Solar Cells
211
device; in contrast, trap-assisted recombination took place in the BTBT/C 3 -PTCDI
and BTBT/C 6 -PTCDI devices. The values of J 00 can be calculated from the measured
n and the slope of the temperature dependence of V OC . The calculated J 00 values were
not very different for the three devices, indicating that the origin of the decreased
V OC loss in the devices using C n -PTCDI with longer side chains was the smaller n.
Figure 8.9f shows a plot of the V OC values under 1 sun illumination versus the E CT
values for various kinds of OSCs reported in the literature, as well as the values for the
BTBT/C n -PTCDI devices. The reported V OC values for the OSCs were smaller than
E CT /e − 0.5 V. The V OC loss for the BTBT/C 8 -PTCDI device was very small, less
than 0.3 V, because trap-assisted recombination was greatly suppressed and ideal
band-to-band recombination was realized, as mentioned above in the quantitative
analysis of V OC loss. The origin of the trap states in organic semiconductor films is
under debate, but a possible candidate is a tailing in the density of states induced by
the thermal motion of the molecules [26]. The D/A materials used in this work have
high charge mobility (BTBT: 43 cm
2 V s
−1 , C 8 -PTCDI: 1.3 cm
2 V s
−1 ). Recently,
the octyl side chains of BTBT have been reported to assist in molecular packing and
to suppress molecular vibration of the π-conjugated cores, which are the origin of
the high hole mobility of BTBT [39]. The origin of the extraordinarily small V OC
loss in the BTBT/C 8 -PTCDI device could be explained by analogy with the origin
of the high hole mobility of BTBT: the highly ordered nature of the molecules at the
D/A interface suppressed the thermal motion of the π-conjugated cores and reduced
the energetic disorder near the D/A interface.
To summarize this section, the model OSC device with high mobility and highly
crystalline donor and acceptor materials was able to reduce the V OC loss to less than
0.3 V and to attain an FF of approximately 0.8. The high crystallinity of the several
molecular layers near the D/A interface is important for reducing the energy loss in
the output voltage and realizing ideal band-to-band recombination.
8.3.6 Controlling Energy Level of Acceptor Dye Molecule
to Reduce Open-Circuit Voltage Loss in Organic Solar
Cells
Tetra-substituted perylenediimide (4SubPDI) with four electron-donating groups as
substituents was synthesized with the aim of developing a novel acceptor molecule
(Fig. 8.10a) [40]. The molecule has a higher-lying LUMO level (~ 0.1 eV) than
the non-substituted reference PDI molecule (RefPDI). The effect of the LUMO
difference on the V OC loss was investigated.
Figure 8.10b depicts the J -V curves of mixed bulk heterojunction OSC devices
with PTB7-Th as the donor and 4SubPDI or RefPDI as the acceptor. The devices
with 4SubPDI showed a very high V OC , reaching 1.0 V, comparable to the highest
value ever found for PTB7-Th based devices [41]. The optical HOMO-LUMO gap
(E
opt
g ) of the device was estimated to be 1.57 eV from the absorption spectral edges
211
device; in contrast, trap-assisted recombination took place in the BTBT/C 3 -PTCDI
and BTBT/C 6 -PTCDI devices. The values of J 00 can be calculated from the measured
n and the slope of the temperature dependence of V OC . The calculated J 00 values were
not very different for the three devices, indicating that the origin of the decreased
V OC loss in the devices using C n -PTCDI with longer side chains was the smaller n.
Figure 8.9f shows a plot of the V OC values under 1 sun illumination versus the E CT
values for various kinds of OSCs reported in the literature, as well as the values for the
BTBT/C n -PTCDI devices. The reported V OC values for the OSCs were smaller than
E CT /e − 0.5 V. The V OC loss for the BTBT/C 8 -PTCDI device was very small, less
than 0.3 V, because trap-assisted recombination was greatly suppressed and ideal
band-to-band recombination was realized, as mentioned above in the quantitative
analysis of V OC loss. The origin of the trap states in organic semiconductor films is
under debate, but a possible candidate is a tailing in the density of states induced by
the thermal motion of the molecules [26]. The D/A materials used in this work have
high charge mobility (BTBT: 43 cm
2 V s
−1 , C 8 -PTCDI: 1.3 cm
2 V s
−1 ). Recently,
the octyl side chains of BTBT have been reported to assist in molecular packing and
to suppress molecular vibration of the π-conjugated cores, which are the origin of
the high hole mobility of BTBT [39]. The origin of the extraordinarily small V OC
loss in the BTBT/C 8 -PTCDI device could be explained by analogy with the origin
of the high hole mobility of BTBT: the highly ordered nature of the molecules at the
D/A interface suppressed the thermal motion of the π-conjugated cores and reduced
the energetic disorder near the D/A interface.
To summarize this section, the model OSC device with high mobility and highly
crystalline donor and acceptor materials was able to reduce the V OC loss to less than
0.3 V and to attain an FF of approximately 0.8. The high crystallinity of the several
molecular layers near the D/A interface is important for reducing the energy loss in
the output voltage and realizing ideal band-to-band recombination.
8.3.6 Controlling Energy Level of Acceptor Dye Molecule
to Reduce Open-Circuit Voltage Loss in Organic Solar
Cells
Tetra-substituted perylenediimide (4SubPDI) with four electron-donating groups as
substituents was synthesized with the aim of developing a novel acceptor molecule
(Fig. 8.10a) [40]. The molecule has a higher-lying LUMO level (~ 0.1 eV) than
the non-substituted reference PDI molecule (RefPDI). The effect of the LUMO
difference on the V OC loss was investigated.
Figure 8.10b depicts the J -V curves of mixed bulk heterojunction OSC devices
with PTB7-Th as the donor and 4SubPDI or RefPDI as the acceptor. The devices
with 4SubPDI showed a very high V OC , reaching 1.0 V, comparable to the highest
value ever found for PTB7-Th based devices [41]. The optical HOMO-LUMO gap
(E
opt
g ) of the device was estimated to be 1.57 eV from the absorption spectral edges