Side-Chain Engineering of Benzodithiophene-Bridged Dimeric Porphyrin Donors for All-Small-Molecule Organic Solar Cells

Two new A-D-A small-molecule donors (C8T-BDTDP and C8ST-BDTDP) are prepared from benzodithiophene (BDT)-linked dimeric porphyrin (DP), which differ in side chains of BDT linkers with 4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b′]dithiophene (C8T-BDT) for the former and 4,8-bis{5-[(2-ethylhexyl)thio]-2-thienyl}benzo[1,2-b:4,5-b′]dithiophene (C8ST-BDT) for the latter. Both dimeric porphyrin donors show strongly UV-visible to near-infrared absorption. Compared to C8T-BDTDP, C8ST-BDTDP with an alkylthiothienyl-substituted BDT linker exhibits more intense absorption bands in the film and a lower highest occupied molecular orbital energy level. The blend film of the electron acceptor 6TIC with the respective dimeric porphyrin donor displays a broad photon response from 400 to 900 nm, unfortunately, with an absorption valley at ca. 600 nm. The device based on C8ST-BDTDP/6TIC demonstrates a promising power conversion efficiency (PCE) of 10.39% with a high short-circuit current density (JSC) of 19.53 mA cm-2, whereas the device based on C8T-BDTDP/6TIC shows a slightly lower PCE of 8.73% with a JSC of 17.75 mA cm-2. The better performance for C8ST-BDTDP/6TIC is mainly attributed to efficient charge dissociation and transportation because of the smooth surface morphology and highly ordered crystalline packing.