How Heteroatom Substitution in Donor-Acceptor Copolymers Affects Excitonic and Charge Photogeneration Processes in Organic Photovoltaic Cells

Recent attention has been drawn to expanding the class of polymer-based OPV materials through heteroatom substitution within the repeating units in the main chain of a polymer backbone. We sought to investigate how heteroatom substitution within a donor-acceptor copolymer causes such large variations in the photovoltaic parameters, which cannot be explained considering the variations in the optical band gap alone. Our study applies broadband transient absorption spectroscopy to a series of low-band gap copolymers, wherein the S-position of the benzothiadiazole in the parent polymer structure is substituted for an oxygen (i.e., benzooxadiazole) and selenium (i.e., benzoselenadiazole). Our thin-film morphology measurements reveal unfavorable packing of the oxygen- and selenium-containing polymers near the interfaces with [6,6]-phenyl-C-71-butyric acid methyl ester (PCBM) or in intermixed regions. We explain the device performance differences based upon a sub-optimal blend morphology, resulting in suppressed dissociation of charge-transfer states and, concomitantly, high geminate recombination rates for these systems. Furthermore, the heavy-atom effect of the selenium-containing polymer facilitates access to the triplet manifold. We find that triplet state formation can initially be circumvented by fast charge photogeneration in the finely intermixed morphology of the polymer:PCBM blend; however, this morphology also prevents charge dissociation and ultimately results in recombination to form triplet exciton states. These results provide valuable insights into how heteroatom substitutions affect the thin-film morphology and severe photocurrent loss pathways in polymer solar cells.