Hole transport layer engineering in high performance quasi-2D perovskite blue light emitting diodes.

Quasi-2D perovskites have emerged as highly promising materials for application in perovskite light-emitting diodes (PeLEDs), garnering significant attention due to their outstanding semiconductor properties. These materials boast an inherent multi-quantum well structure that imparts a robust confinement effect, particularly advantageous for blue emission. However, the development of blue emitters utilizing quasi-2D perovskites encounters challenges, notably colour instability, multipeak emission, and suboptimal fluorescence yield. The hole transfer layer (HTL) on which the perovskite layer is deposited in PeLEDs further affects the performance and efficiency. In this review, we delve into the evolution of blue PeLEDs and elucidate the optical properties of quasi-2D perovskites with the primary focus on HTL materials. We explore different HTL materials like PEDOT:PSS, metal oxides, and conjugated polyelectrolytes as well as ionic liquids, and their role in enhancing the colour stability, minimizing interfacial defects and increasing the fluorescence yield. This review endeavours to provide a holistic perspective of the different HTLs and serve as a valuable reference for researchers navigating the realm of HTL engineering towards the realization of high-performance blue quasi-2D PeLEDs.

Chiral electroluminescence from thin-film perovskite metacavities.

Chiral light sources realized in ultracompact device platforms are highly desirable for various applications. Among active media used for thin-film emission devices, lead-halide perovskites have been extensively studied for photoluminescence due to their exceptional properties. However, up to date, there have been no demonstrations of chiral electroluminescence with a substantial degree of circular polarization (DCP) based on perovskite materials, being critical for the development of practical devices. Here, we propose a concept of chiral light sources based on a thin-film perovskite metacavity and experimentally demonstrate chiral electroluminescence with a peak DCP approaching 0.38. We design a metacavity created by a metal and a dielectric metasurface supporting photonic eigenstates with a close-to-maximum chiral response. Chiral cavity modes facilitate asymmetric electroluminescence of pairs of left and right circularly polarized waves propagating in the opposite oblique directions. The proposed ultracompact light sources are especially advantageous for many applications requiring chiral light beams of both helicities.

Highly luminescent red-emitting In(Zn)P quantum dots using zinc oxo cluster: synthesis and application to light-emitting diodes.

Despite the importance of separating nucleation steps from growth steps for the production of monodisperse highly luminescent In(Zn)P quantum dots (QDs), the practical implementation of this strategy is hindered by the high reactivity and fast depletion of conventional P precursors. This problem can be mitigated through the use of (i) Zn oxo clusters, which effectively regulate the kinetics of QD growth and prevent the fast depletion of conventional P precursors in the nucleation step, or (ii) seed-mediated continuous growth methods, which avoid secondary nucleation in the growth step and yield red-emitting InP QDs. Herein, we combine approaches (i) and (ii) to synthesize red-emitting In(Zn)P QDs with a high photoluminescence quantum yield (>93%) and a low emission bandwidth (full width at half maximum = 38 nm), revealing that our strategy hinders the carboxylate ketonization-induced generation of byproducts and suppresses the surface oxidation of In(Zn)P QDs during growth steps. The prepared In(Zn)P QDs are used to fabricate QD light-emitting diodes with a maximum brightness of 1164 cd m-2 and an external quantum efficiency of 3.61%. Thus, our results pave the way to the replacement of toxic Cd- and Pb-based QDs with more eco-friendly Zn- and In-based analogs for a variety of applications.

Inverted Polymer Solar Cells with Annealing-Free Solution-Processable NiO.

Nickel oxide (NiO) offers intrinsic p-type behavior and high thermal and chemical stability, making it promising as a hole transport layer (HTL) material in inverted organic solar cells. However, its use in this application has been rare because of a wettability problem caused by use of water as base solvent and high-temperature annealing requirements. In the present work, an annealing-free solution-processable method for NiO deposition is developed and applied in both conventional and inverted non-fullerene polymer solar cells. To overcome the wettability problem, the typical DI water solvent is replaced with a mixed solvent of DI water and isopropyl alcohol with a small amount of 2-butanol additive. This allows a NiO nanoparticle suspension (s-NiO) to be deposited on a hydrophobic active layer surface. An inverted non-fullerene solar cell based on a blend of p-type polymer PTB7-Th and non-fullerene acceptor IEICO-4F exhibits the high efficiency of 11.23% with an s-NiO HTL, comparable to the efficiency of an inverted solar cell with a MoOx HTL deposited by thermal evaporation. Conventionally structured devices including this s-NiO layer show efficiency comparable to that of a conventional device with a PEDOT:PSS HTL.

