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

The performance of organic solar cells has been remarkably improved recently, where the cell structures are important for achieving high efficiency and stability. The formation and accumulation of long-lived charges in the cells are critical for the efficiency and stability of the cells; however, their relations with the cell structures have not yet been clarified from a microscopic viewpoint. Here, we report the microscopic mechanism of high efficiency and stability of inverted polymer fullerene solar cells compared to conventional cells even though the same photoactive layers are utilized. We directly observe the formation and accumulation of long-lived charges in these cells with electron spin resonance at a molecular level. We find the reduced effects of the formation and accumulation of long-lived charges in the inverted cells on their efficiency and stability compared to the case of conventional cells. These findings provide a striking advance in fundamental understanding, which is useful for further clarifying the operation mechanism of organic solar cells as well as further improving their efficiency and stability.