Theoretical analysis of Capsicum annuum as natural sensitizer in dye sensitized solar cell using periodic DFT and TD-DFT

Abstract

The dye sensitizers coming from Capsicum annuum, namely capsorubin, capsanthin, and capsaicin, are studied for their application in dye-sensitized solar cells (DSSC). DFT calculations are performed to analyze the DSSC mechanism, especially the photoexcitation and electronic injection step. Absorption spectra describe a band around 450 nm, which originates from a π-π∗ electronic transition associated with the frontier orbitals. This is localized in the large chain of double bonds present in capsorubin and capsanthin. Moreover, the behavior towards adsorption and electron injection of Capsicum annuum to the TiO₂ semiconductor is analyzed through the capsorubin@(TiO₂)₇₂, capsanthin@(TiO₂)₇₂, and capsaicin@(TiO₂)₇₂ models. The last one shows a different geometry compared to capsorubin@(TiO₂)₇₂ and capsanthin@(TiO₂)₇₂. Specifically, the carotenoids capsorubin and capsanthin are bonded to the surface by hydrogen bonds between the methyl substituents and the oxygen atoms of TiO₂, while capsaicin loses linearity around the amide group (C-NH-CO 180°), transforming into an angular rearrangement (C-NH-CO 120°). These structural changes influence the electronic injection of Capsicum annuum into the TiO₂ semiconductor because the free energy change for the electron injection ($\Delta G_{inj})$ is spontaneous for capsaicin but is not for capsorubin and capsanthin. Thus, the efficiency is controlled by the presence of weak interactions in this type of sensitizer.