Ruth Shinar

High-Efficiency Solution-Processed Small Molecule Electrophosphorescent Organic Light-Emitting Diodes

Extensive research on organic light-emitting diodes (OLEDs) continues due to their promise in applications such as fl at panel displays and solid state lighting. [ 1–5 ] Commonly, thermal high-vacuum evaporation technology is used for fabrication of small molecule-based OLEDs (SMOLEDs) and solution processing technology is used for those based on polymers (PLEDs). Thermal evaporation deposition enables complicated multilayer device architectures and renders excellent devices with high effi ciencies. [ 6 , 7 ] In contrast, solution-based deposition limits fabrication of composite device structures because the solvent used for one layer can redissolve or otherwise damage the previous layers. [ 8 ] Therefore, thermally evaporated SMOLEDs are typically more effi cient and longer-lived than solution-processed PLEDs. However, thermal evaporation deposition has its own disadvantages. First, it requires high vacuum and is consequently much more costly. Second, making multidopant OLEDs, such as white OLEDs (WOLEDs), requires precise control of the doping concentration of each dopant in the emitting layer (EML) to obtain the desired emission. [ 9 , 10 ] These reasons usually lead to a fabrication process of greater complexity and higher cost. On the other hand, solution processing, such as spin-coating, inkjet printing, and screen printing, is advantageous over thermal evaporation processing, due to its low-cost and large area manufacturability. [ 10 , 11 ] Additionally, it is possible to realize co-doping of several dopants by mixing the dopants and host material in solution. Hence, the fabrication of SMOLEDs via a solution process is of great importance. To that end, we demonstrate high effi ciency (forward power and luminous effi ciencies up to 60 lm W − 1 and 69 Cd A − 1 , respectively) spin-coated electrophosphorescent SMOLEDs based on greenemitting tris[2-(p-tolyl)pyridine] iridium(III) (Ir(mppy) 3 ) doped into a 4,4’-bis(9-carbazolyl)-biphenyl (CBP) host, probably due to the materials and fi lm morphology. This is the highest reported effi ciency of any solution-processed OLED and among the highest of any OLED without outcoupling enhancement. The

Extremely Efficient Indium–Tin-Oxide-Free Green Phosphorescent Organic Light-Emitting Diodes

This paper demonstrates extremely efficient (η(P,max) = 118 lm W(-1) ) ITO-free green phosphorescent OLEDs (PHOLEDs) with multilayered, highly conductive poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) films as the anode. The efficiency is obtained without any outcoupling-enhancing structures and is 44% higher than the 82 lm W(-1) of similar optimized ITO-anode PHOLEDs. Detailed simulations show that this improvement is due largely to the intrinsically enhanced outcoupling that results from a weak microcavity effect.

Microporous phase-separated films of polymer blends for enhanced outcoupling of light from OLEDs

Thin microporous films were formed by dropcasting a toluene solution containing various ratios of polystyrene:polyethylene glycol blends on a glass substrate, with OLEDs on the ITO that coated the opposite side of that substrate. We demonstrate for the first time that such easily-fabricated films with surface and bulk micropores in the index-matching polystyrene can serve as random microlens-like arrays to improve forward OLED light extraction by up to ~60%. A theoretical interpretation of the angular emission profile of the device, considering the geometrical change at the substrate/air interface and the scattering by the pores within the films, was established in excellent agreement with the experiments. The use of such blended thin films provides an economical method, independent of the OLED fabrication technique, for improving the outcoupling efficiency.

Multiple approaches for enhancing all-organic electronics photoluminescent sensors: Simultaneous oxygen and pH monitoring

