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.