Fine Purification Engineering Enables Efficient Perovskite QLEDs with Efficiency Exceeding 23.

Perovskite quantum dots (PeQDs) have great application prospects in fields such as displays and solar cells due to their adjustable band gap, high absorption coefficient, high carrier mobility, and solution processability. However, the ionic crystal characteristic of PeQDs and their surface ligands have led to problems such as solvent sensitivity, poor crystal stability, and difficulty in adjusting the photoelectric properties, which are challenges in high-quality PeQDs. Here, to solve the problem of fluorescence degradation caused by phase change and loss of surface ligands during the purification process of CsPbI3 QDs, this work develops a purification strategy that finely regulates the polarity of the purification solvent, to obtain high-purity perovskite. This strategy can tune the surface ligand concentration and optoelectronic properties while maintaining the crystal stability. The optimized purification process enables the quantum dots to maintain the same level of luminescence performance as the original solution (PLQY is ∼90%). Meanwhile, the electrical properties are improved to significantly increase the exciton recombination rate under an electrical drive. Finally, a highly efficient QLED with an external quantum efficiency of exceeding 23% can be achieved. This scheme for fine purification of CsPbI3 QDs will provide some inspiration for the development of efficient PeQDs and the realization of high-performance optoelectronic devices.

High-Color-Rendition White QLEDs by Balancing Red, Green and Blue Centres in Eco-Friendly ZnCuGaS:In@ZnS Quantum Dots.

White light-emitting diodes (WLEDs) are the key components in the next-generation lighting and display devices. The inherent toxicity of Cd/Pb-based quantum dots (QDs) limits the further application in WLEDs. Recently, more attention is focused on eco-friendly QDs and their WLEDs, especially the phosphor-free WLEDs based on mono-component, which profits from bias-insensitive color stability. However, the imbalanced carrier distribution between red-green-blue luminescent centers, even the absence of a certain luminescent center, hinders their balanced and stable photoluminescence/electroluminescence (PL/EL). Here, an In3+-doped strategy in Zn-Cu-Ga-S@ZnS QDs is first proposed, and the balanced carrier distribution is realized by non-equivalent substitution and In3+ doping concentration modulation. The alleviation of the green emitter by the In3+-related red emitter and the compensation of blue emitter by the Zn-related electronic states contribute to the balanced red-green-blue emitting with high PL quantum yield (PLQY) of 95.3% and long lifetime (T90) of over 1100 h in atmospheric conditions. Thus, the In3+-doped WLEDs can achieve exceedingly slight proportional variations between red-green-blue EL intensity over time (∆CIE = (0.007, 0.009)), and high champion CRI of 94.9. This study proposes a single-component QD with balanced and stable red-green-blue PL/EL spectrum, meeting the requirements of lighting and display.

A Bifunctional Optoelectronic Device for Photodetection and Photoluminescence Switching Based on Graphene/ZnTe/Graphene van der Waals Heterostructures.

Controlling the dynamic processes, such as generation, separation, transport, and recombination, of photoexcited carriers in a semiconductor is foundational in the design of various devices for optoelectronic applications. One may imagine that if different processes can be manipulated in one single device and thus generate useful signals, a multifunctional device can be realized, and the toolbox for integrated optoelectronics will be expanded. Here, we revealed that in a graphene/ZnTe/graphene van der Waals (vdW) heterostructure, the carriers can be generated by illumination from visible to infrared frequencies, and thus, the detected spectrum range extends to the communication band, well beyond the band gap of ZnTe (2.26 eV). More importantly, we are able to control the competition between separation and recombination of the photoexcited carriers by an electric bias along the thickness-defined channel of the ZnTe flake: as the bias increases, the photodetecting performance, e.g. response speed and photocurrent, are improved due to the efficient separation of carriers; synchronously, the photoluminescence (PL) intensity decreases and even switches off due to the suppressed recombination process. The ZnTe-based vdW heterostructure device thus integrates both photodetection and PL switching functions by manipulating the generation, separation, transport, and recombination of carriers, which may inspire the design of the next generation of miniaturized optoelectronic devices based on the vdW heterostructures made by various thin flakes.

