Interfacial Energy-Level Alignment for High-Performance All-Inorganic Perovskite CsPbBr3 Quantum Dot-Based Inverted Light-Emitting Diodes

Modulation of Charge Transport Layer for Perovskite Light-Emitting Diodes

Perovskite light-emitting diodes (Pero-LEDs) have garnered significant attention due to their exceptional emission characteristics, including narrow full width at half maximum, high color purity, and tunable emission colors. Recent efficiency and operational stability advancements have positioned Pero-LEDs as a promising next-generation display technology. Extensive research and review articles on the compositional engineering and defect passivation of perovskite layers have substantially contributed to the development of multi-color and high-efficiency Pero-LEDs. However, the crucial aspect of charge transport layer (CTL) modulation in Pero-LEDs remains relatively underexplored. CTL modulation not only impacts the charge carrier transport efficiency and injection balance but also plays a critical role in passivating the perovskite surface, blocking ion migration, enhancing perovskite crystallinity, and improving light extraction efficiency. Therefore, optimizing CTLs is pivotal for further enhancing Pero-LED performance. Herein, this review discusses the roles of CTLs in Pero-LEDs and categorizes both reported and potential CTL materials. Then, various CTL optimization strategies are presented, alongside an analysis of the selection criteria for CTLs in high-performance Pero-LEDs. Finally, a summary and outlook on the potential of CTL modulation to further advance Pero-LED performances are provided.

Efficient Quasi-Two-Dimensional Perovskite Light-Emitting Diodes Achieved through the Passivation of Multi-Fluorine Phosphate Molecules

The surface morphology of perovskite films significantly influences the performance of perovskite light-emitting diodes (PeLEDs). However, the thin perovskite thickness (~10 nm) results in low surface coverage on the substrate, limiting the improvement of photoelectric performance. Here, we propose a molecular additive strategy that employs pentafluorophenyl diphenylphosphinate (FDPP) molecules as additives. P=O and Pentafluorophenyl (5F) on FDPP can coordinate with Pb2+ to slow the crystallization process of perovskite and enhance surface coverage. Moreover, FDPP reduces the defect density of perovskite and enhances the crystalline quality. The maximum brightness, power efficiency (PE), and external quantum efficiency (EQE) of the optimal device reached 24,230 cd m-2, 82.73 lm W-1, and 21.06%, respectively. The device maintains an EQE of 19.79% at 1000 cd m-2 and the stability is further enhanced. This study further extends the applicability of P=O-based additives.

Superior white electroluminescent devices using nitrogen-doped carbon dots/TiO2nanorods heterostructures

Nitrogen-doped carbon dots (NCDs), exhibiting strong yellow emission in aqueous solution and solid matrices, have been utilized for fabricating heterostructure white electroluminescence devices. These devices consist of nitrogen-doped carbon dots as an emissive layer sandwiched between an organic hole transport layer (PEDOT:PSS) and an array of rutile TiO2nanorods, acting as an electron transport layer. Under an applied forward bias of 5 V, the device exhibits broadband electroluminescence covering the wavelength range of 390-900 nm, resulting in pure white light emission characteristics at room temperature. The result demonstrates the successful fabrication of all solution-processed, low-cost, eco-friendly NCDs-based LEDs with CIE (Commission Internationale d'Éclairage) coordinate of (0.31, 0.34) and color rendering index (CRI) > 90, which are close to ideal white light emission characteristics. The device functionalities are achieved based on defect-related NIR emission from TiO2nanorods array and visible emission from nitrogen-doped carbon dots. This result paves a new opportunity to develop low-cost, solution-processed nitrogen-doped carbon dots based on warm White light emitting diodes with high CRI for large-area display and lighting applications.

