CsPb2Br5 Plates/Quasi-2D Perovskite Heterojunction for Efficient Sky-Blue Light-Emitting Diodes.

Perovskite light-emitting diodes (PeLEDs) have gained significant attention owing to their remarkable tunability and color stability, and substantial progress has been made with green and red PeLEDs. However, the advancement of blue PeLEDs still lags far behind their red and green counterparts. In this study, we report efficient sky-blue PeLEDs utilizing an in situ fabricated CsPb2Br5 plates/quasi-2D perovskite heterojunction using chelating molecules to modulate the crystallization process of perovskites. The wide bandgap of CsPb2Br5 facilitated the formation of a type-I band alignment at the heterojunction, allowing efficient carrier transfer from CsPb2Br5 to CsPbBr3. This heterojunction leads to a noteworthy enhancement of device efficiency. The PeLEDs exhibit a maximum brightness of 2311 cd m-2, accompanied by a maximum external quantum efficiency of 12.86% at 487 nm. Our tailored design of CsPb2Br5/perovskite heterojunction thin films offers a promising avenue for advancing PeLED performance. This work contributes valuable insights into the burgeoning field of perovskite electroluminescence, paving the way for further optimization of PeLED technologies.

Homogenizing Energy Landscape for Efficient and Spectrally Stable Blue Perovskite Light-Emitting Diodes.

Blue perovskite light-emitting diodes (PeLEDs) have attracted enormous attention; however, their unsatisfactory device efficiency and spectral stability still remain great challenges. Unfavorable low-dimensional phase distribution and defects with deeper energy levels usually cause energy disorder, substantially limiting the device's performance. Here, an additive-interface optimization strategy is reported to tackle these issues, thus realizing efficient and spectrally stable blue PeLEDs. A new type of additive-formamidinium tetrafluorosuccinate (FATFSA) is introduced into the quasi-2D mixed halide perovskite accompanied by interface engineering, which effectively impedes the formation of undesired low-dimensional phases with various bandgaps throughout the entire film, thereby boosting energy transfer process for accelerating radiative recombination; this strategy also diminishes the halide vacancies especially chloride-related defects with deep energy level, thus reducing nonradiative energy loss for efficient radiative recombination. Benefitting from homogenized energy landscape throughout the entire perovskite emitting layer, PeLEDs with spectrally-stable blue emission (478 nm) and champion external quantum efficiency (EQE) of 21.9% are realized, which represents a record value among this type of PeLEDs in the pure blue region.

Efficient Polycrystalline Single-Cation Perovskite Light-Emitting Diodes by Simultaneous Intracrystal and Interfacial Defect Passivation.

Polycrystalline perovskite light-emitting diodes (PeLEDs) have shown great promise with high efficiency and easy processability. However, PeLEDs using single-cation polycrystalline perovskite emitters have demonstrated low efficiency due to defects within the grains and at the interfaces between the perovskite layer and the charge injection contact. Thus, simultaneous defect engineering of perovskites to suppress exciton loss within the grains and at the interfaces is crucial for achieving high efficiency in PeLEDs. Here, 1,8-octanedithiol which is a strong nucleophile, is used to increase the luminescence efficiency of a single-cation perovskite by suppressing non-radiative recombination within the grains of their polycrystalline emitter film as well as at their interface with an anode. The dithiol additive performs a multifunctional role in defect passivation, spatial confinement of excitons, and prevention of exciton quenching at the interface between the perovskite layer and the underlying hole-injection layer. Photoluminescence studies demonstrate that incorporating the dithiol additive significantly enhances the charge carrier dynamics in perovskites, resulting in an external quantum efficiency (EQE) of up to 23.46% even in a simplified PeLED that does not use a hole-injection layer. This represents the highest level of EQE achieved among devices utilizing polycrystalline single-cation perovskites.

Ions-induced Assembly of Perovskite Nanocomposites for Highly Efficient Light-Emitting Diodes with EQE Exceeding 30.

