Bing R. Hsieh

Work function of indium tin oxide transparent conductor measured by photoelectron spectroscopy

We used ultraviolet and x‐ray photoelectron spectroscopy (XPS) and (UPS) techniques to directly measure absolute values of vacuum work function of indium tin oxide (ITO) thin films. We obtained a work function of 4.4–4.5 eV which is lower than the commonly cited value. These values do not change substantially by heating and Ar ion sputtering. The atomic concentrations of each element in ITO, measured with XPS, are also quite stable under heat treatment and ion sputtering.

Fabrication and properties of light‐emitting diodes based on self‐assembled multilayers of poly(phenylene vinylene)

Light‐emitting diodes have been fabricated from self‐assembled multilayers of poly(p‐phenylene vinylene) (PPV) and two different polyanions; polystyrene sulfonic acid (SPS) and polymethacrylic acid (PMA). The type of polyanion used to assemble the multilayer thin films was found to dramatically influence the behavior and performance of devices fabricated with indium tin oxide and aluminum electrodes. Light‐emitting devices fabricated from PMA/PPV multilayers were found to exhibit luminance levels in the range of 20–60 cd/m2, a thickness dependent turn‐on voltage and classical rectifying behavior with rectification ratios greater than 105. In sharp contrast, the devices based on SPS/PPV exhibited near symmetric current–voltage curves, thickness independent turn‐on voltages and much lower luminance levels. The significant difference in device behavior observed between these two systems is primarily due to a doping effect induced either chemically or electrochemically by the sulfonic acid groups of SPS. It w...

Photovoltaic and photoconductive properties of aluminum/poly(p-phenylene vinylene) interfaces

Photoconductive and photovoltaic properties of Al/PPV/ITO sandwich devices were investigated by measuring steady-state photocurrents resulting from illumination through the Al electrode. A built-in potential (Vbi) was detected at the Al/PPV interface. The voltage dependence of the photocurrent in the vicinity of Vbi was measured at 1 mW/cm2 of incident illumination to give an open-circuit voltage and a short-circuit current of 1.2 V and 6 × 10−7 A/cm2, respectively. The dependences of the short-circuit current on excitation wavelength and illumination intensity are presented and the C-V characteristics of the Al/PPV interface are analyzed. The quantum collection efficiency decreased from 5% to 1% as the intensity of illumination increased from 10−5 to 1 mW/cm2. The photovoltaic conversion power efficiency was about 0.07% for intensities approaching 1 mW/cm2. The Vbi value was accounted for by surface band-bending at the Al/PPV interface.

Dramatic photoluminescence quenching of phenylene vinylene oligomer thin films upon submonolayer Ca deposition

We report that Ca atoms severely quench the photoluminescence (PL) of an organic thin film 1,4‐bis[4‐(3,5‐di‐tert‐butylstyryl)stryyl]benzene (4PV). A submonolayer coverage (0.035 monolayer or 0.1 A) of Ca on the 4PV surface reduced the PL of a 300 A layer of 4PV by as much as 50%. Three distinct quenching rates were observed throughout the 200 A Ca deposition. An exciton diffusion length of 200 A in 4PV is estimated assuming that the PL quenching is diffusion controlled.

Interface formation of Ca with poly(p‐phenylene vinylene)

We have investigated the interface formation of Ca with poly(p‐phenylene vinylene) (PPV) using x‐ray photoemission spectroscopy. The most significant part of our findings was the very late occurrence of band bending at the Ca/PPV interface and the lack of strong reaction between the Ca overlayer with the PPV substrate, indicating that the Schottky barrier formation in Ca/PPV was a slow process. The late barrier formation at the Ca/PPV interface may be due to the shielding by surface oxygen impurities, part of which underwent calcium oxide formation at the interface.

Energy level bending and alignment at the interface between Ca and a phenylene vinylene oligomer

We have studied the interface formation of a vinylene phenylene oligomer with a Ca substrate using photoemission spectroscopy. The evolution of core and valence spectra during the deposition of the oligomer on Ca has indicated the molecular energy level bending. The total energy level bending was 0.5 eV and the thickness of the level bending region was about 100 A. We propose an energy level diagram of the oligomer‐Ca interface based on the information obtained from the photoemission spectra.

Space-Charge-Limited Charge Injection From Ito/Ppv Into A Trap-Free Molecularly Doped Polymer

We describe bilayer structures comprised of a poly ( p -phenylene vinylene) (PPV) layer and a trap-free diaryldiamine (TPD) doped in polycarbonate (PC) layer, sandwiched between indium-tin-oxide (ITO) and aluminum (Al) contacts. Two critical phenomena in the operation of polymer based electroluminescent devices, interface injection and carrier range, are investigated. It is established that the ITO/PPV contact is capable of sustaining dark current under trap-free space-charge-limited (TFSCL) conditions into a hole transporting TPD:PC layer. TFSCL currents are not observed in devices without the PPV layer. Upon increasing the thickness of the PPV layer a deviation from the TFSCL regime is observed which is attributed to trapping of the injected holes within PPV. These observations suggest a novel method for estimating the trapping Mobility-lifetime product μτ for holes in PPV. By this means we estimate μτ ∼ 10 −9 cm 2 /V.

Band bending modified tunneling at metal/conjugated polymer interfaces

Metal/polymer interfaces play an important role in polymeric light‐emitting diodes. Recent transport measurement studies by Parker promote a rigid band model with triangular barrier tunneling at the metal/polymer interface. This finding, however, stands in contradiction to the band bending and Schottky barrier formation observed using surface analytical techniques. We found that this apparent contradiction can be nicely reconciled if a modification of the tunneling model by band bending is included in the interpretation of the transport data. Band bending modified tunneling gives a clear physical picture of the tunneling process across the metal/polymer interface and is important for low field tunneling processes.

Electron spectroscopy studies of interface formation between metal electrodes and luminescent organic materials

Using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), we have studied the formation of metal/organic interfaces in organic electroluminescent devices. Oligo(p-phenylenevinylenes) (OPV) and tris-(8-hydroxyquinoline)aluminum (Alq3) were used as the organic materials and Ca was used as the metallic layer. Interfaces are formed differently by depositing organic layer on Ca and Ca on organic substrate. For Ca/OPV, UPS revealed a clear evidence for interface state formation upon Ca deposition. The evolution of XPS core level peak as a function of Ca layer thickness was consistent with the energy level bending picture. The XPS and UPS spectra for OPV/Ca as a function of organic layer thickness also confirmed the energy level bending. The data obtained allowed us to deduce the energy level diagram near the interface. Similar data for Alq3/Ca indicated that no electron injection barrier exists at this interface if the Alq3 optical band gap in the literature was used for estimating the energy positi...

X-ray photoemission investigations of the interface formation of Ca and poly(p-phenylene vinylene)

We have investigated the interface formation of Ca with poly(p‐phenylene vinylene) (PPV) using x‐ray photoemission spectroscopy (XPS). The most astonishing result of the investigations is that the Schottky barrier formation in Ca/PPV is a slow process possibly caused by the oxygen and sulfur impurities segregated on the PPV surface.