4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-one (CPDTO) homopolymer with side chains on every other CPDTO

A soluble 4H-cyclopenta[2,1-b;3,4-b′]dithiophene-4-one (CPDTO)-based polymer (C₆-PCPDTO) has been synthesized from two monomers derived from nonalkylated CPDTO and didodecyl CPDTO (C₁₂-CPDTO). Proton NMR, thermal analysis, UV–vis absorption, cyclic voltammetry, and XRD are used to characterize the polymer in solution and film. The new polymer has an optical bandgap of 1.28 eV in film, and has strong interchain interaction in chloroform solutions. The polymer contains a significant amount of homocoupled segments. The regular segments and homocoupled CPDTO segments render the polymer highly aggregating in solution. The non-planar homocoupled C₁₂-CPDTO segments prevent the polymer from forming regular π-stacks, resulting in a low SCLC hole mobility (3.88 × 10⁻⁷ cm²V⁻¹s⁻¹). CV experiments show that C₆-PCPDTO is stable in its oxidized and reduced states. Solar cell devices were fabricated from C₆-PCPDTO2:PC₆₀BM blends of different weight ratios. High PC₆₀BM loading (80% or greater) was required for the devices to show measurable efficiency, indicating that the limited π-stacking of the polymer is not sufficient to cause effective phase separation. Further development of synthetic method is still needed to eliminate structural defects so that long-range ordered pi-stacking can be realized in the polymer for these applications.