14 Photoenergy Conversion (Dye-Sensitized Solar Cells)
471
n-TiO 2 or n-ZnO-based DSSCs can be actually classified into two types, I and II,
depending on the electron-injection pathway from the dye to the conduction band
(CB) of the TiO 2 or ZnO electrode (Fig. 14.1). In type-I DSSCs, photoexcitation
of the local band of the dye adsorbed on the TiO 2 or ZnO surface occurs, followed
by injection of an electron from the photoexcited dye to the CB of the TiO 2 or
ZnO electrode (Fig. 14.1a). In other words, an electron is excited from the highest
occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital
(LUMO) of the dye sensitizer by sunlight illumination, followed by injection of
the LUMO electron into the CB of TiO 2 or ZnO electrode. Thus, this pathway can
also be called the “two-step” or “indirect” electron-injection pathway. The resultant
oxidized dye is subsequently reduced back to its original neutral state by electron
donation from the I
– ions in the redox mediator. The injected electrons move through
the interconnected network of TiO 2 or ZnO nanoparticles to reach the fluorine-doped
tin oxide (FTO) and then pass through the external circuit to the cathode (Pt-coated
glass). The I
− ion is regenerated by reduction of the I 3
− ion at the cathode through
electron donation from the external circuit, completing the circuit. To achieve efficient
electron injection from the excited dye to the CB of the TiO 2 or ZnO electrode, the
LUMO energy level of the dye in a type-I DSSC must be higher (more negative)
than the energy level (E cb ) of the CB (–0.5 V versus the normal hydrogen electrode
(NHE)) of the TiO 2 or ZnO electrode. Specifically, some researchers have proposed
that an energy gap of over 0.2–0.3 V is necessary for efficient electron injection. In
order to achieve efficient regeneration of the oxidized state by electron transfer from
the I 3
− /I
− ion couple in the electrolyte, the HOMO energy level of the dye must
be lower (more positive) than that of the I 3
− /I
− redox potential (0.4 V vs. NHE).
Specifically, some researchers have proposed that an energy gap of over 0.2–0.3 V
is necessary for efficient regeneration. Thus, electron injection and dye regeneration
must be thermodynamically feasible.
In contrast to type-I DSSCs, the electron injection is direct in type-II DSSCs, i.e.,
“one-step” electron injection occurs from the ground state of the dye to the CB of
the TiO 2 electrode by photoexcitation of the dye-to-TiO 2 charge transfer (DTCT)
band (Fig. 14.1b). In other words, the HOMO electron of the dye is directly injected
into the CB of the TiO 2 electrode upon photoexcitation. Therefore, in type-II dye
sensitizers, a strong new photoabsorption band is observed in a longer wavelength
region due to the DTCT. The light-harvesting capability over the wide spectral range
of sunlight is one of the most advantageous aspects in type-II DSSCs over typeI DSSCs because the direct electron injection enables the creation of the broad
DTCT bands and the easing of restrictions on the LUMO energy levels of the dye
sensitizers relative to the indirect electron injection in type-I DSSCs. Meanwhile, the
photoabsorption coefficient of the DTCT band observed in type-II dye sensitizers
is much lower than that of the photoabsorption band originating from the π–π*
transition in conventional dye sensitizers because of the lower overlap between the
ground (molecular-localized) and the excited state (Ti-localized) wave functions.
In type-I DSSCs, there are electron-loss processes, which lower the photovoltaic
performances: recombination of the electrons injected into the TiO 2 or ZnO electrode
with the oxidized dyes (recombination process) or with the I 3
− ions at the TiO 2 or ZnO
471
n-TiO 2 or n-ZnO-based DSSCs can be actually classified into two types, I and II,
depending on the electron-injection pathway from the dye to the conduction band
(CB) of the TiO 2 or ZnO electrode (Fig. 14.1). In type-I DSSCs, photoexcitation
of the local band of the dye adsorbed on the TiO 2 or ZnO surface occurs, followed
by injection of an electron from the photoexcited dye to the CB of the TiO 2 or
ZnO electrode (Fig. 14.1a). In other words, an electron is excited from the highest
occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital
(LUMO) of the dye sensitizer by sunlight illumination, followed by injection of
the LUMO electron into the CB of TiO 2 or ZnO electrode. Thus, this pathway can
also be called the “two-step” or “indirect” electron-injection pathway. The resultant
oxidized dye is subsequently reduced back to its original neutral state by electron
donation from the I
– ions in the redox mediator. The injected electrons move through
the interconnected network of TiO 2 or ZnO nanoparticles to reach the fluorine-doped
tin oxide (FTO) and then pass through the external circuit to the cathode (Pt-coated
glass). The I
− ion is regenerated by reduction of the I 3
− ion at the cathode through
electron donation from the external circuit, completing the circuit. To achieve efficient
electron injection from the excited dye to the CB of the TiO 2 or ZnO electrode, the
LUMO energy level of the dye in a type-I DSSC must be higher (more negative)
than the energy level (E cb ) of the CB (–0.5 V versus the normal hydrogen electrode
(NHE)) of the TiO 2 or ZnO electrode. Specifically, some researchers have proposed
that an energy gap of over 0.2–0.3 V is necessary for efficient electron injection. In
order to achieve efficient regeneration of the oxidized state by electron transfer from
the I 3
− /I
− ion couple in the electrolyte, the HOMO energy level of the dye must
be lower (more positive) than that of the I 3
− /I
− redox potential (0.4 V vs. NHE).
Specifically, some researchers have proposed that an energy gap of over 0.2–0.3 V
is necessary for efficient regeneration. Thus, electron injection and dye regeneration
must be thermodynamically feasible.
In contrast to type-I DSSCs, the electron injection is direct in type-II DSSCs, i.e.,
“one-step” electron injection occurs from the ground state of the dye to the CB of
the TiO 2 electrode by photoexcitation of the dye-to-TiO 2 charge transfer (DTCT)
band (Fig. 14.1b). In other words, the HOMO electron of the dye is directly injected
into the CB of the TiO 2 electrode upon photoexcitation. Therefore, in type-II dye
sensitizers, a strong new photoabsorption band is observed in a longer wavelength
region due to the DTCT. The light-harvesting capability over the wide spectral range
of sunlight is one of the most advantageous aspects in type-II DSSCs over typeI DSSCs because the direct electron injection enables the creation of the broad
DTCT bands and the easing of restrictions on the LUMO energy levels of the dye
sensitizers relative to the indirect electron injection in type-I DSSCs. Meanwhile, the
photoabsorption coefficient of the DTCT band observed in type-II dye sensitizers
is much lower than that of the photoabsorption band originating from the π–π*
transition in conventional dye sensitizers because of the lower overlap between the
ground (molecular-localized) and the excited state (Ti-localized) wave functions.
In type-I DSSCs, there are electron-loss processes, which lower the photovoltaic
performances: recombination of the electrons injected into the TiO 2 or ZnO electrode
with the oxidized dyes (recombination process) or with the I 3
− ions at the TiO 2 or ZnO