Printing and In Situ Assembly of CdSe/CdS Nanoplatelets as Uniform Films with Unity In-Plane Transition Dipole Moment

Highly-Directional, Highly-Efficient Solution-Processed Light-Emitting Diodes of All-Face-Down Oriented Colloidal Quantum Well Self-Assembly

Semiconductor colloidal quantum wells (CQWs) provide anisotropic emission behavior originating from their anisotropic optical transition dipole moments (TDMs). Here, solution-processed colloidal quantum well light-emitting diodes (CQW-LEDs) of a single all-face-down oriented self-assembled monolayer (SAM) film of CQWs that collectively enable a supreme level of IP TDMs at 92% in the ensemble emission are shown. This significantly enhances the outcoupling efficiency from 22% (of standard randomly-oriented emitters) to 34% (of face-down oriented emitters) in the LED. As a result, the external quantum efficiency reaches a record high level of 18.1% for the solution-processed type of CQW-LEDs, putting their efficiency performance on par with the hybrid organic-inorganic evaporation-based CQW-LEDs and all other best solution-processed LEDs. This SAM-CQW-LED architecture allows for a high maximum brightness of 19,800 cd m^-2 with a long operational lifetime of 247 h at 100 cd m^-2 as well as a stable saturated deep-red emission (651 nm) with a low turn-on voltage of 1.7 eV at a current density of 1 mA cm^-2 and a high J₉₀ of 99.58 mA cm^-2 . These findings indicate the effectiveness of oriented self-assembly of CQWs as an electrically-driven emissive layer in improving outcoupling and external quantum efficiencies in the CQW-LEDs.

2D II-VI Semiconductor Nanoplatelets: From Material Synthesis to Optoelectronic Integration

The field of colloidal synthesis of semiconductors emerged 40 years ago and has reached a certain level of maturity thanks to the use of nanocrystals as phosphors in commercial displays. In particular, II-VI semiconductors based on cadmium, zinc, or mercury chalcogenides can now be synthesized with tailored shapes, composition by alloying, and even as nanocrystal heterostructures. Fifteen years ago, II-VI semiconductor nanoplatelets injected new ideas into this field. Indeed, despite the emergence of other promising semiconductors such as halide perovskites or 2D transition metal dichalcogenides, colloidal II-VI semiconductor nanoplatelets remain among the narrowest room-temperature emitters that can be synthesized over a wide spectral range, and they exhibit good material stability over time. Such nanoplatelets are scientifically and technologically interesting because they exhibit optical features and production advantages at the intersection of those expected from colloidal quantum dots and epitaxial quantum wells. In organic solvents, gram-scale syntheses can produce nanoparticles with the same thicknesses and optical properties without inhomogeneous broadening. In such nanoplatelets, quantum confinement is limited to one dimension, defined at the atomic scale, which allows them to be treated as quantum wells. In this review, we discuss the synthetic developments, spectroscopic properties, and applications of such nanoplatelets. Covering growth mechanisms, we explain how a thorough understanding of nanoplatelet growth has enabled the development of nanoplatelets and heterostructured nanoplatelets with multiple emission colors, spatially localized excitations, narrow emission, and high quantum yields over a wide spectral range. Moreover, nanoplatelets, with their large lateral extension and their thin short axis and low dielectric surroundings, can support one or several electron-hole pairs with large exciton binding energies. Thus, we also discuss how the relaxation processes and lifetime of the carriers and excitons are modified in nanoplatelets compared to both spherical quantum dots and epitaxial quantum wells. Finally, we explore how nanoplatelets, with their strong and narrow emission, can be considered as ideal candidates for pure-color light emitting diodes (LEDs), strong gain media for lasers, or for use in luminescent light concentrators.

