ESR studies of photogenerated polarons in regioregular poly(3-alkylthiophene)–fullerene composite

Electron spin resonance (ESR) studies of photogenerated polarons in composite of regioregular poly(3-octylthiophene) (PAT8) and fullerene (C60) using variable photoexcitation energy up to 4.1 eV are reported. Two transient light-induced ESR (LESR) signals of photogenerated positive and negative polarons on PAT8 (g1=2.002) and C60 (g2=1.999), respectively, are observed below 200 K, which are caused by photoinduced electron transfer between PAT8 and C60. A remarkable enhancement of the LESR signals in excitation spectrum at around 1.8 eV is observed compared with the case of pure regioregular PAT8, which is consistent with the enhancement of the photoconductivity at around 1.8 eV where optically forbidden transition of C60 occurs. Spin–lattice relaxation time of the polaron spins of PAT8 is obtained considering inhomogeneous broadening of the ESR linewidth due to the hyperfine interactions, which is at least approximately 18 times longer than that of C60. Prompt and persistent LESR signal components are observed by transient response of the LESR signal. The prompt contribution shows a monotonic increase with increasing excitation-light intensity, while the persistent contribution is found to be excitation intensity independent due to deep traps of the photogenerated polarons.