Density Functional Theory Studies of Ruthenium Dye …
145
to account for the whole absorption spectrum [21]. The UV-Vis spectra, maximum
excitation wavelength, excitation energies, oscillator strength and light harvesting
efficiency of the dyes were computed. Their matching with the solar spectrum is discussed. The light harvesting efficiency (LHE) at the maximum spectrum wavelength
(λ max ) was computed for the ruthenium (N3) complex using Eq. 1,
L H E = 1 − 10
− f
(1)
where “f” is the absorption strength of the dye associated with the maximum
absorption of the dye (also called the oscillator strength) [15, 22, 23].
The absolute values of the adsorption energies of ruthenium (N3) complex on
TiO 2 were computed using Eq. 2.
E ads = E dye + E T i O 2 −
E dye+T i O 2
(2)
where E ads is the adsorption energy, E dye is the energy of the dye, E (T i O 2 ) is the
energy of the TiO 2 slab and E (dye+T i O 2 ) is the total energy of the dye-TiO 2 complex.
A positive value of E ads indicates stable adsorption [14, 24, 25].
The energies of the HOMO and LUMO, the HOMO-LUMO energy gap and
the isodensity surfaces of the molecular orbitals involved in the excitation of the
ruthenium (N3) complex were identified from the fchk file obtained from the
simulations.
The bulk structure of brookite TiO 2 that was used for this study was optimized
using the CASTEP module in Materials Studio BIOVIA [26] to obtain the ground
state structure of the TiO 2 brookite semiconductor. The convergence energies and
k-points were 650 eV and 4 × 7 ×7 respectively where k-points is the radius of
convergence in the brillouin zone. The optimized structure of the ruthenium (N3)
complex was exported into an atomic simulation environment via Avogadro Software
[27]. DFT with the PBE functional was used through GPAW [28] and AVOGADRO
[27] computational software within the atomic simulation environment to explore
the optical properties of two modelled TiO 2 brookite clusters, that is Ti 8 O 16 and
Ti 68 O 136 . The optical properties of the interplay of ruthenium (N3) dye molecules
with brookite Ti 8 O 16 and Ti 68 O 136 clusters were investigated using DFT, in order to
optimize photon current densities in DSSCs.
3 Results and Discussions
3.1 Geometric Properties of the Ruthenium (N3) Complex
The molecular structure of the ruthenium (N3) complex studied in this work is shown
in Fig. 1 and its optimized geometry in Fig. 2. Figure 2 shows that the ruthenium
atom is octahedral coordinated to six nitrogen atoms pertaining to the two thiocyanate
145
to account for the whole absorption spectrum [21]. The UV-Vis spectra, maximum
excitation wavelength, excitation energies, oscillator strength and light harvesting
efficiency of the dyes were computed. Their matching with the solar spectrum is discussed. The light harvesting efficiency (LHE) at the maximum spectrum wavelength
(λ max ) was computed for the ruthenium (N3) complex using Eq. 1,
L H E = 1 − 10
− f
(1)
where “f” is the absorption strength of the dye associated with the maximum
absorption of the dye (also called the oscillator strength) [15, 22, 23].
The absolute values of the adsorption energies of ruthenium (N3) complex on
TiO 2 were computed using Eq. 2.
E ads = E dye + E T i O 2 −
E dye+T i O 2
(2)
where E ads is the adsorption energy, E dye is the energy of the dye, E (T i O 2 ) is the
energy of the TiO 2 slab and E (dye+T i O 2 ) is the total energy of the dye-TiO 2 complex.
A positive value of E ads indicates stable adsorption [14, 24, 25].
The energies of the HOMO and LUMO, the HOMO-LUMO energy gap and
the isodensity surfaces of the molecular orbitals involved in the excitation of the
ruthenium (N3) complex were identified from the fchk file obtained from the
simulations.
The bulk structure of brookite TiO 2 that was used for this study was optimized
using the CASTEP module in Materials Studio BIOVIA [26] to obtain the ground
state structure of the TiO 2 brookite semiconductor. The convergence energies and
k-points were 650 eV and 4 × 7 ×7 respectively where k-points is the radius of
convergence in the brillouin zone. The optimized structure of the ruthenium (N3)
complex was exported into an atomic simulation environment via Avogadro Software
[27]. DFT with the PBE functional was used through GPAW [28] and AVOGADRO
[27] computational software within the atomic simulation environment to explore
the optical properties of two modelled TiO 2 brookite clusters, that is Ti 8 O 16 and
Ti 68 O 136 . The optical properties of the interplay of ruthenium (N3) dye molecules
with brookite Ti 8 O 16 and Ti 68 O 136 clusters were investigated using DFT, in order to
optimize photon current densities in DSSCs.
3 Results and Discussions
3.1 Geometric Properties of the Ruthenium (N3) Complex
The molecular structure of the ruthenium (N3) complex studied in this work is shown
in Fig. 1 and its optimized geometry in Fig. 2. Figure 2 shows that the ruthenium
atom is octahedral coordinated to six nitrogen atoms pertaining to the two thiocyanate