Polymer solar cells are one of the promising energy sources because
of the easy solution-processable production with large area at a low cost without
toxicity. Among the polymer materials, a donor?acceptor conjugated copolymer
PTB7 has been extensively studied because of the typical high-performance
polymer solar cells. Here, we show operando direct observation of charge
accumulation in PTB7:PC71BM blend solar cells from a microscopic viewpoint
using electron spin resonance spectroscopy. The accumulation of ambipolar
charges in the PTB7-based cells is directly observed for the first time, which shows
a clear correlation with the performance deterioration during device operation. The
sites of the ambipolar charge accumulation are elucidated at the molecular level,
whose information would be useful for improving the cell durability in addition to
the performance improvement.