Bionic electroluminescent perovskite light-emitting device.

A perovskite light-emitting-diode (PeLED) displaying green color is combined with a brown fluorescent coating (FC) layer to form a hybrid FC-PeLED system. The FC-PeLED system can simulate the natural process of bionics of plant colors from green to brown through a low energy (<0.6 mW) input, promoting the development of future low-cost and low-power consumption bionics technology.

Full solution-processed heavy-metal-free mini-QLEDs for flexible display applications.

Micro/mini displays play an extremely significant role in the modern information society, which is treated as a promising technology for a range of applications. Here, utilizing the full solution process with the electrode array mask, we successfully achieved passive-matrix and active-matrix mini-quantum dot light-emitting diodes (PM/AM-m-QLEDs) based on heavy-metal-free blue ZnTeSe/ZnS QDs. The pixels per inch (PPI) of m-QLEDs fabricated in this study can reach 36, 90, 180, and 360, which meet the requirements of televisions, computers, and mobile phones. Moreover, by adjusting the electrodes, m-QLEDs achieved patterned display applications based on both flexible and solid substrates. These results imply that heavy-metal-free blue m-QLEDs show a wide display application potential, i.e., AM/PM displays. Given the low-cost advantage of solution-processed QDs, our proposed techniques could pave the way for low-cost displays.

A Universal Ternary-Solvent-Ink Strategy toward Efficient Inkjet-Printed Perovskite Quantum Dot Light-Emitting Diodes.

Toward next-generation electroluminescent quantum dot (QD) displays, inkjet printing technique has been convinced as one of the most promising low-cost and large-scale manufacturing of patterned quantum dot light-emitting diodes (QLEDs). The development of high-quality and stable QD inks is a key step to push this technology toward practical applications. Herein, a universal ternary-solvent-ink strategy is proposed for the cesium lead halides (CsPbX3 ) perovskite QDs and their corresponding inkjet-printed QLEDs. With this tailor-made ternary halogen-free solvent (naphthene, n-tridecane, and n-nonane) recipe, a highly dispersive and stable CsPbX3 QD ink is obtained, which exhibits much better printability and film-forming ability than that of the binary solvent (naphthene and n-tridecane) system, leading to a much better qualitied perovskite QD thin film. Consequently, a record peak external quantum efficiency (EQE) of 8.54% and maximum luminance of 43 883.39 cd m-2 is achieved in inkjet-printed green perovskite QLEDs, which is much higher than that of the binary-solvent-system-based devices (EQE = 2.26%). Moreover, the ternary-solvent-system exhibits a universal applicability in the inkjet-printed red and blue perovskite QLEDs as well as cadmium (Cd)-based QLEDs. This work demonstrates a new strategy for tailor-making a general ternary-solvent-QD-ink system for efficient inkjet-printed QLEDs as well as the other solution-processed electronic devices in the future.

Perovskite White Light Emitting Diodes: Progress, Challenges, and Opportunities.

As global warming, energy shortages, and environment pollution have intensified, low-carbon and energy-saving lighting technology has attracted great attention worldwide. Light emitting diodes (LEDs) have been around for decades and are considered to be the most ideal lighting technology currently due to their high luminescence efficiency (LE) and long lifespan. Besides, along with the development of modern technology, lighting technologies with higher performance and more functions are desired. Perovskite based LEDs (PeLEDs) have recently emerged as ideal candidates for lighting technology owing to the extraordinary photoelectric properties of perovskite, such as high photoluminescence quantum yields (PLQYs), easy wavelength tuning, and low-cost synthesis. Herein, we open this review by introducing the background of white LEDs (WLEDs), including their light-emitting mechanism, typical characteristics, and key indicators in applications. Then, four main approaches to fabricate WLEDs are discussed and compared. After that, in accordance with the four categories, we focus on the recent progress of white PeLEDs (Pe-WLEDs), followed by the challenges and opportunities for Pe-WLEDs in practical application. Meanwhile, some pertinent countermeasures to their challenges are put forward. Finally, the development promise of Pe-WLEDs is explored.

