D-π-A Sensitizers for Dye-Sensitized Solar Cells: Linear vs Branched Oligothiophenes

Two donor-π-acceptor (D-π-A) dyes, coded as L-3T-DPA 1 and B-5T-DPA 2, were synthesized for application in dye-sensitized solar cells. These D-π-A sensitizers use diphenylamine as donor, an oligothiophene as π-bridge, and cyanoacrylic acid as an acceptor group that can be anchored to the surface of TiO2. While the two dyes comprise the same donor and acceptor units, the bridging oligothiophene is linear in one case and branched in the other case. Photophysical and electrochemical properties of the dyes were investigated by UV−vis spectrometry and cyclic voltammetry. The dyes were subsequently implemented as sensitizers in dye-sensitized solar cells. Photovoltaic devices with dye 1 showed a maximum monochromatic incident photon to current efficiency (IPCE) of 80% and an overall conversion efficiency of 6.8% under full sunlight (AM 1.5G, 100 mW cm−2) irradiation. The photovoltaic performance of branched dye 2 was lower because of less dye loading on the TiO2 surface. The dyes were also tested in ionic liquid and solid-state devices and showed good efficiencies. Long-term stability measurements were performed over 1000 h at full sunlight and at 60 °C in ionic liquid devices. Branched dye 2 thereby showed excellent stability retaining 96% of its initial efficiency, while linear dye 1 retained 73% after 1000 h of irradiation.