In ternary organic solar cells (TOSCs), three different components are mixed to form the photoactive layer, opening up opportunities to boost the power conversion efficiency — for example, by broadening the absorption range, improving the blend morphology or tuning the exciton splitting and charge extraction. Because of these possibilities, ternary systems are among the best performing OSCs and will have a crucial role in the future of organic photovoltaics. Owing to the interplay of three different components, the mechanisms in TOSCs are complex. Multiple models for those mechanisms currently exist, which differ mainly in the description of the composition dependence of the open-circuit voltage. However, these models are not defined precisely, they are based on narrow presuppositions and they frequently contradict each other. Moreover, although the state of knowledge has evolved since the development of models, new TOSCs are still assigned to them. This Review describes the existing models and concepts, highlights their inconsistencies and summarizes newer results on electronic and morphological properties of TOSCs. Subsequently, the conventional models are revisited in the light of these new insights, with the aim of pointing out existing gaps and providing the stimulus for challenging old concepts.
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The authors thank M. Kemerink, A. Aspuru-Guzik and A. Weis for the fruitful discussions. The authors acknowledge funding of this work by the RSE International Joint Project (1787), the National Science Foundation (CBET-1803063), SolTech, and DFG Clusters of Excellence e-conversion (EXC 2089/1 – 390776260).
The authors declare no competing interests.
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- Bulk heterojunction
(BHJ). Term describing the morphology of an absorbing layer that consists of a nanoscopic blend of donor and acceptor, allowing simultaneously a maximized interface area and continuous charge transport pathways.
- Fill factor
Ratio of the maximum power output of a solar cell and the product of short-circuit current and open-circuit voltage.
- Frenkel excitons
Electron–hole pair with strong Coulomb interaction, with a typical binding energy of 0.1–1 eV.
- Marcus theory
Theory for the description of electron transfer reaction rates established by Rudolph A. Marcus.
- Non-fullerene acceptors
(NFAs). Organic molecules with electron-accepting properties that do not belong to the group of fullerenes, which have long been the dominating acceptor materials. Non-fullerene materials usually have a higher absorption capability than fullerenes.
- Open-circuit voltage
(VOC). Maximum voltage of a solar cell that occurs at zero current.
- Power conversion efficiencies
(PCEs). Ratio of the electrical power output and optical power input of a solar cell.
- Short-circuit current
Current through the solar cell when the applied voltage is zero.
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Günther, M., Kazerouni, N., Blätte, D. et al. Models and mechanisms of ternary organic solar cells. Nat Rev Mater (2023). https://doi.org/10.1038/s41578-023-00545-1