Transient Monolayer Structure of Rubrene on Graphite: Impact on Hole–Phonon Coupling

Abstract

Charge transport in molecular thin films is often dominated by incoherent hopping processes and charge-carrier phonon coupling plays a major role in defining mobilities. Our high resolution angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) study reveals the influence of molecular configuration and packing on the hole-phonon coupling in vacuum-sublimed thin films of rubrene on graphite and allows determining charge reorganization energies. In the contact layer to the substrate rubrene is well-ordered with, for low coverages, the tetracene backbone being almost parallel to the graphite surface forming a loose-packed monolayer. Increasing the coverage leads to an orientational transition and a more tilted orientation of rubrene in a close-packed monolayer. This transition results in dramatic changes of spectral features in ARUPS including the photoelectron angular distribution of the highest occupied molecular orbital derived intensity. The charge reorganization energy, however, only changes slightly. The transient monolayer structure of rubrene on graphite allows thus to demonstrate that hole-phonon coupling in organic thin films does not depend very critically on the packing structure. IntroductionLow charge-carrier mobilities are often limiting the performance of organic (opto-)electronic devices as charge transport in organic semiconductor thin films is often dominated by incoherent hopping processes 1-4 . Hopping mobilities depend critically on the charge-carrier phonon coupling 5-8 , which determines the charge reorganization energy λ. In most molecular thin films intermolecular interactions are weak and λ is consequently dominated by intramolecular vibrations and is often taken from gas-phase calculations 5,9-11 or gas-phase ultraviolet photoelectron spectroscopy (UPS) data 9,12-14 . However, the relevant quantities for device application are the solid state values, which depend on the molecular surrounding, i.e. the interaction with neighboring molecules and/or a substrate 7,15 . Due to the similarities of ionization (followed by neutralization) during localized hopping transport and photoionization, UPS is the method of choice to measure charge reorganization energies of organic thin films by an analysis of the vibrational fine structure of the highest occupied molecular orbital (HOMO)-derived peak 7,8,[15][16][17][18] . The relevant vibrational energies (hν i ) of a molecule are often centered closely and in good approximation the analysis can be done by a single mode fit with one main vibrational energy (hν) 19 . Moreover, in most molecular thin films the relaxation energies for ionization and neutralization of a molecule are rather similar and the charge reorganization energy is thus simply given by λ=2Shν with S being the Huang-Rhys factor, which is determined by the relative intensity contribution to the vibrational progressions 7,13 .

Role of molecular conformations in rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures

“…For the third group of rubrene thick film deposited with T sub  = 170 °C, the peaks assigned to a twisted rubrene (2.366 and 2.520 eV) almost vanished, but the two weak peaks corresponding to a planar rubrene23 were again observed at T  > 100 K. Because the calculated energy difference of twisted and planar conformations is about 163~210 meV81718, the thermal energy corresponding to T sub during the deposition process is too small for the required transformation energy. Thus, the energy needed to planarize the twisted backbone is proposed to be compensated by the lattice energy from a more efficient packing of the planar rubrene molecules in the bulk81718, which can be induced either from the critical thickness of the film13 or from the interaction with substrate23. Our experimental findings reveal that the third group of thick rubrene films fabricated with T sub  > 170 °C with crystalline grain size > 5 μm (as estimated from Fig.…”

“…Motivated by a high hole mobility1234 and a long exciton diffusion length56 observed in rubrene (C 42 H 28 ) single crystal, an intense research effort has been devoted to the preparation and characterization of various rubrene molecular solid films suitable for applications in organic electronic and optoelectronic devices78910111213141516. Rubrene molecule consists of a tetracene backbone with four phenyl side rings, and each side ring lies in a plane nearly perpendicular to the tetracene backbone.…”

“…For a free rubrene molecule, the total energy of a twisted rubrene is smaller than that of a planar rubrene by about 163~210 meV based on the electronic structures calculation81718. Thus, in vapor phase or in thin amorphous films, rubrene molecules possess the low energy state of twisting conformation; however in highly crystalline films or single crystals, rubrene molecules adopt a planar conformation in which the energy required to planarize the backbone is compensated by the lattice energy81317181920212223. The rubrene molecular solids are formed by weak intermolecular interactions, and hence different growth methods can lead to different polymorphs24.…”

Ab initio study of electron-phonon coupling in rubrene

“…1). It often serves as model compound for experiments [18][19][20][21][22][23][24][25][26][27] and simulations [28][29][30][31][32][33][34][35][36] , including advances in growth by means of van der Waals epitaxy 37 , the detailed analysis of defect formation 38 , or investigation of electron-phonon coupling effects. 27,39,40 Finally, rubrene has one of the highest carrier mobilities, which can reach few tens of cm 2 /Vs for holes.…”

Binding and Electronic Level Alignment of $\boldsymbolπ$-Conjugated Systems on Metals

“…Moreover, even for largely angle-integrated measurements the photoelectron angular distribution (PAD) provides insight into, e.g., the orientation of COMs on the surface 115,236,281,282 . The vibrational fine structure of HOMO-peaks allows to assess charge reorganization energies and thus to estimate hopping mobilities by a 'firstprinciple' experiment 279,[333][334][335][336] . Moreover, the development of instrumentation over the last decades has made it possible to measure photoelectron reciprocal-space maps, often termed "orbital tomography", which can be used to reconstruct molecular orbitals in real space and/or to precisely assign photoemission intensities to a particular molecular orbital 286,337115,236,[338][339][340] .…”