Highly Bright and Stable Lead-Free Double Perovskite White Light-Emitting Diodes

Lead-free double perovskites (DPs) are emerging highly stable emitters with efficient broadband self-trapped exciton (STE) photoluminescence (PL), but their low electroluminescent (EL) efficiency is a critical shortcoming. This work promotes the external quantum efficiency (EQE) and luminance of DP-based white light-emitting diode (wLED) with a normal device structure to 0.76% and 2793 cd m-2 via two modifications: This work prevents the formation of adverse metallic silver, spatially confined STE, and lowers local site symmetry in Cs2 Na0.4 Ag0.6 In0.97 Bi0.03 Cl6 DP by terbium doping; and this work develops a guest-host strategy to improve film morphology, reduce defect density, and increase carrier mobility. These alterations cause substantial increase in STE radiative recombination and charge injection efficiency of perovskite layer. Finally, pure white EL with ideal color coordinates of (0.328, 0.329) and a record-breaking optoelectronic performance is achieved by introducing additional green carbon dots in LED to fill the deficient green component.

Controlled Synthesis of Lead-Free Double Perovskite Colloidal Nanocrystals for Nonvolatile Resistive Memory Devices

Although lead-free double perovskites such as Cs2AgBiBr6 have been widely explored, they still remain a daunting challenge for the controlled synthesis of lead-free double perovskite nanocrystals with highly tunable morphology and band structure. Here, we report the controlled synthesis of lead-free double perovskite colloidal nanocrystals including Cs2AgBiBr6 and Cs2AgInxBi1-xBr6 via a facile wet-chemical synthesis method for the fabrication of high-performance nonvolatile resistive memory devices. Cs2AgBiBr6 colloidal nanocrystals with well-defined cuboidal, hexagonal, and triangular morphologies are synthesized through a facile wet-chemical approach by tuning the reaction temperature from 150 to 190 °C. Further incorporating indium into Cs2AgBiBr6 to synthesize alloyed Cs2AgInxBi1-xBr6 nanocrystals not only can induce the indirect-to-direct bandgap transition with enhanced photoluminescence but also can improve its structural stability. After optimizing the active layers and device structure, the fabricated Ag/polymethylene acrylate@Cs2AgIn0.25Bi0.75Br6/ITO resistive memory device exhibits a low power consumption (the operating voltage is ∼0.17 V), excellent cycling stability (>10 000 cycles), and good synaptic property. Our study would enable the facile wet-chemical synthesis of lead-free double perovskite colloidal nanocrystals in a highly controllable manner for the development of high-performance resistive memory devices.

Ultralow-Power Machine Vision with Self-Powered Sensor Reservoir

A neuromorphic visual system integrating optoelectronic synapses to perform the in-sensor computing is triggering a revolution due to the reduction of latency and energy consumption. Here it is demonstrated that the dwell time of photon-generated carriers in the space-charge region can be effectively extended by embedding a potential well on the shoulder of Schottky energy barrier. It permits the nonlinear interaction of photocurrents stimulated by spatiotemporal optical signals, which is necessary for in-sensor reservoir computing (RC). The machine vision with the sensor reservoir constituted by designed self-powered Au/P(VDF-TrFE)/Cs2 AgBiBr6 /ITO devices is competent for both static and dynamic vision tasks. It shows an accuracy of 99.97% for face classification and 100% for dynamic vehicle flow recognition. The in-sensor RC system takes advantage of near-zero energy consumption in the reservoir, resulting in decades-time lower training costs than a conventional neural network. This work paves the way for ultralow-power machine vision using photonic devices.