Highly Stable Bulk Perovskite for Blue LEDs with Anion-Exchange Method.

To date, the light emitting diode (LED) based halide perovskite was rapidly developed due to the outstanding property of perovskite materials. However, the blue perovskite LEDs based on the bulk halide perovskites have been rarely researched and showed low efficiencies. The bulk blue perovskite LEDs suffered from insufficient coverage on the substrate due to the low solubility of the inorganic Cl sources or damaged by the structural instability with participation of organic cations. Here, we show the new method of fabricating stable inorganic bulk blue perovskite LEDs with the anion exchange approach to avoid use of insoluble Cl precursors. The devices showed nice operational spectral stability at the desired blue emission peak. The bulk perovskite blue LEDs showed a maximum luminance of 1468 and 494 cd m-2 for the 490 and 470 nm emission peaks, respectively.

Origin of the luminescence spectra width in perovskite nanocrystals with surface passivation.

Though halide perovskite nanocrystal (PeNC) based blue light emitting devices have been improved in the last few years, and the reasons for the improvements have been successfully explained, the origin of the narrow emission spectra of PeNCs have not been studied much. Here, the factors that affect the width of the emission spectra of PeNCs are analyzed with controlled synthesis and surface passivation treatment. The overall spectra are governed by the size of PeNCs; however, the width could be narrowed by surface passivation treatment. The anion passivation effect of the surface passivation improved most of optoelectronic properties, but had less effect on the emission spectra width. The narrower emission spectra of PeNCs are obtained by ligand passivation effect of the surface passivation. Light emitting devices with enhanced optoelectronic properties are successfully fabricated and narrow (0.094 eV, 16.72 nm) blue electroluminescence emission spectra (∼470 nm) are obtained.

High colloidal stability ZnO nanoparticles independent on solvent polarity and their application in polymer solar cells.

Significant aggregation between ZnO nanoparticles (ZnO NPs) dispersed in polar and nonpolar solvents hinders the formation of high quality thin film for the device application and impedes their excellent electron transporting ability. Herein a bifunctional coordination complex, titanium diisopropoxide bis(acetylacetonate) (Ti(acac)2) is employed as efficient stabilizer to improve colloidal stability of ZnO NPs. Acetylacetonate functionalized ZnO exhibited long-term stability and maintained its superior optical and electrical properties for months aging under ambient atmospheric condition. The functionalized ZnO NPs were then incorporated into polymer solar cells with conventional structure as n-type buffer layer. The devices exhibited nearly identical power conversion efficiency regardless of the use of fresh and old (2 months aged) NPs. Our approach provides a simple and efficient route to boost colloidal stability of ZnO NPs in both polar and nonpolar solvents, which could enable structure-independent intense studies for efficient organic and hybrid optoelectronic devices.

Functionalized PFN-X (X = Cl, Br, or I) for Balanced Charge Carriers of Highly Efficient Blue Light-Emitting Diodes.

All-inorganic perovskite nanocrystals (PeNCs), CsPbX3 (X = Cl, Br, or I), have been considered as one of the prospective emissive materials for display applications, which showed superior photoluminescence quantum yield and high color purity with narrow spectral line width. Recently, high-performance green and red perovskite light-emitting diodes (PeLEDs) were introduced; however, the efficiency of blue PeLEDs still lagged owing to PeNCs' deep HOMO energy level (∼6.0 eV), which is in discord with the adjacent organic interlayer. In this work, we demonstrated an interfacial engineering strategy with conjugated polyelectrolytes, functionalized PFN (poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]) with halide anions, between the hole injection layer and PeNCs. By introducing PFN-X (X = Cl, Br, or I), they exhibit well-balanced charge carriers and resultant effective radiative recombination in the PeNC layer with reduced hole injection barrier and electron blocking behavior. Among them, in particular, the PFN-Cl-treated PeLEDs display a maximum external quantum efficiency of 1.34% at 470 nm electroluminescence emission with enhanced spectral operating stability.

