AbstractThe power conversion efficiency of ternary organic solar cells (TOSCs), consisting of one host binary blend and one guest component, remains limited by large voltage losses. The fundamental understanding of the open-circuit voltage (V_OC) in TOSCs is controversial, limiting rational design of the guest component. In this study, we systematically investigate how the guest component affects the radiative and non-radiative related parts of V_OC of a series of TOSCs using the detailed balanced principle. We highlight that the thermal population of charge-transfer and local exciton states provided by the guest binary blend (that is, the guest-component-based binary blend) has a significant influence on the non-radiative voltage losses. Ultimately, we provide two design rules for enhancing the V_OC in TOSCs: high emission yield for the guest binary blend and similar charge-transfer-state energies for host/guest binary blends; high miscibility of the guest component with the low gap component in the host binary blend.