3.2. Optical/Electronic Properties of Dyes
The increase in the length of π-spacer is expected to enhance the
oscillator strength and lessen the excitation energy to induce
transitions. A detailed analysis of energy levels, especially HOMO and
LUMO, is made to understand the effects of structural modification on
optical and electronic properties of the dyes. The
(TiO2)96 was optimized with mean
diameter as 2.7 nm whereas the calculated value of HOMO-LUMO gap was 1.9
eV.
The HOMO and the LUMO energy levels of dye for a competent injection of
electron should be compatible with the semiconductor. The LUMO of the
dyes should be more negative than the CB of semiconductor (frequently
used TiO2) [60,61]
and energy of HOMO ought to be lower than the oxidation and reduction
potential of electrolyte (frequently
I-1/I-3 redox couple is utilized)
[62]. Moreover, a small band gap is preferred to excite electron
from HOMO of dye to its LUMO in order to make the transition by
providing the small amount of energy. Table 1 clearly shows that the
LUMO of all the dyes simulated in this work is more negative than the CB
of the QD. In addition to this the HOMO of these dyes are less negative
than the redox potential of electrolyte, which results in competent
renewal of dyes.
The energy of HOMO level of both dyes D1 and D1A is found less than HOMO
of redox electrolyte I3- (-4.8eV) that
results in effective regeneration of dye whereas the energy of LUMO of
both dyes is greater than energy of LUMO of TiO2 (-4.3
eV) which may cause easy injection of electron from dye to
TiO2. The energies of LUMO and HOMO of D1 and D1A,
obtained by utilizing ADF/TD-DFT with the XC functional set as GGA-PBE
Hybrid- B3LYP and SCC-DFTB. The respective values of HOMO of D1 is at
-5.0 eV, -5.6eV [23], 5.2 eV and LUMO
at -3.9 eV, -3.2 eV [23] and -3.9 eV
with band gap -1.1 eV, -2.4 eV and -1.3 eV calculated using GGA-PBE,
Hybrid-B3LYP and SCC-DFTB. In case of D1A, the respective values of HOMO
and LUMO are -4.9 eV, -5.6 eV, -5.1 eV and -3.8eV, -3.2eV, -3.8 eV with
same band gap as that of D1 calculated for all three functionals. The
comparison indicates that the energy of HOMO increases from D1to D1A
which shows a clear increase in the energy of HOMO level with increase
in length of π-bridge in agreement with literature [63]. Likewise,
the changes in energy of LUMO level are noted but very negligible change
in case of Hybrid-B3LYP are observed for both HOMO and LUMO
[63-65].
The HOMO of D2 is found at -5.0 eV, -5.6 eV
[23], -5.2 eV and LUMO at -4.1 eV,
-3.5eV [23], -3.9 eV with band gap of
0.9 eV, -2.1 eV and -1.3 eV calculated using-PBE, Hybrid-B3LYP and
SCC-DFTB respectively. On the other hand, the HOMO of D2A is found at
-5.0 eV, -5.6 eV, -5.2 eV and LUMO level is at -4.0 eV, -3.4 eV, -3.9 eV
with a band gap of 1.0 eV, -2.2 eV, -1.3 eV calculated for three
respective functionals. The addition of electron withdrawing group
oxadiazole in π-bridge has led to change in LUMO energy level because of
change in charge transfer, while the energy of HOMO remains same as the
values of band gaps for D1 are larger than D2. The decrease in value of
band gap of D2 upon addition of electron withdrawing oxadiazole group is
due to lowering of its LUMO level
[64]. A slight increase in band gap
has taken place when π-bridge length was increased for D2A.
The HOMO of D3 is found at -5.0 eV, 5.6 eV
[64], -5.1 eV and LUMO is at -4.2 eV,
3.5 eV [64], -3.9 eV with a band gap
of 0.8 eV, -2.1 eV and -1.2 eV when calculated using the respective
functionals. On the other hand, in case of D3A the HOMO level is found
at -5.0 eV, -5.6 eV, -5.1 eV and LUMO level is at -4.1 eV, -3.5 eV, -3.9
eV with a band gap of 0.9 eV, -2.1 eV and -1.3 eV when calculated using
GGA-PBE, Hybrid-B3LYP and SCC-DFTB respectively. A decrease in value of
band gap is observed on addition of electron withdrawing oxadiazole in
D3 whereas a slight increase in band gap occurs when π-bridge length
increases for D3A[64]. The
comparative analysis of calculated values of HOMO, LUMO and their gap
calculated using three functionals is sketched in figure 4.