Intra- and interchain luminescence in amorphous and semicrystalline films of phenyl-substituted polythiophene

We report time-resolved photoluminescence (PL) measurements of spin-cast am orphous films of a regioregular polythiophene derivative poly[3-(4-octylphe nyl)thiophene] and of annealed semicrystalline films of the same polymer wi th an interchain stacking distance of 5 Angstrom. Red-shifted PL appears at long delay times in both pristine and annealed films, which we assign to i nterchain aggregates populated by excitation energy migration. Aggregate lu minescence in annealed films exhibits a pronounced vibronic structure indic ating the coupling to a C=C bond stretch with a Huang-Rhys factor S = 2. Tw o types of aggregates are distinguished in annealed films: in a few picosec onds most excitations are trapped by aggregates with a large energy gap (ab out 1.83 eV) between the lowest excited singlet state and the ground state. Excitation energy is transferred or the aggregates relax structurally with a time constant of about 200 ps to aggregates with a smaller energy gap (a bout 1.75 eV). The radiative lifetime of aggregates with smaller energy gap is estimated to be about 35 ns, two times longer than that of aggregates w ith the larger energy gap. In spin-cast amorphous films, emission from intr achain singlet excitons has a longer lifetime than in annealed films and on ly high-energy-gap aggregates are populated.