Paul A. Lee

Interface electronic structure of organic semiconductors with controlled doping levels

Abstract We investigate the properties of inorganic–organic interfaces by ultraviolet and X-ray photoemission spectroscopy (UPS and XPS) and transport experiments. In particular, we study the interface between inorganic conductive substrates and organic layers that are intentionally p-type doped by co-evaporation of a matrix material and acceptor molecules. The photoemission spectra clearly show that the Fermi levels shift due to the doping and that the space charge layer width changes with doping (high doping – small width). The changes in the electronic structure of the interface due to doping agree well with results of transport experiments.

Photoemission spectroscopy of LiF coated Al and Pt electrodes

Thin lithium fluoride (LiF) interlayers between the low work function electrode and the electron transport layer in organic light emitting diodes (OLED) result in improved device performance. We investigated the electronic structure of LiF coated Al and Pt electrodes by x-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). Thin LiF films were grown in several steps onto Ar+ sputtered Al and Pt foils. After each growth step the surfaces were characterized in situ by XPS and UPS measurements. After evaluating band bending, work function and valence band offset for both samples, their band lineups were determined. Our measurements indicate that despite the insulating character of LiF in both samples, band bending is present in the LiF layer. The difference in band bending between the samples allows the conclusion that the driving force for the development of the band bending results from the contact potential between the metal and the LiF overlayer. The band bending is most...

Energy and charge transfer in organic light-emitting diodes: A soluble quinacridone study

A soluble derivative of quinacridone, N,N′-di-isoamyl quinacridone (DIQA), has been synthesized and used to study the mechanisms of Forster energy transfer and charge transfer in organic light-emitting diodes (OLEDs) based on 8-hydroxyquinoline (Alq3). Quantum efficiencies and spectra were measured for both photoluminescence (PL) and electroluminescence (EL) for films of poly(9-vinylcarbazole) (PVK) doped with Alq3 and DIQA. Both PL and EL showed an efficiency enhancement in films of PVK:Alq3:DIQA compared to films of PVK:Alq3. However, the optimal DIQA doping concentration was found to be lower for EL than for PL. Examination of the spectra revealed that more emission originated from DIQA for EL than for PL at a given doping level. We conclude that Forster energy transfer from Alq3 to DIQA occurs in both cases of PL and EL, but that charge transfer to DIQA occurs in the operation of the OLED resulting in additional pathways to DIQA emission. Ultraviolet photoelectron spectroscopy measurements showed that...

Observation of strong band bending in perylene tetracarboxylic dianhydride thin films grown on SnS2

Perylene tetracarboxylic dianhydride (PTCDA) thin films were grown in several steps on tin disulfide (SnS2) single crystals and characterized by combined x-ray and ultraviolet photoemission spectroscopy (XPS), (UPS) in order to characterize the frontier orbital line-up and the interface dipole at their interface. Due to the large difference between the work functions of PTCDA (4.26 eV) and SnS2 (5.09 eV) this experiment represents a model system for the investigation of band bending related phenomena in organic semiconductor heterojunctions. Our results show that the equilibration between the Fermi levels of both materials in contact is achieved almost solely by band bending (bulk charge redistribution) in the PTCDA layer. No significant interface dipole was detected which means that the PTCDA molecular orbitals and the SnS2 bands align at the vacuum level corresponding to the electron affinity rule. Our experiments clearly demonstrate the importance of an additional XPS measurement which (in most cases) ...

Progress in high work function TCO OLED anode alternatives and OLED nanopixelation

As organic light-emitting diodes (OLEDs) increase in sophistication and our understanding of building block-structure-luminous response mechanism increases, the remarkable properties of these heterostructures raise intriguing possibilities for future optoelectronics. In this contribution, we address two complimentary areas of interest in OLED science and engineering: (i) development and application of new transparent conducting oxide (TCO) materials for OLED anodes; (ii) effective OLED patterning strategies for nanofabrication.

Determination of frontier orbital alignment and band bending at an organic semiconductor heterointerface by combined x-ray and ultraviolet photoemission measurements

The alignment of the highest occupied molecular orbitals (HOMO) at the tris (8-hydroxy quinoline) aluminum (Alq3)/N,N′-di-(3-methylphenyl)-N,N′diphenyl-4,4′-diaminobiphenyl (TPD) heterojunction, used in organic light-emitting diodes (OLED), was determined by growing a TPD layer in several steps on a thick Alq3 substrate layer. After each growth step the sample was characterized in situ by x-ray and ultraviolet photoemission spectroscopy. The offset of the HOMO maxima at the interface was determined to be −0.13 eV from Alq3 to TPD. By including the known HOMO–lowest occupied molecular orbital (LUMO) gaps for both molecules into the evaluation, the offset of the LUMO minima was determined to be −0.33 eV from Alq3 to TPD. These values are consistent with previous assumptions that this interface represents a higher barrier for electron injection from Alq3 to TPD than for hole injection from TPD to Alq3.

