Synthesis and characterization of benzo- and naphtho[2,1-b:3,4-b′] dithiophene-containing oligomers for photovoltaic applications

Dicyanovinyl (DCV) end-capped oligomers 1-7 containing fused benzo[2,1-b: 3,4-b'] dithiophene (BDT) and naphtho[2,1-b: 3,4-b'] dithiophene (NDT) as central cores have been synthesized and characterized. These oligomers show excellent thermal stability due to the insertion of the central fused ring system thus allowing purification by gradient sublimation in high yield. With respect to the reference compound, non-fused tetramer DCV4T, the new oligomers showed hypsochromic shifts in absorption and emission spectra and larger band gaps in thin films. Due to high lying LUMO energy levels, they exhibit sufficient energy offset with respect to C-60 to allow for efficient charge transfer in organic solar cells. At the same time, low-lying HOMO energy levels result in high open-circuit voltages (V-OC). As a result, planar heterojunction (PHJ) solar cells derived from the novel oligomers and C60 provide very high open circuit voltages (VOC) of up to 1.21 V and power conversion efficiencies (PCEs) of up to 2.7%. Bulk-heterojunction (BHJ) devices comprising oligomers 1-4 and C60 display a slightly lower VOC of 1 V leading to efficiencies as high as 3.6%.