182
T. Fujita
7.5 Pentacene/C 60 Planar Heterojunction
In this section, we present preliminary results for a PEN/C 60 interface structure as
a model for organic solar cells. We investigate interfacial CT states comprising an
electron on a C 60 molecule and a hole on a PEN molecule, with a focus on the effect
of induced polarization on the e–h energies.
Because PEN and C 60 molecules are typical p-type and n-type organic semiconductor molecules, their blends have been studied from several aspects. The electronic
levels at the PEN/C 60 interfaces have been determined by the photoelectron spectroscopy [80, 95, 119]. The CT absorption for the standing-up orientation of the
PEN/C 60 planar heterojunction was revealed by Rand and coworkers [13] using the
polarized external quantum efficiency measurements. Later, they investigated the
interfacial CT states of the PEN/C 60 interfaces under different conditions of blend
morphologies and D/A ratios [70], observing the CT energy variations as large as
0.3 eV. In the computational approaches, excited-state calculations for PEN/C 60 have
been performed for model D/A systems [12, 72, 75, 121, 124, 125]. In particular,
Bredas and coworkers performed TDDFT excited-state calculations for PEN/C 60
clusters of up to ten molecules [125]. Although interfacial CT states can be modeled
by using a small D/A cluster, CS states cannot be defined in such a small system. We
note that in some first-principles studies for D/A systems, interfacial CT states are not
distinguished from CS states. Recently, we performed the excited-state calculations
for the local interface structures of the PEN/C 60 planar heterojunction based on the
FMO method [44]. However, the e–h interactions were described as the bare Coulomb
interactions, and the screening of e–h interactions by the excited-state polarization
was not included. This is seen in the relative overestimation of the CS state energies
relative to the short-range CT states [44]. The importance of the polarizable environments has been emphasized by Beljonne and coworkers [24, 115], who treated the
state-specific polarization using microelectrostatic calculations.
The computational details are briefly provided here. According to the X-ray
diffraction measurement [13], we prepared the edge-on orientation of the PEN/C 60
planar heterojunctions, as shown in Fig. 7.11. The details of the modeling and subsequent molecular dynamics simulations are presented elsewhere [44]. Although the
molecular dynamics simulations performed by Fu et al. [35] have indicated the coexistence of the edge-on and face-on orientations of PEN-C 60 pairs, we used the edge-on
orientation as a limiting case. The FMO-GW calculations were performed for the
local interface structure Fig. 7.11b, which includes 36 PEN and 15 C 60 molecules.
The remaining molecules in the total structure in Fig. 7.11a were incorporated as
the external point charges. The B3LYP was used as a starting point for the one-shot
FMO-GW calculation with 6-31G* basis set. The electronic structure calculations
were performed using the ABINIT-MP software [76, 77, 107]. Other details for the
FMO-GW calculations can be found in [45].
The HOMO and LUMO energies along the direction perpendicular to the PEN/C 60
interface are shown in Fig. 7.12. Near the interface, the averaged HOMO/LUMO
energies are −4.26/− 0.86 eV for PEN molecules and −5.56/− 0.74 eV for C 60
T. Fujita
7.5 Pentacene/C 60 Planar Heterojunction
In this section, we present preliminary results for a PEN/C 60 interface structure as
a model for organic solar cells. We investigate interfacial CT states comprising an
electron on a C 60 molecule and a hole on a PEN molecule, with a focus on the effect
of induced polarization on the e–h energies.
Because PEN and C 60 molecules are typical p-type and n-type organic semiconductor molecules, their blends have been studied from several aspects. The electronic
levels at the PEN/C 60 interfaces have been determined by the photoelectron spectroscopy [80, 95, 119]. The CT absorption for the standing-up orientation of the
PEN/C 60 planar heterojunction was revealed by Rand and coworkers [13] using the
polarized external quantum efficiency measurements. Later, they investigated the
interfacial CT states of the PEN/C 60 interfaces under different conditions of blend
morphologies and D/A ratios [70], observing the CT energy variations as large as
0.3 eV. In the computational approaches, excited-state calculations for PEN/C 60 have
been performed for model D/A systems [12, 72, 75, 121, 124, 125]. In particular,
Bredas and coworkers performed TDDFT excited-state calculations for PEN/C 60
clusters of up to ten molecules [125]. Although interfacial CT states can be modeled
by using a small D/A cluster, CS states cannot be defined in such a small system. We
note that in some first-principles studies for D/A systems, interfacial CT states are not
distinguished from CS states. Recently, we performed the excited-state calculations
for the local interface structures of the PEN/C 60 planar heterojunction based on the
FMO method [44]. However, the e–h interactions were described as the bare Coulomb
interactions, and the screening of e–h interactions by the excited-state polarization
was not included. This is seen in the relative overestimation of the CS state energies
relative to the short-range CT states [44]. The importance of the polarizable environments has been emphasized by Beljonne and coworkers [24, 115], who treated the
state-specific polarization using microelectrostatic calculations.
The computational details are briefly provided here. According to the X-ray
diffraction measurement [13], we prepared the edge-on orientation of the PEN/C 60
planar heterojunctions, as shown in Fig. 7.11. The details of the modeling and subsequent molecular dynamics simulations are presented elsewhere [44]. Although the
molecular dynamics simulations performed by Fu et al. [35] have indicated the coexistence of the edge-on and face-on orientations of PEN-C 60 pairs, we used the edge-on
orientation as a limiting case. The FMO-GW calculations were performed for the
local interface structure Fig. 7.11b, which includes 36 PEN and 15 C 60 molecules.
The remaining molecules in the total structure in Fig. 7.11a were incorporated as
the external point charges. The B3LYP was used as a starting point for the one-shot
FMO-GW calculation with 6-31G* basis set. The electronic structure calculations
were performed using the ABINIT-MP software [76, 77, 107]. Other details for the
FMO-GW calculations can be found in [45].
The HOMO and LUMO energies along the direction perpendicular to the PEN/C 60
interface are shown in Fig. 7.12. Near the interface, the averaged HOMO/LUMO
energies are −4.26/− 0.86 eV for PEN molecules and −5.56/− 0.74 eV for C 60