Phenanthrodithiophene-isoindigo copolymers: Effect of side chains on their molecular order and solar cell performance

The synthesis, characterization, and solar cell application of newly developed semiconducting polymers containing phenanthro[1,2-b:8,7-b′]dithiophene (PDT) combined with a bis(thienyl)isoindigo (IID) unit are described. The polymers with longer alkyl chains are sufficiently soluble to be compatible with the processes required to manufacture solar cells. In conventional solar cell devices, polymers with all branched alkyl chains tend to form a higher proportion of a well-ordered face-on crystallite in the π-stack direction than those with both linear and branched alkyl chains, which significantly improves the fill factor (FF), resulting in higher power conversion efficiency (PCE). In terms of optimizing the alkyl chain lengths, the installation of longer alkyl side chains on the polymer backbone leads to low molecular weight polymer, which may promote a large phase separation. As a result, the polymers 12OD and BOBO, bearing shorter alkyl groups, performed better, and a BOBO polymer-based solar cell (PSC) showed the best PCE value up to 3.83%. In the inverted PSCs, the polymers with all branched alkyl chains have a higher face-on ratio than those with both linear and branched alkyl chains. Because of their improved J_sc, inverted PSCs with BOBO/PC₇₁BM gave the best performance, with a PCE up to 5.28%. Although an obvious dependence of photovoltaic properties on molecular order was observed in conventional solar cell devices, no trend was observed in inverted cells, possibly attributable to their amorphous nature, which arises from the axisymmetrical structure of PDT, leading to less effective π-π overlap and low crystallinity.