The influence of metal work function on the barrier heights of metal/pentacene junctions

Abstract

The electronic structure of Cu(111)/pentacene and Ag(111)/pentacene interfaces were investigated with photoelectron spectroscopy and the hole barrier heights were determined to be 0.74 and 0.90 eV, respectively. When combined with previous measurements of the Au(111)/pentacene interface, the slope of the plot of metal work function against barrier height for Schottky barrier formation was determined to be 0.36, in agreement with current-voltage (I−V) measurements in the literature. However, the absolute barrier heights from photoemission measurements are 0.16 eV higher. The offset between the I−V measurement and the x-ray and ultraviolet photoelectron spectroscopy measurements was attributed to differences in how the highest occupied molecular orbital position is determined. Photoemission data indicates that at low coverages the pentacene molecules lie flat on the metal substrates, whereas at higher coverages the molecular orientation changes to orient the long molecular axis normal to the surface. Thicker films then tend to form three-dimensional islands, resulting in structured rather than flat films forming on clean Ag(111) and Cu(111) at room temperature.

A High Molecular Weight Donor for Electron Injection Interlayers on Metal Electrodes

“…Similarly, F changes from 4.90 eV [pristine Cu(111)] to 3.75 eV (12 NMA) on copper. In both cases, the change is 0.3 eV larger than reported values [Ag(111)~0.7 eV, Cu(111)0 .9 eV [44,45] for weakly interacting molecules on silver and copper. Thus, a (weak) charge transfer that decreases F below the reported values for the "push-back" effect happens most likely in all three cases, despite the fact that the valence band spectra show no direct evidence for a new density of states (intragap states) or pronounced differential shifts of the occupied molecular orbitals.…”

Integer versus Fractional Charge Transfer at Metal(/Insulator)/Organic Interfaces: Cu(/NaCl)/TCNE

“… 17 , 18 In organic (opto-)electronic devices in particular, charge transfer between a metallic electrode and layers of semiconducting molecules governs the injection of electrons or holes into the active organic device component or the collection of electrons or holes from the active organic device region. 19 − 23 Optimizing this charge injection or extraction can significantly improve the performance of, e.g. , organic light-emitting diodes (OLEDs) for display and lighting applications or organic photovoltaic cells (OPVCs).…”

Structural, Optical and Electrical Analysis of Pentacene Thin Film

“…The modulus decreases suddenly when the ac frequency is below 80khz. f. metal-insulator-metal model : Fig.10 shows energy level diagrams and it is drawn in terms of work function of the electrode metals and pentacene [18]. The work function of aluminium is 4.7eV and that of silver is 4.2eV.…”