Sebastian Reineke

White organic light-emitting diodes with fluorescent tube efficiency

The development of white organic light-emitting diodes (OLEDs) holds great promise for the production of highly efficient large-area light sources. High internal quantum efficiencies for the conversion of electrical energy to light have been realized. Nevertheless, the overall device power efficiencies are still considerably below the 60–70 lumens per watt of fluorescent tubes, which is the current benchmark for novel light sources. Although some reports about highly power-efficient white OLEDs exist, details about structure and the measurement conditions of these structures have not been fully disclosed: the highest power efficiency reported in the scientific literature is 44 lm W-1 (ref. 7). Here we report an improved OLED structure which reaches fluorescent tube efficiency. By combining a carefully chosen emitter layer with high-refractive-index substrates, and using a periodic outcoupling structure, we achieve a device power efficiency of 90 lm W-1 at 1,000 candelas per square metre. This efficiency has the potential to be raised to 124 lm W-1 if the light outcoupling can be further improved. Besides approaching internal quantum efficiency values of one, we have also focused on reducing energetic and ohmic losses that occur during electron–photon conversion. We anticipate that our results will be a starting point for further research, leading to white OLEDs having efficiencies beyond 100 lm W-1. This could make white-light OLEDs, with their soft area light and high colour-rendering qualities, the light sources of choice for the future.

External quantum efficiency above 100% in a singlet-exciton-fission-based organic photovoltaic cell.

Splitting Singlets Solar cell efficiency is limited because light at wavelengths shorter than the cells absorption threshold does not channel any of its excess energy into the generated electricity. Congreve et al. (p. 334) have developed a method to harvest the excess energy in carbon-based absorbers through a process termed “singlet fission.” In this process, high-energy photons propel two current carriers, rather than just one, by populating a singlet state that spontaneously divides into a pair of triplet states. Although it works in a functioning organic solar cell, the efficiency needs improving. Single photons are shown to propel more than one carrier in a carbon-based solar cell. Singlet exciton fission transforms a molecular singlet excited state into two triplet states, each with half the energy of the original singlet. In solar cells, it could potentially double the photocurrent from high-energy photons. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in a portion of the visible spectrum. Using a fullerene acceptor, a poly(3-hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, we show a peak external quantum efficiency of (109 ± 1)% at wavelength λ = 670 nanometers for a 15-nanometer-thick pentacene film. The corresponding internal quantum efficiency is (160 ± 10)%. Analysis of the magnetic field effect on photocurrent suggests that the triplet yield approaches 200% for pentacene films thicker than 5 nanometers.

Selective Turn-On Ammonia Sensing Enabled by High-Temperature Fluorescence in Metal–Organic Frameworks with Open Metal Sites

We show that fluorescent molecules incorporated as ligands in rigid, porous metal-organic frameworks (MOFs) maintain their fluorescence response to a much higher temperature than in molecular crystals. The remarkable high-temperature ligand-based fluorescence, demonstrated here with tetraphenylethylene- and dihydroxyterephthalate-based linkers, is essential for enabling selective and rapid detection of analytes in the gas phase. Both Zn2(TCPE) (TCPE = tetrakis(4-carboxyphenyl)ethylene) and Mg(H2DHBDC) (H2DHBDC(2-) = 2,5-dihydroxybenzene-1,4-dicarboxylate) function as selective sensors for ammonia at 100 °C, although neither shows NH3 selectivity at room temperature. Variable-temperature diffuse-reflectance infrared spectroscopy, fluorescence spectroscopy, and X-ray crystallography are coupled with density-functional calculations to interrogate the temperature-dependent guest-framework interactions and the preferential analyte binding in each material. These results describe a heretofore unrecognized, yet potentially general property of many rigid, fluorescent MOFs and portend new applications for these materials in selective sensors, with selectivity profiles that can be tuned as a function of temperature.

Influence of charge balance and exciton distribution on efficiency and lifetime of phosphorescent organic light-emitting devices

We discuss the importance of appropriate charge carrier confinement and exciton management for the realization of highly efficient and stable organic light-emitting diodes (OLEDs). As an example, we choose red p-i-n-type OLEDs based on the iridium-based electrophosphorescent emitter Ir(MDQ)2(acac) doped in α-NPD as host material. We show how an appropriate choice of the hole blocking layer material allows external quantum efficiencies as high as 20% for this emitter. At the same time, the display-relevant brightness of 100 cd/m2 is reached at an operation voltage of only 2.4 V, which is close to the thermodynamic limit. As a result, a high total power efficiency of 37.5 lm/W at 100 cd/m2 is reached. In a further step, we study the influence of the blocker materials on device lifetime. We investigate the chemical reactions causing the degradation process by use of matrix assisted laser desorption time-of-flight mass spectrometry. It can be shown that discovered degradation reactions can be suppressed by an...

