Rusli Daik

Indium–tin oxide treatments for single- and double-layer polymeric light-emitting diodes: The relation between the anode physical, chemical, and morphological properties and the device performance

We report combined studies of the influence of chemical and physical treatments on the properties of indium–tin oxide (ITO) thin films. The ITO films were also used as transparent anodes of polymeric light-emitting diodes (LEDs) incorporating poly(p-phenylene vinylene) (PPV) as the emitter material, with, or without, doped poly(3,4-ethylene dioxythiophene) (PEDOT) as a hole-injection/transport layer. Structures based on a soluble green derivative of PPV, poly(4,4′-diphenylene diphenylvinylene) were also tested. We studied chemical (aquaregia, degreasing, RCA protocol) and physical (oxygen and argon plasmas, Teflon, and paper rubbing) treatments and, in contrast to recently published work, we find that for Balzer Baltracon ITO, oxygen plasma and not aquaregia yields the highest efficiencies and luminances and the lowest drive voltages. For oxygen-plasma-treated anodes, the device efficiency clearly correlates with the value of the ITO surface work function, which in turn depends on the time of treatment. I...

BUILT-IN FIELD ELECTROABSORPTION SPECTROSCOPY OF POLYMER LIGHT-EMITTING DIODES INCORPORATING A DOPED POLY(3,4-ETHYLENE DIOXYTHIOPHENE) HOLE INJECTION LAYER

We report electroabsorption measurements of polymer light-emitting diodes, (LEDs), fabricated with poly(4-4′-diphenylene diphenylvinylene), PDPV, as the emissive layer, Ca–Al cathodes, and indium tin oxide (ITO) anodes, with and without a doped conducting polymer hole injection/transport layer, namely poly(3,4-ethylene dioxythiophene), PEDOT, doped with poly(styrene sulfonate), PSS−. In these structures, the bias at which the electroabsorption signal is null corresponds to the difference between the electrodes’ work functions. We find that such a built-in voltage increases by 0.5 V when a PEDOT:PSS film is incorporated between the ITO electrode and the emissive layer. This leads to a marked reduction of the anode barrier height at the hole-injecting interface, and accounts for a variety of improvements brought about by the PEDOT insertion, namely: (a) the increase of luminescence efficiency, (b) the reduction of the turn-on voltage, and (c) the increase of the device lifetime.

Characterisation of the properties of surface-treated indium-tin oxide thin films

Abstract We studied the surface properties of indium-tin oxide (ITO) modified by wet (aquaregia, ultrasonication, RCA) and dry (oxygen and argon plasmas) treatments. The influence of surface treatment on ITO was investigated by surface energy, surface roughness, sheet resistance, and workfunction measurements. We report that oxygen plasma treatment induces: (1) the highest surface energy with the highest polarity, (2) the smoothest surface, (3) the lowest sheet resistance, and (4) the highest workfunction. We also observe that the PL efficiencies of PPV, PDPV and PEDOT/PPV on ITO are affected by the surface treatment of ITO.

Efficient green light‐emitting diodes from a phenylated derivative of poly(p‐phenylene–vinylene)

We have used polyvinylcarbazole, PVK, in combination with poly(p‐phenylene vinylene), PPV and a green‐emitting, soluble derivative, poly(4,4′‐diphenylene diphenylvinylene) or briefly PDPV, to fabricate triple‐layer light‐emitting diodes PPV/PVK/PDPV/metal, with indium tin oxide as the anode. We report luminances in excess of 1300 cd/m2 with Al cathodes, turn‐on voltages of ∼8 V in structures 300 nm thick, and internal efficiencies in the range 0.4%–0.8% for Al and Ca cathodes; at least ten times higher than for single‐layer PDPV devices. Quantum efficiencies for Ca or Al electrodes are only a factor 1.5 different. The improvements over single‐layer PDPV devices are considered to be due to better hole injection by PPV and to the formation of a hole barrier at the PPV/PVK interface, which forces a better balance of the electron and hole currents.

A review of organic small molecule-based hole-transporting materials for meso-structured organic-inorganic perovskite solar cells

This review summarizes the current designs and development of new types of organic small molecules as a hole-transporting material (HTM) in a meso-structured perovskite solar cell (PSC). The roles of each layer in the meso-structured perovskite device architecture are elaborated and the employment of new types of organic HTMs in the device is compared with the commercially available HTM spiro-OMeTAD in terms of the properties, device performance and stability. The studies found that nearly half of the new synthesized and pristine HTMs have comparable or better photovoltaic properties than those of doped spiro-OMeTAD. These HTMs have the characteristics of a fused planar core structure with extended π-conjugated lengths and electron-donating functional groups, which are believed to contribute to their high intrinsic conductivity and help make them an alternative to spiro-OMeTAD as a better HTM in meso-structured PSCs. Some of the devices based on the new synthesized HTMs even have longer device lifetimes than their spiro-OMeTAD-based PSC counterparts. Moreover, studies found that the cost per gram (Cg) and cost-per-peak Watt (Cw) of synthesized HTMs can be reduced via minimizing the number of synthesis steps and by optimization of the starting materials in order to yield low-cost HTMs for meso-structured PSC applications.

