Deuteration-enhanced neutron contrast to probe amorphous domain size in organic photovoltaic bulk heterojunction films

An organic photovoltaic bulk heterojunction is comprised of a mixture of organic donor (D) and acceptor (A) materials, generally forming a semi-crystalline thin film with both crystalline and amorphous domains. The sizes of the donor and acceptor domains critically impact the device performance; however, conventional X-ray scattering techniques lack the ability to detect the D/A contrast within the amorphous D/A intermixing regions. In this study, we employed neutron scattering and targeted deuteration to enhance the D/A contrast by nearly one order of magnitude. Remarkably, the PM6:deuterated Y6 ( d -Y6) system revealed a new length scale, indicating short-range aggregation of d -Y6 molecules in the amorphous intermixing regions. All-atom molecular dynamics simulations confirmed that this short-range aggregation is an inherent morphological advantage of Y6 which effectively assists charge extraction and suppresses charge recombination as demonstrated by capacitance spectroscopy. For the first time, our findings uncovered the amorphous nanomorphology of organic photovoltaic thin films, providing crucial insights into the morphology-driven device performance.