Colored Silicon Heterojunction Solar Cells Exceeding 23.5% Efficiency Enabled by Luminescent Down-Shift Quantum Dots

Colored solar panels, realized by depositing various reflection layers or structures, are emerging as power sources for building with visual aesthetics. However, these panels suffer from reduced photocurrent generation due to the less efficient light harvesting from visible light reflection and degraded power conversion efficiency (PCE). Here, color-patterned silicon heterojunction solar cells are achieved by incorporating luminescent quantum dots (QDs) with high quantum yields as light converters to realize an asthenic appearance with high PCE. It is found that large bandgap (blue) QD layers can convert UV light into visible light, which can notably alleviate the parasitic absorption by the front indium tin oxide and doped amorphous silicon. Additionally, a universal optical path model is proposed to understand the light transmission process, which is suitable for luminescent down-shift devices. In this study, solar cells with a PCE exceeding 23.5% are achieved using the combination of a blue QD layer and a top low refractive index anti-reflection layer. Based on our best knoledge,the obtained PCE is the highest for a color-patterned solar cell. The results suggest an enhanced strategy involving incorporation of luminescent QDs with an optical path design for high-performance photovoltaic panels with visual aesthetics.

Controlled Core/Crown Growth Enables Blue-Emitting Colloidal Nanoplatelets with Efficient and Pure Photoluminescence

Colloidal semiconductor CdSe nanoplatelets (NPLs) feature ultranarrow and anisotropic emissions. However, the optical performance of blue-emitting NPLs is deteriorated by trap states, currently exhibiting tainted emissions and inferior photoluminescence quantum yields (PLQYs). Here, near trap-free blue-emitting NPLs are achieved by the controlled growth of the core/crown. Deep trap states in NPLs can be partially suppressed with the asymmetrical crown growth and are further suppressed with the growth of the small core and the alloyed symmetrical crown, yielding NPLs with pure blue emissions and near-unity PLQYs. Exciton dynamic research based on these NPLs indicates that the trap emission stems from surface traps. Besides, light-emitting diodes exhibiting ultranarrow emission centered around 461 nm with full-width-at-half-maximums down to 11 nm are fabricated using these NPLs.

CdSe/CdSeS Nanoplatelet Light-Emitting Diodes with Ultrapure Green Color and High External Quantum Efficiency

Colloidal II-VI group nanoplatelets (NPLs) possess ultranarrow emission line widths, for which they have great promise in achieving the purest display color in solution-processed light-emitting diodes (LEDs). Red NPL-LEDs have shown extremely saturated red color with high efficiency, while the green and blue ones lag far behind. Herein, we report green NPL-LEDs with the purest color in accordance with the Rec. 2020 standard and the peak external quantum efficiency (EQE) of 9.78%. By carefully controlling the aspect ratio, capping ligands, and purifications of CdSe/CdSeS core/alloyed-crown NPLs, NPL films with excellent flatness and unity photoluminescence quantum yields (PLQYs) are realized, laying a solid foundation for improving LED performance. Furthermore, via tuning the carrier injection balance, the record-high EQE for green NPL-LEDs is achieved. The electroluminescence (EL) exhibits an extremely saturated green color with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.163 0.786), which demonstrates their great potential in applications of ultrahigh-definition display technology. Our findings would help to further improve the performance of all NPL-LEDs.

Advances in Perovskite Light-Emitting Diodes Possessing Improved Lifetime

Recently, perovskite light-emitting diodes (PeLEDs) are seeing an increasing academic and industrial interest with a potential for a broad range of technologies including display, lighting, and signaling. The maximum external quantum efficiency of PeLEDs can overtake 20% nowadays, however, the lifetime of PeLEDs is still far from the demand of practical applications. In this review, state-of-the-art concepts to improve the lifetime of PeLEDs are comprehensively summarized from the perspective of the design of perovskite emitting materials, the innovation of device engineering, the manipulation of optical effects, and the introduction of advanced encapsulations. First, the fundamental concepts determining the lifetime of PeLEDs are presented. Then, the strategies to improve the lifetime of both organic-inorganic hybrid and all-inorganic PeLEDs are highlighted. Particularly, the approaches to manage optical effects and encapsulations for the improved lifetime, which are negligibly studied in PeLEDs, are discussed based on the related concepts of organic LEDs and Cd-based quantum-dot LEDs, which is beneficial to insightfully understand the lifetime of PeLEDs. At last, the challenges and opportunities to further enhance the lifetime of PeLEDs are introduced.