Metal halide perovskites, a cost-effective class of semiconductos, hold great promise for display technologies that demand high-efficiency, color-pure light-emitting diodes (LEDs). Early research on three-dimensional (3D) perovskites showed low radiative efficiencies due to modest exciton binding energies. To inprove luminescence, reducing dimensionality or grain size has been a common approach. However, dividing the perovskite lattice into smaller units may hinder carrier transport, compromising electrical performance. Moreover, the increased surface area introduce additional surface trap states, leading to greater non-radiative recombination. Here, an ions-induced growth method is employed to assembe lattice-anchored perovskite nanocomposites for efficient LEDs with high color purity. This approach enables the nanocomposite thin films, composed of 3D CsPbBr3 and its variant of zero-dimensional (0D) Cs4PbBr6, to feature significant low trap-assisted nonradiative recombination, enhanced light out-coupling with a corrugated surface, and well-balanced charge carrier transport. Based on the resultant 3D/0D perovskite nanocomposites, the perovskite LEDs (PeLEDs) achieving an remarkable external quantum efficiency of 31.0% at the emission peak of 521 nm with a narrow full width at half-maximum of only 18 nm. This sets a new benchmark for color purity in high performance PeLED research, highlighting the significant advantage of this approach.

Bidirectional Inhibiting Interfacial Ion Migration in the Inorganic Hole Transport Layer for Perovskite Light-Emitting Diodes.

Cu2ZnSnS4 (CZTS) is strong candidate for hole transport in perovskite light emitting diodes (PeLEDs) due to their cost-effectiveness, deep highest occupied molecular orbital (HOMO), and high hole mobility. However, its inherent polymetallic ions usually deteriorate the quality of the perovskite emission layer (EML) affecting device performance. In this study, a bidirectional anchoring strategy is proposed by adding 15-crown-5 ether (15C5) into CZTS hole transport layer (HTL) to suppress the reaction between HTL and EML. The 15C5 molecule interacts with Cu+, Zn2+ and Sn2+ cations forming host-guest complexes to impede their migration, which is elucidated by density functional theory calculations. Additionally, 15C5 can neutralize lead (Pb) defects by the abundant oxygen (O) and high electronegative cavities to reduce the nonradiative recombination of FAPbBr3 film. This bidirectional anchoring strategy effectively improves hole charge transport efficiency and suppresses nonradiative recombination at the HTL/EML interface. As a result, the optimized PeLEDs present a 3.5 times peak external quantum efficiency (EQE) from 3.12% to 11.08% and the maximum luminance (Lmax) increased from 24495 to 50584 cd m-2. These findings offer innovative insights into addressing the metal ion migration issue commonly observed in inorganic HTLs.

Efficient Quasi-2D Perovskite Based Blue Light-Emitting Diodes with Carbon Dots Modified Hole Transport Layer.

Quasi-2D perovskites based blue light-emitting diodes (LEDs) suffer from its poor electroluminescence performance, mainly caused by the nonradiative recombination in in defect-rich low-n phases and the unbalanced hole-electron injection in the device. Here, we developed a highly efficient quasi-2D perovskite based sky-blue LEDs behaving recorded external quantum efficiency (EQE) of 21.07% by employing carbon dots (CDs) as additives in the hole transport layer (HTL). We ascribe the high EQE to the effective engineering of CDs: (1) The CDs at the interface of HTLs can suppress the formation of low-efficient n = 1 phase, resulting a high luminescence quantum yield and energy transfer efficiency of the mixed n-phase quasi-2D perovskites. (2) The CDs additives can reduce the conductivity of HTL, partially blocking the hole injection, and thus making more balanced hole-electron injection. The CDs-treated devices have excellent Spectral stability and enhanced operational stability and could be a new alternative additive in the perovskite optoelectronic devices.

Chemical Reaction Modulated Low-Dimensional Phase Toward Highly Efficient Sky-Blue Perovskite Light-Emitting Diodes.

Blue perovskite light-emitting diodes (PeLEDs) are crucial avenues for achieving full-color displays and lighting based on perovskite materials. However, the relatively low external quantum efficiency (EQE) has hindered their progression towards commercial applications. Quasi-two-dimensional (quasi-2D) perovskites stand out as promising candidates for blue PeLEDs, with optimized control over low-dimensional phases contributing to enhanced radiative properties of excitons. Herein, the impact of organic molecular dopants on the crystallization of various n-phase structures in quasi-2D perovskite films. The results reveal that the highly reactive bis(4-(trifluoromethyl)phenyl)phosphine oxide (BTF-PPO) molecule could effectively restrain the formation of organic spacer cation-ordered layered perovskite phases through chemical reactions, simultaneously passivate those uncoordinated Pb2+ defects. Consequently, the prepared PeLEDs exhibited a maximum EQE of 16.6 % (@ 490 nm). The finding provides a new route to design dopant molecules for phase modulation in quasi-2D PeLEDs.

