A comparative study of porphyrin dye sensitizers YD2-o-C8, SM315 and SM371 for solar cells: the electronic structures and excitation-related properties

Understanding the electronic structures and excitation properties of dye sensitizers has significant importance to improve the photon-energy conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs). Here, based upon the results calculated using density functional theory, the electronic structures and excitation related properties of porphyrin dye sensitizers YD2-o-C8, SM315, and SM371 were analyzed. It was found that the similar electronic structures of YD2-o-C8 and SM371 result in similar absorption spectra, excitation, and free energy variation for electron injection (EI) and dye regeneration. However, since the electronic structure of the benzothiadiazole unit is well-coupled to that of the porphyrin ring, introducing benzothiadiazole into porphyrin dyes generates a decrease in the lowest unoccupied molecular orbital energy, red-shift and splitting of absorption bands. Meanwhile, remarkably it increases the transferred charges of excitation, which is responsible for the superior short-circuit current density of SM315 sensitized DSSCs. Furthermore, the transition configurations and molecular orbitals indicate the diarylamine group acts as an electronic donor, and the different EI modes with different timescales coexist in excited states due to the multi-configurations of transition. The results of structure-property relationships are favorable to develop novel dye sensitizers for DSSCs. Graphical abstract