Key issues in using organic light emitting diodes (OLEDs) as excitation sources in structurally integrated photoluminescence (PL)-based sensors are the low forward light outcoupling, the OLEDs broad electroluminescence (EL) bands, and the long-lived remnant EL that follows an EL pulse. The outcoupling issue limits the detection sensitivity (S) as only ~20% of the light generated within standard OLEDs can be forward outcoupled and used for sensor probe excitation. The EL broad band interferes with the analyte-sensitive PL, leading to a background that reduces S and dynamic range. In particular, these issues hinder designing compact sensors, potentially miniaturizable, that are devoid of optical filters and couplers. We address these shortcomings by introducing easy-to-employ multiple approaches for outcoupling improvement, PL enhancement, and background EL reduction leading to novel, compact all-organic device architectures demonstrated for simultaneous monitoring of oxygen and pH. The sensor comprises simply-fabricated, directionally-emitting, narrower-band, multicolor microcavity OLED excitation and small molecule- and polymer-based organic photodetectors (OPDs) with a more selective spectral response. Additionally, S and PL intensity for oxygen are enhanced by using polystyrene (PS):polyethylene glycol (PEG) blends as the sensing film matrix. By utilizing higher molecular weight PS, the ratio τ0/τ100 (PL decay time τ at 0% O2/τ at 100% O2) that is often used to express S increases ×1.9 to 20.7 relative to the lower molecular weight PS, where this ratio is 11.0. This increase reduces to ×1.7 when the PEG is added (τ0/τ100=18.2), but the latter results in an increase ×2.7 in the PL intensity. The sensors response time is <10s in all cases. The microporous structure of these blended films, with PEG decorating PS pores, serves a dual purpose. It results in light scattering that reduces the EL that is waveguided in the substrate of the OLEDs and consequently enhances light outcoupling from the OLEDs by ~60%, and it increases the PL directed toward the OPD. The multiple functional structures of multicolor microcavity OLED pixels/microporous scattering films/OPDs enable generation of enhanced individually addressable sensor arrays, devoid of interfering issues, for O2 and pH as well as for other analytes and biochemical parameters.

Transient electroluminescence dynamics in small molecular organic light-emitting diodes

Intriguing electroluminescence (EL) spikes, following a voltage pulse applied to small molecular OLEDs, are discussed, elucidating carrier and exciton quenching dynamics and their relation to device structure. At low temperatures, all devices exhibit spikes at ∼70–300u2002ns and μs-long tails. At 295 K only those with a hole injection barrier, carrier-trapping guest-host emitting layer, and no strong hole-blocking layer exhibit the spikes. They narrow and appear earlier under post-pulse reverse bias. The spikes and tails are in agreement with a revised model of recombination of correlated charge pairs (CCPs) and initially unpaired charges. Decreased post-pulse field-induced dissociative quenching of singlet excitons and CCPs, and possibly increased post-pulse current of holes that “turn back” toward the recombination zone after having drifted beyond it are suspected to cause the spikes’ amplitude, which exceeds the dc EL.

Oxygen and relative humidity monitoring with films tailored for enhanced photoluminescence

Approaches to generate porous or doped sensing films, which significantly enhance the photoluminescence (PL) of oxygen optical sensors, and thus improve the signal-to-noise (S/N) ratio, are presented. Tailored films, which enable monitoring the relative humidity (RH) as well, are also presented. Effective porous structures, in which the O2-sensitive dye Pt octaethylporphyrin (PtOEP) or the Pd analog PdOEP was embedded, were realized by first generating blend films of polyethylene glycol (PEG) with polystyrene (PS) or with ethyl cellulose (EC), and then immersing the dried films in water to remove the water-soluble PEG. This approach creates pores (voids) in the sensing films. The dielectric contrast between the films constituents and the voids increases photon scattering, which in turn increases the optical path of the excitation light within the film, and hence light absorption by the dye, and its PL. Optimized sensing films with a PEG:PS ratio of 1:4 (PEGs molecular weight Mw ∼8000) led to ∼4.4× enhancement in the PL (in comparison to PS films). Lower Mw ∼200 PEG with a PEG:EC ratio of 1:1 led to a PL enhancement of ∼4.7×. Film-dependent PL enhancements were observed at all oxygen concentrations. The strong PL enhancement enables (i) using lower dye (luminophore) concentrations, (ii) reducing power consumption and enhancing the sensors operational lifetime when using organic light emitting diodes (OLEDs) as excitation sources, (iii) improving performance when using compact photodetectors with no internal gain, and (iv) reliably extending the dynamic range. The effect of RH on O2 sensing is also presented. Dye:EC films are sensitive to the RH, as shown by the change of the dyes PL decay time with RH at a given O2 concentration. Surprisingly, this RH sensitivity vanishes by adding PEG to EC, including by washing PEG off. In contrast, doping EC with TiO2 nanoparticles maintains the RH effect with the advantage of significant PL enhancement. This enhancement enables differentiation of <10% changes in the RH, which is unattained with the dye:EC sensing films. The results are discussed in terms of the composition, thickness, and microstructure, whether porous or nanoparticle doped, of the composite films.