Electroluminescence Studies Research Articles

Optimizing charge transport in hybrid GaN-PEDOT:PSS/PMMADevice for advanced application

Organic–inorganic hybrid light-emitting devices have garnered significant attention in the last few years due to their potential. These devices integrate the superior electron mobility of inorganic semiconductors with the remarkable optoelectronic characteristics of organic semiconductors. The inquiry focused on analyzing the optical and electrical properties of a light-emitting heterojunction that combines p-type GaN with organic materials (PEDOT, PSS, and PMMA). This heterojunction is an organic–inorganic hybrid. The procedure entailed utilizing a spin-coating technique to apply a layer of either poly(methyl methacrylate) (PMMA) or a mixture of PMMA and poly(3,4ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT: PSS) onto an indium tin oxide (ITO) substrate. Subsequently, different Nd:YAG laser pulses (200, 250, and 300 pulses) were used to administer a GaN inorganic layer onto the prepared organic layer using a pulsed laser deposition approach. Subsequently, the thermal evaporation technique was employed to deposit an aluminum electrode on the top of the organic and inorganic layers, while laser pulses were fine-tuned for optimal performance. The Hall effect investigation verifies the p-type conductivity of the GaN material. The electroluminescence studies confirmed the production of blue light by the GaN-based devices throughout a range of voltage situations, spanning from 45 to 72 V.

A Broadened Class of Donor-Acceptor Stacked Macrometallacyclic Adducts of Different Coinage Metals.

A yet-outstanding supramolecular chemistry challenge is isolation of novel varieties of stacked complexes with finely-tuned donor-acceptor bonding and optoelectronic properties, as herein reported for binary adducts comprising two different cyclic trinuclear complexes (CTC@CTC'). Most previous attempts focused only on 1-2 factors among metal/ligand/substituent combinations, resulting in heterobimetallic complexes. Instead, here we show that, when all 3 factors are carefully considered, a broadened variety of CTC@CTC' stacked pairs with intuitively-enhanced intertrimer coordinate-covalent bonding strength and ligand-ligand/metal-ligand dispersion are attained (dM-M' 2.868(2) Å; ΔE>50 kcal/mol, an order of magnitude higher than aurophilic/metallophilic interactions). Significantly, CTC@CTC' pairs remain intact/strongly-bound even in solution (Keq 4.67×105 L/mol via NMR/UV-vis titrations), and the gas phase (mass spectrometry revealing molecular peaks for the entire CTC@CTC' units in sublimed samples), rather than simple co-crystal formation. Photo-/electro-luminescence studies unravel metal-centered phosphorescence useful for novel all metal-organic light-emitting diodes (MOLEDs) optoelectronic device concepts. This work manifests systematic design of supramolecular bonding and multi-faceted spectral properties of pure metal-organic macrometallacyclic donor/acceptor (inorganic/inorganic) stacks with remarkably-rich optoelectronic properties akin to well-established organic/organic and organic/inorganic analogues.

Size‐Dependent Characteristics of InGaN‐Based Blue and Green Micro‐Light‐Emitting Diodes

This study focuses on investigating the size‐dependent characteristics (chip diameter: 100–10 μm) of InGaN‐based blue and green micro‐light‐emitting diodes (μLEDs) with respect to their electrical and optical properties. The LED's epistructure is grown using the metal–organic chemical vapor deposition technique, preceded by simulation and optimization using SiLENSe LED simulator. The cross‐sectional transmission electron microscopy study confirms the epistructure. The Photoluminescence mapping of the epistructure reveals that the blue emission is at 461 nm (full width at half maximum [FWHM] ≈ 15.5 nm) and the green emission is at 510 nm (FWHM ≈ 17.5 nm). The study analyzes the size‐dependent external quantum efficiency (EQE), optical power, and peak wavelength shift between blue and green μLEDs. The electroluminescence (EL) study reveals that the redshift of the EL peak wavelength of μLEDs is attributed to the release of strain at reduced chip sizes. The size‐dependent EQE study shows a reduction of EQE with shrunken chip size, attributed to the increase of nonradiative Shockley–Read–Hall recombination. Finally, the study estimates the characteristics temperature of the μLEDs using temperature‐dependent EL measurement, revealing that blue μLEDs exhibit a significantly large value of characteristic temperature ≈1926 K.