Research progress of full electroluminescent white light-emitting diodes based on a single emissive layer.

Carbon neutrality, energy savings, and lighting costs and quality have always led to urgent demand for lighting technology innovation. White light-emitting diodes (WLEDs) based on a single emissive layer (SEL) fabricated by the solution method have been continuously researched in recent years; they are advantageous because they have a low cost and are ultrathin and flexible. Here, we reviewed the history and development of SEL-WLEDs over recent years to provide inspiration and promote their progress in lighting applications. We first introduced the emitters and analysed the advantages of these emitters in creating SEL-WLEDs and then reviewed some cases that involve the above emitters, which were formed via vacuum thermal evaporation or solution processes. Some notable developments that deserve attention are highlighted in this review due to their potential use in SEL-WLEDs, such as perovskite materials. Finally, we looked at future development trends of SEL-WLEDs and proposed potential research directions.

Long-Term Stable Near-Infrared-Short-Wave-Infrared Photodetector Driven by the Photothermal Effect of Polypyrrole Nanostructures.

Polypyrrole (PPy) is a conductive polymer and widely applied in different applications owing to its broadband absorption in the UV-visible, near-infrared (NIR), and short-wave-infrared (SWIR) spectrum, excellent conductivity, and strong photothermal effect. In this work, we explored for the first time the photothermal effect of PPy nanoparticles (PPy-NPs) in a photothermal-induced detector structure and developed a new type of air-stable hybrid PPy-NPs/Pt photodetector (PD) with NIR/SWIR sensitivity. By combining PPy-NPs with a platinum (Pt)-resistive pattern, we fabricated PPy-NPs/Pt PDs that are sensitive to illumination in the wavelength range from 800 to 2000 nm. Under the illumination of λ = 1.5 µm, the maximum photoresponsivity was measured to be ∼1.3 A/W with a 131 µs photoresponse rise time. Owing to the excellent material stability from both PPy-NPs and the Pt pattern, the current photodetectors show long-term stable photoresponsivity when they were stored in air without encapsulation. The results suggest that the PPy-NPs/Pt hybrid PDs are promising candidates for a new type of low-cost and broadband due to their multiple advantages such as free of toxic heavy metals, air stability, and solution processing.

Colloidal upconversion nanocrystals enable low-temperature-grown GaAs photoconductive switch operating atλ = 1.55µm.

Microwave photoconductive switches, allowing an optical control on the magnitude and phase of the microwave signals to be transmitted, are important components for many optoelectronic applications. In recent years, there are significant demands to develop photoconductive switches functional in the short-wave-infrared spectrum window (e.g.λ = 1.3-1.55µm) but most state-of-the-art semiconductors for photoconductive switches cannot achieve this goal. In this work, we propose a novel approach, by the use of solution-processed colloidal upconversion nanocrystals deposited directly onto low-temperature-grown gallium arsenide (LT-GaAs), to achieve microwave photoconductive switches functional atλ = 1.55µm illumination. Hybrid upconversion Er3+-doped NaYF4nanocrystal/LT-GaAs photoconductive switch was fabricated. Under a continuous waveλ = 1.55µm laser illumination (power density âˆ¼ 12.9 mWµm-2), thanks to the upconversion energy transfer from the nanocrystals, a more than 2-fold larger value in decibel was measured for the ON/OFF ratio on the hybrid nanocrystal/LT-GaAs device by comparison to the control device without upconversion nanoparticles. A maximum ON/OFF ratio reaching 20.6 dB was measured on the nanocrystal/LT-GaAs hybrid device at an input signal frequency of 20 MHz.

Enhancing the Efficiency and Stability of Triple-Cation Perovskite Solar Cells by Eliminating Excess PbI2 from the Perovskite/Hole Transport Layer Interface.

