Microscopic Mechanism of High Efficiency and Stability of Inverted Polymer Fullerene Solar Cells

The performance of organic solar cells has been remarkablyimprovedrecently, where the cell structures are important for achieving highefficiency and stability. The formation and accumulation of long-livedcharges in the cells are critical for the efficiency and stabilityof the cells; however, their relations with the cell structures havenot yet been clarified from a microscopic viewpoint. Here, we reportthe microscopic mechanism of high efficiency and stability of invertedpolymer fullerene solar cells compared to conventional cells eventhough the same photoactive layers are utilized. We directly observethe formation and accumulation of long-lived charges in these cellswith electron spin resonance at a molecular level. We find the reducedeffects of the formation and accumulation of long-lived charges inthe inverted cells on their efficiency and stability compared to thecase of conventional cells. These findings provide a striking advancein fundamental understanding, which is useful for further clarifyingthe operation mechanism of organic solar cells as well as furtherimproving their efficiency and stability.