Organic Dyes Containing Thieno[3,2-b]indole Donor for Efficient Dye-Sensitized Solar Cells

Three new organic dyes, comprising thieno[3,2-b]indole moiety as an electron donor, n-hexyl substituted oligothiophene units as a π-conjugated bridge, and cyanoacrylic acid group as an electron acceptor and anchoring group, have been designed and synthesized for nanocrystalline TiO2 dye-sensitized solar cells (DSSCs). Compared with our previous carbazole-based MK dyes, in the similar donor-π-acceptor structure, thieno[3,2-b]indole not only has stronger electron donating ability but also could hold the dye molecule in a more planar conformation than carbazole. The newly synthesized thieno[3,2-b]indole-based dyes exhibit their major electronic absorption in the range of 400−600 nm in the visible region in solution. Solar energy to electricity conversion efficiency (η) up to 7.8% was obtained with a DSSC using 2-cyano-3-[5′′-(4-ethyl-4H-thieno[3,2-b]indol-2-yl)-3′,3′′,4-tri-n-hexyl-[2,2′,5′,2′′]terthiophen-5-yl]acrylic acid (MKZ-40) as light harvesting sensitizer under simulated AM 1.5 G irradiation (100 mW cm−2) with an aperture mask: short-circuit photocurrent density (Jsc) = 14.6 mA cm−2, open-circuit voltage (Voc) = 0.70 V, and fill factor (FF) = 0.76, suggesting that thieno[3,2-b]indole-based organic dyes are promising candidates for DSSCs. However, the DSSCs based on thieno[3,2-b]indole dyes showed shorter electron lifetime compared to that for the solar cell with a conventional carbazole dye, resulting in lower Voc.