Surface and bulk phenomena in conjugated polymers devices

Abstract We report a number of investigations into a variety of surface and bulk phenomena which are particularly relevant to the design of polymer optoelectronic devices, and in particular to light-emitting diodes (LEDs). First, we consider the surface properties of the most common anode, a thin layer of indium tin oxide (ITO), as modified by chemical and physical treatments. We show how the treatments affect the electrode workfunction, surface energy, morphology, sheet resistance and therefore, the LEDs efficiency and lifetime. We will also consider the effect of a conducting polymer hole-transport layer, such as poly(styrene sulfonate) doped poly(3,4-ethylene dioxythiophene) (PEDOT), incorporated between the ITO and the active layer. Secondly, we turn our attention to the optimisation of the photoluminescence (PL) efficiency, and report examples of how the polymer microstructure and possibly the engineering of disorder in a class of poly( p -phenylene vinylene)s and poly- or oligo-thiophenes can determine a significant increase of the PL efficiencies in the solid state.

Enhanced photostability of poly(1,3-phenylene diphenylvinylene)-derivatives by diphenyl-substitution

Abstract We report measurements of the UV-photodegradation of the poly( para -phenylene vinylene) derivatives poly-(4,4′-diphenylene diphenylvinylene) (PDPV) and poly-(1,3-phenylene diphenylvinylene) m -PPV-DP in the presence of oxygen. We find that degradation of both PDPV and m -PPV-DP is slower than that of PPV, and attribute this to the replacement of the vinylene hydrogens by phenyl rings. The photoluminescence decay and the photoluminescence excitation spectra were measured as a function of UV-irradiation time. Initially a reversible luminescence quenching process was observed for films of either PDPV or m -PPV-DP in moist air in the presence of oxygen and/or water. We propose that this arises from conformational changes of the polymer chain produced following optical excitation. At longer irradiation times, FTIR studies reveal that the degradation process is oxidative. This oxidation is rate limited by oxygen diffusion into the film. Models for the photoluminescence properties are presented.

Optical probing of sample heating in scanning near-field experiments with apertured probes

We have used the inherent thermochromism of conjugated polymers to investigate substrate heating effects in scanning near-field experiments with metal-coated “apertured” probes. Chemically etched and pulled fibers were used to provide near-field excitation of fully converted films of poly(p-phenylene vinylene), PPV, and of poly(4,4′-diphenylene diphenylvinylene). We detect no significant blueshift of the photoluminescence spectra generated with near-field excitation, in comparison to those collected with far-field excitation. We conclude that polymer heating in the region contributing to the luminescence is less than 40K. We also demonstrate that thermolithography of the PPV precursor is not significant by comparing UV (325nm) and red (670nm) illumination.

Effect of poly(3,4-ethylene dioxythiophene) on the built-in field in polymer light-emitting diodes probed by electroabsorption spectroscopy

Abstract Here we report electroabsorption (EA) measurements on light-emitting diodes (LEDs), fabricated with poly(4-4′-diphenylene diphenylvinylene) (PDPV) as the emissive layer in indium–tin oxide (ITO)/poly(3,4-ethylene dioxythiophene) (PEDOT):polystyrene sulfonic acid (PSS)/PDPV/Ca–Al and ITO/PDPV/Ca–Al structures. In the latter structure, the built-in potential, determined from nulling the EA signal, corresponds to the difference between the work functions of the electrodes. By incorporating a PEDOT:PSS film between the ITO electrode and the emissive layer we find that such a built-in voltage increases by 0.5 V. The correspondent lowering of the anodic barrier height at the PDPV interface is likely to be responsible for the improvement in device performance.

Electroluminescence lifetime and efficiency of polymer LEDs with surface-treated anodes

Abstract We studied the influence of wet (aquaregia, ultrasonication, RCA) and dry (oxygen- and argon-plasmas) treatments of ITO anodes on single- and double-layer LEDs based on PPV or PDPV, in terms of the electroluminescence (EL) lifetimes, efficiencies (Cd/A and Im/W), luminances and driving voltages. Oxygen plasma treatment yields the highest efficiencies, luminances, and the lowest driving voltages. We measured the lifetimes (time to half luminance at constant V) of ITO/PPV/Ca devices, all driven with an initial current density of 10 mA/cm 2 . Oxygen plasma treatment increases significantly the EL lifetime by an order of magnitude with respect to the untreated ITO, and removes the occasional occurrence of low-voltage current anomalies.