Evidence of Molecular Structure Dependent Charge Transfer between Isoindigo-Based Polymers and Fullerene

The effects of the oligothiophene length of two thiophene-isoindigo copolymers on film morphology, charge transfer, and photovoltaic device performance are reported. Despite the similarities in their repeat unit structures, the two polymers show distinctly different film morphologies and photovoltaic performance upon blending with PC₇₁BM. We found that there is a significant increase in the dielectric constant of the photoactive film upon blending fullerene with the polymer that exhibits a higher power conversion efficiency. Blend photoluminescence transients revealed a fast dissociation route in the better performing polymer followed by a slower decay. The fast decay in transient PL is attributed to a higher charge transfer efficiency when blending with the fullerene. We suggest that the charge transfer efficiency is determined not only by the microscopic morphology but also whether the polymer can accommodate the fullerene molecules in close proximity to the acceptor moiety to facilitate electronic coupling between the isoindigo acceptor and the fullerene molecule. We propose that the fast decay component seen in transient PL for the better performing polymer, along with the increase in dielectric constant, is a signature of enhanced electronic coupling between the polymer and the fullerene. The enhanced electronic coupling is thought to originate from a polymer chemical structure which allows the fullerene molecules to come to closer proximity for more efficient charge transfer.