Excited State Dynamics of a Self‐Doped Conjugated Polyelectrolyte

Abstract

The growing number of applications of doped organic semiconductors drives the development of highly conductive and stable materials. Lack of understanding about the formation and properties of mobile charges limits the ability to improve material design. Thus the largely unexplored photophysics of doped systems are addressed here to gain insights about the characteristics of doping‐induced polarons and their interactions with their surroundings. The study of the ultrafast optical processes in a self‐doped conjugated polyelectrolyte reveals that polarons not only affect their environment via Coulomb effects but also strongly couple electronically to nearby neutral sites. This is unambiguously demonstrated by the simultaneous depletion of both the neutral and polaronic transitions, as well as by correlated excited state dynamics, when either transition is targeted during ultrafast experiments. The results contrast with the conventional picture of localized intragap polaron states but agree with revised models for the optical transitions in doped organic materials, which predict a common ground level for polarons and neighboring neutral sites. Such delocalization of polarons into the frontier transport levels of their surroundings could enhance the electronic connectivity between doped and undoped sites, contributing to the formation of conductive charges.

Charge-Transfer Intermediates in the Electrochemical Doping Mechanism of Conjugated Polymers

“…In contrast to the findings of Tsokkou et al, who evaluated acid doping of the donor-acceptor system PCPDTBT, 38 we do not observe a broad bleaching feature corresponding to the energy of the polaron (SI Figure S18). In fact, for ProDOT(OE3)-DMP, we see no evidence for a shared ground state between the exciton and polaron.…”

“…What seems to be common to both the acid-doped PCPDTBT 38 and our analysis of electrochemically doped ProDOT(OE3)-DMP is the effect of charge concentration on the dynamics of the PB. In both cases, doping is associated with a quenching of the PB, which is to be expected given the general phenomenon of exciton-charge annihilation.…”

“…64 Tsokkou et al reported dynamics spanning ~1 ps for the fully p-doped polyelectrolyte. 38 For ProDOT(OE3)-DMP, we also note that the excited state dynamics are sensitive to the nature of the electrolyte. In a polar aprotic electrolyte (0.5 M TBAPF6 in propylene carbonate), the recovery of the ground state bleach is notably reduced, even for similar oxidation states or doping levels (SI Figure S20).…”

“…We highlight the significance of these interpretations as they relate to the optoelectronic properties of electrochemically induced charges and their interaction with neutral excitons, especially in the context of recent literature concerning the excited state dynamics of an acid-doped polyelectrolyte. 38 These foundational studies demonstrate the utility and application of in situ ultrafast and nonlinear spectroscopy in understanding redox processes in OMIECs. evolution with doping.…”

An Organic Borate Salt with Superior p‐Doping Capability for Organic Semiconductors

“…At present, it is not clear why the formation of bipolarons with B(C 6 F 5 ) 3 as dopant (or other molecular dopants, as discussed in the Introduction) does not happen. We speculate that this could be attributed i) to a higher effective oxidation strength of Mes 2 B + [B(C 6 F 5 ) 4 ] − compared to other dopants (facilitating the creation of further holes in the already highly doped P3HT), ii) to the bulkiness of the [B(C 6 F 5 ) 4 ] − anion and thus a different electrostatic potential landscape on the molecular length scale in films, [76] and iii) to dopant-induced changes in morphology that render bipolaron formation more favorable. We recall at this point our observations in absorption spectra, which indicated that beyond ≈5% dopant concentration aggregation of P3HT was effectively suppressed.…”