T J Whitcher

Enhancement of the work function of indium tin oxide by surface modification using caesium fluoride

The work function of indium tin oxide (ITO) was modified using caesium fluoride (CsF). Various concentrations of CsF was spin-coated on top of ITO and baked while the residual CsF was washed away with DI water. The work function of all the ITO samples was measured using ultraviolet photoelectron spectroscopy and it was found that the work function of ITO reaches as high as 5.75 eV. The work function rapidly increases with small concentrations of CsF solution and then decreases for higher concentrations. Using atomic force microscopy and x-ray photoelectron spectroscopy, the cause was determined to be the change in surface roughness and the oxygen concentration, with the former having a much greater influence on the work function than the latter. The current density of ITO/poly(vinylcarbazole)/Al hole-only devices using the modified ITO increases by more than seven orders of magnitude compared with the control device.

Electrostatic model of the energy-bending within organic semiconductors: experiment and simulation.

UNLABELLED The interfacial properties between electrodes and the various organic layers that comprise an organic electronic device are of direct relevance in understanding charge injection, extraction and generation. The energy levels and energy-bending of three interfaces; indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate ( PEDOT PSS), ITO/poly(N-vinylcarbazole) (PVK) and PEDOT PSS/PVK were measured using ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). By decoupling the vacuum shift and energy-bending, the energy-bending at these interfaces can be simulated using an electrostatic model that takes into account the energetic disorder of the polymers. The model is further extended to include blended mixtures of semiconductors at differing concentrations and it was found that a very good agreement exists between the experiment and theory for all interfaces. This suggests that the electrostatic model can be used to describe energy-bending at the interface between any organic semiconductors. Further investigation into the effect of the Gaussian density of states width on energy-bending is warranted.

Interfacial behavior of resistive switching in ITO–PVK–Al WORM memory devices

Understanding the mechanism of resistive switching in a memory device is fundamental in order to improve device performance. The mechanism of current switching in a basic organic write-once read-many (WORM) memory device is investigated by determining the energy level alignments of indium tin oxide (ITO), poly(9-vinylcarbazole) (PVK) and aluminum (Al) using x-ray and ultraviolet photoelectron spectroscopy, current–voltage characterization and Auger depth profiling. The current switching mechanism was determined to be controlled by the interface between the ITO and the PVK. The electric field applied across the device causes the ITO from the uneven surface of the anode to form metallic filaments through the PVK, causing a shorting effect within the device leading to increased conduction. This was found to be independent of the PVK thickness, although the switch-on voltage was non-linearly dependent on the thickness. The formation of these filaments also caused the destruction of the interfacial dipole at the PVK–Al interface.

The efficiency enhancement of single-layer solution-processed blue phosphorescent organic light emitting diodes by hole injection layer modification

Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) PEDOT : PSS is extensively used as a hole injection layer (HIL) in solution-processed organic light emitting diodes (OLEDs). The high work function of a HIL is crucial in improving OLED efficiency. The work function of PEDOT : PSS is usually around 5.1–5.3 eV. By adding perfluorinated ionomer (PFI), the work function of PEDOT : PSS has been reported to reach as high as 5.95 eV. We investigated the effects of PFI-modified PEDOT : PSS in a single-layer solution-processed blue phosphorescent OLED (PHOLED). We observed that high concentrations of a PFI in PEDOT : PSS has detrimental effects on the device efficiency due to the low conductivity of the PFI. Using this approach, blue PHOLEDs with efficiencies of 9.4 lm W−1 (18.2 cd A−1) and 7.9 lm W−1 (20.4 cd A−1) at 100 cd m−2 and 1000 cd m−2, respectively, were demonstrated.

Determination of energy levels at the interface between O2 plasma treated ITO/P3HT?:?PCBM and PEDOT?:?PSS/P3HT?:?PCBM using angular-resolved x-ray and ultraviolet photoelectron spectroscopy

Interfacial energy alignments at the anode of solution processable organic photovoltaics are rarely studied. Here we use blended regio-regular poly(3-hexylthiophene) (P3HT) : phenyl-C61-butyric acid methyl ester (PCBM) deposited on top of O2 plasma cleaned indium tin oxide (ITO) and poly(3,4-ethylene-dioxythiophene) : polystyrene sulfonic acid (PEDOT : PSS) as a platform to obtain the interfacial energy alignment using angular-resolved x-ray and ultraviolet photoelectron spectroscopy. A strong downward vacuum level bending of 1.3 eV at the interface for plasma-ITO/P3HT : PCBM is observed. This results in an interfacial energetic barrier as high as 2.5 eV for holes and a reduction of barrier for electrons. This could be one of the contributing factors that result in lower device efficiency in O2 plasma-ITO/P3HT : PCBM compared to O2 plasma-ITO/PEDOT : PSS/P3HT : PCBM. The full interfacial energy diagram is determined for O2 plasma-ITO/P3HT : PCBM and PEDOT : PSS/P3HT : PCBM. Such methods can be extended to study various interfacial properties of solution processable organic semiconducting materials.

Highly efficient processable molybdenum trioxide as a hole blocking interlayer for super-yellow organic light emitting diode

By inserting lithium fluoride (LiF) between solution-processed MoO3 with optimal thickness on top of super yellow poly-(p-phenylenevinylene) (SY-PPV), the efficiency of the SY-PPV fluorescent-based devices can be significantly improved by more than two-fold. Despite the increased driving voltage, the device showed a current and a luminance efficiency up to 22.8 cd A−1 and 14.3 lm W−1 respectively, which is a more than a two-fold increase in efficiency compared to the control device using LiF/Al at a brightness of 1000 cdm−2. Ultraviolet photoelectron spectroscopy (UPS) is used to analyze the energy alignment between SY-PPV and the solution processed MoO3 and MoO3/LiF/Al interfaces. We found that the solution processed MoO3 using diluted sodium hydroxide has relatively low ionization energy (IA), electron affinity (EA) and work function decreasing with increasing thickness of MoO3. However, the optical bandgap increases with increasing spin-speed. A large energetic barrier is always present between the SY-PPY and deep lying valence band of MoO3. This is supported by suppression of hole current in hole dominating devices. The ability of thin MoO3 (~2 nm) acting as a hole blocking layer while allowing electrons to be transported across the layer and a large upward vacuum shift appeared to be the origin of efficiency enhancement of SY-PPV light-emitting diode when MoO3/LiF/Al is used.