Identifying the origin of delayed electroluminescence in a polariton organic light-emitting diode.

Modifying the energy landscape of existing molecular emitters is an attractive challenge with favourable outcomes in chemistry and organic optoelectronic research. It has recently been explored through strong light-matter coupling studies where the organic emitters were placed in an optical cavity. Nonetheless, a debate revolves around whether the observed change in the material properties represents novel coupled system dynamics or the unmasking of pre-existing material properties induced by light-matter interactions. Here, for the first time, we examined the effect of strong coupling in polariton organic light-emitting diodes via time-resolved electroluminescence studies. We accompanied our experimental analysis with theoretical fits using a model of coupled rate equations accounting for all major mechanisms that can result in delayed electroluminescence in organic emitters. We found that in our devices the delayed electroluminescence was dominated by emission from trapped charges and this mechanism remained unmodified in the presence of strong coupling.

Ultraviolet-A electroluminescence from polymorphic CaSiO3:Ce3+ in electrolyte-thin film semiconductor

The realization of ultraviolet (UV) radiation lighting is emerging as a crucial method for water disinfection and particular fine matters. In particular, using UV sources as a small-scale disinfection option for water purification in remote areas is a growing trend. Herein, the study of UV electrolyte-assisted electroluminescence has been presetting by using Ce3+-activated polymorphic CaSiO3 as an emitting layer. It is prepared through an all-air atmosphere processes sol-gel-based spin coating method followed by a high-temperature rapid thermal annealing. Under an AC source in an aqueous electrolyte, the film is capable of emitting a UV-A spectrum attributed to the spin-allowed transition of Ce3+ ions. According to the spectrum, this device exhibits identical emission spectra in voltage and frequency dependency in AC sinusoidal driving voltage. With such a new method to obtain UV sources, this study could provide further development in silicon semiconductors, especially in the optoelectronics matter.

Forthcoming hyperfluorescence display technology: relevant factors to achieve high-performance stable organic light emitting diodes.

Over the decade, there have been developments in purely organic thermally activated delayed fluorescent (TADF) materials for organic light-emitting diodes (OLEDs). However, achieving narrow full width at half maximum (FWHM) and high external quantum efficiency (EQE) is crucial for real display industries. To overcome these hurdles, hyperfluorescence (HF) technology was proposed for next-generation OLEDs. In this technology, the TADF material was considered a sensitizing host, the so-called TADF sensitized host (TSH), for use of triplet excitons via the reverse intersystem crossing (RISC) pathway. Since most of the TADF materials show bipolar characteristics, electrically generated singlet and triplet exciton energies can be transported to the final fluorescent emitter (FE) through Förster resonance energy transfer (FRET) rather than Dexter energy transfer (DET). This mechanism is possible from the S1 state of the TSH to the S1 state of the final fluorescent dopant (FD) as a long-range energy transfer. Considering this, some reports are available based on hyperfluorescence OLEDs, but the detailed analysis for highly efficient and stable devices for commercialization was unclear. So herein, we reviewed the relevant factors based on recent advancements to build a highly efficient and stable hyperfluorescence system. The factors include an energy transfer mechanism based on spectral overlapping, TSH requirements, electroluminescence study based on exciplex and polarity system, shielding effect, DET suppression, and FD orientation. Furthermore, the outlook and future positives with new directions were discussed to build high-performance OLEDs.

Probing charge carrier and triplet dynamics in TADF-based OLEDs using transient electroluminescence studies

Thermally activated delayed fluorescence (TADF) systems exhibit high emissive yield due to efficient back-conversion of nonemissive triplet states to emissive singlet states via reverse intersystem crossing (RISC). In this paper, both the charge carrier and triplet exciton dynamics are explored using transient electroluminescence (TrEL) measurements in the TADF molecule, 2,3,4,6-Tetra(9H-carbazol-9-yl)-5-fluorobenzonitrile (4CzFCN)-based devices. The analysis of the rising edge of the TrEL pulse indicates that the carriers follow multiple trapping, de-trapping, and exciton recombination dynamics. The trailing edge of the TrEL pulse provides insight into the monomolecular and bimolecular exciton dynamics. These studies along with a kinetic model reveal triplet harvesting processes in a 4CzFCN molecule via both RISC and triplet–triplet annihilation (TTA). Furthermore, at high temperatures, the analysis suggests that TADF processes are dominant with negligible contribution from TTA. The presence of bimolecular triplet processes acts as bottlenecks for accessing higher efficiencies in TADF organic light emitting diodes.