Reduced efficiency roll-off in high-efficiency hybrid white organic light-emitting diodes

White organic light emitting diodes harvesting triplet excitons from the fluorescent blue emitter N,N′-di-1-naphthalenyl-N,N′-diphenyl-[1,1′:4′,1″:4″,1‴-quaterphenyl]-4,4‴-diamine (4P-NPD) are presented. Direct doping of the phosphorescent orange iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) into 4P-NPD results in a strongly reduced efficiency roll-off as compared to separate emission layers, and yields 49.3lmW−1 total external power efficiency (24.1% quantum efficiency) at a luminance of 1000cdm−2 [CIE 1931 chromaticity coordinates (0.49,0.41)], measured in an integrating sphere. Introduction of an exciton balancing interlayer improves the chromaticity (0.43,0.43) toward the CIE illuminant A warm white point and keeps a high efficiency of 40.7lmW−1, 20.3%.

Reduced efficiency roll-off in phosphorescent organic light emitting diodes by suppression of triplet-triplet annihilation

The authors investigate phosphorescent organic light emitting diodes comprising mixed films of fac tris(2-phenylpyridine) iridium dispersed in 4,4′,4′-tris(N-carbazolyl)-triphenylamine (TCTA) as emission layer (EML). Based on the results of photoluminescence experiments, they intermit the EML with thin neat layers of TCTA acting as an exciton blocking layer inside the EML which suppresses triplet-triplet annihilation. They show that this EML structure leads to an improved roll-off behavior: Starting at the initial external quantum efficiencies (IEQE) of 15.8% and 14.4% at low brightness for the reference and interlayer device, respectively, those structures yield critical current densities jc of 140mA∕cm2 and 270mA∕cm2 defining the current density of half-value IEQE.

Room temperature triplet state spectroscopy of organic semiconductors

Organic light-emitting devices and solar cells are devices that create, manipulate, and convert excited states in organic semiconductors. It is crucial to characterize these excited states, or excitons, to optimize device performance in applications like displays and solar energy harvesting. This is complicated if the excited state is a triplet because the electronic transition is ‘dark’ with a vanishing oscillator strength. As a consequence, triplet state spectroscopy must usually be performed at cryogenic temperatures to reduce competition from non-radiative rates. Here, we control non-radiative rates by engineering a solid-state host matrix containing the target molecule, allowing the observation of phosphorescence at room temperature and alleviating constraints of cryogenic experiments. We test these techniques on a wide range of materials with functionalities spanning multi-exciton generation (singlet exciton fission), organic light emitting device host materials, and thermally activated delayed fluorescence type emitters. Control of non-radiative modes in the matrix surrounding a target molecule may also have broader applications in light-emitting and photovoltaic devices.

Highly efficient white organic light-emitting diodes based on fluorescent blue emitters

Beside inorganic LEDs and fluorescent lamps, organic light-emitting diodes (OLEDs) are evolving into a serious alternative to incandescent lamps. Up to now, it was assumed that all-phosphorescent OLEDs are required for reaching sufficiently high efficiencies. However, the stability of phosphorescent blue emitters is a major challenge. We present a novel approach to achieve highly efficient (up to 90 lm/W at 1000 cd/m2 using a macroextractor) white light emission from OLEDs. The here presented combination of a fluorescent blue and a phosphorescent red emitter simultaneously allows for a strong blue emission and efficient triplet transfer to the phosphor. The spectrum is extended in the green and yellow region by a full phosphorescent unit stacked on top of the triplet harvesting device. This superposition of four different emitters results in color coordinates close to illuminant A and a color rendering index of 80. Furthermore, color stability is given with respect to varying driving conditions and estima...

Highly phosphorescent organic mixed films: The effect of aggregation on triplet-triplet annihilation

The efficiency roll-off at high brightness levels is a key factor limiting the application of organic light emitting diodes. We investigate triplet-triplet annihilation in an archetype phosphorescent host-guest system. We show that the currently used host-guest systems are not at the physical limit set by intrinsic annihilation, but have an increased roll-off due to aggregate formation. The existence of these aggregates is directly proven by transmission electron microscopy.

Recent advances in light outcoupling from white organic light-emitting diodes

Abstract. Organic light-emitting diodes (OLEDs) have been successfully introduced to the smartphone display market and have geared up to become contenders for applications in general illumination where they promise to combine efficient generation of white light with excellent color quality, glare-free illumination, and highly attractive designs. Device efficiency is the key requirement for such white OLEDs, not only from a sustainability perspective, but also because at the high brightness required for general illumination, losses lead to heating and may, thus, cause rapid device degradation. The efficiency of white OLEDs increased tremendously over the past two decades, and internal charge-to-photon conversion can now be achieved at ∼100% yield. However, the extraction of photons remains rather inefficient (typically <30%). Here, we provide an introduction to the underlying physics of outcoupling in white OLEDs and review recent progress toward making light extraction more efficient. We describe how structures that scatter, refract, or diffract light can be attached to the outside of white OLEDs (external outcoupling) or can be integrated close to the active layers of the device (internal outcoupling). Moreover, the prospects of using top-emitting metal–metal microcavity designs for white OLEDs and of tuning the average orientation of the emissive molecules within the OLED are discussed.