Dissertation (MSc Chemistry)Dye-sensitized solar cells (DSSC) have received considerable attention for transferring clean solar energy into electricity over the previous decade. The Fluorescein dye is one among the metal-free organic dyes having a high potential for application in DSSC because of its environmentally friendly, less cost, easy fabrications. The aim of this study was to apply computational quantum calculation to understand the structural, molecular, electronic and photophysical properties of the fluorescein dye derivatives. The organic D-π-A dye based on Fluorescein (FS) and five derivatives of FS were investigated through attaching different donor group amine (A), ethene (E), methane (M), methoxy (O) and thiophene (T). The optimization of the geometry was done in gas and solvent phases. The excited state energies, electron absorption spectra and oscillator strengths (f) were calculated using TDDFT/B3LYP method with 6-311G bases set calculations on fully DFT optimized geometries. The result shows that FS attached with thiophene (FST) compound have small energy gap compared to other derivatives. The increasing order of energy gap between HOMO and LUMO for the fluorescein and their derivatives is FST<FSE<FSM<FS<FSO<FSA. In addition, the light harvesting efficiencies (LHE) were calculated and this parameter depends upon the oscillator strength of the molecules, the results show that the FST has highest and FS have lowest LHE value. The order of decreasing light harvesting efficiency is FST>FSA>FSO>FSM>FSE>FS. The result indicates that, the FST and FSA have the highest and lowest electron injection efficiency respectively. It is suggested that FST has better properties for application in DSSC as compared with the rest derivatives.