114
I. Osaka
N
O
N
N
O
N
S
S
S
C 10 H 21
C 12 H 25
C 10 H 21
C 12 H 25
n
S
PNOz4T
b
a
400
500
600
700
800
900
PNTz4T
PNOz4T
Absorbance (a.u.)
Wavelength (nm)
-2 -1.5 -1 -0.5 0
0.5
1
1.5
PNTz4T
PNOz4T
PC 61 BM
PC 71 BM
Current (a.u.)
Potential (V vs Fc/Fc + )
PC 61 BM
or
PC 71 BM
LUMO
HOMO
E (eV)
–3.65
–5.48
PNOz4T
–3.77
ca. –5.8
ΔE L = 0.12
PNTz4T
–3.46
–5.14
d
c
Fig. 5.23 a Chemical structure of PNOz4T. b UV-vis absorption spectra of PNOz4T in the thin
film in comparison with PNTz4T. c Cyclic voltammograms of PNTz4T, PNOz4T, PC 61 BM, and
PC 71 BM. d Energy diagrams of PNTz4T, PNOz4T, PC 61 BM, and PC 71 BM. The HOMO and LUMO
energy levels were determined by onset redox potentials in the cyclic voltammograms. Reproduced
with permission [51]. Copyright (2015) Nature Publishing Group
(Fig. 5.23d). Importantly, whereas L for PNTz4T (between PC 71 BM) was 0.31 eV,
that for PNOz4T was 0.12 eV, which is much smaller than the widely referenced
empirical threshold value of 0.3 eV [45]. This indicates that the driving force of the
photoinduced charge separation is reduced in the PNOz4T system compared with
the PNTz4T system. Note that, however, the estimation of the energetics based on
the LUMO energy level evaluated by the measurements on the isolated polymer
and PCBMs is considered to neglect the influence of exciton binding energy and
other interfacial effects [52, 53]. Alternatively, the energy offset can be estimated
by the difference of the excited state energy of the polymer and the charge transfer
(CT) state energy (E CT ) of the polymer/PCBM blend [41, 54]. Thus, we attempted to
determine E CT of the blend films by means of the electroluminescence measurement.
However, for the PNOz4T/PC 71 BM system, the emission band of the pure PNOz4T
and the CT emission band of the PNOz4T/PC 71 BM blend system measured by the
electroluminescence cannot be distinguished, and thus E CT cannot be evaluated. This
suggests, however, that the excited state of the polymer and the CT state of the blend
are energetically very close, and thereby the energy offset is very small [55]. This is
I. Osaka
N
O
N
N
O
N
S
S
S
C 10 H 21
C 12 H 25
C 10 H 21
C 12 H 25
n
S
PNOz4T
b
a
400
500
600
700
800
900
PNTz4T
PNOz4T
Absorbance (a.u.)
Wavelength (nm)
-2 -1.5 -1 -0.5 0
0.5
1
1.5
PNTz4T
PNOz4T
PC 61 BM
PC 71 BM
Current (a.u.)
Potential (V vs Fc/Fc + )
PC 61 BM
or
PC 71 BM
LUMO
HOMO
E (eV)
–3.65
–5.48
PNOz4T
–3.77
ca. –5.8
ΔE L = 0.12
PNTz4T
–3.46
–5.14
d
c
Fig. 5.23 a Chemical structure of PNOz4T. b UV-vis absorption spectra of PNOz4T in the thin
film in comparison with PNTz4T. c Cyclic voltammograms of PNTz4T, PNOz4T, PC 61 BM, and
PC 71 BM. d Energy diagrams of PNTz4T, PNOz4T, PC 61 BM, and PC 71 BM. The HOMO and LUMO
energy levels were determined by onset redox potentials in the cyclic voltammograms. Reproduced
with permission [51]. Copyright (2015) Nature Publishing Group
(Fig. 5.23d). Importantly, whereas L for PNTz4T (between PC 71 BM) was 0.31 eV,
that for PNOz4T was 0.12 eV, which is much smaller than the widely referenced
empirical threshold value of 0.3 eV [45]. This indicates that the driving force of the
photoinduced charge separation is reduced in the PNOz4T system compared with
the PNTz4T system. Note that, however, the estimation of the energetics based on
the LUMO energy level evaluated by the measurements on the isolated polymer
and PCBMs is considered to neglect the influence of exciton binding energy and
other interfacial effects [52, 53]. Alternatively, the energy offset can be estimated
by the difference of the excited state energy of the polymer and the charge transfer
(CT) state energy (E CT ) of the polymer/PCBM blend [41, 54]. Thus, we attempted to
determine E CT of the blend films by means of the electroluminescence measurement.
However, for the PNOz4T/PC 71 BM system, the emission band of the pure PNOz4T
and the CT emission band of the PNOz4T/PC 71 BM blend system measured by the
electroluminescence cannot be distinguished, and thus E CT cannot be evaluated. This
suggests, however, that the excited state of the polymer and the CT state of the blend
are energetically very close, and thereby the energy offset is very small [55]. This is