Stability improvement mechanism due to less charge accumulation in ternary polymer solar cells

Ternary polymer solar cells based on a thiazolothiazole-based polymer donor (PTzBT) and a fullerene acceptor (PC61BM) have attracted attention because they show high efficiency and stability by addition of a non-fullerene acceptor (ITIC). However, the performance improvement mechanism is not completely elucidated. Here, we show the stability improvement mechanism due to less charge accumulation in the PTzBT cells with ITIC using operando electron spin resonance from a microscopic viewpoint. We observed two correlations between device performance and number of spins (Nspin) under solar irradiation. One correlation is the decrease in short-circuit current and the Nspin increase in electrons on PC61BM and holes in PTzBT, where the ITIC addition causes the less these Nspin. The other correlation is the increase in open-circuit voltage and the Nspin decrease in holes in ZnO. These findings explain the stability improvement mechanism, showing the correlation between less charge accumulation and higher stability, which is valuable for the development of further efficient and stable polymer solar cells.