Investigation into aggregation types in a benzodithiophene-isoindigo copolymer.

The type of aggregation in conjugated polymers determines their use in electronic devices. H-type aggregates are most suitable for solar cell applications, while J-type aggregates are recommended for light-emitting diodes. In this work, we used three methods to determine the type of aggregates in a benzodithiophene-isoindigo-based (PBDTI-DT) copolymer, namely, Huang-Rhys factor evolution with temperature, Franck-Condon analysis, and relative quantum yield (QY) calculation. All three methods indicate that both aggregation types are present, and the QY calculation clearly indicates that H-aggregates are more dominant. Time-dependent density functional theory was used to identify the two absorption bands of PBDTI-DT as local π - π* and intramolecular charge-transfer transitions.