Inverted-type polymer solar cells with poly[naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole-5,10-diyl[3,4′-bis(2-butyloctyl)[2,2′-bithiophene]-5,5′-diyl]thiazolo[5,4-d]thiazole-2,5-diyl[3′,4-bis(2-butyloctyl)[2,2′-bithiophene]-5,5′-diyl]] (PTzNTz) have attracted attention because of their high power conversion efficiency and thermal stability. However, the internal deterioration mechanism has not yet been completely clarified. Here, we report operand electron spin resonance (ESR) spectroscopy of the highly durable PTzNTz polymer solar cells to investigate the mechanism of the device-performance deterioration at a molecular level. We identify the layer with accumulated charges in the PTzNTz cells by the analyses of light-induced components in the ESR spectra of the thin films and cells with PTzNTz. We observe the correlation between the increase in the number of spins and the decrease in the device performance. Notably, compared to the previously reported ESR studies of polymer solar cells, the PTzNTz solar cells have less charge accumulation and higher durability. This correlation holds even if another hole-transport layer of PTzNTz solar cells is utilized. The calculation with density functional theory is performed to analyze the charge-accumulation states in the cells. The present results are useful to develop further highly efficient and durable polymer solar cells.