Symmetrical molecules of low band gap with a central spacer connected via ether bond with terminal 4-nitro-α-cyanostilbene units: Synthesis and application for bulk heterojunction solar cells

Two new soluble and symmetrical molecules M1 and M2 of low band gap with a central spacer which was connected at both sides via ether bond with terminal 4-nitro-α-cyanostilbene units were synthesized. The spacer of M1 and M2 consisted of dihexyloxyphenylene and n-hexylene, respectively. Their long-wavelength absorption maximum was at 590–640 nm. The thin film absorption onset for both molecules was located at 742 nm which corresponds to an optical band gap of 1.67 eV. We have fabricated bulk heterojunction (BHJ) photovoltaic devices using these molecules as donor and PCBM as acceptor. We have investigated the solvent vapor treatment effect of these blends on their morphology and photovoltaic properties. We found that the overall power conversion efficiency (PCE) for the devices based on the solvent vapor treated M1:PCBM and M2:PCBM blends is 3.05% and 1.90%, respectively, higher than those of pristine blends. This increase of PCE has been attributed to the increase in surface roughness of the blend and better balance in charge transport. The PCE has been further increased up to 3.57% and 2.42% with the thermally annealed solvent treated M1:PCBM and M2:PCBM blend, respectively. This increase of PCE may be attributed to the enhanced crystallinity of the blend and reduction of the space charge effect, improving the charge transport and collection efficiency.