Coumarin-based D–π–A dyes for efficient DSSCs: DFT and TD-DFT study of the π-spacers influence on photovoltaic properties

A series of D–π–A architectures dyes with Coumarin-based derivatives as difluorenylaminocoumarin (DF) and diphenylaminocoumarin (DP) have been designed by modifying the π-bridge for potential application in dye-sensitized solar cells (DSSCs). Three different π-bridges viz. benzothiadiazole (BTD), diketopyrrolopyrrole (DPP) and quinoxaline (QX) with are used to design six different dye systems DFi (i = 1–3) on difluorenylaminocoumarin-thiophene-cyanoacrylic acid (R1) and DPi (i = 1–3) on diphenylaminocoumarin-thiophene-cyanoacrylic acid (R2). The electronic, optical and photovoltaic properties have been evaluated, using the density functional theory (DFT), and time dependant DFT methods to study the effect of these spacers on the cell performance. In addition, we have used four functionals to establish our calculation method, in order to compare the theoretical with the experimental results of R1 and R2 dyes. Thus, the CAM-B3LYP functional was sufficient to predict the UV–visible spectra. Moreover, the gap energy (Egap), Voc, the electronic injection free energy (ΔGinject), light harvesting efficiency (LHE), total reorganization energy (λtotal), and absorption spectra of all dyes were discussed. In addition the DF2, DP2 dyes are displayed smaller Egap, negative ΔGinject and absorption spectra shifted to red compared to R1 and R2, respectively. The results of these dyes reveal that the DF2 and DP2 dyes molecules are favourable candidates for DSSCs application.