Impact of the Torsion Angle in Y6-Backbone Acceptors on the Open-Circuit Voltage in Organic Solar Cells

In organic solar cells (OSCs), optimizing the molecular geometry is crucial for improving device efficiency by reducing recombination rates and maximizing charge transfer (CT) state energy. Understanding the structure-property relationship regarding molecular geometry, electronic structure, and open-circuit voltage (V_oc) is essential. By employing molecular dynamics simulations and density functional theory calculations, we explored how intramolecular torsion angles (θ) between conjugated moieties impact V_oc. Small θ promotes molecular orbital energy degeneracy, reducing the CT energy (E_CT) and its energetic disorder (σ_CT). While a low E_CT can increase non-radiative energy losses (ΔE_nr), a small σ_CT decreases ΔE_nr. Balancing these effects is essential to maximize the value of E_CT - ΔE_nr for high V_oc. L8-BO exhibits large θ, resulting in high E_CT of 1.17 eV in PM6/L8-BO compared to 1.04 eV in PM6/Y6, while the latter has 0.17 eV lower ΔE_nr. Consequently, PM6/L8-BO achieved a V_oc of 0.87 V, surpassing 0.81 V of PM6/Y6. These findings were consistent with experimental 0.89 V in PM6/L8-BO and 0.84 V in PM6/Y6. This study demonstrates the crucial role of intramolecular dihedral angles on OSC material design, as they significantly influence the conjugation effect and CT state distribution.