Heteroquinoid Merocyanine Dyes with High Thermal Stability as Absorber Materials in Vacuum-Processed Organic Solar Cells

A series of heteroquinoid merocyanine dyes with an extended pi-system as well as lower homologues (bearing varied donor units such as benzothiazolylidene and 4H-pyran-4-ylidene) were synthesized, and their thermal, optical and electrochemical properties were investigated. The dyes exhibit a variety of features that are desirable for solar cell production. These include strong absorption coefficients (up to 67.000 L M-1 cm(-1)), broad Vis/NIR absorption bands, frontier molecular orbital energies suitable for electron transfer to C-60, and electronic gaps ranging from 0.90 to 1.74 eV. Owing to their high thermal stability, the dyes can be sublimated in high yields. For three of the dyes, the packing arrangements in the solid state by single-crystal X-ray analysis were examined. The photovoltaic characteristics of the most promising thienoquinoid merocyanine dye was investigated in devices using C-60 as acceptor material. In an optimized vacuum-processed device, a PCE of 3.6 % was achieved.