Spin-dependent photophysics in polymers lightly doped with fullerene derivatives : photoluminescence and electrically detected magnetic resonance
Faculty of Sciences. Physics
Physical review : B : condensed matter and materials physics. - Lancaster, Pa, 1998 - 2015
, 11 p.
University of Antwerp
Lightly fullerene-doped polymers are suitable composite systems to study spin-dependent bimolecular interactions among charge excitations due to their long lifetimes in these systems. These interactions can affect the photocurrent as well as the open-circuit voltage in an organic solar cell. Combining photoluminescence detected magnetic resonance (PLDMR) and electrically detected magnetic resonance (EDMR) spectroscopies we study films and devices of poly(p-phenylene vinylene) polymers poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) and superyellow PPV (SY-PPV) lightly doped with various fullerene derivatives [6,6]-phenyl C-61 butyric acid methyl ester (PCBM), bisPCBM (bis-PCBM), indene-C-60 bisadduct (ICBA), and [6,6]-phenyl C-71 butyric acid methyl ester (PC70BM). (i) We demonstrate strong fullerene triplet exciton (TE) production in SY-PPV: fullerene blends, whereas this is absent in MDMO-PPV:PCBM and only very weak in MDMO-PPV:ICBA. The low TE production in blends with MDMO-PPV is attributed to a weaker singlet-singlet energy-transfer coupling and an unfavorable triplet level alignment between the blend components. (ii) The fullerene TE spectra are analyzed on the basis of a single type of triplet excitation in PCBM, bis-PCBM, and ICBA, and two triplet species in PC70BM which are attributed to the alpha- and beta-type isomers of the latter molecule. (iii) The sign change with increasing temperature of the g similar to 2 sharp central line in photo-EDMR, which is observed both in pristine SY-PPV and in blends with fullerene, is correlated to a transition from dominant TE-polaron annihilation to nongeminate polaron recombination processes.