Efficient Perovskite Light-Emitting Diodes with Chemically Bonded Contact and Regulated Charge Behavior

Efficient charge injection and radiative recombination are essential to achieving high-performance perovskite light-emitting diodes (Pero-LEDs). However, the perovskite emission layer (EML) and the electron transport layer (ETL) form a poor physically interfacial contact and non-negligible charge injection barrier, limiting the device performance. Herein, we utilize a phosphine oxide, 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), to treat the perovskite/ETL interface and form a chemically bonded contact. Specifically, PO-T2T firmly bonds on the perovskite's surface and grain boundaries through a dative bond, effectively passivating the uncoordinated lead defects. Additionally, PO-T2T has high electron mobility and establishes an electron transport highway to bridge the ETL and EML. As a result, a maximum external quantum efficiency (EQEmax) of 22.06% (average EQEmax of 20.02 ± 1.00%) and maximum luminance (Lmax) of 103286 cd m-2 have been achieved for the champion device. Our results indicate that EML/ETL interface modifications are crucial for the fabrication of highly efficient Pero-LEDs.

Near-Infrared Photoluminescence from Ytterbium- and Erbium-Codoped CsPbCl3 Perovskite Quantum Dots with Negative Thermal Quenching

Near-infrared (NIR) luminescent phosphors hold promise for a wide range of applications, from bioimaging to light-emitting diodes (LEDs), but are typically confined to wavelengths <1300 nm and manifest substantial thermal quenching pervasive in luminescent materials. Here we observed the thermally enhanced NIR luminescence of Er³⁺ (1540 nm), a 2.5-fold enhancement with increasing temperature from 298 to 356 K, from Yb³⁺- and Er³⁺-codoped CsPbCl₃ perovskite quantum dots (PQDs) (photoexcited at ∼365 nm). Mechanistic investigations revealed that thermally enhanced phenomena originated from combined effects of thermally stable cascade energy transfer (from a photoexcited exciton to a pair of Yb³⁺ and then to surrounding Er³⁺) and minimized quenching of surface-adsorbed water molecules on the ⁴I_13/2 state of Er³⁺ induced by the temperature increase. Importantly, these PQDs enable producing phosphor-converted LEDs emitting at 1540 nm with inherited thermally enhanced properties, having implications for a wide range of photonic applications.

All-Inorganic Perovskite Quantum Dot-Based Blue Light-Emitting Diodes: Recent Advances and Strategies

Light-emitting diodes (LEDs) based on all-inorganic lead halide perovskite quantum dots (PQDs) have undergone rapid development especially in the past five years, and external quantum efficiencies (EQEs) of the corresponding green- and red-emitting devices have exceeded 23%. However, the blue-emitting devices are facing greater challenges than their counterparts, and their poor luminous efficiency has hindered the display application of PQD-based LEDs (PeQLEDs). This review focuses on the key challenges of blue-emitting PeQLEDs including low EQEs, short operating lifetime, and spectral instability, and discusses the essential mechanism by referring to the latest research. We then systematically summarize the development of preparation methods of blue emission PQDs, as well as the current strategies on alleviating the poor device performance involved in composition engineering, ligand engineering, surface/interface engineering, and device structural engineering. Ultimately, suggestions and outlooks are proposed around the major challenges and future research direction of blue PeQLEDs.

Recent Progress on Perovskite Surfaces and Interfaces in Optoelectronic Devices

Surfaces and heterojunction interfaces, where defects and energy levels dictate charge-carrier dynamics in optoelectronic devices, are critical for unlocking the full potential of perovskite semiconductors. In this progress report, chemical structures of perovskite surfaces are discussed and basic physical rules for the band alignment are summarized at various perovskite interfaces. Common perovskite surfaces are typically decorated by various compositional and structural defects such as residual surface reactants, discrete nanoclusters, reactions by products, vacancies, interstitials, antisites, etc. Some of these surface species induce deep-level defect states in the forbidden band forming very harmful charge-carrier traps and affect negatively the interface band alignments for achieving optimal device performance. Herein, an overview of research progresses on surface and interface engineering is provided to minimize deep-level defect states. The reviewed subjects include selection of interface and substrate buffer layers for growing better crystals, materials and processing methods for surface passivation, the surface catalyst for microstructure transformations, organic semiconductors for charge extraction or injection, heterojunctions with wide bandgap perovskites or nanocrystals for mitigating defects, and electrode interlayer for preventing interdiffusion and reactions. These surface and interface engineering strategies are shown to be critical in boosting device performance for both solar cells and light-emitting diodes.

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.

