On the Importance of Additive Flexibility for Efficient Organic Solar Cells

Additive engineering offers a simple and efficient way to enhance the structural order of organic semiconductors, yet the interplays between their molecular structural and interactions remains unclear. Herein, two sets of additives, including conformationally rigid DTCO-2Br and DTC and conformationally flexible INF-TC and BT-TC, featuring opposite electrostatic potentials (ESPs), are designed. For PM6, both ESP complementarity and conformational flexibility are effective in tuning the molecular packing, while the conformational flexible INF-TC with opposite ESP not only enhances the structural order but also shortens the π–π stacking distance. For the relatively rigid L8-BO, conformational flexibility is essential to adjust the intermolecular interaction. As such, only conformational flexible INF-TC and BT-TC improve the lamellar stacking of L8-BO, and the ESP complementary BT-TC demonstrates the strongest effect. By applying INF-TC in the donor layer and BT-TC in the acceptor layer, improved efficiency and morphological stability were achieved, with a maximum efficiency of 20.4% achieved in the D18/L8-BO system.