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H. Ohkita
In addition, new approaches recently proposed have effectively boosted the PCE of
polymer solar cells furthermore. Ternary blend solar cells are promising approaches
to expand the light-harvesting wavelength range from visible to near-IR region. For
example, we have demonstrated that the short-circuit current density (J SC ) can be
effectively enhanced by addition of near-IR dye molecules into RR-P3HT/PCBM
binary blend solar cells [9, 10]. The key to success of this approach is location of
dye molecules in ternary blend films: Dye molecules should be located at a RRP3HT/PCBM interface in blend films so that they can contribute to photocurrent
generation effectively [11, 12]. Interestingly, most dye molecules are spontaneously
located at the RR-P3HT/PCBM interface in ternary blend films, which is revealed
by transient absorption spectroscopy [13], as described in Sect. 6.4.4.
In order to improve photovoltaic performance rationally, it is of particular importance to gain in-depth understanding of photovoltaic conversion mechanism in
polymer solar cells. Thus, we need to directly observe dynamics of transient species
such as exciton and charge carriers generated in polymer solar cells. In polymer
solar cells, as described in Sect. 6.2, the exciton and charge carrier dynamics
ranges from femtoseconds up to microseconds on a temporal scale [14–16]. Thus,
time-resolved measurement techniques are essential for understanding photovoltaic
conversion mechanism. In particular, transient absorption spectroscopy is one of
the most powerful methods for studying the exciton and charge carrier dynamics
directly. This chapter describes recent progress in understanding of photovoltaic
conversion mechanism in polymer solar cells studied by time-resolved optoelectronic
measurements.
6.2 Photovoltaic Conversion in Polymer Solar Cells
This section describes elementary processes of photovoltaic conversion in polymer
solar cells. Figure 6.1 shows a schematic illustration of the photovoltaic conversion
process in bilayered polymer solar cells based on hole-transporting (donor) and
electron-transporting (acceptor) materials. Under the solar illumination, a photon is
absorbed by either donor or acceptor material. The photon absorption efficiency (η A )
is defined as the ratio of the number of absorbed photons to the number of incident
photons at a wavelength. Here, it is considered that a photon is absorbed by the donor
material. Roughly speaking, the photon absorption excites an electron in the highest
occupied molecular orbital (HOMO) of the donor material into the lowest unoccupied
molecular orbital (LUMO), resulting in a hole in the HOMO and an electron in the
LUMO. This electron–hole pair is tightly bound because of large Coulomb interaction
in organic materials. This bound electron–hole pair is called a singlet exciton. This
state is also called singlet excited state, which consists of several different electron
configurations more strictly. In polymer solar cells, excitons cannot dissociate into
free charge carriers at room temperature because the exciton binding energy is much
larger than thermal energy k B T. This is partly because relative permittivity ε r is as
small as 3–4 in most organic materials, and hence a critical distance r C at which the
H. Ohkita
In addition, new approaches recently proposed have effectively boosted the PCE of
polymer solar cells furthermore. Ternary blend solar cells are promising approaches
to expand the light-harvesting wavelength range from visible to near-IR region. For
example, we have demonstrated that the short-circuit current density (J SC ) can be
effectively enhanced by addition of near-IR dye molecules into RR-P3HT/PCBM
binary blend solar cells [9, 10]. The key to success of this approach is location of
dye molecules in ternary blend films: Dye molecules should be located at a RRP3HT/PCBM interface in blend films so that they can contribute to photocurrent
generation effectively [11, 12]. Interestingly, most dye molecules are spontaneously
located at the RR-P3HT/PCBM interface in ternary blend films, which is revealed
by transient absorption spectroscopy [13], as described in Sect. 6.4.4.
In order to improve photovoltaic performance rationally, it is of particular importance to gain in-depth understanding of photovoltaic conversion mechanism in
polymer solar cells. Thus, we need to directly observe dynamics of transient species
such as exciton and charge carriers generated in polymer solar cells. In polymer
solar cells, as described in Sect. 6.2, the exciton and charge carrier dynamics
ranges from femtoseconds up to microseconds on a temporal scale [14–16]. Thus,
time-resolved measurement techniques are essential for understanding photovoltaic
conversion mechanism. In particular, transient absorption spectroscopy is one of
the most powerful methods for studying the exciton and charge carrier dynamics
directly. This chapter describes recent progress in understanding of photovoltaic
conversion mechanism in polymer solar cells studied by time-resolved optoelectronic
measurements.
6.2 Photovoltaic Conversion in Polymer Solar Cells
This section describes elementary processes of photovoltaic conversion in polymer
solar cells. Figure 6.1 shows a schematic illustration of the photovoltaic conversion
process in bilayered polymer solar cells based on hole-transporting (donor) and
electron-transporting (acceptor) materials. Under the solar illumination, a photon is
absorbed by either donor or acceptor material. The photon absorption efficiency (η A )
is defined as the ratio of the number of absorbed photons to the number of incident
photons at a wavelength. Here, it is considered that a photon is absorbed by the donor
material. Roughly speaking, the photon absorption excites an electron in the highest
occupied molecular orbital (HOMO) of the donor material into the lowest unoccupied
molecular orbital (LUMO), resulting in a hole in the HOMO and an electron in the
LUMO. This electron–hole pair is tightly bound because of large Coulomb interaction
in organic materials. This bound electron–hole pair is called a singlet exciton. This
state is also called singlet excited state, which consists of several different electron
configurations more strictly. In polymer solar cells, excitons cannot dissociate into
free charge carriers at room temperature because the exciton binding energy is much
larger than thermal energy k B T. This is partly because relative permittivity ε r is as
small as 3–4 in most organic materials, and hence a critical distance r C at which the