Singlet‐exciton optics and phonon‐mediated dynamics in oligoacene semiconductor crystals

Abstract

Organic semiconductor crystals stand out as an efficient, cheap, and diverse platform for realizing optoelectronic applications. The optical response of these crystals is governed by a rich tapestry of exciton physics. So far, little is known on the phonon-driven singlet-exciton dynamics in this class of materials. In this joint theory-experiment work, we combine the fabrication of a high-quality oligoacene semiconductor crystal and characterization via photoluminescence measurements with a sophisticated approach to the microscopic modeling in these crystals. This allows us to investigate singlet-exciton optics and dynamics. We predict phonon-bottleneck effects in pentacene crystals, and find that dark excitons act as crucial phonon-mediated relaxation scattering channels in both pentacene and tetracene. While the efficient singlet fission in pentacene crystals hampers the experimental observation of this bottleneck effect, we reveal both in theory and experiment a distinct polarization and temperature dependence in absorption and photoluminescence spectra of tetracene crystals, including microscopic origin of exciton linewidths, the activation of the higher Davydov states at large temperatures, and polarization-dependent quenching of specific exciton resonances. Our joint theory-experiment study represents a significant advance in microscopic understanding of singlet-exciton optics and dynamics in oligoacene crystals.

Microscopic modelling of phonon-mediated exciton propagation in organic semiconductors

“…[34][35][36] Our previous studies have extracted the magnitude of this excitonphonon coupling from the experimentally observed temperature-dependence of exciton linewidths. 19 We find that the exciton diffusion is anisotropic due to the anisotropy of the group velocity (band structure), exciton occupation and also the phonon scattering. 29 We do not take into account singlet-fission processes, as they occur on a longer timescale 37,38 in Tc crystals.…”

“…17 These excitons form around different positions in the electronic band structure and have nearly perpendicular optical dipole moments. 18,19 Starting from the electronic band structure of tetracene, we employ the Wannier equation [20][21][22][23] to derive the excitonic band structure E µ Q and wavefunctions φ µ k . The excitons in acene crystals, such as pentacene and tetracene are described as charge transfer excitons, somewhere between extremely localised Frenkel excitons and delocalised Wannier excitons, 18,19,24 with exciton wavefunctions spread over nearest neighbour molecules and large binding energies.…”

“…18,19 Starting from the electronic band structure of tetracene, we employ the Wannier equation [20][21][22][23] to derive the excitonic band structure E µ Q and wavefunctions φ µ k . The excitons in acene crystals, such as pentacene and tetracene are described as charge transfer excitons, somewhere between extremely localised Frenkel excitons and delocalised Wannier excitons, 18,19,24 with exciton wavefunctions spread over nearest neighbour molecules and large binding energies. We find exciton binding energies of ∼ 800 meV owing to the large effective mass of the underlying band structure.…”

“…18,25 The excitons are well-confined to individual tetracene layers due to the weak interlayer coupling within the crystal 15,18,26 and hence we use a screened 2D Coulomb potential to derive the excitonic binding energies. 19 To microscopically describe the excitonic propagation, we calculate the spatio-temporal evolution of the exciton distribution 27,28 (cf. Fig.…”

“…The energy conservation is determined by the delta function δ(∆E µ,µ ′ q,Q ± ℏω q ), while the exciton-phonon coupling G µµ ′ ν Qq is described in more detail in previous studies. 19,33 We take into account inter-and intramolecular, as well as both acoustic and optical phonon modes. [34][35][36] Our previous studies have extracted the magnitude of this excitonphonon coupling from the experimentally observed temperature-dependence of exciton linewidths.…”