All-solution-processed perovskite light-emitting diodes with all metal oxide transport layers

Challenges and Opportunities for the Blue Perovskite Quantum Dot Light-Emitting Diodes

Perovskite quantum dots (PQDs), as the promising materials for the blue light-emitting diodes (LEDs), own huge advantages as ultra-high color purity, flexibility and whole-spectrum tunability. Through dimensional and compositional engineering, PQD-LEDs have shown superiority in deep-blue light emission. However, compared with the fast development of red and green PeLEDs, the electroluminescent performance of PQD-LEDs has faced more obstacles. In this review, we aim to explore and state the uniqueness and the possible solutions for the bottleneck problems of the PQD-LEDs.

Perovskite Quantum Dots with Ultralow Trap Density by Acid Etching-Driven Ligand Exchange for High Luminance and Stable Pure-Blue Light-Emitting Diodes

The research on metal halide perovskite light-emitting diodes (PeLEDs) with green and infrared emission has demonstrated significant progress in achieving higher functional performance. However, the realization of stable pure-blue (≈470 nm wavelength) PeLEDs with increased brightness and efficiency still constitutes a considerable challenge. Here, a novel acid etching-driven ligand exchange strategy is devised for achieving pure-blue emitting small-sized (≈4 nm) CsPbBr₃ perovskite quantum dots (QDs) with ultralow trap density and excellent stability. The acid, hydrogen bromide (HBr), is employed to etch imperfect [PbBr₆ ]^4- octahedrons, thereby removing surface defects and excessive carboxylate ligands. Subsequently, didodecylamine and phenethylamine are successively introduced to bond the residual uncoordinated sites of the QDs and attain in situ exchange with the original long-chain organic ligands, resulting in near-unity quantum yield (97%) and remarkable stability. The QD-based PeLEDs exhibit pure-blue electroluminescence at 470 nm (corresponding to the Commission Internationale del'Eclairage (CIE) (0.13, 0.11) coordinates), an external quantum efficiency of 4.7%, and a remarkable luminance of 3850 cd m^-2 , which is the highest brightness reported so far for pure-blue PeLEDs. Furthermore, the PeLEDs exhibit robust durability, with a half-lifetime exceeding 12 h under continuous operation, representing a record performance value for blue PeLEDs.

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.

Recent Advances and Challenges in Halide Perovskite Crystals in Optoelectronic Devices from Solar Cells to Other Applications

Organic-inorganic hybrid perovskite materials have attracted tremendous attention as a key material in various optoelectronic devices. Distinctive optoelectronic properties, such as a tunable energy band position, long carrier diffusion lengths, and high charge carrier mobility, have allowed rapid progress in various perovskite-based optoelectronic devices (solar cells, photodetectors, light emitting diodes (LEDs), and lasers). Interestingly, the developments of each field are based on different characteristics of perovskite materials which are suitable for their own applications. In this review, we provide the fundamental properties of perovskite materials and categorize the usages in various optoelectronic applications. In addition, the prerequisite factors for those applications are suggested to understand the recent progress of perovskite-based optoelectronic devices and the challenges that need to be solved for commercialization.

Boosting Efficiency and Curtailing the Efficiency Roll-Off in Green Perovskite Light-Emitting Diodes via Incorporating Ytterbium as Cathode Interface Layer

Perovskite light-emitting diodes (PeLEDs) exhibit high external quantum efficiencies (EQEs), emerging as a next-generation lighting and display technology. Nevertheless, they suffer from severe efficiency roll-off at high luminance, particularly in the case of blue and green emissions, which is one of the major bottlenecks in their industrial applications. Here, we attack this problem using a rare-earth metal, Yb, as cathode interface layer (CIL) for green PeLEDs. By adopting a new device configuration of ITO/TFB/FA-based quasi-2D perovskite/TPBi/Yb/Ag, we achieved a peak current efficiency (CE) of 22.3 cd/A with a corresponding EQE of 5.28% and a high maximum luminance of 19 160 cd/m². Importantly, the maximum CE of 22.0 cd/A at 2000 cd/m² slightly decreased to 16.8 cd/A at 5000 cd/m² and maintained a still-decent value of 12.0 cd/A at a high luminance of 10 000 cd/m², exhibiting a remarkably low efficiency roll-off. Our Yb-incorporated devices significantly outperformed the PeLEDs containing conventional CILs, including Mg and Liq, in terms of peak efficiency, efficiency roll-off, and operational lifetime. We attribute this encouraging performance to barrier-free, efficient electron injection enabled by the low work function of Yb (2.6 eV), which led to a high electron current, nearly approaching the hole current in hole-dominant PeLEDs, as confirmed by the single-carrier device measurements. In addition, we also present Yb-incorporated PeLEDs containing Cs-based quasi-2D perovskite as the emissive layer, which displayed an impressive CE of 51.3 cd/A with a corresponding EQE of 16.4% and a maximum luminance of 14 240 cd/m², and still demonstrated a reduced efficiency roll-off comparing to that of the Liq-based equivalent. These results unveil the inspiring prospects of Yb as an efficient CIL for PeLEDs toward high efficiency with curtailed roll-off.

Critical Role of Organic Spacers for Bright 2D Layered Perovskites Light-Emitting Diodes

Light-emitting diodes (LEDs) made with quasi-2D/3D and layered perovskites have undergone an unprecedented surge as their external quantum efficiency (EQE) is rapidly approaching other lighting technologies. Manipulating the charge recombination pathway in semiconductors is highly desirable for improving the device performance. This study reports high-performance layered perovskites LEDs with benzyl ring as spacer where radiative recombination lifetime is longer, compared with much shorter alkyl chain spacer yields. Based on detailed optical and X-ray absorption spectroscopy measurements, direct signature of charges localization is observed near the band edge in exchange with the shallow traps in benzyl organics containing layered perovskites. As a result, it boosts the photoluminescence intensity by 7.4 times compared to that made with the alkyl organics. As a demonstration, a bright LED made with the benzyl organics with current efficiency of 23.46 ± 1.52 cd A-1 is shown when the device emits at a high brightness of 6.6 ± 0.93 × 104 cd m-2. The average EQE is 9.2% ± 1.43%, two orders of magnitude higher than the device made with alkyl organics. The study suggests that the choices of organic spacers provide a path toward the manipulation of charge recombination, essential for efficient optoelectronic device fabrications.

Efficient Two-Dimensional Tin Halide Perovskite Light-Emitting Diodes via a Spacer Cation Substitution Strategy

Lead halide perovskites have attracted tremendous attention because of their impressive optoelectronic properties. However, the toxicity of lead remains a bottleneck for further commercial development. Two-dimensional Ruddlesden-Popper tin-based perovskites are lead-free and more stable compared to their three-dimensional counterparts, which have great potential in the optoelectronic device field. Herein, we demonstrate high-quality two-dimensional phenylethylammonium tin-iodide perovskite (PEA₂SnI₄) thin films by using toluene as the antisolvent. Furthermore, the PeLED performance is greatly improved by replacing the PEAI spacer cation with 2-thiopheneethyllamine iodide (TEAI). As a result, a TEA-based PeLED device is achieved with a low turn-on voltage of 2.3 V, a maximum luminance of 322 cd m^-2, and maximum external quantum efficiency of 0.62%, which are the highest efficiency and brightness for pure red (emission peak = 638 nm) tin-based PeLEDs to date.

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.

Widely applicable phosphomolybdic acid doped poly(9-vinylcarbazole) hole transport layer for perovskite light-emitting devices

Perovskite light-emitting devices using a PVK:PMA hole transport layer show robust performance, allowing the wide range selection of antisolvents and hole injection layers.