The influence of the vibrational properties on charge transport in oligoacenes

Over recent years there has been a growing interest in molecular semiconductors of the oligoacene family. The interest has been spurred by the unique properties of these materials for applications for electronic devices, namely being °exible, cheap and exhibit- ing relatively high charge-carrier mobilities. Despite this interest, the nature of charge transport in the materials has remained the subject of intense debate. When these oligoacenes are used for device applications, they will often be evaporated as a ¯lm onto a substrate, since the fabrication process and the handling of these thin ¯lms is easier and cheaper than using single crystals. Besides the fundamental bulk properties, for the usage in devices it is also of high importance to know how the properties of these materials di®er at the interface with metals, since the substrates and contacts are often metallic. Therefore in this work both the vibrational and electronic properties of thin ¯lms of pentacene have been investigated as well as the intrinsic charge transport mechanism for the single-crystalline bulk material of pentacene and rubrene. The ¯rst main focus in this study was on the electronic and vibrational properties of thin layers of pentacene (ranging from 1 monolayer to 6 layers of pentacene molecules) evaporated on metal surfaces. These pentacene layers have been investigated using High Resolution Electron Energy Loss Spectroscopy (HREELS), which is a surface sensitive technique that probes both the vibrational and electronic states of the top layers of the sample by detecting the loss in energy of a beam of monochromatic electrons. The main advantages of this technique in comparison to techniques like IR and Raman spectroscopy are the surface sensitivity, the high spectral resolution (FWHM of 13 cm¡1), the broad energy range (from lattice phonons to surface plasmons and interband transitions) and the ability to measure as a function of wave vector transfer. Pentacene was grown upon Ag(111) and Au(100) surfaces, forming thin layers of var- ious thicknesses. The obtained vibrational spectra were compared to the vibrational modes for an isolated molecule calculated using DFT methods. The spectra of ¯lms of over 4 molecular layers showed contributions from all the optical active modes predicted by the calculation as is expected for a non-frustrated Van Der Waals bonded crystalline material. Furthermore the spectra showed low energetic features related to the lattice phonons. Both inter- and intramolecular modes showed little to no dispersion. The spec- tra for a monolayer deviated strongly from the vibrational structure of the multi-layers. Not only were the intermolecular modes absent, all intramolecular optical active modes with a dipole moment parallel to the interface were screened by the metal surface. In the case of a monolayer of pentacene on Ag(111) the interaction with the surface is so strong that the symmetric Ag modes became optically active as a result of charge transfer from the metallic substrate. For the intermediate regime (layers of 2 to 4 molecules) the spectra showed several contributions that cannot be explained using the calculated data on the symmetry and energy of the intramolecular modes. With HREELS not only the vibrational aspects of the thin ¯lms were studied but also the electronic transitions. The spectra for layers of more than four molecular layers could be modeled by combining calculations on the position of both excitonic and band levels and calculations of the vibronic ¯ne structure. For a monolayer of pentacene, states within the electronic gap were observed, the so-called Charge Neutrality Levels, which are substrate dependent. With increasing thickness the observed presence and position of the exciton states di®ered, thereby giving more insight in the structure of pentacene near the interface. Terahertz Time Domain Spectroscopy (THz TDS) is a technique that allows determin- ing the complex frequency dependent conductivity and is a powerful tool to investigate the charge transport mechanism in materials. THz TDS was used to study the charge trans- port as a function of temperature for single crystals of both pentacene and rubrene. For pentacene the conductivity spectra at various temperatures showed a strong absorption for the low energetic vibrational modes and thereby gave evidence for the model proposed by A. Troisi and G. Orlandi. This model describes the charge transport behaviour by one single mechanism: the large °uctuations in the intermolecular coupling of electronic states caused by thermal molecular motions. Furthermore it is shown that the charge density, and as a result the interaction between charges, plays an important role in the conductivity of these materials. In conclusion, this work shows that vibrational aspects are a key factor in under- standing the charge transport in oligoacene materials, and that good knowledge of the properties of these materials at the interface is essential, since di®erences in both struc- ture and electronic interaction with the substrate give rise to di®erent vibrational and electronic states in oligoacene materials.