Monoammonium Modified Dion-Jacobson Quasi-2D Perovskite for High Efficiency Pure-Blue Light Emitting Diodes.

Quasi-2D perovskites exhibit impressive optoelectronic properties and hold significant promise for future light-emitting devices. However, the efficiency of perovskite light-emitting diodes (PeLEDs) is seriously limited by defect-induced nonradiative recombination and imbalanced charge injection. Here, the defect states are passivated and charge injection balance is effectively improved by introducing the additive cyclohexanemethylammonium (CHMA) to bromide-based Dion-Jacobson (D-J) structure quasi-2D perovskite emission layer. CHMA participates in the crystallization of perovskite, leading to high quality film composed of compact and well-contacted grains with enhanced hole transportation and less defects. As a result, the corresponding PeLEDs exhibit stable pure blue emission at 466 nm with a maximum external quantum efficiency (EQE) of 9.22%. According to current knowledge, this represents the highest EQE reported for pure-blue PeLEDs based on quasi-2D bromide perovskite thin films. These findings underscore the potential of quasi-2D perovskites for advanced light-emitting devices and pave the way for further advancements in PeLEDs.

Synergistic Steric Effect of Precursor And Antisolvent Enables Strongly Confined Perovskite Films with Efficient and Spectral Stable Blue Emission.

Reducing the crystal size of perovskites to the strong quantum confinement regime is an effective way to realize blue luminescence for light-emitting applications. However, challenges remain in directly constraining the crystal growth during film preparation to achieve three-dimensional quantum confinement, and the widely used long-chain ligands may bring difficulties for charge transport and unfavorably affect the device performance. Herein, we report a novel strategy for fabricating strongly confined blue-emitting perovskite nanocrystalline films via synergistic steric effect modulation by precursors and antisolvents. We synthesize cesium pentafluoropropanoate (CsPFPA) as a new type of precursor agent, where the steric effect of the PFPA group can help constrain the growth of perovskite crystals and passivate the defects. Furthermore, different types of antisolvents with varied molecular sizes and steric hindrance are used to regulate the size of perovskite crystals and improve film quality. Consequently, highly emissive blue perovskite films are realized with the emission wavelength effectively tuned in the blue region by varying the concentration of CsPFPA as well as the type of antisolvents. Based on the strongly confined perovskite films, blue light-emitting diodes (LEDs) are constructed, showing good spectral tunability and stability in the electroluminescence. This work demonstrates a novel pathway for developing bright perovskite blue emitters for LEDs, which may potentially advance their future applications in display and lighting.

Enhanced Efficiency and Stability of Sky Blue Perovskite Light-Emitting Diodes via Introducing Lead Acetate.

All-inorganic metal halide perovskite is promising for highly efficient and thermally stable perovskite light-emitting diodes (PeLEDs). However, there is still great room for improvement in the film quality, including low coverage and high trap density, which play a vital role in achieving high-efficiency PeLEDs. In this work, lead acetate (Pb(Ac)2) was introduced into the perovskite precursor solution as an additive. Experimental results show that perovskite films deposited from a one-step anti-solvent free solution process with increased surface coverage and reduced trap density were obtained, leading to enhanced photoluminescence (PL) intensity. More than that, the valence band maximum (VBM) of perovskite films was reduced, bringing about a better energy level matching the work function of the hole-injection layer (HIL) poly (3,4-ethylenedioxythiophene)-poly (styrene sulfonate) (PEDOT: PSS), which is facilitated for the hole injection, leading to a decrease in the turn-on voltage (Vth) of PeLEDs from 3.4 V for the control device to 2.6 V. Finally, the external quantum efficiency (EQE) of the sky blue PeLEDs (at 484 nm) increased from 0.09% to 0.66%. The principles of Pb(Ac)2 were thoroughly investigated by using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). This work provides a simple and effective strategy for improving the morphology of perovskite and therefore the performance of PeLEDs.

