Ternary organic solar cells: optimizing the third components

Ternary organic solar cells (T-OSCs) have emerged as a promising strategy for overcoming efficiency limitations in binary organic photovoltaic systems by enabling enhanced photon harvesting, tuneable electronic properties, and optimized morphology. Recent advances have pushed stabilized power conversion efficiencies beyond 21%, highlighting the growing potential of ternary architectures. However, several key questions remain regarding the rational design of the third component, the mechanisms governing energy and charge transfer, and the control of morphology-performance relationships in multicomponent blends. This tutorial review addresses these challenges by critically examining donor/donor/acceptor and donor/acceptor/acceptor ternary configurations and their role in improving device performance. We systematically analyse more than 400 studies published up to December 2025, with particular emphasis on non-fullerene acceptor (NFA)-based T-OSCs. A decade-long perspective on fused-ring small-molecule design strategies is presented, highlighting their influence on frontier molecular orbital alignment, charge transport, and morphological stability. The review further discusses the incorporation of NFAs into binary OSC platforms to form high-performance ternary blends involving polymer donor/NFA/fullerene and polymer donor/NFA1/NFA2 systems. Key aspects including donor-acceptor interactions, photophysical processes, charge dynamics, interface engineering, and processing strategies are critically analysed. Special attention is given to chemical modification strategies for benchmark NFAs such as PDI, Y6, ITIC, and related BDT, DPP, and IDT-based derivatives. Finally, we outline the remaining challenges in T-OSC design - particularly third-component selection, morphology control, and long-term stability - and discuss future opportunities for scalable materials, device engineering, and commercial deployment.