Enhancing extraction efficiency of quantum dot light-emitting diodes by surface engineering (original) (raw)
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30% External Quantum Efficiency From Surface Textured, Thin-Film Light-Emitting Diodes
Applied Physics Letters, 1993
There is a significant gap between the internal efficiency of light-emitting diodes (LEDs) and their external efficiency. The reason for this shortfall is the narrow escape cone for light in high refractive index semiconductors. We have found that by separating thin-film LEDs from their substrates (by epitaxial lift-off, for example), it is much easier for light to escape from the LED structure and thereby avoid absorption. Moreover, by nanotexturing the thin-film surface using "natural lithography," the light ray dynamics becomes chaotic, and the optical phase-space distribution becomes "ergodic," allowing even more of the light to find the escape cone. We have demonstrated 30% external efficiency in GaAs LEDs employing these principles.
Light-extraction mechanisms in high-efficiency surface-textured light-emitting diodes
IEEE Journal of Selected Topics in Quantum Electronics, 2002
In this paper, we present a detailed quantitative analysis of the light extraction and loss mechansisms in high-efficiency GaAs-AlGaAs surface-textured thin-film light-emitting diodes (LEDs). The analysis is based on a Monte Carlo simulation. Most input parameters, including scattering of photons at the textured surface, sub-bandgap absorption, and absorption at the metal mirror are obtained from experiments or from literature. The simulation also takes into account the effect of photon recycling and the realistic geometry of the diodes. The only remaining fitting parameter is the internal quantum efficiency, which is deduced to be about 80% at room temperature for the experimentally realized 850-nm LEDs with an external quantum efficiency of 44%. The analysis shows further that the most important loss mechanism is reabsorption in the active layer, and in particular in those parts of the active layer that are not electrically pumped. This conclusion is also valid for other types of high-efficiency LEDs. We could furthermore verify the validity of the Monte-Carlo simulation results by conducting experiments at low temperatures, where nonradiative recombination processes are reduced, resulting in the internal quantum efficiency approaching unity. The measured external quantum efficiency at 90 K is 68%, which is close to the theoretically predicted efficiency for a perfect active layer. The results demonstrate that the light extraction from surface-textured LEDs is fully understood and can be quantitatively modeled by a simple raytracing algorithm.
Optics Express, 2014
A new approach to surface roughening was established and optimized in this paper for enhancing the light extraction of high power AlGaInP-based LEDs, by combining ultraviolet (UV) assisted imprinting with dry etching techniques. In this approach, hexagonal arrays of coneshaped etch pits are fabricated on the surface of LEDs, forming gradient effective-refractive-index that can mitigate the emission loss due to total internal reflection and therefore increase the light extraction efficiency. For comparison, wafer-scale FLAT-LEDs without any surface roughening, WET-LEDs with surface roughened by wet etching, and DRY-LEDs with surface roughened by varying the dry etching time of the AlGaInP layer, were fabricated and characterized. The average output power for waferscale FLAT-LEDs, WET-LEDs, and DRY3-LEDs (optimal) at 350 mA was found to be 102, 140, and 172 mW, respectively, and there was no noticeable electrical degradation with the WET-LEDs and DRY-LEDs. The light output was increased by 37.3% with wet etching, and 68.6% with dry etching surface roughening, respectively, without compromising the electrical performance of LEDs. A total number of 1600 LED chips were tested for each type of LEDs. The yield of chips with an optical output power of 120 mW and above was 0.3% (4 chips), 42.8% (684 chips), and 90.1% (1441 chips) for FLAT-LEDs, WET-LEDs, and DRY3-LEDs, respectively. The dry etching surface roughening approach developed here is potentially useful for the industrial mass production of wafer-scale high power LEDs.
Light extraction mechanisms in surface-textured light-emitting diodes
Light-emitting diodes (LEDs) with high efficiencies can be fabricated by a combination of surface texturing and the application of a rear reflector. We demonstrate an external quantum efficiency of 43% for unencapsulated surface-textured thin-film LEDs, which increases to 54% after encapsu1ation At low temperatures, the efficiency of unencapsulated devices increases up to 68%. We investigate the light extraction mechanism from such LEDs employing a Monte Carlo simulation of the light propagation inside the LED structure. One essential input parameter for the simulation are the light scattering properties of the textured surface, which have been investigated experimentally. For light incidence below the critical angle of total internal reflection, the transmission through a textured surface is reduced compared to a flat surface. However, due to surface texturing, transmission becomes possible for incident angles above the critical angle. As a result, the internal scattering during internal reflection at the textured surface is not necessary for an efficient extraction of the light generated inside the LED structureS In addition, the Monte Carlo simulation also explains the strong increase of the LED efficiency at low temperatures quantitatively by photon reycling effects. Photon recycling is also demonstrated to be partially responsible for the shift of the emission wavelength in thin-film LEDs, as compared to conventional LEDs.
