Organic solar cells are a type of photovoltaic cell that uses organic electronics to produce electricity from sunlight by the photovoltaic effect. Organic electronics deal with conductive organic polymers or small organic molecules for light absorption and charge transport. The optical absorption coefficient of organic molecules is high, so a large amount of light can be absorbed with a small amount of materials, usually on the order of hundreds of nanometers. They have an ability of a conductor, too. This allows the creation of an extremely lightweight, flexible, and thin-film solar cells.

      In general, p-n heterojunction structure is adopted. When light is absorbed by the semiconductor, it creates electron-hole pairs. The electric field at the p-n junction separates the electrons and holes, creating a current that can be collected by electrodes. Bulk heterojunction (BHJ) is a structure used in organic solar cells that comprises a bicontinuous interpenetrating network of the donor and acceptor material. he bulk heterojunction structure is composed of a blend of two different materials, usually a polymer donor and a fullerene acceptor, forming a complex interpenetrating network. (1)

        Researchers are exploring new materials and manufacturing processes to improve the efficiency and stability of bulk heterojunction solar cells. For example, graded bulk-heterojunction (G-BHJ) with well-defined vertical phase separation has been demonstrated using nonfullerene acceptor (NFA) OSCs, delivering an outstanding 17.48% power conversion efficiency (PCE) Graded bulk-heterojunction (G-BHJ) is a structure with well-defined vertical phase separation that has the potential to surpass the classical bulk-heterojunction in organic solar cells (OSCs). The difference between G-BHJ and BHJ is that G-BHJ has a more defined vertical phase separation, which benefits charge transport and enables outstanding thick OSC power conversion efficiency. The paper discusses an effective G-BHJ strategy via nonhalogenated solvent sequential deposition, which is different from the preparation method of classical BHJ. The nonhalogenated solvent enables G-BHJ OSC via open-air blade coating and achieves a record 16.77% PCE. The blade-coated G-BHJ has drastically different D-A crystallization kinetics, which suppresses the excessive aggregation induced unfavorable phase separation in BHJ.       In terms of the stability and material innovation, non-fullerene acceptors (NFAs) have been searched. New NFAs have emerged, which have contributed to the stability of organic solar cells. Innovations in material design have contributed to the progress of organic solar cells, and various NFAs have been developed and showed promising results. (3)

(1) Chem. Rev. 2018, 118, 7, 3447–3507. https://doi.org/10.1021/acs.chemrev.7b00535
(2) Nat Commun 12, 4815 (2021). https://doi.org/10.1038/s41467-021-25148-8
(3) Angew. Chem. Int. Ed. 2022, 61, e202209021. https://doi.org/10.1002/anie.202209021