Quantifying the Relationship between the Maximum Achievable Voltage and Current Levels in Low-Bandgap Polymer Photovoltaics

Abstract

A critical problem in the design of materials for organic photovoltaics is quantifying the driving force needed for efficient charge separation without losses associated with a large overpotential. Here, we directly measured the effect of the molecular driving force on the charge transfer rate in films of low-bandgap push–pull type polymers mixed with a series of fullerene-based molecular acceptors using broadband near-infrared transient absorption spectroscopy. By systematically tuning the absolute energy levels of the donor and acceptor, as well as the relative offset between them, we determine the minimum voltage loss required to achieve a high short circuit current. A molecular donor–acceptor framework provides a quantitative description of the charge transfer rate constants in our system and describes the scaling of the photogenerated current with S1–LUMO energy offset. These results point to potential efficiency gains for high performing polymer devices through recovery of additional voltage without sacrificing current output.