Boron–dibenzopyrromethene-based organic dyes for application in dye-sensitized solar cells

Abstract

Novel boron–dibenzopyrromethene dyes with thienyl-cyanoacrylic acid units were synthesized and characterized for application in dye-sensitized solar cells (DSSCs); the dyes feature intense absorption bands in the longer wavelength region with λ_max values of 647 nm (ε = 1.57 × 10⁵ M⁻¹ cm⁻¹) for 1, which has two anchoring units, 660 nm (ε = 1.09 × 10⁵ M⁻¹ cm⁻¹) for regioisomer 2, and 644 nm (ε = 1.39 × 10⁵ M⁻¹ cm⁻¹) for 3, which has a single anchoring unit. Density functional theory (DFT) analysis revealed that these absorption properties are mainly characterized by intramolecular charge transfer from the dibenzopyrromethene core to the thienyl-cyanoacrylic acid unit that depend on both the number of anchoring groups and the position of the thienyl-cyanoacrylic acid unit on the isoindole ring. The relationship between the chemical structures and cell properties of these dyes was investigated. Although the short circuit photocurrent density (J_sc) value of a 1-loaded cell is larger than that of a 3-loaded cell, which reflects the results of the incident photon-to-charge carrier efficiency (IPCE) spectra, the 1-loaded cell has a lower open circuit voltage (V_oc) and fill factor (FF). Accordingly, overall power-to-current conversion efficiencies of 5.24 and 5.48% were obtained for a 1-loaded cell and 3-loaded cell, respectively, under 100 mW cm⁻² AM1.5G simulated light. On the other hand, the cell containing butterfly-shaped regioisomer 2, which showed improved intramolecular charge transfer, has an overall power-to-current conversion efficiency of 6.06%; this value is the highest published for BODIPY dyes even though the sensitizer does not contain any strong donor units, such as arylamines.