Synthetic Strategy of Low-Bandgap Organic Sensitizers and Their Photoelectron Injection Characteristics

Low-bandgap organic sensitizers are designed and synthesized for high photocurrent generation in dye-sensitized solar cell. Introduction of the -conjugated benzothiadiazole (BTD) unit in K1, designated as K2, decreases the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap energy from 2.63 to 2.10 eV, whereas the photovoltaic performance is not improved although the absorption threshold in incident photon-to-current conversion efficiency spectrum is extended from 650 nm to approximately 800 nm. Low photovoltaic performance is overcome by introduction of a phenylenevinylene moiety next to the BTD unit in K2, designated as K3 that shows similar HOMO-LUMO gap energy and absorbance to those of K2. As a result, a K3-sensitized 5 m-thick TiO solar cell demonstrates photocurrent density of 12.24 mA/cm, voltage of 0.549 V, and conversion efficiency of 3.8%, which is better than the conversion efficiency of 2.49% for K2 with photocurrent density of 8.13 mA/cm and voltage of 0.470 V. According to nanosecond transient absorption spectroscopic study, photoexcited electron injection efficiency of K3 is found to be three times higher than that of K2, which is attributed to the higher photocurrent of K3.