Figure 7: The schematic of charge transfer from dye to Semiconductor TiO2
The direct photoinjection is usually indicated by appearance of an additional peak in excitation spectrum where no additional peaks are observed in indirect photoinjection [39, 40, 81-83]. In this work a (TiO2)96QD having 288 atom and size 2.7 nm is utilized to model the photoinjection in (TiO2)96-D complex where D = D1, D1A, D2, D2A, D3 and D3A. In order to prepare the series of complexes, the dyes were adsorbed on the surface of (TiO2)96 in dissociative monodentate mode in which OH and O of the anchoring group were attached with respective oxygen and titanium atoms on surface of (TiO2)96[39, 84]. SCC-DFTB approach was utilized in order to get optimized geometries of dye-(TiO2)96 complex with parameters utilized in these simulations are org/trio 0-1. The relaxed geometries of the complex were obtained by allowing the dye and its anchoring TiO2 units to take part in geometry optimization whereas the rest of TiO2 units were kept fixed in order to lessen the computational cost. UV-Vis excitation spectra were then obtained for the optimized complex at SCC-DFTB level of theory in order to model the photoinjection in the mentioned complexes. The spectra obtained for all six complexes show small red shift and broadening of peaks in comparison to the bare dyes. The behavior of all dyes is changed after absorption on (TiO2)96whereas the extent of red shift in the adsorbed dyes is also different. The values of red shift for D1 and D1A are 0.26 eV and 0.21 eV respectively. D2 exhibited largest red shift of 0.85 eV whereas D2A, D3 and D3A show red shift of 0.14 eV, 0.04 eV and 0.1 eV respectively. The spectra of the adsorbed dyes shows a red shift that attribute to hybridization of orbits in density orbit stabilization [84]. This behavior points to the indirect photoinjection in these dyes. The excitation spectra of bare dyes and dyes attached to TiO2QD are given in figure 8.