Sensitivity of the hidden TADF to the linking topology of di-tert-butyl-carbazolyl and benzonitrile moieties in the molecules of emitters or hosts intended for efficient blue OLEDs

With the aim of detecting hidden thermally activated delayed fluorescence (TADF), six organic compounds were synthesized and characterized as emitters and hosts of blue organic light-emitting diodes (OLEDs). The effect of the linking topology of di-tert-butyl-carbazolyl and benzonitrile moieties in donor–acceptor-donor type molecules on device efficiencies was investigated. Just by altering the linking position of di-tert-butyl-carbazolyl moieties at benzonitrile fragment of the emitters and hosts of the blue TADF OLEDs, their maximum external quantum efficiencies (EQEs) can be changed from 1.13 to 5.17 % as well as from 13.8 to the record-high 22.8%. After the comprehensive investigations of the properties including the charge mobility measurements by time-of-flight technique, steady-state and time-resolved photoluminescence and electroluminescence studies, good correlation between EQE of OLEDs and the properties of the isomers with the differently linked donor and acceptor moieties were observed.

Star-shaped Blue Emitting Truxene-based Discotic Liquid Crystalline Molecules: Synthesis, Self-assembly, and Electroluminescence Studies

Two new C3 symmetrical star-shaped truxene-based discotic liquid crystals have been designed to study their optoelectronic properties. Both the truxene derivatives exhibited columnar hexagonal self-assembly, confirmed by a thorough analysis using POM, DSC, and XRD. A multilayered OLED device was structured using these mesogens as blue emitters for electroluminescent studies. The best-performing configuration was perceived for devices based on emitter 1b at a dopant concentration of 1 wt% in a well-aligned CBP host, giving a maximum EQE of 2.9%.

SiV Centers Electroluminescence in Diamond Merged Diode

The results of a study of electroluminescence of silicon vacancy color centers (SiV centers) in a diamond merged diode with a structure involving parallel connection of a p–i–n diode and a Schottky diode are presented. To create color centers, the inner region of the diode is doped with silicon. In the luminescence spectrum, only a line at a wavelength of 738 nm is detected, corresponding to the emission of a SiV color center in a negative charge state. Emission at a wavelength of 946 nm, corresponding to the SiV color center in the neutral charge state, is not detected. The electroluminescence of color centers is observed only in the p–i–n region of the diode, that is, it is experimentally demonstrated that both types of charge carriers are necessary to excite electroluminescence. A pronounced dependence of the photoluminescence intensity of SiV centers on the applied voltage is found.

Luminescence study of ZnS/PVK nanocomposites

Sulphide-based luminescence materials have attracted more curiosity for their extensive applications in electro and photoluminescence devices. One of the sulphides, such as zinc sulphide (ZnS), is a favourable material for phosphor growth in various bands for visible emission. The composition of the polymer matrix with ZnS demonstrates its special functions and stabilizes the nanoparticles. We have used chemical methods for the preparation of ZnS/PVK nanocomposites and reported their synthesis, photoluminescence (PL), and electroluminescence (EL) studies. The samples were analyzed through SEM (Scanning Electron Microscope), AFM (Atomic Force Microscope), and XRD (X-ray Diffraction). The XRD study shows the formation of ZnS nanocrystals with a cubic crystal structure, having a size between 3 and 12 nm and the broadening of peaks shows the formation of an amorphous compound. The AFM and SEM show clusters of particles in the range of a few tens to a few hundred nm. The results from all three studies show an increase in the size of particles with the ZnS loading in PVK (poly N-vinylcarbazole). The PL of a PVK sample excited at 400 nm shows a single PL peak at 451 nm and covers the spectral range from 400 to 600 nm. ZnS/PVK samples excited by 370 nm, give a PL peak due to PVK near 450 nm and when excited by 425 nm, the PL peak is found near 470 nm due to defect-related transitions in ZnS. In both cases, PL intensity increases with increasing ZnS loading in PVK. This may be because of interfacial charge transfer and radiative recombination. The EL studies show that corresponding to the applied voltage, the brightness is increased. The EL emission starts at a lower threshold voltage with a higher loading of ZnS in PVK and at a higher threshold voltage for higher frequencies. EL intensity of the composites increases with increasing ZnS loading in PVK. The voltage-current curve shows the linear relationship between voltage and current that is ohmic in nature. The frequency of applied voltage increases as impedance decreases.

