Natural pigments in dye-sensitized solar cell (DSSC): a DFT-TDDFT study

Abstract

The molecular structures and photophysical properties of twelve types of the dyes from natural pigments of chlorophyll, flavonoid, carotenoid and anthocyanin were calculated with density functional theory (DFT) and time-dependent DFT (TD-DFT). Based on the comparison and better agreement between experimental wavelengths of maximum absorption and their calculated values with functionals of B3LYP, CAM-B3LYP and BHand-H, the B3LYP/6-31G** level was chosen for the process of our studies. The dyes’ properties in solvent environment were carried out using conductor-like polarizable continuum model methods (CPCM-DFT and TD-CPCM-DFT). The results show that pigment type affects the photophysical properties, and the dyes of a specific pigment type have nearly the same properties. Anthocyanins have the highest values of ionization potential (IP), electron affinity (EA), electronic chemical potential (µ) (their absolute values), chemical hardness (η), electrophilicity index (ω), electroaccepting power (ω+) and electrodonating power (ω−), which can lead to high efficiency of these dyes type for dye-sensitized solar cells (DSSCs). Energy gap of ELUMO and EHOMO of the dyes respectively with the conduction band edge of TiO2 (ECB) and Eredox of $$\left(\tt {I^ - }/I_{3}^{ - }\right)$$ couple (ΔEL and ΔEH), the free energy difference for electron injection (ΔGinject) and the driving force for dye regeneration (ΔGreg), the maximum absorption wavelengths of peaks (λmax) and oscillator strength values (f) for all the dyes in gas, acetonitrile and water phases were calculated and explained. These parameters show that carotenoids, chlorophylls and anthocyanins are more appropriate pigments than flavonoid pigment and the estimated open-circuit photo-voltage values (eVoc) showed that flavonoids are the desirable pigment. We expect these results could be helpful for DSSC producers to choose highly efficient natural dyes according to their optical and electronic properties.