Recent Advances in Carbazole-Based Self-Assembled Monolayer for Solution-Processed Optoelectronic Devices.

Self-assembled molecules (SAMs) have shown great potential in the application of optoelectronic devices due to their unique molecular properties. Recently, emerging phosphonic acid-based SAMs, 2-(9Hcarbazol-9-yl)ethyl]phosphonic acid (2PACz), have successfully applied in perovskite solar cells (PSCs), organic solar cells (OSCs) and perovskite light emitting diodes (PeLEDs). More importantly, impressive results based on 2PACz SAMs are reported recently in succession. Therefore, it is essential to provide an insightful summary to promote it further development. In this review, the molecule design strategies about 2PACz are first concluded. Subsequently, this work systematically reviews the recent advances of 2PACz and its derivatives for single junction PSCs, tandem PSCs, OSCs and PeLEDs. Finally, this work concludes and discusses future challenges for 2PACz and its derivatives to further develop in optoelectronic devices.

High Efficiency and Low Roll-Off Pure-Red Perovskite LED Enabled by Simultaneously Inhibiting Auger and Trap Recombination of Quantum Dots.

CsPbI3 perovskite quantum dots (QDs) could achieve pure-red emission by reducing their size, but the increased exciton binding energy (EB) and surface defects for the small-sized QDs (SQDs) cause severe Auger and trap recombinations, thus worsening their electroluminescence (EL) performance. Herein, we utilize the dangling bonds of the SQDs as a driving force to accelerate KI dissolution to solve its low solubility in nonpolar solvents, thereby allowing K+ and I- to bond to the surface of SQDs. The EB of the SQDs was decreased from 305 to 51 meV because of the attraction of K+ to electrons, meanwhile surface vacancies were passivated by K+ and I-. The Auger and trap recombinations were simultaneously suppressed by this difunctional ligand. The SQD-based light-emitting diode showed a stable pure-red EL peak of 639 nm, an external quantum efficiency of 25.1% with low roll-off, and a brightness of 5934 cd m-2.

Enhancing the Performance of Perovskite Light-Emitting Diodes via Synergistic Effect of Defect Passivation and Dielectric Screening.

Metal halide perovskites, particularly the quasi-two-dimensional perovskite subclass, have exhibited considerable potential for next-generation electroluminescent materials for lighting and display. Nevertheless, the presence of defects within these perovskites has a substantial influence on the emission efficiency and durability of the devices. In this study, we revealed a synergistic passivation mechanism on perovskite films by using a dual-functional compound of potassium bromide. The dual functional potassium bromide on the one hand can passivate the defects of halide vacancies with bromine anions and, on the other hand, can screen the charged defects at the grain boundaries with potassium cations. This approach effectively reduces the probability of carriers quenching resulting from charged defects capture and consequently enhances the radiative recombination efficiency of perovskite thin films, leading to a significant enhancement of photoluminescence quantum yield to near-unity values (95%). Meanwhile, the potassium bromide treatment promoted the growth of homogeneous and smooth film, facilitating the charge carrier injection in the devices. Consequently, the perovskite light-emitting diodes based on this strategy achieve a maximum external quantum efficiency of ~ 21% and maximum luminance of ~ 60,000 cd m-2. This work provides a deeper insight into the passivation mechanism of ionic compound additives in perovskite with the solution method.

Z-Type Ligand Enables Efficient and Stable Deep-Blue Perovskite Light-Emitting Diodes.

During the synthesis of deep-blue perovskite quantum dots (PQDs), they generally emerge as a two-dimensional byproduct with poor yield and low photoluminescence quantum yield (PLQY) due to amine ligand enrichment-induced abundant surface defects. Herein, we provide a colloidal synthesis method to prepare deep-blue CsPbBr3 PQDs in a green nontoxic solvent via strategic Z-type ligand engineering. Z-type ligands of zinc octanoate enable the formation of robust coordination bonds with surface bromide ions of PQDs, maintaining acid-base equilibrium and reducing excess amine enrichment on the PQDs surface. Consequently, homogeneous and monodispersed PQDs with improved PLQY of 73% are successfully synthesized, achieving efficient deep-blue LEDs with a peak EQE of 5.46%, a maximum luminance of 847.6 cd/m2, and an operational half-lifetime of 14 min. The devices exhibit color coordinates of (0.137, 0.049), closely approximating the Rec. 2020 blue standard. Our work offers a potentially eco-friendly and viable route for realizing high-performance LEDs in the deep-blue region.

