Pyridomethene–BF2 complex/phenothiazine hybrid sensitizer with high molar extinction coefficient for efficient, sensitized solar cells

Abstract

A series of new pyridomethene–BF₂/phenothiazine hybrid metal-free organic sensitizers K1–K8 containing different π-spacers were synthesized and applied in dye-sensitized solar cells (DSSC). The introduction of the pyridomethene–BF₂ complex unit to the phenothiazine chromophore resulted in a high molar extinction coefficient in favor of light harvesting. Quantum chemical calculations were performed using the density functional theory (DFT) at the B3LYP/6-31G(d,p) level to investigate the structural properties and electron density distributions of these dyes. The effect of dyes K1–K8 on the performance of DSSC was investigated systematically with comparisons to the plain phenothiazine dyes R1 and R2. Upon co-adsorption with deoxycholic acid, the dye K3, with a thiophene unit between the phenothiazine and pyridomethene–BF₂ units, exhibited the best photovoltaic performance. The short-circuit current density (J_sc) was 15.43 mA cm⁻² with an open-circuit voltage (V_oc) of 0.69 V and a fill factor (FF) of 0.62, which correspond to a power-conversion efficiency (η) of 6.58% under AM 1.5G irradiation (100 mW cm⁻²). The n-hexyl chain attached to the thiophene in K4–K5 improved the V_oc value. The presence of the phenyl pyridomethene–BF₂ moiety at the N(10) atom of phenothiazine in K6–K8 reduced π–π aggregation. These results reveal the advantage of incorporating a pyridomethene–BF₂ group in the dyes for high-performance DSSC cells.