Metal halide perovskites are promising contenders for next-generation photovoltaic applications due to their remarkable photovoltaic efficiency and their compatibility with solution-processed fabrication. Among the various strategies to control the crystallinity and the morphology of the perovskite active layer and its interfaces with the transport layers, fabrication of perovskite solar cells from precursor solutions with a slight excess of PbI2 has become very common. Despite this, the role of such excess PbI2 is still rather controversial, lacking consensus on its effect on the bulk and interface properties of the perovskite layer. In this work, we investigate the effect of removing the excess PbI2 from the surface of a triple-cation mixed-halide Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 perovskite layer by four different organic salts on their photovoltaic performance and stability. We show that treatments with iodide salts such as methylammonium iodide (MAI) and formamidinium iodide (FAI) can lead to the strongest beneficial effects on solar cell efficiency, charge recombination suppression, and stability while non-iodide salts such as methylammonium bromide (MABr) and methylammonium chloride (MACl) can also provide improvement in terms of charge recombination suppression and stability to a moderate extent in comparison to the untreated sample. Under optimized conditions and continuous solar illumination, the MAI- and FAI-treated devices maintained 81 and 86% of their initial power conversion efficiency (PCEs), respectively, after 100 h of continuous illumination (versus 64% for the untreated solar cell with excess PbI2). Our study demonstrates that eliminating excess PbI2 at the perovskite/hole transport layer (HTL) interface by treating the perovskite surface with organic salts is a simple and efficient route to enhance the efficiency, and in particular the stability of perovskite solar cells.

Upconversion nanoparticles extending the spectral sensitivity of silicon photodetectors to λ = 1.5 µm.

The telecommunication wavelength of λ = 1.5 µm has been playing an important role in various fields. In particular, performing photodetection at this wavelength is challenging, demanding more performance stability and lower manufacturing cost. In this work, upconversion nanoparticle (UCNP)/Si hybrid photodetectors (hybrid PDs) are presented, made by integrating solution-processed Er3+-doped NaYF4 upconversion nanoparticles (UCNPs) onto a silicon photodetector. After optimization, we demonstrated that a layer of UCNPs can well lead to an effective spectral sensitivity extension without sacrificing the photodetection performance of the Si photodetector in the visible and near-infrared (near-IR) spectrum. Under λ = 1.5 µm illumination, the hybrid UCNPs/Si-PD exhibits a room-temperature detectivity of 6.15 × 1012 Jones and a response speed of 0.4 ms. These UCNPs/Si-PDs represent a promising hybrid strategy in the quest for low-cost and broadband photodetection that is sensitive in the spectrum from visible light down to the short-wave infrared.

Heavy-Metal-Free Flexible Hybrid Polymer-Nanocrystal Photodetectors Sensitive to 1.5 µm Wavelength.

Photodetection in the short-wave infrared (SWIR) wavelength window represents one of the core technologies allowing for many applications. Most current photodetectors suffer from high cost due to the epitaxial growth requirements and the ecological issue due to the use of highly toxic heavy-metal elements. Toward alternative SWIR photodetection strategies, in this work, high-performance heavy-metal-free flexible photodetectors sensitive to λ = 1.5 µm photons are presented based on the formation of a solution-processed hybrid composed of a conjugated diketopyrrolopyrrole-base polymer/PC70BM bulk heterojunction organic host together with inorganic guest NaYF4:15%Er3+ upconversion nanoparticles (UCNPs). Under the illumination of λ = 1.5 µm SWIR photons, optimized hybrid bulk-heterojunction (BHJ)/UCNP photodetectors exhibit a photoresponsivity of 0.73 and 0.44 mA/W, respectively, for devices built on rigid indium tin oxide (ITO)/glass and flexible ITO/polyethylene terephthalate substrates. These hybrid photodetectors are capable of performing SWIR photodetection with a fast operation speed, characterized by a short photocurrent rise time down to 80 µs, together with an excellent mechanical robustness for flexible applications. Exhibiting simultaneously multiple advantages including solution-processability, flexibility, and the absence of toxic heavy metal elements together with a fast operation speed and good photoresponsivity, these hybrid BHJ(DPPTT-T/PC70BM)/UCNP photodetectors are promising candidates for next-generation low-cost and high-performance SWIR photodetectors.

