Displacement of polarons by vibrational modes in doped conjugated polymers

Abstract

Organic pi-conjugated polymers are deemed to be soft materials with strong electron-phonon coupling, which results in the formation of polarons, i.e., charge carriers dressed by self-localized distortion of the nuclei. Universal signatures for polarons are optical resonances below the band gap and intense vibrational modes (IVMs), both found in the infrared (IR) spectral region. Here, we study p-doped conjugated homo-and copolymers by combining first-principles modelling and optical spectroscopy from the far-IR to the visible. Polaronic IVMs are found to feature absorption intensities comparable to purely electronic transitions and, most remarkably, show only loose resemblance to the Raman or IR-active modes of the neutral polymer. The IVM frequency is dramatically scaled down (up to 50%) compared to the backbone carbon-stretching modes in the pristine polymers. The very large intensity of IVMs is associated with displacement of the excess positive charge along the backbone driven by specific vibrational modes. We propose a quantitative picture for the identification of these polaron shifting modes that solely based on structural information, directly correlates with their IR intensity. This finding finally discloses the elusive microscopic mechanism behind the huge IR intensity of IVMs in doped polymeric semiconductors.

Delocalisation softens polaron electronic transitions and vibrational modes in conjugated polymers

“…Whilst some of them can be traced back to ground state modes, the distinct assignment to polaron-related modes has recently been invoked for the most prominent features. 10,11,16 Of the smaller features, some shift to lower energy (e.g. at 0.102 eV).…”

“…Also this classical explanation has recently been contested and it was suggested that pronounced vibrational signals should be considered as related to polarons instead of the ground state of the polymer. 10,11 The low energy signatures of polymer polarons have lately moved back into the focus of research interest as a means to extract information about the chain conformation in the environment of the charge carrier. The groups of Schwartz 12 and Salleo, 13 for example, studied the spectra of doped films of P3HT to investigate the location of doping-induced charge carriers.…”

Charge transport in high-mobility conjugated polymers and molecular semiconductors

“…However, while the carriers remain on a particular site, the electronic coupling to the vibrational modes also causes fast oscillations of the carrier wavefunction along the polymer backbone driven by the structural dynamics. These fast oscillations of the carrier wavefunction along the backbone around the sites defined by static disorder can be directly observed in optical spectroscopy as they lead to large enhancements of the oscillator strength for optical absorption by infrared vibrational modes 87 . They are similar to the charge carrier diffusion driven by the structural dynamics in the TL regime of molecular crystals.…”

Molecular Doping of PCPDT-BT Copolymers: Comparison of Molecular Complexes with and Without Integer Charge Transfer

“…In experiment, these two mechanisms can be distinguished using optical spectroscopy, as the electronic excitations are qualitatively different and free charges on the polymer chains show up as distinct polaron absorption features [9][10][11][12][13][14]. Also different vibrational modes exist with unique optical fingerprints in the infrared spectral range [15][16][17][18][19]. In conductivity measurements generally only a limited frac- Transfer of a charge from the oligomer to the dopant molecule, leaving the system in a broken symmetry electronic ground state with a positively charged polaron on the oligomer chain and a negative charge on the dopant.…”