RESUMEN
Molecular dopants are often added to semiconducting polymers to improve electrical conductivity. However, the use of such dopants does not always produce mobile charge carriers. In this work, ultrafast spectroscopy is used to explore the nature of the carriers created following doping of conjugated push-pull polymers with both F4 TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) and FeCl3 . It is shown that for one particular push-pull material, the charge carriers created by doping are entirely non-conductive bipolarons and not single polarons, and that transient absorption spectroscopy following excitation in the infrared can readily distinguish the two types of charge carriers. Based on density functional theory calculations and experiments on multiple push-pull conjugated polymers, it is argued that the size of the donor push units determines the relative stabilities of polarons and bipolarons, with larger donor units stabilizing the bipolarons by providing more area for two charges to co-reside.
RESUMEN
It is generally presumed that the vast majority of carriers created by chemical doping of semiconducting polymer films are coulombically trapped by the counteranion, with only a small fraction that are free and responsible for the increased conductivity essential for organic electronic applications. At higher doping levels, it is also possible for bipolarons to form, which are expected to be less conductive than single polarons. Unfortunately, there is no simple way to distinguish free polarons, trapped polarons and bipolarons using steady-state spectroscopy. Thus, in this work, we use ultrafast transient absorption spectroscopy to study the dynamics of polarons in 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TNCQ)-doped films of poly(3-hexylthiophene-2,5-diyl) (P3HT) as a function of dopant concentration and excitation wavelength. When exciting on the red side of the polaron P1 transition, our transient absorption spectra and kinetics match well with what is expected for free 2-D-delocalized polarons; the measurements are not consistent with a recent theory of doped conjugated polymer electronic structure that suggests that the half-filled state lies deeper in the conduction band rather than in the bandgap. As we tune the excitation wavelength to the blue, our measurements reveal an increasing amount of slower transient kinetics that are consistent with the presence of coulombically-trapped polarons rather than bipolarons. Taking advantage of their distinct ultrafast relaxation kinetics as a type of action spectroscopy, we are able to extract the steady-state absorption spectra of free and trapped polarons as a function of dopant concentration. By comparing the results to theoretical models, we determine that in F4TCNQ-doped P3HT films, trapped polarons sit â¼0.4 nm away from the anion while free polarons reside between 0.7 and 0.9 nm from the counteranion. Perhaps counterintuitively, the ratio of trapped to free polarons increases at higher doping levels, an observation that is consistent with a plateau in the concentration-dependent conductivity of F4TCNQ-doped P3HT films.
RESUMEN
Laser-induced optoacoustic spectroscopy (LIOAS), diffuse reflectance laser flash photolysis (DRLFP), and laser-induced luminescence (LIL) have been applied in conjunction to the determination of triplet state quantum yields of Rose Bengal (RB) supported on microcrystalline cellulose, a strongly light-scattering solid. Among the three used methods, the only one capable of providing absolute triplet quantum yields is LIOAS, but DRLFP and LIL aid in demonstrating that the LIOAS signal arises in fact from the triplet state and confirm the trend found with RB concentration. The coherence found for the three techniques demonstrates the usefulness of the approach. Observed triplet quantum yields are nearly constant within a limited concentration range, after which they decay strongly due to the generation of inactive dye aggregates or energy trapping centers. When quantum yields are divided by the fraction of absorbed light exciting the dye, the quotient falls off steadily with concentration, following the same trend as the observed fluorescence quantum yield. The conditions that maximize triplet formation are determined as a compromise between the rising light absorption and the decrease of quantum yield with RB concentration.