208
S. Izawa
fullerene acceptors (PCBM and bisPCBM), four energy landscapes (cascade, normal
with PCBM, normal with bis PCBM, and trap) were constructed at the D/A interface
in the bilayer OSCs (Fig. 8.8b). These four devices differ only in the energy level of
the monolayer at the D/A interface.
The J-V characteristics of the four devices with Poly(3-hexylthiophene-2,5-diyl)
(P3HT) as the donor layer are depicted in Fig. 8.8c. The cascade device exhibited
V OC and FF values that were larger than those of the normal (PCBM) device. The
changes in V OC and FF were +0.09 V and +0.13, respectively. In contrast, the trap
device exhibited much lower V OC , FF, and J SC values than the normal (bisPCBM)
device. The changes in V OC and FF were −0.12 V and −0.18, respectively.
To explain the large V OC changes caused by different energy landscapes at the
D/A interface, E CT of the four types of devices was measured by electroluminescence
(EL). The EL spectra from the CT state emission of the bilayer devices are shown in
Fig. 8.8d. A shift of approximately +0.1 eV in the EL spectra was observed for the
cascade device compared to the case of the normal (PCBM) device. In contrast, the
peak top for the trap device shifted by approximately −0.1 eV compared to the case
of the normal (bisPCBM) device. Shifts in E CT were also observed in the temperature dependence of V OC . The energy changes in E CT , extrapolated from the lowtemperature limits for the cascade (+0.09 eV) and trap (−0.12 eV) compared to the
corresponding normal devices, generally agreed with those from EL measurements.
The differences in E CT were shown to be the main reason for the change in V OC in
the cascade and trap devices. E CT is expressed as the energy-level difference between
the HOMO of the donor and the LUMO of the acceptor, and the Coulomb binding
is dominated by the electron-hole distance, as explained by Eq. 8.2 in the previous
section. The energy level of the LUMO at the first layer of the D/A interface in the
cascade device was higher than that of the normal (PCBM) device. The energy of
the electrons in the first layer of the acceptor was raised by a higher LUMO, and the
spatially separated charge pairs that were further away than the first monolayer from
the D/A interface experienced a weaker Coulomb attraction. E CT was destabilized
in both cases; thus, V OC in the cascade device became larger than V OC in the normal
device.
The cascade energy structure is beneficial for increasing both V OC and FF. To
investigate the charge separation processes, the temperature dependence of J SC was
measured. The temperature dependence of J SC is expressed by the Arrhenius equation, and the slope in Fig. 8.8e is related to the activation energy for charge separation. When the temperature decreased, J SC decreased more sharply in the trap
device than in the normal (bisPCBM) device. Interestingly, the J SC of the cascade
device was almost independent of temperature. The origin of the activation energy
was attributed to the trapping of the Coulomb attraction at the D/A interface. Charge
pairs were deeply trapped at the D/A interface in the trap devices, promoting geminate recombination. In contrast, the temperature independence of J SC in the cascade
devices indicated free charge generation at the D/A interface without thermal activation. These mechanisms of promotion and suppression of geminate recombination
produced changes in FF. This result suggests that the Coulomb binding in the CT
state was weak enough to form free charges in the cascade device.
S. Izawa
fullerene acceptors (PCBM and bisPCBM), four energy landscapes (cascade, normal
with PCBM, normal with bis PCBM, and trap) were constructed at the D/A interface
in the bilayer OSCs (Fig. 8.8b). These four devices differ only in the energy level of
the monolayer at the D/A interface.
The J-V characteristics of the four devices with Poly(3-hexylthiophene-2,5-diyl)
(P3HT) as the donor layer are depicted in Fig. 8.8c. The cascade device exhibited
V OC and FF values that were larger than those of the normal (PCBM) device. The
changes in V OC and FF were +0.09 V and +0.13, respectively. In contrast, the trap
device exhibited much lower V OC , FF, and J SC values than the normal (bisPCBM)
device. The changes in V OC and FF were −0.12 V and −0.18, respectively.
To explain the large V OC changes caused by different energy landscapes at the
D/A interface, E CT of the four types of devices was measured by electroluminescence
(EL). The EL spectra from the CT state emission of the bilayer devices are shown in
Fig. 8.8d. A shift of approximately +0.1 eV in the EL spectra was observed for the
cascade device compared to the case of the normal (PCBM) device. In contrast, the
peak top for the trap device shifted by approximately −0.1 eV compared to the case
of the normal (bisPCBM) device. Shifts in E CT were also observed in the temperature dependence of V OC . The energy changes in E CT , extrapolated from the lowtemperature limits for the cascade (+0.09 eV) and trap (−0.12 eV) compared to the
corresponding normal devices, generally agreed with those from EL measurements.
The differences in E CT were shown to be the main reason for the change in V OC in
the cascade and trap devices. E CT is expressed as the energy-level difference between
the HOMO of the donor and the LUMO of the acceptor, and the Coulomb binding
is dominated by the electron-hole distance, as explained by Eq. 8.2 in the previous
section. The energy level of the LUMO at the first layer of the D/A interface in the
cascade device was higher than that of the normal (PCBM) device. The energy of
the electrons in the first layer of the acceptor was raised by a higher LUMO, and the
spatially separated charge pairs that were further away than the first monolayer from
the D/A interface experienced a weaker Coulomb attraction. E CT was destabilized
in both cases; thus, V OC in the cascade device became larger than V OC in the normal
device.
The cascade energy structure is beneficial for increasing both V OC and FF. To
investigate the charge separation processes, the temperature dependence of J SC was
measured. The temperature dependence of J SC is expressed by the Arrhenius equation, and the slope in Fig. 8.8e is related to the activation energy for charge separation. When the temperature decreased, J SC decreased more sharply in the trap
device than in the normal (bisPCBM) device. Interestingly, the J SC of the cascade
device was almost independent of temperature. The origin of the activation energy
was attributed to the trapping of the Coulomb attraction at the D/A interface. Charge
pairs were deeply trapped at the D/A interface in the trap devices, promoting geminate recombination. In contrast, the temperature independence of J SC in the cascade
devices indicated free charge generation at the D/A interface without thermal activation. These mechanisms of promotion and suppression of geminate recombination
produced changes in FF. This result suggests that the Coulomb binding in the CT
state was weak enough to form free charges in the cascade device.