Green Solvent-Processed Organic Solar Cells Approaching 20.4% Efficiency via Active Layer Pre-Solidification

Green solvent fabrication of efficient organic solar cells (OSCs) is essential for their industrial scale extension and ecological sustainability, but there is typically an obvious efficiency drop during the transition from halogenated to green solvents due to the severe molecular aggregation. Here, an innovative strategy of active layer pre-solidification by liquid nitrogen freezing process is proposed to accelerate molecular precipitation and crystallization, and therefore suppress the excessive phase separation, as demonstrated by PiFM and GISAXs results. Moreover, pre-solidification process allows more solvents to carry acceptor molecules for an orderly upward migration during rapid volatilization, facilitating an ideal longitudinal gradient arrangement of photovoltaic materials that is favorable for charge transport and extraction. Consequently, PM6:L8BO-X:BO-8F OSCs fabricate with o-xylene yielded a record efficiency of 20.38%, which is comparable to 20.56% for chloroform-processed devices. The inconspicuous efficiency gaps are observed in various photovoltaic materials by using green solvents, proving the outstanding universality. In addition, the pre-solidification process effectively inhibits the coffee-ring effect in large-area active layer films and boosts the OSC modules to an exciting 17.06% efficiency. This work virtually eliminates the photovoltaic performance losses caused by green solvents, and charts a sustainable avenue to large-scale commercialization of organic photovoltaics.