Hybrid plasmonic gold-nanorod-platinum short-wave infrared photodetectors with fast response.

Short-wave infrared (SWIR) photodetectors, sensitive to the wavelength range between 1 and 3 µm, are essential components for various applications, which constantly demand devices with a lower cost, a higher responsivity and a faster response. In this work, a new hybrid device structure is presented for SWIR photodetection composing a coupling between solution-processed colloidal plasmonic gold (Au) NRs and a morphology-optimized resistive platinum (Pt) microwire. Pt microwires harvest efficiently the photothermal effect of Au NRs and in return generating a change of device resistance. A fast photon-heat-resistance conversion happens in these Au-NRs/Pt photodetectors exhibiting a response (rise) time of 97 µs under the illumination of a λ = 1.5 µm laser. Clear photoresponse can be observed in these devices at a laser illumination with a modulation frequency up to 50 kHz. The photoresponsivity of the current devices reached 4500 Ω W-1 under a laser power of 0.2 mW, which is equivalent to a responsivity of 340 mA W-1 under a DC bias of 1 V. A series of mapping experiments were performed providing a direct correlation between Au NRs and the device zone where resistance change happens under a laser illumination modulated at different frequencies.

Multifunctional Silver Nanoparticle Interlayer-Modified ZnO as the Electron-Injection Layer for Efficient Inverted Organic Light-Emitting Diodes.

The insufficient electron injection constitutes the major obstacle to achieving high-performance inverted organic light-emitting diodes (OLEDs). Here, a facile electron-injection architecture featuring a silver nanoparticle (AgNPs) interlayer-modified sol-gel-derived transparent zinc oxide (ZnO) ultrathin film is proposed and demonstrated. The optimized external quantum efficiencies of the developed inverted fluorescent and phosphorescent OLEDs capitalized on our proposed electron-injection structure reached 4.0 and 21.2% at a current density of 20 mA cm-2 and increased by a factor of 1.90 and 2.86 relative to a reference device without the AgNP interlayer, while simultaneously reducing the operational voltage and substantially ameliorating the device efficiency. Detailed analyses reveal that the local surface plasmon resonance emanated from AgNPs plays three meaningful roles simultaneously: suppressing the surface plasmon polariton mode loss, aiding in energy-level alignments, and inducing and reinforcing the local exciton-plasmon coupling electric field. Among these interesting and multifunctional roles, the enhanced local exciton-plasmon coupling electric field dominates the electron injection enhancement and substantial increases the device efficiency. Additionally, the light-scattering effect also helps in recovering the trapped light energy flux and thus improves the device efficiency. The proposed approach and findings provide an alternative path to fabricate high-performance inverted OLEDs and other related organic electronic or optoelectronic devices.

Probing charge transfer states at organic and hybrid internal interfaces by photothermal deflection spectroscopy.

In organic and hybrid photovoltaic devices, the asymmetry required for charge separation necessitates the use of a donor and an acceptor material, resulting in the formation of internal interfaces in the device active layer. While the core objective of these interfaces is to facilitate charge separation, bound states between electrons and holes may form across them, resulting in a loss mechanism that diminishes the performance of the solar cells. These interfacial transitions appear in organic systems as charge transfer (CT) states and as bound charge pairs (BCP) in hybrid systems. Despite being similar, the latter are far less investigated. Herein, we employ photothermal deflection spectroscopy and pump-push-probe experiments in order to determine the characteristics and dynamics of interfacial states in two model systems: an organic P3HT:PCBM and hybrid P3HT:ZnO photovoltaic layer. By controlling the area of the internal interface, we identify CT states between 1.4 eV and 1.8 eV in the organic bulk-heterojunction (BHJ) and BCP between 1.1 eV and 1.4 eV in the hybrid BHJ. The energetic distribution of these states suggests that they not only contribute to losses in photocurrent, but also significantly limit the possible maximum open circuit voltage obtainable from these devices.