Crown-Assisted CsCu2I3 Growth and Trap Passivation for Perovskite Light-Emitting Diodes.

Copper (Cu)-based perovskites are promising for lead-free perovskite light-emitting diodes (PeLEDs). However, it remains a significant challenge to achieve high performance devices due to the nonradiative loss caused by the disordered crystallization and lack of passivation. Crown ethers are known to form host-guest complexes by the interaction between C-O-C groups and certain cations, and 18-crown-6 (18C6) with an appropriate complementary size can interact with Cs+ and Cu+ cations. Herein, we studied the interaction between CsCu2I3 and two crowns with the same cyclic size, 18C6 and dibenzo-18-crown-6 (D18C6). Particularly, D18C6 can reduce the nonradiative recombination rate of CsCu2I3 film by passivating the defects and optimizing the film morphology effectively. The room mean square (RMS) decreased from 5.06 to 2.95 nm, and the PLQY was promoted from 4.71% to 19.9%. Besides, D18C6 can also decrease the barrier of hole injection. The PeLEDs based on D18C6-modified CsCu2I3 realized noticeable improvement with a maximum luminance and EQE of 583 cd/m2 and 0.662%, respectively.

Eliminating the Adverse Impact of Composition Modulation in Perovskite Light-Emitting Diodes toward Ultra-High Brightness and Stability.

Excess ammonium halides as composition additives are widely employed in perovskite light-emitting diodes (PeLEDs), aiming to achieve high performance by controlling crystallinity and passivating defects. However, an in-depth understanding of whether excess organoammonium components affect the film physical/electrical properties and the resultant device instability is still lacking. Here, the trade-off between the performance and stability in high-efficiency formamidinium lead iodide (FAPbI3)-based PeLEDs with excess ammonium halides is pointed, and the underlying mechanism is explored. Systematic experimental and theoretical studies reveal that excess halide salt-induced ion-doping largely alters the PeLEDs properties (e.g., carrier injection, field-dependent ion-drifting, defect physics, and phase stability). A surface clean assisted cross-linking strategy is demonstrated to eliminate the adverse impact of composition modulation and boost the operational stability without sacrificing the efficiency, achieving a high efficiency of 23.6%, a high radiance of 964 W sr-1 m-2 (The highest value for FAPbI3 based PeLEDs), and a prolong lifetime of 106.1 h at large direct current density (100 mA cm-2), concurrently. The findings uncovered an important link between excess halide salts and the device performance, providing a guideline for rational design of stable, bright, and high efficiency PeLEDs.

Strongly-Confined CsPbBr3 Perovskite Quantum Dots with Ultralow Trap Density and Narrow Size Distribution for Efficient Pure-Blue Light-Emitting Diodes.

The development of pure-blue perovskite light-emitting diodes (PeLEDs) faces challenges of spectral stability and low external quantum efficiency (EQE) due to phase separation in mixed halide compositions. Perovskite quantum dots (QDs) with strong confinement effects are promising alternatives to achieve high-quality pure-blue PeLEDs, yet their performance is often hindered by the poor size distribution and high trap density. A strategy combining thermodynamic control with a polishing-driven ligand exchange process to produce high-quality QDs is developed. The strongly-confined pure-blue (≈470 nm) CsPbBr3 QDs exhibit narrow size distribution (12% dispersion) and are achieved in Br-rich ion environment based on growth thermodynamic control. Subsequent polishing-driven ligand exchange process removes imperfect surface sites and replaces initial long-chain organic ligands with short-chain benzene ligands. The resulting QDs exhibit high photoluminescence quantum yield (PLQY) to near-unity. The resulting PeLEDs exhibit a pure-blue electroluminescence (EL) emission at 472 nm with narrow full-width at half-maximum (FWHM) of 25 nm, achieving a maximum EQE of 10.7% and a bright maximum luminance of 7697 cd m-2. The pure-blue PeLEDs show ultrahigh spectral stability under high voltage, a low roll-off of EQE, and an operational half-lifetime (T50) of 127 min at an initial luminance of 103 cd m-2 under continuous operation.

