Figure-03: Carbazole-Donor Based Modified Organic Dyes: (a) D1A
(b) D2A (c)D3A
The molecular structures of dyes comprise of 91 atoms in D1, 89 atoms in
D2, 96 atoms in D3, 98 atoms in D1A, 96 atoms in D2A and 103 atoms in
D3A. The Slater type basis and fit functions were used with number of 4
for H, 9 for C, 9 for N, 13 for S, 9 for O. The entire calculations were
done without enforcing any symmetry. The structures were pre-optimized
using universal force field and then geometries were optimized to ensure
complete relaxation of the molecules/structures at minimum possible
energy. In the course of structural relaxation, the geometry updates
involved ‘delocalized’ optimization coordinates with BFGS
(Broyden-Fletcher-Goldfarb-Shanno) as Hessian update algorithm.
The basis functions were in the form of Slater Type Orbitals (STOs)of
quality TZP (core double zeta, valence triple zeta, single polarized
basis set) for all atoms used in the dye molecules. The exchange and
correlation functional (XC) was separately taken as standard GGA-PBE and
hybrid BELYP for all cases studied in this work. The iterative geometry
improvements involved convergence criteria with changes in energy,
gradients and bond length changes as 0.02 eV, 0.02 eV/Å and 0.01Å
whereas the SCF updates of geometry were carried out with convergence
criterion of 2.7x 10-5 eV. The numerical integrations
were carried out with Becke grid quality and ZlmFit density fit Quality
as ‘Good’. The precision parameters involved in numerical integration
was 6.0000000000 whereas the basis and fit neglect functions were
0.1x10-7. The linear scaling parameters of cut-off
radii density fit, overlap cut-off criterion for Coulomb potential and
multipole terms were 0.1x10-9,
0.1x10-7, 0.1x10-9 and
0.1x10-9 respectively. The entire calculations were of
all-electron type in such a way that no of core orbitals were considered
for finding the orbital energies. The ADF calculates the value of bond
energy by finding the difference of energy between molecule and its
fragments in the form of spherically symmetric and spin restricted atoms
[54].
(TiO2)96 quantum dot having 96 atoms of
Ti and 192 atoms of O was prepared using DFTB by employing SCC-DFTB
model. The dispersion and periodicity were set as ‘none’ and the
parameter set trio/org 0-1 was used thereby setting Fermi temperature at
5 K. The default values of occupation and initial hessian were selected.
The energy convergence, gradient convergence and step convergence are
10-5 eV, 10-3 eV/Å and
10-3 Ǻ respectively. The optimization method and
optimization space were employed as quasi-newton method and cartesian
space respectively.
After the geometry optimization of the structures, all electrons single
point calculations were done, on the optimized neutral dye structures
without considering symmetry, in order to study electronic and optical
characteristics of photosensitizes. The UV-Vis excitation spectrum was
calculated using ADF-Response code by considering the singlet-singlet
allowed transitions by utilizing Davidson method [55]. The
properties of the structures optimized at corresponding levels of theory
were calculated using XC functionals GGA-PBE and Hybrid-B3LYP. The
criterion of convergence was adopted as 10-6 eV
thereby setting the numerical quality ‘good’ with basis set TZP. The
excitation spectra were are also calculated using DFTB with model
SCC-DFTB by selecting singlet-singlet transitions with both Davidson and
exact methods.
In order to study the photoinjection, the already optimized dye
molecules were adsorbed on the surface of optimized
(TiO2)96 quantum dot (QD) by performing
geometry optimization at DFTB level of theory using Canonical DFTB
algorithm. A complete SCC convergence was ensured with gradient
convergence of 0.01 Hartree/Å and charge convergence of
10-8e at Fermi temperature of 300 K.The atoms of the
QD were kept fixed except the unit involved in binding with dye
acceptor. Further, the entire atoms of the dyes were allowed to relax
during the optimization of QD-Dye complex. The excitation spectra on the
optimized QD-Dye complexes were calculated in single point runs to study
the photoinjection by employing the stated parameters. The electron
occupation in shells and sub shells are set as ‘default’ that include
both fermi and aufbau principal and with K-space sampling as ‘gamma’.
Transition charges in these calculations are pre-calculated. From
excitation spectra, photoinjection energy, recombination energy and LHE
were calculated.
The effects of solvation were also studied by optimizing the dyes
structure in two solvents; water and methanol. The number of molecules
of solvents and the polarity of solvents are important factors that
should be kept in mind during these calculations. The parameters that
are used in geometry optimization of dyes and dyes-TiO2were also utilized for relaxation of dyes in solvents. The solvent
radius was set as 10.0 where the solute factor was set as 2.7 that
comprise almost 27 solvent molecules relative to one molecule of dye. As
the dyes get relaxed in solvents then their UV-Vis excitation spectrums
of all the dyes were calculated in order to study the spectral shift in
dyes due to solvation.