Excited-State Lifetime Modulation by Twisted and Tilted Molecular Design in Carbene-Metal-Amide Photoemitters.

Carbene–metal–amides (CMAs) are an emerging class of photoemitters based on a linear donor–linker–acceptor arrangement. They exhibit high flexibility about the carbene–metal and metal–amide bonds, leading to a conformational freedom which has a strong influence on their photophysical properties. Herein we report CMA complexes with (1) nearly coplanar, (2) twisted, (3) tilted, and (4) tilt-twisted orientations between donor and acceptor ligands and illustrate the influence of preferred ground-state conformations on both the luminescence quantum yields and excited-state lifetimes. The performance is found to be optimum for structures with partially twisted and/or tilted conformations, resulting in radiative rates exceeding 1 × 106 s–1. Although the metal atoms make only small contributions to HOMOs and LUMOs, they provide sufficient spin–orbit coupling between the low-lying excited states to reduce the excited-state lifetimes down to 500 ns. At the same time, high photoluminescence quantum yields are maintained for a strongly tilted emitter in a host matrix. Proof-of-concept organic light-emitting diodes (OLEDs) based on these new emitter designs were fabricated, with a maximum external quantum efficiency (EQE) of 19.1% with low device roll-off efficiency. Transient electroluminescence studies indicate that molecular design concepts for new CMA emitters can be successfully translated into the OLED device.

New deep blue fluorophores based on benzo[d]thiazole group as acceptor core: Theoretical, synthesis, photophysical and electroluminescent investigation

Deep-blue organic emitters have attracted considerable attention from both industrial as well as academic research in organic light-emitting diodes (OLEDs). In this context, three novel deep blue fluorophores based on benzothiazole 2-(4'-(1,4,5-triphenyl-1H-imidazole-2-yl)-[1,1′-biphenyl]-4-yl)benzo [d]thiazole (PHBISN), 2-(4'-(1-(4-(tert-butyl)phenyl)-4,5-diphenyl-1H-imidazole-2-yl)-[1,1′-biphenyl]-4-yl)benzo [d]thiazole (PTBISN) and 2-(4'-(4,5-diphenyl-1-(3-(trifluoromethyl)phenyl)-1H-imidazole-2-yl)-[1,1′-biphenyl]-4-yl)benzo [d]thiazole (m-CFBISN) were designed and synthesized. The fluorophores were structurally characterized and a detailed computational study reveals the compounds with wide energy gap (>3eV). The thermal properties, photophysical and electronic properties were systematically investigated. Photoluminescence and electroluminescence study shows fluorophores with deep blue emission. Novel synthesized fluorophores exhibit good performance for deep-blue solution-processed OLEDs. The doped device based on PTBISN shows the highest performance with deep blue electroluminescent and maximum EQE 1.5% with CIE color coordinates of (0.15, 0.08), which is near to the blue standard (0.14, 0.08) of the National Television System Committee (NTSC).

Rare Earth Complexes with 5d-4f Transition: New Emitters in Organic Light-Emitting Diodes.

Organic light-emitting diodes (OLEDs) are considered as next-generation displays and lighting technologies. During the past three decades, various luminescent materials such as fluorescence, phosphorescence, and thermally activated delayed fluorescence materials have been subsequently investigated as emitters. To date, blue OLEDs are still the bottleneck as compared to red and green ones because of the lack of efficient emitters with simultaneous high exciton utilization efficiency and long-term stability. Recently, d-f transition rare earth complexes have been reported as new emitters in OLEDs with potential high efficiency and stability. In this Perspective, we present a brief introduction to OLEDs and an overview of the previous electroluminescence study on d-f transition rare earth complexes. This is followed by our recent developments in cerium(III) complex- and europium(II) complex-based OLEDs. We finally discuss the challenges and opportunities for OLED study based on d-f transition rare earth complexes.

