The Effect of Fluorination on Pentacene/Gold Interface Energetics and Charge Reorganization Energy

Abstract

The energy level alignment at interfaces between conjugated organic semiconductors and metals is recognized as a key factor determining the performance of organic-based (opto)-electronic devices. [1,2] Experimentally, the hole-injection barriers (HIBs) at organic/metal interfaces can be directly determined by using ultraviolet photoelectron spectroscopy (UPS).In addition, angle-resolved UPS (ARUPS) allows molecular orientations at surfaces to be assessed. In the work reported here, we used this method to investigate interfaces between two prototypical organic semiconductors, pentacene (PEN) and perfluoropentacene (PFP), and Au. PEN can be used as the active material in organic field-effect transistors (OFETs) with a high hole mobility of up to 5.5 cm 2 V -1 s -1 .[3] The fabrication of integrated circuits also requires high-electron-mobility OFETs; this is difficult to achieve with PEN, which has reported electron mobilities of up to 0.04 cm 2 V -1 s -1 .[4] One approach to improving n-type performance of OFETs is to use PFP, resulting in high electron mobilities of more than 0.2 cm 2 V -1 s -1 . [5] Commonly, Au is used as the source and drain contact metal in OFETs. Therefore, the interface energetics between organic semiconductors and Au are of interest, since charges have to be transported across these interfaces and minimized contact resistance is aimed at. While a number of reports are already available on PEN/Au interfaces, nothing is yet known about PFP/Au. Moreover, published work on PEN/Au interfaces reveals controversial values for the HIB, ranging from ca. 0.5 to 0.9 eV for various polycrystalline [6][7][8][9] and single-crystal [10,11] Au surfaces. Even larger differences of the HIB were found for PEN on pristine and molecularly modified Au [9] and Ag [12] surfaces. At present, molecular-layer thickness and orientation, polarization, and thin-film polymorphism-and combinations thereof, as these parameters are interdependent-are being discussed as possible reasons for the observed variation. [8,9,12] The molecular-charge reorganization energy is yet another important parameter for efficient transport of charge carriers across interfaces and can also be assessed with UPS. [13][14][15] The present comparative ARUPS study for PEN and PFP on Au(111) highlights with unprecedented accuracy i) the effect of perfluorination (of PEN) on the interface energy levels, and ii) the effect of molecular-layer thickness on organic semiconductor/metal interface energetics. In short, the HIB for a monolayer of each organic material on the metal was almost identical, despite the very different ionization energies of PEN and PFP. In addition, we found distinctly different molecular level energies as a function of layer thickness, attributed to different polarization energies. The photoemission spectra for monolayers of the two organic materials corroborated that the charge-reorganization energy of PEN/Au (111) is significantly larger than for PEN in the gas phase, and even larger for PFP/Au(111).First, we discuss the inte...

Influence of intramolecular polar bonds on interface energetics in perfluoro-pentacene on Ag(111)

“…[13][14][15] However, with increasing film thickness the molecules often undergo an orientational transition. [16][17][18][19] In a previous ultraviolet photoelectron spectroscopy ͑UPS͒ study of PFP on Au͑111͒ a transition from a flat lying monolayer to herringbonelike multilayers was proposed, 12 and concomitant changes in the IE were reported, however, they could not be explained. The IPBs of PFP are expected to have a similar impact on the electronic structure of a PFP thin film irrespective whether it is composed of standing molecules or of molecules with their short axes tilted with respect to the substrate surface ͓Fig.…”

“…Thus, it may be speculated that the IE for lying molecules in the herringbone structure of PFP is larger compared to that in the flat lying conformation and, indeed, an increase of the IE was found with increasing film thickness. 12 As the fluorination of pentacene ͑PEN͒ significantly reduces the adsorbate-substrate interaction, 12,20,21 leading to physisorption on various metal substrates, a flat lying monolayer and tilted multilayer can be expected also on the Ag͑111͒ substrate. Thus PFP/Ag͑111͒ is a useful prototypical system to unambiguously elucidate the impact of IPBs on the electronic structure in herringbonelike arrangements.…”

“…1͑a͔͒ is 0.65 eV higher if PFP is nearly upright standing on the substrate in comparison to flat lying PFP. 7,10,12 On metallic substrates conjugated organic molecules preferentially adsorb with their system parallel to the surface ͑i.e., flat lying orientation͒. [13][14][15] However, with increasing film thickness the molecules often undergo an orientational transition.…”

Controlling energy level offsets in organic/organic heterostructures using intramolecular polar bonds

“…This results in surface dipoles of opposite sign that significantly impact the IE of the respective standing layers. Consequently, PEN and PFP exhibit a moderate difference in the IE ͑⌬IE͒ of 0.45 eV for films of lying molecules, 13 but a substantially increased ⌬IE of 1.85 eV for films of standing molecules. 8 Both molecules grow in an almost uprightstanding orientation in thin films on SiO 2 substrates [14][15][16][17][18][19] and this orientation was also reported for thin PFP films on top of PEN layers.…”

Fingerprint of Fractional Charge Transfer at the Metal/Organic Interface

“…[3,4,[14][15][16][17][18][19][20] These properties can be tuned, according to specific technological needs, for example, by fluorination, turning, e.g., pentacene into the ntype semiconductor perfluoropentacene. [13,21,22] In this Letter, we investigate F4PEN molecules deposited on Au(110) single crystals by using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The simultaneous use of this variety of techniques, in combination with controlled in-situ deposition, gives the opportunity not only to explore the complete electronic structure of the systems (occupied and unoccupied states, or orbitals) but to avoid possible artefacts and discrepancies due to slightly different preparations that could impact the morphology and the structure of the assemblies and consequently their electronic structure.…”