Influence of the additional electron-withdrawing unit in β-functionalized porphyrin sensitizers on the photovoltaic performance of dye-sensitized solar cells

The β-functionalized porphyrin containing an additional electron-withdrawing unit, 2,3-diphenylquinoxaline(DPQ) for LP-5 or 2,1,3-benzothiadiazole (BTD) for LP-6 with different electron-withdrawing abilities, between the porphyrin core and the anchoring group and the reference porphyrin dye (LP-4) have been designed and synthesized for DSCs. The influence of the additional electron-withdrawing units on molecular properties as well as photovoltaic performance of the corresponding DSCs was investigated systematically. Compared with LP-4, the introduction of additional electron-deficient unit at the porphyrin β π-linker in LP-5 and LP-6 decreases the lowest unoccupied molecular orbital (LUMO) energy levels, resulting in the broader absorption spectra and significantly improved IPCE spectra in the region 350-500 nm, which ensures the better light-harvesting properties and the higher short-circuit current density (Jsc). On the other hand, the introduction of additional acceptors of LP-5 and LP-6 induces dye aggregation and reduces the lifetime of the charge-separated states, which decreases the open-circuit voltage (Voc). Interestingly, the loss in Voc is overcompensated by the improvement in Jsc. The study provides not only an alternative approach to design novel porphyrin sensitizers, but also an insight into how to manipulate the LUMO energy levels of porphyrin sensitizers via the β-linker modifications for the optimal photovoltaic applications.