New starburst sensitizer with carbazole antennas for efficient and stable dye-sensitized solar cells

A new starburst organic sensitizer (I) with carbazole units as antennas and the corresponding dyes (II and III) with starburst triphenylamine or one triphenylamine moiety (for the purpose of comparison) were designed and synthesized, in which carbazole or triphenylamine moieties were used as the electron donor, thiophene units as the pi-conjugated bridge, and a cyanoacrylic acid group as the electron acceptor. The photophysical and electrochemical properties of the dyes were investigated by UV-vis spectrometry and cyclic voltammetry. Electrochemical measurement data indicate that the tuning of the HOMO and LUMO energy levels can be conveniently accomplished by alternating the donor moiety. Photovoltaic devices with carbazole dye I showed a maximum monochromatic incident photon to current efficiency (IPCE) of 89% and an overall conversion efficiency of 4.4% under full sunlight (AM 1.5G, 100mWcm(-2)) irradiation. The dyes were also tested in quasi-solid-state devices and showed good efficiencies. The stability of devices was performed over 1200 h in full sunlight and at 50 degrees C. The sensitizer I with carbazole antennas exhibited good stability, retaining 95% of its initial efficiency, while dyes II and III retained 82% and 46% after 1200 h of irradiation, respectively. This result means that carbazole is a photostable hole-transporting moiety for use in dye-sensitized solar cells.