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.