Excited-states spectroscopies and its magnetic field effect of π-conjugated polymer-fullerene blends with below-gap excitation

•Below-gap photon excited polarons exhibit trap limited bimolecular recombination.•Below gap and above gap polarons are intrinsically same in millisecond time region.•Below gap polaron presents much weaker magnetic field effect than above gap one.•Much longer spin decoherent time could be achieved using below gap excitation.

We used continuous wave photoinduced absorption (PIA) spectroscopy to investigate long-lived polarons in blend of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly (3-hexylthiophene) (RR-P3HT) with below-gap excitation. Compared to above-gap excitation, the polarons exhibited thermally activated and multi-trap limited bimolecular recombination too, but with narrower distribution profile in polaron states in millisecond time regime. However, magnetophoto-induced absorption (MPA) of polarons presented much weaker magnetic field effect in below-gap excitation (∼0.5 mT) than in above-gap excitation (order of ∼10 mT), although the underline mechanism is same as spin mixing of polaron pairs by the hyperfine interaction at small field region.