Discovery of Lead-Free Perovskites for High-Performance Solar Cells via Machine Learning: Ultrabroadband Absorption, Low Radiative Combination, and Enhanced Thermal Conductivities

Exploring lead-free candidates and improving efficiency and stability remain the obstacle of hybrid organic-inorganic perovskite-based devices commercialization. Traditional trial-and-error methods seriously restrict the discovery especially for large search space, complex crystal structure and multi-objective properties. Here, the authors propose a multi-step and multi-stage screening scheme to accelerate the discovery of hybrid organic-inorganic perovskites A2 BB'X6 from a large number of candidates through combining machine learning with high-throughput calculations for pursuing excellent efficiency and thermal stability in solar cells. Followed by a series of screenings, the structure-property relationships mapping A2 BB'X6 properties are built and the predictions are close to reported experimental results. Successfully, four experimental-feasibly candidates with good stability, high Debye temperature and suitable band gap are screened out and further verified by density-functional theory calculations, in which the predicted efficiency for three lead-free candidates ((CH3 NH3 )2 AgGaBr6 , (CH3 NH3 )2 AgInBr6 and (C2 NH6 )2 AgInBr6 ) achieves 20.6%, 19.9% and 27.6% due to ultrabroadband absorption region ranging from UVC to IRC with excitonic radiative combination rates as low as 10 ps, large or intermediate polarons form with properties similar to CH3 NH3 PbI3 and the calculated thermal conductivities are 5.04, 4.39 and 5.16 Wm-1 K-1 , respectively, with Debye temperatures larger than 500 K, beneficial for suppression of both nonradiative combination and heat-induced degradation.

Mixed-Halide Double Perovskite Cs2 AgBiX6 (X=Br, I) with Tunable Optical Properties via Anion Exchange

Lead-free double perovskites, A2 M+ M'3+ X6 , are considered as promising alternatives to lead-halide perovskites, in optoelectronics applications. Although iodide (I) and bromide (Br) mixing is a versatile tool for bandgap tuning in lead perovskites, similar mixed I/Br double perovskite films have not been reported in double perovskites, which may be due to the large activation energy for ion migration. In this work, mixed Br/I double perovskites were realized utilizing an anion exchange method starting from Cs2 AgBiBr6 solid thin-films with large grain-size. The optical and structural properties were studied experimentally and theoretically. Importantly, the halide exchange mechanism was investigated. Hydroiodic acid was the key factor to facilitate the halide exchange reaction, through a dissolution-recrystallization process. In addition, the common organic iodide salts could successfully perform halide-exchange while retaining high mixed-halide phase stability and strong light absorption capability.

Codoping of Lead-Free Double Perovskites Promotes Near-Infrared Photoluminescence

In the rapidly developing smart era, near-infrared luminescent materials have important applications in various fields that are closely related to people. Nag and co-workers provided a first codoping strategy to achieve efficient near-infrared photoluminescence in lead-free double perovskite materials. Through the introduction of Bi³⁺ ions, a new energy state is formed that leads to the absorption of lower-energy light. The excited state formed by this light absorption subsequently excites f-electrons of Er³⁺ or Yb³⁺ ions, and the relaxation of these f-electrons results in near-infrared photoluminescence. This may open a new chapter in the application of perovskites for infrared detection and human sensing.

Lead-Free Halide Double Perovskite Cs2 AgBiBr6 with Decreased Band Gap

Environmentally friendly halide double perovskites with improved stability are regarded as a promising alternative to lead halide perovskites. The benchmark double perovskite, Cs₂AgBiBr₆, shows attractive optical and electronic features, making it promising for high‐efficiency optoelectronic devices. However, the large band gap limits its further applications, especially for photovoltaics. Herein, we develop a novel crystal‐engineering strategy to significantly decrease the band gap by approximately 0.26 eV, reaching the smallest reported band gap of 1.72 eV for Cs₂AgBiBr₆ under ambient conditions. The band‐gap narrowing is confirmed by both absorption and photoluminescence measurements. Our first‐principles calculations indicate that enhanced Ag–Bi disorder has a large impact on the band structure and decreases the band gap, providing a possible explanation of the observed band‐gap narrowing effect. This work provides new insights for achieving lead‐free double perovskites with suitable band gaps for optoelectronic applications.