Ternary polymer solar cells (PSCs) have been widely demonstrated as an effective method to improve device performance. Meanwhile, the simple fabrication technology can be well kept with a single bulk heterojunction active layer, which is beneficial to the potential industrialization of PSCs. The photovoltaic parameters, such as short-circuit current density, open-circuit voltage, and fill factor, of ternary PSCs can be simultaneously improved by appropriately incorporating the third component, resulting in the increased power conversion efficiency (PCE) compared with the corresponding binary PSCs. To date, the PCE of ternary PSCs has exceeded 17% with the development of effective donors and non-fullerene acceptors. The fundamental designing rules of ternary PSCs are summarized to give some available guidelines for material selection. Next, the recent progress of efficient ternary PSCs based on various types is investigated. Then, some methods are introduced to investigate the working mechanisms of ternary PSCs. At length, the perspective toward the future development of ternary PSCs is discussed.

Organic semiconductors have shown magnetic field responses in electrical currents. This phenomenon has been generally attributed to hyperfine interaction (HFI) and spin–orbit coupling (SOC). For the first time, we report the experimental and theoretical studies on hyperfine interaction vs. spin–orbit coupling in one thin film of organic semiconductor poly [9,9-di-n-hexyl-fluorenyl-2,7-diyl] and the dramatic influence of doping the PFO with complexes of bis [2-(2′-benzothienyl)-pyridinato-N, C3′] iridium (acetyl-acetonate). The intra-molecular HFI and SOC are only dependent on the intra-molecular interaction among electrons, nuclei and orbital field in a given molecule. Phosphorescence and fluorescence spectroscopies were used to measure the spin–orbit coupling strength. Furthermore, HFI is a short-range interaction and inter-molecular HFI is negligible. Inter-molecular SOC depends not only on molecular structure but also distance between adjacent molecules. Inter-molecular SOC strength increases strongly when the distance between adjacent molecules is decreased.