High-Performance Perovskite Light-Emitting Diodes with Surface Passivation of CsPbBrxI3-x Nanocrystals via Antisolvent-Triggered Ion-Exchange.

Inorganic lead halide perovskite nanocrystals (PeNCs) have intensively drawn attention as efficient light-emitting materials for optoelectronic applications due to their fine optoelectronic properties with a high photoluminescence quantum yield and easily tunable saturated emission color. However, the poor stability of the red-emitting PeNCs has become an obstacle because of the uncontrollable iodine substitution from the PeNCs due to weak Pb-I bonding. In this work, we have demonstrated a ligand-mediated post-treatment (LMPT) method using a halide ion-pair ligand, tridodecylmethyl ammonium iodide (TrDAI), for the air stable and high-quality red-emitting PeNCs. Through the LMPT method, the optoelectronic properties of red-emitting PeNCs are dramatically improved resulting in a PLQY of 88.7% at 637 ± 2 nm emission with an increased carrier lifetime from 20.77 to 31.52 ns. We achieve highly efficient red perovskite light-emitting diodes exhibiting a maximum current efficiency of 7.69 cd A-1 and an external quantum efficiency of 6.36% at 637 ± 2 nm electroluminescence emission with a sharp full-width at half maximum of 31 nm.

Vivid and Fully Saturated Blue Light-Emitting Diodes Based on Ligand-Modified Halide Perovskite Nanocrystals.

CsPbX3 (X = I, Br, Cl) perovskite nanocrystals (NCs) have recently emerged as emitting materials for optoelectronic and display applications owing to their easily tunable emissions, high photoluminescence quantum yield (PLQY), and vivid color purity (full width at half maximum of approximately 20 nm). However, the lagging quantum yields of blue-emitting perovskite NCs have resulted in low efficiency compared to green or red perovskite light-emitting diodes (PeLEDs); moreover, the long insulating ligands (such as oleylamine and oleic acid) inhibit charge carrier injection. In this study, we demonstrated a facile ligand-mediated post-treatment (LMPT) method for high-quality perovskite NCs with changing optical properties to allow fine-tuning of the target emission wavelength. This method involves the use of a mixed halide ion-pair ligand, di-dodecyl dimethyl ammonium bromide, and chloride, which can induce a reconstruction through a self-anion exchange. Using the LMPT method, the PLQY of the surface-passivated blue-emitting NCs was dramatically enhanced to over 70% within the 485 nm blue emission region and 50% within the 467 nm deep-blue emission region. Through this treatment, we achieved highly efficient blue-PeLED maximum external quantum efficiencies of 0.44 and 0.86% within the 470 and 480 ± 2 nm electroluminescence emission regions, respectively.

Morphology-Dependent Hole Transfer under Negligible HOMO Difference in Non-Fullerene Acceptor-Based Ternary Polymer Solar Cells.

In the field of organic solar cells, it has been generally accepted until recently that a difference in band energies of at least 0.3 eV between the highest occupied molecular orbital (HOMO) level of the donor and the HOMO of the acceptor is required to provide adequate driving force for efficient photoinduced hole transfer due to the large binding energy of excitons in organic materials. In this work, we investigate polymeric donor:non-fullerene acceptor junctions in binary and ternary blend polymer solar cells, which exhibit efficient photoinduced hole transfer despite negligible HOMO offset and demonstrate that hole transfer in this system is dependent on morphology. The morphology of the organic blend was gradually tuned by controlling the amount of ITIC and PC70BM. High external quantum efficiency was achieved at long wavelengths, despite ITIC-to-PC70BM ratio of 1:9, which indicates efficient photoinduced hole transfer from ITIC to the donor despite an undesirable HOMO energy offset. Transient absorption spectra further confirm that hole transfer from ITIC to the donor becomes more efficient upon optimizing the morphology of the ternary blend compared to that of donor:ITIC binary blend.