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

A series of new pyridomethene-BF2/phenothiazine hybrid metal-free organic sensitizers K1-K8 containing different pi-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 (eta) 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 Voc value. The presence of the phenyl pyridomethene-BF2 moiety at the N(10) atom of phenothiazine in K6-K8 reduced pi-pi aggregation. These results reveal the advantage of incorporating a pyridomethene-BF2 group in the dyes for high-performance DSSC cells.