Electroluminescence study of InGaN/GaN QW based p-i-n and inverted p-i-n junction based short-wavelength LED device using laser MBE technique

In the present work, the Laser Molecular Beam Epitaxy (Laser MBE) technique has been used for the fabrication of InGaN/GaN quantum well LEDs. A comparative study was performed for analysing the performance of LEDs with two different device structures i.e conventional (p-GaN/QW/i-GaN/n-GaN/Substrate) and inverted (n-GaN/QW/i-GaN/p-GaN/Substrate). The fabricated QW based Inverted p-i-n junction and conventional (p-i-n) LEDs exhibited non-linear rectifying current-voltage characteristics with a forward threshold voltage of about 0.7 V and 0.5 V, respectively. Room temperature electroluminescence was observed with increasing injection current in both conventional and inverted junction LED, where LEDs with 5 QW periods showed carrier localization effect and higher external quantum efficiency (EQE) at fixed injection current. The emission in fabricated QW LEDs was found to exhibit a spectral blue shift with increasing injection current. Convention LEDs (topmost layer: p-type GaN) exhibited higher luminous efficiency (i.e., 2.83 lm/W) than the Inverted LEDs (top most layer: n-type GaN) luminous efficiency (i.e., 1.56 lm/W). The efficiency droop observed is 14% in the conventional and 8% in the inverted LED device, respectively. This study focuses on the demonstration of room temperature electroluminescence in InGaN/GaN based LEDs grown using Laser MBE technique. The successful utilization of Laser MBE technique for growth of InGaN/GaN quantum wells can be a revolutionary step towards the establishment of a reliable, cost-effective, and ammonia-free process for fabrication of LED.

Intermedial annealing process applied during the growth of quantum wells and its influence on the performance of GaN-based laser diodes.

An intermedial annealing treatment is adopted during epitaxial growth of InGaN/GaN multiple quantum well (MQW) by the metal-organic chemical vapor deposition (MOCVD), which is employed after each GaN cap layer growth is finished. Optical power, threshold current and slope efficiency of GaN-based laser diodes is improved through an appropriate intermedial annealing process. A further investigation about the influence of annealing duration on the luminescence characteristics of light-emitting diodes and the surface topography evolution of single quantum well layers is conducted through the study of electroluminescence, temperature dependent photoluminescence and atomic force microscopy. It is found that the improvement of GaN-based laser diode is attributed to reduction of nonradiative recombination centers in MQW, which is due to a better interface quality between well and barrier layers after an intermedial annealing process.

Influence of atomistic protrusion on the substrate on molecular luminescence in tunnel junctions

Scanning tunneling microscope (STM) induced luminescence can be used to study various optoelectronic phenomena of single molecules and to understand the fundamental photophysical mechanisms involved. To clearly observe the molecule-specific luminescence, it is important to improve the quantum efficiency of molecules in the metallic nanocavity. In this work, we investigate theoretically the influence of an atomic-scale protrusion on the substrate on the emission properties of a point dipole oriented parallel to the substrate in a silver plasmonic nanocavity by electromagnetic simulations. We find that an atomic-scale protrusion on the substrate can strongly enhance the quantum efficiency of a horizontal dipole emitter, similar to the situation with a protrusion at the tip apex. We also consider a double-protrusion junction geometry in which there is an atomic-scale protrusion on both the tip and the substrate, and find that this geometry does provide significantly enhanced emission compared with the protrusion-free situation, but does not appear to improve the quantum efficiency compared to the mono-protrusion situation either at the tip apex or on the substrate. These results are believed to be instructive for future STM induced electroluminescence and photoluminescence studies on single molecules.

