Analysis of the excited states of regioregular polythiophene P3HT

Abstract

Poly(3-hexylthiophene) (P3HT) [90–93% regioregular, M_w ∼55 kDa, PDI <2] is studied by transient absorption spectroscopy and the properties of excited state P3HT in solution and thin film contrasted. In solution, excitation of P3HT yields the first singlet state which has a characteristic lifetime of ∼600 ps, and a measured high quantum yield of fluorescence in chlorobenzene solution of 0.33 ± 0.07. The long lived (∼μs) species in solution is ascribed to the P3HT triplet state, formed by intersystem crossing from the singlet state with a lifetime of around 300 ns in aerated chlorobenzene solution. By contrast the properties of P3HT in the solid state are very different to that in solution. The quantum yield of fluorescence is found to be reduced to only 0.02 ± 0.001 and transient absorption data show the presence of two species in P3HT on a ∼500 ps timescale, one with a lifetime of less than 500 ps and the other a longer lived nanosecond time region decay which follows a bimolecular recombination pattern. Alongside the different kinetics, both short and long lived species also show contrasting transient absorption spectra and therefore are assigned to two different species in P3HT thought to be the singlet emissive state of P3HT and charged species/polaron state respectively. Analysis of the decay kinetics suggest that P3HT singlet emissive states in the film do not decay into polaron states and therefore polaron formation must originate on earlier timescales. The competitive formation of polarons compared to emissive states in P3HT film could be exploited to generate power in organic solar cell devices more efficiently than is currently possible with donor–acceptor junction organic solar cells.