Charge Injection and Auger Recombination Modulation for Efficient and Stable Quasi-2D Perovskite Light-Emitting Diodes.

The inefficient charge transport and large exciton binding energy of quasi-2D perovskites pose challenges to the emission efficiency and roll-off issues for perovskite light-emitting diodes (PeLEDs) despite excellent stability compared to 3D counterparts. Herein, alkyldiammonium cations with different molecular sizes, namely 1,4-butanediamine (BDA), 1,6-hexanediamine (HDA) and 1,8-octanediamine (ODA), are employed into quasi-2D perovskites, to simultaneously modulate the injection efficiency and recombination dynamics. The size increase of the bulky cation leads to increased excitonic recombination and also larger Auger recombination rate. Besides, the larger size assists the formation of randomly distributed 2D perovskite nanoplates, which results in less efficient injection and deteriorates the electroluminescent performance. Moderate exciton binding energy, suppressed 2D phases and balanced carrier injection of HDA-based PeLEDs contribute to a peak external quantum efficiency of 21.9%, among the highest in quasi-2D perovskite based near-infrared devices. Besides, the HDA-PeLED shows an ultralong operational half-lifetime T50 up to 479 h at 20 mA cm‒2, and sustains the initial performance after a record-level 30 000 cycles of ON-OFF switching, attributed to the suppressed migration of iodide anions into adjacent layers and the electrochemical reaction in HDA-PeLEDs. This work provides a potential direction of cation design for efficient and stable quasi-2D-PeLEDs.

Enhancing Light Outcoupling Efficiency via Anisotropic Low Refractive Index Electron Transporting Materials for Efficient Perovskite Light-Emitting Diodes.

Thanks to the extensive efforts toward optimizing perovskite crystallization properties, high-quality perovskite films with near-unity photoluminescence quantum yield are successfully achieved. However, the light outcoupling efficiency of perovskite light-emitting diodes (PeLEDs) is impeded by insufficient light extraction, which poses a challenge to the further advancement of PeLEDs. Here, an anisotropic multifunctional electron transporting material, 9,10-bis(4-(2-phenyl-1H-benzo[d]imidazole-1-yl)phenyl) anthracene (BPBiPA), with a low extraordinary refractive index (ne) and high electron mobility is developed for fabricating high-efficiency PeLEDs. The anisotropic molecular orientations of BPBiPA can result in a low ne of 1.59 along the z-axis direction. Optical simulations show that the low ne of BPBiPA can effectively mitigate the surface plasmon polariton loss and enhance the photon extraction efficiency in waveguide mode, thereby improving the light outcoupling efficiency of PeLEDs. In addition, the high electron mobility of BPBiPA can facilitate balanced carrier injection in PeLEDs. As a result, high-efficiency green PeLEDs with a record external quantum efficiency of 32.1% and a current efficiency of 111.7 cd A-1 are obtained, which provides new inspirations for the design of electron transporting materials for high-performance PeLEDs.

In Situ Hydrolysis of Phosphate Enabling Sky-Blue Perovskite Light-Emitting Diode with EQE Approaching 16.32.

The performance of blue perovskite light-emitting diodes (PeLEDs) lags behind the green and red counterparts owing to high trap density and undesirable red shift of the electroluminescence spectrum under operation conditions. Organic molecular additives were employed as passivators in previous reports. However, most commonly have limited functions, making it challenging to effectively address both efficiency and stability issues simultaneously. Herein, we reported an innovatively dynamic in situ hydrolysis strategy to modulate quasi-2D sky-blue perovskites by the multifunctional passivator phenyl dichlorophosphate that not only passivated the defects but also underwent in situ hydrolysis reaction to stabilize the emission. Moreover, hydrolysis products were beneficial for low-dimensional phase manipulation. Eventually, we obtained high-performance sky-blue PeLEDs with a maximum external quantum efficiency (EQE) of 16.32% and an exceptional luminance of 5740 cd m-2. More importantly, the emission peak of devices located at 485 nm remained stable under different biases. Our work signified the significant advancement toward realizing future applications of PeLEDs.