Novel Surface Passivation Scheme by Using p-Type AlTiO to Mitigate Dynamic ON Resistance Behavior in AlGaN/GaN HEMTs—Part II

The underlying mechanism responsible for the unique dynamic ON-resistance behavior is unified by demonstrating the presence of critical drain stress voltage, above which dynamic ON-resistance increases significantly, in different gate stacks. Metal–insulator–semiconductor (MIS)- and Schottky-gated HEMTs show similar dependence of critical voltage on various parameters, which establishes that gate-stack design has negligible impact on the observed phenomena. Furthermore, using the physical insights developed, this work proposes a novel surface passivation scheme to improve the dynamic performance of the device. The proposed surface passivation scheme uses p-type Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> O (AlTiO), which is deposited over SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> passivation (or GaN cap), and is shown to be an effective tool in improving the dynamic ON-resistance of the device by modulating the electric field in the GaN buffer. The proposed passivation scheme has avoided the critical voltage to appear for the entire drain stress voltage, stress time, and substrate bias range. Detailed computational analysis in conjunction with electroluminescence (EL) and photoluminescence (PL) studies revealed an electric field redistribution due to the p-type nature of AlTiO deposited over the surface passivation/capping layer, which is responsible for relaxed electric field profile in GaN buffer and observed improvement in dynamic performance. Besides, the new observations have further helped to understand the interplay between surface conditions and GaN buffer, defining its collective role in governing the dynamic performance of GaN HEMTs. Finally, various findings and the proposal in this work have been validated for buffers having higher carbon doping and devices with p-type AlTiO deposited over GaN cap instead of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> cap.

Recycling of Triplets into Singlets for High‐Performance Organic Lasers

AbstractAchieving continuous‐wave (CW) lasing in organic semiconductors is known to be a difficult task because long‐lived triplets quench radiative singlets via singlet–triplet annihilation (STA). To avoid STA and operate organic lasers in CW or long‐pulse photoexcitation, the triplets need to be removed from an organic laser gain medium. However, this triplet removal leads to a loss of excitons. In addition to removing the detrimental triplets, here it is reported a triplet recycling process, which includes triplet scavenging and successive triplet upconversion via triplet–triplet annihilation (TTA) to regenerate emissive singlet excitons in a laser medium. An anthracene derivative of 9‐(1‐naphthalenyl)‐10‐(4‐(2‐naphthalenyl)phenyl)anthracene (NaNaP‐A) and a laser dye of 4,4′‐bis[4‐(diphenylamino)styryl]biphenyl (BDAVBi) are used as the triplet recycling sensitizer and the emitting laser dye, respectively. In this laser system, NaNaP‐A can efficiently scavenge the triplets formed on BDAVBi because the triplet level is deeper for NaNaP‐A than for BDAVBi, and then NaNaP‐A successively recycles the triplets into the BDAVBi's singlet state via TTA. The TTA compensates and overcomes the STA in this laser system. Hence, these laser devices can be operated with long pulse widths of up to 10 ms. This unique triplet recycling behavior is confirmed by transient photoluminescence (PL) and electroluminescence (EL) studies.

Versatile azaryl-ketone-based blue AIEgens for efficient organic light-emitting diodes

We report two donor-acceptor type blue luminophores based on an azaryl-ketone derivative, phenyl(3-phenylimidazo[1,2-a]pyridin-2-yl)methanone (pBZ), as respectively emitter and host materials for organic light-emitting diodes (OLEDs). The two new compounds pBZ-TPA and pBZ-DPA having varying lengths of phenylene linkers are prepared through a facile synthetic method and systematically investigated with thermal, photophysical, electroluminescent, and theoretical studies. They exhibit obvious aggregation-induced emission (AIE) characters. The electron-deficiency pBZ and the electron-rich arylamine (triphenylamine or diphenylamine) respectively serve as electron acceptor and donor, rendering bipolar electronic properties. The two AIE luminophores (AIEgens) exhibit obvious charge transfer character evidenced by positive solvatochromic effect in their photoluminescence. It is also found that the (bi)phenylene π-linker attached in donor would tune the molecular conjugation and degree of charge transfer. The versatility of pBZ-TPA and pBZ-DPA is demonstrated in blue OLEDs as emitters and in red phosphorescent devices as hosts with decent performance and mild efficiency roll-offs.