Ultrafast charge carrier dynamics in organic (opto)electronic materials

First-Principles Study of the Nuclear Dynamics of Doped Conjugated Polymers

“…A thorough understanding of the relationship between molecular structure and optoelectronic properties is essential to design organic functional materials and optimize their device performance. − Key structural characteristics, such as molecular order, bonds pattern, and spatial extent of charged and excited states, are intimately related to the free carrier mobility and exciton lifetime, which ultimately determine the efficiency of organic devices, such as polymer field effect transistors and solar cells. Because of the large electron–phonon coupling, changes in the electronic structure of π-conjugated polymers lead to significant variations in the nuclear structure and dynamics. − Thus, vibrational (IR and Raman) spectroscopy, which is sensitive to variations of local dipole moments and polarizability within the nuclear oscillations, has been the technique of choice to understand electronic and nuclear structures, charge generation, and charge/energy transfer mechanisms in these systems. − …”

Time-resolved infrared absorption spectroscopy applied to photoinduced reactions: how and why

“…The nature of primary photo-excitations in organic semiconductors, particularly in conjugated polymers, has been the subject of an active debate in the scientific community for more than 3 decades [176][177][178][179]. The mechanisms underlying photogeneration of charge carriers as well as thermalization promptly after photoexcitation are indicative of the strength of electron-electron interactions relative to the bandwidth (as manifested by the exciton binding energy), while carriers transport, trapping, and recombination processes are intimately related to the degree of order, purity, and morphology of the materials.…”

“…As the field of organic semiconductors evolved through the initial fundamental studies of molecular crystals to the more application-oriented investigation of conjugated polymers, donor-acceptor and hybrid organic-inorganic systems, ultrafast transient IR techniques have been widely employed toward the experimental characterization of the charge generation and transport properties of new materials and material combinations [13,180]. If on one side, the measurement of charge carriers generation and transportation behavior in polydiacetylene revealed itself a good model for subsequent highly ordered organic semiconductors [176], on the other side theoretical description of organic semiconductors was and is still controversial. Indeed, rather than direct generation of free electrons and holes by inter-band photoexcitation as in conventional inorganic semiconductors, the optical excitation in organic semiconductors is generally believed to be excitonic, leading to the formation of spatially localized bound electron-hole pairs (excitons).…”

Plasmon–Polaron Coupling in Conjugated Polymer on Infrared Nanoantennas

“…Various theoretical studies describe the appearance of strong IRAV modes when even parity symmetric Raman-active vibrational modes (A g modes) are converted into IR-active modes by the local symmetry breaking of polymer chains produced by charge localization. ,, Experimental infrared absorption spectra induced by photo- or chemical-doping show peaks with one-to-one correspondence to the strongest Raman-active modes of the polymer observed in resonance Raman scattering . The intensity of these IRAV modes is directly proportional to charge carrier concentration, making them a unique optical probe for charge carrier density and dynamics in conjugated polymers. , Instead of manifesting as PIA peaks, IRAV modes of P3HT films possess a Fano-type antiresonance line shape (Figure b), created by the superposition of the narrow IRAV modes with the broadband delocalized polaron absorption. , In Figure b, we compare the IRAV modes of the pristine polymer film (black curve) with those obtained from the hybrid IR-nanoantennas/P3HT sample with orthogonal probe polarizations. For 90° polarization (blue curve), the IRAV modes appear in a spectral region not strongly modulated by the plasmon–polaron coupling resonance at ∼0.3 eV; nevertheless, their intensity is enhanced by more than a factor of 2 and all modes are preserved without spectral shift with respect to the pristine film.…”