Modification of TiO ₂ (1 1 0)/organic hole transport layer interface energy levels by a dipolar perylene derivative

Abstract

Our photoemission study reveals that the work function of TiO 2 (1 1 0) decreases by up to 1.5 eV upon deposition of 9-(bis-(p-(tert-octyl)phenyl)amino)-perylene-3,4-dicarboxylic anhydride (BOPA-PDCA). This effect is attributed to a chemical reaction of TiO 2 (1 1 0) and the molecular anhydride group, as well as the molecular dipole. Analysis of the film thickness dependent photoemission and metastable atom electron spectroscopy data reveals that for low coverage the perylene backbone of BOPA-PDCA is almost parallel to the substrate surface and higher coverage leads to an orientational transition to essentially upright standing molecules. Comparing the energylevel alignment between TiO 2 (1 1 0) and the hole transport materials N,N′-bis(1-naphthyl)-N,N′diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) without and with the BOPA-PDCA interlayer, we find that the perylene derivative has a positive impact on the level alignment for dye-sensitized solar cells with high open-circuit voltages.

Inorganic–organic interfaces in hybrid solar cells

“…This was observed, e.g. at organic/ZnO [249,330,345,406] organic/H-Si [136,380], and organic/TiO 2 [258,[407][408][409] interfaces. A change in ELA can be used to increase (decrease) ΔG as well as the ρ overlap between the OSC and ISC for electron injection (recombination), thereby enhancing exciton dissociation [410].…”

“…The opposite trend in interface chemistry strength can be induced by covalently binding the organic and inorganic materials at the hybrid interface. The ELA at these interfaces has been studied as well and we list examples for Si [63,257] and TiO 2 [258,259].…”

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

“…For example, F 4 TCNQ is also a popular molecular dopant 53, [464][465][466][467][468][469] . Furthermore, work function modification is not restricted to metal surfaces 271,[470][471][472][473][474][475][476][477] and electron donating COMs can also lower effective work functions 62,65,448,478,479 by the reversed process as electron accepting molecules, i.e., by an electron transfer from the adsorbate to the substrate. Strongly interacting organic-metal systems have thus a significant relevance for applications.…”

“…In some other rare cases the molecular surface unit cell includes two molecules with one of them lying flat and the other one being tilted [546][547][548] . On other surfaces, e.g., on metal oxides, standing orientations of (polar) COMs in monolayers are successfully used for ELA engineering 475,549,550 . The question arises how this can be achieved for organicmetal interfaces, i.e., what are the driving forces for a molecular semiconductor to adopt a tilted or standing orientation on a clean metal surface?…”

Photoelectron spectroscopy reveals molecular diffusion through physisorbed template layers on Au(111)

“…Organic heterostructures on inorganic substrates are inherent to virtually all organic (opto-)electronic devices and are of key importance for their performance [1][2][3][4][5]. For example, monolayers of molecular donors or acceptors have been shown to effectively tune electrode work functions and, consequently, charge injection barriers into subsequently deposited organic layers [6][7][8][9]. The energy-level alignment tuning at these interfaces relies on interfacial dipole layers, which can be also introduced by dipolar self-assembled monolayers (SAMs) [10,11].…”