Multifunctional Charge Transporting Materials for Perovskite Light-Emitting Diodes

Despite their low exciton-binding energies, metal halide perovskites are extensively studied as light-emitting materials owing to narrow emission with high color purity, easy/wide color tunability, and high photoluminescence quantum yields. To improve the efficiency of perovskite light-emitting diodes (PeLEDs), much effort has been devoted to controlling the emitting layer morphologies to induce charge confinement and decrease the nonradiative recombination. The interfaces between the emitting layer and charge transporting layer (CTL) are vulnerable to various defects that deteriorate the efficiency and stability of the PeLEDs. Therefore, the establishment of multifunctional CTLs that can improve not only charge transport but also critical factors that influence device performance, such as defect passivation, morphology/phase control, ion migration suppression, and light outcoupling efficiency, are highly required. Herein, the fundamental limitations of perovskites as emitters (i.e., defects, morphological and phase instability, high refractive index with poor outcoupling) and the recent developments with regard to multifunctional CTLs to compensate such limitations are summarized, and their device applications are also reviewed. Finally, based on the importance of multifunctional CTLs, the outlook and research prospects of multifunctional CTLs for the further improvement of PeLEDs are discussed.

Identifying Carrier Behavior in Ultrathin Indirect-Bandgap CsPbX3 Nanocrystal Films for Use in UV/Visible-Blind High-Energy Detectors

High-energy radiation detectors such as X-ray detectors with low light photoresponse characteristics are used for several applications including, space, medical, and military devices. Here, an indirect bandgap inorganic perovskite-based X-ray detector is reported. The indirect bandgap nature of perovskite materials is revealed through optical characterizations, time-resolved photoluminescence (TRPL), and theoretical simulations, demonstrating that the differences in temperature-dependent carrier lifetime related to CsPbX₃ (X = Br, I) perovskite composition are due to the changes in the bandgap structure. TRPL, theoretical analyses, and X-ray radiation measurements reveal that the high response of the UV/visible-blind yellow-phase CsPbI₃ under high-energy X-ray exposure is attributed to the nature of the indirect bandgap structure of CsPbX₃ . The yellow-phase CsPbI₃ -based X-ray detector achieves a relatively high sensitivity of 83.6 μCGy_air^-1 cm^-2 (under 1.7 mGy_air s^-1 at an electron field of 0.17 V μm^-1 used for medical diagnostics) although the active layer is based solely on an ultrathin (≈6.6 μm) CsPbI₃ nanocrystal film, exceeding the values obtained for commercial X-ray detectors, and further confirming good material quality. This CsPbX₃ X-ray detector is sufficient for cost-effective device miniaturization based on a simple design.

CsPbBr3 nanowire polarized light-emitting diodes through mechanical rubbing

CsPbBr₃ nanowire polarized light-emitting diodes with low turn-on voltage were obtained through mechanical rubbing combined with an optimal device structure.

Enhanced Performance of Perovskite Light-Emitting Diodes via Diamine Interface Modification

Interfacial engineering between charge transport layers and perovskite light-emitting layers has been applied as an effective strategy to enhance performance of perovskite light-emitting diodes (PeLEDs). Herein, we introduce a Lewis base diamine molecule [2,2-(ethylenedioxy)bis(ethylammonium), EDBE] to modify the interface between the ZnMgO electron transport layer (ETL) and perovskite light-emitting layer in PeLEDs. With two amino groups in EDBE, one amine can interact with ZnMgO beneath to tune the growth of perovskite films, resulting in improved electron injection and suppressed current leakage. Meanwhile, the other amine can passivate the surface trap states of the polycrystalline perovskite films, which would eliminate trap-mediated nonradiative recombination. An enhanced performance for near-infrared PeLEDs is achieved with external quantum efficiency from 9.15 to 12.35% after incorporating the EDBE interfacial layer. This work demonstrated that the introduction of Lewis base diamine molecules as the ETL/perovskite interfacial agent is a promising way for developing high-performance PeLEDs.