All-Solution-Processed Inverted Quantum-Dot Light-Emitting Diodes
ACS Applied Materials & Interfaces, 2014
Quantum dots are a promising new candidate for the emissive material in light-emitting devices for display applications. The fabrication of such devices by solution processing allows considerable cost reduction and is therefore very attractive for industrial manufacturers. We report all solution-processed colloidal quantum-dot light-emitting diodes (QLEDs) with an inverted structure. The red, green, and blue devices showed maximum luminances of 12 510, 32 370, and 249 cd/m 2 and turn-on voltages of 2.8, 3.6, and 3.6 V, respectively. We investigate the effect of a surfactant addition in the hole injection layer (HIL), with the aim of facilitating layer deposition and thereby enhancing device performance. We demonstrate that in the device structure presented in this study, a small amount of surfactant in the HIL can significantly improve the performance of the QLED.
Hybrid Quantum Dot Light-Emitting Diodes: Design, Fabrication, and Characterization
2008 IEEE PhotonicsGlobal@Singapore, 2008
The authors have designed and fabricated the colloidal quantum dot (QD) light-emitting diodes based on a thin CdSe/ZnS QD layer sandwiched by organic hole and electron transport layers. The device configuration can be depicted as ITO/PEDOT:PSS/poly-TPD/QD/BCP/Alq 3 /LiF/Al. The optimized light turn-on voltage is about 7 V and the maximum brightness is 141 cd/m 2 at 11.5 V. It has been observed the second organic layer, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), not only served for the electron transporting but also for hole blocking. It can greatly improve the device performance by enhancing the probabilities of electron and hole injection into the QDs. In device characterization, room temperature photoluminescence (PL) and electroluminescence (EL) peaks are at 625 and 628.6 nm, respectively. The red-shift may be from the local Joule heating induced by the injection current. Emission from Alq 3 at 516 nm is also observed in all EL spectra. Specifically, the QD-dominant EL spectra indicate the QD emission is mainly from the direct injection and radiative recombination of carriers rather than the radiative and nonradiative (Förster) energy transfers from excitons in organic materials shown in single-layered hybrid devices. Since the emission spectra are dominated by the inorganic QDs, the improvement of reliability of hybrid devices can thus be expected. However, the luminance efficiency (~0.014 cd/A) is still not as high as that of pure organic or polymer LEDs. To increase the brightness and efficiency, the device structure has to be modified and equivalent injection of electrons and holes is necessary.
The CdSe/ZnS quantum dots (QDs) have got researcher attention owing to their superior photophysical properties and their applications in QD-based light-emitting diode (QLED). The conventional CdSe/ZnS-based QLED uses highly conductive electron transport layer, low mobility hole transporting layers (HTL) and vacuum-deposited opaque metal electrode at the top. This structure renders unbalanced charge injection into the emissive layer and also allows the device to emit light at the bottom side only, which affects the device output luminance and stability. Moreover, in the vacuum-deposition technique, the fabrication process is more complex, expensive and time consuming. To address all these issues, we have fabricated an allsolution processable double-sided emitting QLED by non-vacuum technique using high mobility multi-HTLs with cascade structure, an insulating layer and transparent silver nanowire (AgNW) electrode for balanced charge injection for obtaining higher luminance at the top-side AgNW
Nature Communications, 2013
Layered assembly structures composed of nanomaterials, such as nanocrystals, have attracted considerable attention as promising candidates for new functional devices whose optical, electromagnetic and electronic behaviours are determined by the spatial arrangement of component elements. However, difficulties in handling each constituent layer in a materialspecific manner limit the 3D integration of disparate nanomaterials into the appropriate heterogeneous electronics. Here we report a pick-and-place transfer method that enables the transfer of large-area nanodot assemblies. This solvent-free transfer utilizes a lifting layer and allows for the reliable transfer of a quantum dot (QD) monolayer, enabling layer-by-layer design. With the controlled multistacking of different bandgap QD layers, we are able to probe the interlayer energy transfer among different QD monolayers. By controlling the emission spectrum through such designed monolayer stacking, we have achieved white emission with stable optoelectronic properties, the closest to pure white among the QD lightemitting diodes reported so far.
Advanced Functional Materials, 2019
In the study of hybrid quantum dot light-emitting diodes (QLEDs), even for state-of-the-art achievement, there still exists a long-standing charge balance problem, i.e., sufficient electron injection versus inefficient hole injection due to the large valence band offset of quantum dots (QDs) with respect to the adjacent carrier transport layer. Here the dedicated design and synthesis of high luminescence Zn 1−x Cd x Se/ZnSe/ZnS QDs is reported by precisely controlled shell growth, which have matched energy level with the adjacent hole transport layer in QLEDs. As emitters, such Zn 1−x Cd x Se-based QLEDs exhibit peak external quantum efficiencies (EQE) of up to 30.9%, maximum brightness of over 334 000 cd m −2 , very low efficiency roll-off at high current density (EQE ≈25% @ current density of 150 mA cm −2), and operational lifetime extended to ≈1 800 000 h at 100 cd m −2. These extraordinary performances make this work the best among all solution-processed QLEDs reported in literature so far by achieving simultaneously high luminescence and balanced charge injection. These major advances are attributed to the combination of an intermediate ZnSe layer with an ultrathin ZnS outer layer as the shell materials and surface modification with 2-ethylhexane-1-thiol, which can dramatically improve hole injection efficiency and thus lead to more balanced charge injection.