210
S. Izawa
The J-V characteristics of the devices under dark conditions and on irradiation
with simulated solar light are presented in Fig. 8.9b. The V OC was found to increase
as the side-chain length of C n -PTCDI increased, with values of 0.76 V, 0.93 V,
and 1.01 V for C 3 -PTCDI, C 6 -PTCDI, and C 8 -PTCDI, respectively. The FF of the
BTBT/C 8 -PTCDI device was 0.79, which is higher than those of the C 3 -PTCDI and
C 6 -PTCDI devices, and is the highest FF value reported so far for OSCs fabricated
by thermal evaporation [38].
To elucidate the effect on device performance of crystallinity near the D/A interface and in the bulk, trilayer devices were fabricated with the active layer composed
of BTBT/C 8 -PTCDI/C 3 -PTCDI. The D/A interface of the trilayer devices consisted
of BTBT and thin C 8 -PTCDI layers, with the majority of the layer, apart from the
interface, made up of the less crystalline C 3 -PTCDI. The trilayer devices based on
10 and 6 nm C 8 -PTCDI layers exhibited almost the same V OC values, 1.01 and
0.99 V, as the BTBT/C 8 -PTCDI bilayer device, whereas the device with the 4 nm
C 8 -PTCDI layer exhibited a decreased V OC of 0.94 V, as shown in Fig. 8.9c. The
J-V characteristics of the devices with the C 8 -PTCDI layer thicker than 6 nm were
almost the same, indicating that the device parameters were not affected by the less
crystalline C 3 -PTCDI layer except at the D/A interface. This result suggests that the
performance of the BTBT/C 8 -PTCDI bilayer device was determined by the region
within 6 nm (i.e., three molecular layers of C 8 -PTCDI) of the D/A interface, and
that highly ordered donor and acceptor materials in this region are very important
for obtaining high V OC and FF.
To quantitatively evaluate the energy loss in the output voltage, E CT was determined from measurements of the temperature dependence of V OC (Fig. 8.9d). E CT /e
can be identified with the intercept of the linear plot of V OC versus temperature at
T = 0 (Eq. 8.6). The E CT values of C 3 -PTCDI, C 6 -PTCDI, and C 8 -PTCDI were
1.24 eV, 1.24 eV, and 1.27 eV, respectively. The difference in eV OC at room temperature between C 3 -PTCDI and C 8 -PTCDI was 0.25 eV; in contrast, the difference
in E CT (i.e., eV OC at 0 K) was very small, 0.03 eV. These results indicated that the
energy loss between E CT and eV OC was strongly affected by the difference in the
C n -PTCDI side-chain length. The energy loss exhibited the smallest value, 0.26 eV,
in the BTBT/C 8 -PTCDI device, which contained the D/A interface with the highest
crystallinity.
According to Eq. 8.6, the origin of the large energy-level difference between E CT
and eV OC for the three devices is a change in either n, which is related to the type
of charge recombination, or J 00 , which is related to the rate of charge recombination. To clarify the dominant factors, the ideality factor n was determined from
the light-intensity dependence of V OC . The value of n is equal to 1 when ideal
band-to-band recombination occurs [15]. When additional pathways such as trapassisted recombination are also involved, V OC displays a stronger dependence on
light intensity and n is greater than 1. Figure 8.9e shows the light-intensity dependence of V OC for the BTBT/C n -PTCDI devices. The values of n for C 3 -PTCDI,
C 6 -PTCDI, and C 8 -PTCDI were 1.90, 1.37, and 1.00, respectively. As the side-chain
length of C n -PTCDI increased, n decreased, and the BTBT/C 8 -PTCDI device was
an ideal diode. Only band-to-band recombination occurred in the BTBT/C 8 -PTCDI
S. Izawa
The J-V characteristics of the devices under dark conditions and on irradiation
with simulated solar light are presented in Fig. 8.9b. The V OC was found to increase
as the side-chain length of C n -PTCDI increased, with values of 0.76 V, 0.93 V,
and 1.01 V for C 3 -PTCDI, C 6 -PTCDI, and C 8 -PTCDI, respectively. The FF of the
BTBT/C 8 -PTCDI device was 0.79, which is higher than those of the C 3 -PTCDI and
C 6 -PTCDI devices, and is the highest FF value reported so far for OSCs fabricated
by thermal evaporation [38].
To elucidate the effect on device performance of crystallinity near the D/A interface and in the bulk, trilayer devices were fabricated with the active layer composed
of BTBT/C 8 -PTCDI/C 3 -PTCDI. The D/A interface of the trilayer devices consisted
of BTBT and thin C 8 -PTCDI layers, with the majority of the layer, apart from the
interface, made up of the less crystalline C 3 -PTCDI. The trilayer devices based on
10 and 6 nm C 8 -PTCDI layers exhibited almost the same V OC values, 1.01 and
0.99 V, as the BTBT/C 8 -PTCDI bilayer device, whereas the device with the 4 nm
C 8 -PTCDI layer exhibited a decreased V OC of 0.94 V, as shown in Fig. 8.9c. The
J-V characteristics of the devices with the C 8 -PTCDI layer thicker than 6 nm were
almost the same, indicating that the device parameters were not affected by the less
crystalline C 3 -PTCDI layer except at the D/A interface. This result suggests that the
performance of the BTBT/C 8 -PTCDI bilayer device was determined by the region
within 6 nm (i.e., three molecular layers of C 8 -PTCDI) of the D/A interface, and
that highly ordered donor and acceptor materials in this region are very important
for obtaining high V OC and FF.
To quantitatively evaluate the energy loss in the output voltage, E CT was determined from measurements of the temperature dependence of V OC (Fig. 8.9d). E CT /e
can be identified with the intercept of the linear plot of V OC versus temperature at
T = 0 (Eq. 8.6). The E CT values of C 3 -PTCDI, C 6 -PTCDI, and C 8 -PTCDI were
1.24 eV, 1.24 eV, and 1.27 eV, respectively. The difference in eV OC at room temperature between C 3 -PTCDI and C 8 -PTCDI was 0.25 eV; in contrast, the difference
in E CT (i.e., eV OC at 0 K) was very small, 0.03 eV. These results indicated that the
energy loss between E CT and eV OC was strongly affected by the difference in the
C n -PTCDI side-chain length. The energy loss exhibited the smallest value, 0.26 eV,
in the BTBT/C 8 -PTCDI device, which contained the D/A interface with the highest
crystallinity.
According to Eq. 8.6, the origin of the large energy-level difference between E CT
and eV OC for the three devices is a change in either n, which is related to the type
of charge recombination, or J 00 , which is related to the rate of charge recombination. To clarify the dominant factors, the ideality factor n was determined from
the light-intensity dependence of V OC . The value of n is equal to 1 when ideal
band-to-band recombination occurs [15]. When additional pathways such as trapassisted recombination are also involved, V OC displays a stronger dependence on
light intensity and n is greater than 1. Figure 8.9e shows the light-intensity dependence of V OC for the BTBT/C n -PTCDI devices. The values of n for C 3 -PTCDI,
C 6 -PTCDI, and C 8 -PTCDI were 1.90, 1.37, and 1.00, respectively. As the side-chain
length of C n -PTCDI increased, n decreased, and the BTBT/C 8 -PTCDI device was
an ideal diode. Only band-to-band recombination occurred in the BTBT/C 8 -PTCDI