Dissertation (Msc. Chemistry)Dye sensitized solar cells (DSSCs) are the one among the important device which established a consideration attention for transferring a fresh solar energy to electricity in the most recent decade. Coumarin dye is one among metal free organic dyes which hold a higher potential for operating in DSSC because of its environmentally friendly, inexpensive and easy to fabricate. The aim of this study was to apply computational quantum calculations which persuade to appreciate the molecular structure, optical absorption spectra, electronic and photo physical properties of coumarin dye derivatives calculation by using DFT and TD-DFT computational method. The organic dye D-π-A dye based on coumarin and their derivatives were investigated by attaching various donors and acceptors to coumarin dyes (D-CM-A). The geometry optimization was done in gas and solvent phases. Electron absorption spectra in excited state energies and oscillator strength (f) were calculated through TD-DFT/B3LYP method with 6-3IG basis set calculations of fully DFT optimized geometries. The results show that, D4-CM-A4 in third and fourth positions have the smallest energy gap (‒1.98 and ‒1.49 eV respectively). On the contrary, the light harvesting efficiencies (LHE) were calculated and this parameter depends upon the oscillator strength of the dye molecules. The results depicted that, in both positions D1-CM-A1 and D2-CM-A2 have the highest LHE (0.85 and 0.91 respectively) than other dye derivatives. Therefore, it is suggested that, D1CM-A1 and D2-CM-A2 have better properties for application in DSSC as compared with rest dye derivatives.