Organic Dyes with Well-Defined Structures for Highly Efficient Dye-Sensitised Solar Cells Based on a Cobalt Electrolyte

Seven SGT organics dyes, containing bis-dimethyl-fluoreneyl amino groups with a dialkoxyphenyl unit as an electron donor and a cyanoacrylic acid group as an anchoring group, connected with oligothiophenes, fused thiophenes and benzothiadiazoles as pi-bridges, were designed and synthesised for applications in dye-sensitised solar cells (DSSCs). The photovoltaic performance of DSSCs based on organic dyes with oligothiophenes depends on the molecular structure of the dyes, in terms of the length change of the pi-bridging units. The best performance was found with a pi-bridge length of about 6 angstrom. To further enhance the photovoltaic performance associated with this concept, cyclopenta[ 1,2-b:5,4-b'] dithiophene (CPDT) and benzothiadiazole were introduced into the pi-bridge unit. As a result, the DSSC based on the organic dye containing the CPDT moiety showed the best photovoltaic performance with a short-circuit photocurrent density (J(sc)) of 14.1 mAcm(-2), an open-circuit voltage (V-oc) of 0.84 V and a fill factor (FF) of 0.72, corresponding to an overall conversion efficiency (eta) of 8.61% under standard AM 1.5 irradiation.