Tuning hole-injection barriers at organic/metal interfaces exploiting the orientation of a molecular acceptor interlayer

Ultraviolet photoelectron spectroscopy was used to demonstrate organic/metal-contact charge injection barrier tuning by exploiting the orientation-dependent work function ϕ of a molecular acceptor [hexaazatriphenylene-hexanitrile (HATCN)] interlayer on Ag(111). The work function ϕ of a flat-lying HATCN monolayer on Ag was 4.6 eV (similar to a pristine Ag electrode), whereas a layer of edge-on HATCN on Ag exhibited ϕ of 5.5 eV (comparable to a pristine Au electrode). The hole-injection barriers (HIBs) between HATCN-modified electrodes and the organic semiconductors tris(8-hydroxyquinoline)aluminum (Alq${}_3$) and N,N${}^{\prime }$-bis(1-naphtyhl)-N,N${}^{\prime }$-diphenyl-1,1${}^{\prime }$-biphenyl-4.4${}^{\prime }$-diamine (α-NPD) were reduced by more than 1 eV compared to pristine Ag and Au electrodes. Noteworthy, the HIBs determined with the flat-lying HATCN interlayer were lower than those obtained for pristine Ag substrates (ϕ of both electrodes is 4.6 eV), and the HIBs with the edge-on HATCN on Ag were lower than those found for pristine Au (ϕ of both electrodes ca. 5.4 eV). This shows that acceptor interlayers are beneficial for charge injection in electronic devices even when the molecularly modified electrode ϕ is comparable to that of a pristine metal surface. It is argued that the molecularly modified electrodes are electronically more rigid than their pristine metal counterparts, i.e., the electron spill-out at the organic-terminated surface is less pronounced compared to Ag and Au surfaces.

Figure 1

UPS valence and SECO spectra for (a) α-NPD and (b) Alq${}_3$ (thickness θ${}_{\alpha -\mathrm{NPD}}$ and θ${}_{\mathrm{Alq}3}$ respectively) deposited on (from top to bottom) pristine Ag(111), flat-lying HATCN interlayer on Ag(111), and edge-on HATCN interlayer on Ag(111). The flat-lying and edge-on HATCN interlayer spectra are labeled 0${}^{*}$ and 0${}^{**}$, respectively. The spectrum of 10 Å α-NPD/Ag(111) is also shown shifted (dashed gray) to illustrate the HOMO shift from mono- to multilayer. The insets for (b) show how the energy position of the SECO and the low binding energy onset of emission from the HOMO were determined at the example of 30 Å Alq${}_3$/Ag(111). The chemical structures of HATCN, α-NPD, and Alq${}_3$ are shown in the upper row.

Figure 2

Schematic energy level diagrams of (a) α-NPD and (b) Alq${}_3$ on (columns from left to right) pristine Ag(111), flat-lying HATCN interlayer on Ag(111), edge-on HATCN interlayer on Ag(111), and Au [polycrystalline Au for α-NPD (data taken from Ref. 16) and Au(111) for Alq${}_3$ (data taken from Ref. 5)]. Note that to minimize effects of the permanent molecular dipole moment of Alq${}_3$, a nominal coverage of 30 Å was chosen as multilayer for the respective systems. This is in contrast to the α-NPD results for which the multilayers have a nominal coverage of 100 Å.