Atom-Varied Side Chains in Conjugated Polymers Affect Efficiencies of Photovoltaic Devices Incorporating Small Molecules

We synthesized three conjugated polymers involving sulfur (S)-inserted and chlorine (Cl)-substituted side chains of the parent two-dimensional conjugated polymer—poly(benzodithiophene–thiophene–benzooxadiazole) (BO)—to form S-inserted (BO2S), Cl-substituted (BO2Cl), and S-inserted with Cl-substituted (BO2S2Cl) polymers for tuning their surface energies and, thus, interaction with IT-4F small molecule in their binary blends. In the BO:IT-4F blend, the bulk heterojunction (BHJ) structure was constituted mainly from the long-rod BO domains along with a few IT-4F domains that dispersed well in the blend; in contrast, favorable networks for charge transport existed in the BO2S or BO2S2Cl with IT-4F blend. The disk sizes of IT-4F in the BO2S2Cl:IT-4F blend were larger than that in the BO2S:IT-4F blend (23.3 vs 18.1 nm). As the extent of atom variation increased from BO to BO2S to BO2S2Cl, the induced IT-4F crystallinity increased, and the orientation of molecular packing of the polymer varied. The highest PCE (12.06%) was that for the device based on the double sulfur-inserted/chlorine-substituted side chain polymer and IT-4F acceptor (BO2S2Cl:IT-4F), owing to the more balanced hole-to-electron mobility, being consistent with the value predicted (11.8%) using the random forest machine learning model. This study not only provides insight into the photovoltaic performance of the polymers with atom-inserted or -substituted side chain but also reveals that the effects of molecular packing.