Light-induced electron spin resonance (LESR) study of polymer solar cells has been performed to investigate accumulated hole carriers in these devices under device operation. We analyzed clear correlation between the number of accumulated holes in regioregular poly(3-hexylthiophene) (P3HT) evaluated by LESR and the deterioration of device performance (V-oc, J(sc)) observed using the same device under simulated solar irradiation. The effects of hole accumulation with deep trapping levels formed in P3HT at the organic interfaces on the performance are examined by considering interfacial electric dipole layers and charge-carrier scattering by accumulated holes.