Abstract:
The optical and electronic properties of four conjugated organic molecules, Indigo, Alizarin, Emodin and Purpurin, as potential organic solar cells were studied theoretically. The electronic properties, reorganization energy (λh and λe), adiabatic ionization potential (IP), adiabatic electron affinity (EA), chemical hardness (n), highest Occupied Molecular Orbital(HOMO) and Lowest Unoccupied Molecular Orbital(LUMO) energies and HOMO-LUMO energy gap (Eg) were calculated using density functional theory (DFT). The optical properties as the maximum absorption (λmax) along with oscillator strengths (f ) at the excited states in vacuum and ethanol as a solvent were also calculated using time-dependent density functional theory (TD-DFT). The impact of functionalization, of these molecules with electronegative functional groups, on their optical and electronic properties was explored. The results show that the values of λh and λe for Indigo were 0.195 eV and 0.175 eV and for Alizarin 0.366 eV and 0.482 eV, respectively.
The functionalization of Indigo and Alizarin has resulted in overall conversion of the materials to better n-type molecules.
The introduction of functional groups to Emodin converted the molecule from p-type into n-type material. while functionalization of Purpurin, which is considered as an n-type material, with NO2 and F- resulted in a slight increase of λe with values of 0.42 eV for each. This is considered as detrimental for the charge-transport process.
The functionalized molecules have shown an increase of EA and decrease in LUMO energy level, indicating their potential use as n-type materials. Overall, the (Eg) for studied molecules has been generally reduced upon introduction of electronegative functional groups. This indicates the possibility of tuning the optical and electronic properties of these organic molecules by introducing suitably selected functional groups. Also, the studied molecules along with their functionalized molecules show properties that fall among organic semi-conductors and thereby, can potentially be used in solar cells.
The intermolecular interaction studies on Emodin and Purpurin, using Hirschfeld surface analysis and energy framework, showed that the major intermolecular interaction occurs at the O– H/H–O with 31.8% for Emodin and 41% for Purpurin.