Short-Wave Infrared Sensor by the Photothermal Effect of Colloidal Gold Nanorods.

Photodetection in the short-wave infrared (SWIR) spectrum is a challenging task achieved often by costly low bandgap compound semiconductors involving highly toxic elements. In this work, an alternative low-cost approach is reported for SWIR sensors that rely on the plasmonic-induced photothermal effect of solution-processed colloidal gold nanorods (Au NRs). A series of uniform solution-processed Au NRs of various aspect ratios are prepared exhibiting a strong and well-defined longitudinal localized surface plasmon resonance (L-LSPR) maximum from 900 nm to 1.3 µm. A hybrid device structure is fabricated by applying Au NRs on the surface of a thermistor. Under a monochromatic illumination, hybrid Au-NR/thermistor devices exhibit a clear photoresponse in the form of photoinduced resistance drop in the wavelength window from 1.0 to 1.8 µm. The photoresponsivity of such hybrid devices reaches a maximum value of 4.44 × 107 Ω W-1 at λ = 1.4 µm (intensity = 0.28 mW cm-2 ), a wavelength in agreement with the L-LSPR of the Au NRs applied. Colloidal Au NRs, capable to perform fast conversion between photon absorption and thermal energy, thus open an interesting avenue for alternative low-cost SWIR photodetection.

Compact layer free mixed-cation lead mixed-halide perovskite solar cells.

Thickness-tunable and compact FA0.83Cs0.17Pb(I0.6Br0.4)3 perovskite thin films are achieved with a large grain size up to 12 microns. They are then employed to fabricate functional solar cells with a simplified planar structure without the use of electron-transport (ETL) layers. These results are highly encouraging for the future large-scale fabrication of FA0.83Cs0.17Pb(I0.6Br0.4)3-based solar cells.

Efficient Color-Stable Inverted White Organic Light-Emitting Diodes with Outcoupling-Enhanced ZnO Layer.

Inverted organic light-emitting diode (OLED) has attracted extensive attention due to the demand in active-matrix OLED display panels as its geometry enables the direct connection with n-channel transistor backplane on the substrate. One key challenge of high-performance inverted OLED is an efficient electron-injection layer with superior electrical and optical properties to match the indium tin oxide cathode on substrate. We here propose a synergistic electron-injection architecture using surface modification of ZnO layer to simultaneously promote electron injection into organic emitter and enhance out-coupling of waveguided light. An efficient inverted white OLED is realized by introducing the nanoimprinted aperiodic nanostructure of ZnO for broadband and angle-independent light out-coupling and inserting an n-type doped interlayer for energy level tuning and injection barrier lowering. As a result, the optimized inverted white OLEDs have an external quantum efficiency of 42.4% and a power efficiency of 85.4 lm W1-, which are accompanied by the superiority of angular color stability over the visible wavelength range. Our results may inspire a promising approach to fabricate high-efficiency inverted OLEDs for large-scale display panels.

Microcavity-Free Broadband Light Outcoupling Enhancement in Flexible Organic Light-Emitting Diodes with Nanostructured Transparent Metal-Dielectric Composite Electrodes.

Flexible organic light-emitting diodes (OLEDs) hold great promise for future bendable display and curved lighting applications. One key challenge of high-performance flexible OLEDs is to develop new flexible transparent conductive electrodes with superior mechanical, electrical, and optical properties. Herein, an effective nanostructured metal/dielectric composite electrode on a plastic substrate is reported by combining a quasi-random outcoupling structure for broadband and angle-independent light outcoupling of white emission with an ultrathin metal alloy film for optimum optical transparency, electrical conduction, and mechanical flexibility. The microcavity effect and surface plasmonic loss can be remarkably reduced in white flexible OLEDs, resulting in a substantial increase in the external quantum efficiency and power efficiency to 47.2% and 112.4 lm W(-1).