Large molecule epitaxy on single crystal metals, insulators and single crystal and MBE-grown layered semiconductors

We review the packing structures for a series of aromatic hydrocarbons, deposited by vacuum deposition methods as ordered monolayers→→multilayers, on a variety of metal, semiconductor and insulator surfaces. New results are presented for the adsorption of monolayers of perylenes, phthalocyanines, coronene, and pentacenes on the Cu(100) surface, along with the implications of these studies for the formation of ordered multilayers of these molecular systems. Aromatic molecules without heteroatoms appear to pack in a flat-lying motif, and exhibit approximately hexagonal close packing, even on a substrate with four-fold symmetry such as Cu(100). In general, aromatic systems whose bulk structures lend themselves to layer-by-layer growth during vacuum deposition appear to be the best candidates for ordered multilayer growth.

Orientation and structure of monolayer →→ multilayer phthalocyanine thin films on layered semiconductor (MoS2 and SnS2) surfaces

Thin films of both chloroindium and copper pthalocyanines have been vacuum deposited onto metal dichalcogenide surfaces such as MoS2 and SnS2, with ordering achieved for these four‐fold symmetric molecules ranging from below monolayer to multilayers. Reflection high‐energy electron diffraction suggests that square lattice geometries are adopted for low coverages of each phthalocyanine (Pc), but with multiple domains. Low‐energy electron diffraction confirms the presence of three square lattice domains, each domain rotated by 60° with respect to the other. Basal plane defects, and especially terrace sites in the metal dichalcogenide surface, are implicated as the nucleation sites for the growth of these square lattice domains. Optical spectroscopies have been used to characterize submonolayer to multilayer deposits of chloroindium phthalocyanine on SnS2 thin films, where the packing geometries of the adjacent Pcs cause perceptible changes in the position and width of the absorbance band in the visible/near...

RHEED and optical characterization of ordered multilayers of phthalocyanine⧸C60 and phthalocyanine/perylene-tetracarboxylicdianhydride (PTCDA)

Ordered monolayers → multilayers of trivalent metal phthalocyanines, C60, and the perylenetetracarboxylic dianhydride (PTCDA) have been formed by molecular beam epitaxy processes on both single crystal MoS2 and SnS2, and on MBE-deposited SnS2 thin films. The bulk packing structures for the trivalent metal chloride phthalocyanines lend themselves to layered growth during deposition, as do the structures for C60 and PTCDA. RHEED data collected during the formation of [(InPc-Cl)1−4 ML/(C60)1−4 ML]1−20/SnS2/mica multilayers suggests that the structures observed during the formation of single component thin films can be sustained during multilayer formation, up to several lattice periods. Absorbance spectra of Pc/C60 and Pc/PTCDA multilayers show Pc Q-bands with maxima and full widths consistent with the formation of ordered Pc layers, provided that the Pc layer thickness is kept at 1–2 monolayers throughout the formation of the multilayer assembly.

Titanium dioxide electron-selective interlayers created by chemical vapor deposition for inverted configuration organic solar cells

We demonstrate the use of chemical vapor deposition (CVD) to create unique thin (12–36 nm) and conformal TiO2 interlayers on indium-tin oxide (ITO) electrodes, for use as electron collection contacts in inverted bulk heterojunction P3HT/PC61BM organic photovoltaics (OPVs). Optimized CVD formation of these oxide films is inherently scalable to large areas, and may be a viable non-contact alternative to electron-selective interlayer formation. Oxide-based electron-selective interlayers, such as TiO2, need to be thin, conformal and sufficiently electronically conducting films without sacrificing electron harvesting selectivity. Using volatile titanium-tetraisopropoxide (TTIP) precursors in a flowing N2 gas stream, the CVD process provides nanometer control of film thickness to produce 12–36 nm thickness device-quality films. The best performing CVD films, processed at substrate temperatures of ca. 210 °C, characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were found to be amorphous but stoichiometric TiO2. Solution electrochemistries (voltammetry) of probe molecules were shown to be easily accessible indicators of film porosity and are predictive for electron harvesting selectivity (and hole-blocking) in an inverted configuration OPV platform. Small molecules whose redox potentials placed them energetically above the conduction band edge energy (ECB) were reduced/oxidized at nearly the same rates as for bare ITO. Probe molecules whose redox potentials place them energetically within the band gap region, below ECB, show almost complete blocking of their oxidation/reduction processes, for optimized conformal (and nonporous) TiO2 films. In addition, background oxidation current densities for solution probe molecules correlate inversely with the shunt resistance (RP) measured in OPVs. OPVs with the configuration: ITO/CVD-TiO2/P3HT:PC61BM/MoO3/Ag, using TiO2 films of 12, 24 and 36 nm, were evaluated for short-circuit photocurrent (JSC), open-circuit photopotential (VOC), and fill-factor (FF), versus bare ITO. OPVs using amorphous, conformal 24 nm TiO2 interlayers showed the highest fill factors, lowest RS, highest RP and power conversion efficiencies of ca. 3.7%.