Ultrafast photogeneration of charged polarons on conjugated polymer chains in dilute solution

Abstract

Ultrafast photoinduced absorption by infrared-active vibrational modes is used to study the photogeneration of polarons on semiconducting polymer chains in dilute solutions and in solid films of a soluble derivative of poly(para-phenylene vinylene). In dilute solutions, polaron pairs are photogenerated on the conjugated polymer within less than 250 fs with quantum efficiencies ch ϳ 3%, about one-third of that for solid films of the same polymer. The excitation spectra of ch for both solutions and films show that ch is weakly dependent on photon energy between 2.2 eV (the onset of absorption) and 4.7 eV. The recombination dynamics of polarons is very fast and highly dependent on the excitation density for polymer films, but it is significantly slower and less sensitive to pump intensity for the semiconducting polymer in dilute solution. We conclude that the positive and negative polarons on a single chain in solution are typically separated by hundreds of monomer repeat units and that their one-dimensional diffusion along the chain is inhibited by the intervening excitons. This, together with the suppression of interchain recombination, explains the surprisingly slower polaron recombination in isolated chains.

Experimental determination of the charge/neutral branching ratioηin the photoexcitation ofπ-conjugated polymers by broadband ultrafast spectroscopy

“…10. The variations in value among the different MEH-PPV films and solutions can be tracked back to their nanomorphologies.…”

“…3 However, so far only limited spectral ranges have been studied with the aim of measuring in polymer films and solutions. [3][4][5][6][7][8][9][10] It is well known that the main polaron absorption band in -conjugated polymers lies in the mid-IR spectral range, 2,11 which has not been studied by ultrafast spectroscopy since it is not easily accessible using femtosecond ͑fs͒ pulses. This may be the main reason that controversies in the obtained values in polymer films and solutions have flourished in the last decade.…”

Charge carrier photogeneration and recombination in ladder-type poly(para-phenylene): Interplay between impurities and external electric field

“…The direct carrier generation concept was mainly developed by Heeger and co-workers on the basis of investigations of poly͑p-phenylene vinylene͒ type of polymers. [1][2][3][4][5] According to this model, charge carriers are generated directly by photon absorption; very rapid and intensity dependent recombination taking place on a femtosecond to picosecond time scale 6,7 is a consequence of this picture. Another, two-step carrier generation model grounds on the molecular exciton picture.…”

Optical Studies of Photoexcitations in Polymer/Fullerene Blends for Organic Photovoltaic Applications

“…These processes include singlet exciton fission and exciton dissociation [45]. The former process creates two triplet excitons with opposite spins from one singlet exciton, in time faster than the usual intersystem crossing time from the singlet to the triplet manifold [46,47]; whereas the latter process directly generates charge carriers on two adjacent chains [48,49]. However in PCPs these two processes happen with much smaller probability than the internal conversion process, and although important for applications, they are not included in our present discussion.…”