Hybrid light emitting diodes based on stable, high brightness all-inorganic CsPbI3 perovskite nanocrystals and InGaN

Despite important advances in the synthesis of inorganic perovskite nanocrystals (NCs), the long-term instability and degradation of their quantum yield (QY) over time need to be addressed to enable the further development and exploitation of these nanomaterials. Here we report stable CsPbI3 perovskite NCs and their use in hybrid light emitting diodes (LEDs), which combine in one system the NCs and a blue GaN-based LED. Nanocrystals with improved morphological and optical properties are obtained by optimizing the post-synthesis replacement of oleic acid ligands with iminodibenzoic acid: the NCs have a long shelf-life (>2 months), stability under different environmental conditions, and a high QY, of up to 90%, in the visible spectral range. Ligand replacement enables the engineering of the morphological and optical properties of the NCs. Furthermore, the NCs can be used to coat the surface of a GaN-LED to realize a stable diode where they are excited by blue light from the LED under low current injection conditions, resulting in emissions at distinct wavelengths in the visible range. The high QY and fluorescence lifetime in the nanosecond range are key parameters for visible light communication, an emerging technology that requires high-performance visible light sources for secure, fast energy-efficient wireless transmission.

Device Engineering for All-Inorganic Perovskite Light-Emitting Diodes

Recently, all-inorganic perovskite light-emitting diodes (PeLEDs) have attracted both academic and industrial interest thanks to their outstanding properties, such as high efficiency, bright luminance, excellent color purity, low cost and potentially good operational stability. Apart from the design and treatment of all-inorganic emitters, the device engineering is another significant factor to guarantee the high performance. In this review, we have summarized the state-of-the-art concepts for device engineering in all-inorganic PeLEDs, where the charge injection, transport, balance and leakage play a critical role in the performance. First, we have described the fundamental concepts of all-inorganic PeLEDs. Then, we have introduced the enhancement of device engineering in all-inorganic PeLEDs. Particularly, we have comprehensively highlighted the emergence of all-inorganic PeLEDs, strategies to improve the hole injection, approaches to enhance the electron injection, schemes to increase the charge balance and methods to decrease the charge leakage. Finally, we have clarified the issues and ways to further enhance the performance of all-inorganic PeLEDs.

Structure optimization of perovskite quantum dot light-emitting diodes

Although all-inorganic perovskite light emitting diodes (PeLED) have satisfactory stability under an ambient atmosphere, producing devices with high performance is challenging. A device architecture with a reduced energy barrier between adjacent layers and optimized energy level alignment in the PeLED is critical to achieve high electroluminescence efficiency. In this study, we report the optimization of a CsPbBr3-based PeLED device structure with Li-doped TiO2 nanoparticles as the electron transport layer (ETL). Optimal Li doping balances charge carrier injection between the hole transport layer (HTL) and ETL, leading to superior performance in both devices. The turn-on voltages for devices with Li-doped TiO2 nanoparticles were significantly reduced from 7.7 V to 4.9 V and from 3 V to 2 V in the direct and inverted PeLED structures, respectively. The low turn-on voltage for green emission is one of the lowest values among the reported CsPbBr3-based PeLEDs. Further investigations show that the device with an inverted structure is superior to the device with a direct structure because the energy barrier for carrier injection was minimized. The inverted structure devices exhibited a current efficiency of 5.6 cd A-1 for the pristine TiO2 ETL, while it was 15.2 cd A-1 for the Li-doped TiO2 ETL, a factor of ∼2.7 enhancement at 5000 cd m-2.

All-Solution-Processed Perovskite Quantum Dots Light-Emitting Diodes Based on the Solvent Engineering Strategy

Perovskite quantum dots (PeQDs) have emerged as a new kind of nanomaterial in various applications, especially light-emitting diodes (LEDs). However, the synthesis of PeQDs is relatively complicated and the electron transport layer (ETL) is usually fabricated in a vacuum because of the dissolution of PeQDs films in organic solvents, which will increase the difficulty and cost in mass production. Here, a simple one-step "ultrasonic bath" treatment to synthesis PeQDs is adopted and applied into the PeQDs-LEDs. Meanwhile, an all-solution process is developed to fabricate PeQDs-LEDs based on the solvent engineering strategy. By using methyl acetate (MeOAc) as the solvent of ETL, the all-solution-processed PeQDs-LEDs exhibit bright luminance with the maximum current efficiency of 3.26 cd/A. This work is simple and easy to be scaled up, which will pave a new way to the low-cost all-solution processable PeQDs-LEDs.