Narrow-bandgap Sn-Pb mixed perovskite single crystals for high-performance near-infrared photodetectors

Single Crystal Sn-Based Halide Perovskites

Sn-based halide perovskites are expected to be the best replacement for toxic lead-based counterparts, owing to their similar ionic radii and the optimal band gap for use in solar cells, as well as their versatile use in light-emitting diodes and photodetection applications. Concerns, however, exist about their stability under ambient conditions, an issue that is exacerbated in polycrystalline films because grain boundaries present large concentrations of defects and act as entrance points for oxygen and water, causing Sn oxidation. A current thriving research area in perovskite materials is the fabrication of perovskite single crystals, promising improved optoelectronic properties due to excellent uniformity, reduced defects, and the absence of grain boundaries. This review summarizes the most recent advances in the fabrication of single crystal Sn-based halide perovskites, with emphasis on synthesis methods, compositional engineering, and formation mechanisms, followed by a discussion of various challenges and appropriate strategies for improving their performance in optoelectronic applications.

Recent Advances in Perovskite Single-Crystal Thin Film Optoelectronic Devices

Among novel semiconductors, perovskites have gained significant attention due to their versatility, combining tunable optoelectronic properties with relatively easy fabrication processes. However, certain issues still hinder their widespread use, often related to the presence of defects and traps within the material. Beyond defect passivation in polycrystalline thin films, an alternative approach to enhancing material quality lies in the fabrication of single crystals. This review aims to provide an overview of the promising approaches explored to address specific challenges of perovskites that benefit from the single crystal nature, restricting our analysis to perovskite single crystal thin films (PSC-TF). We will discuss novel fabrication techniques and highlight recent achievements in devices, such as photodetectors, solar cells, and transistors. By examining the fundamental properties already discovered and showcasing the latest advancements, we aim to provide an overview of the perspectives and open challenges for PSC-TF in next-generation optoelectronic devices.

Day-Night imaging without Infrared Cutfilter removal based on metal-gradient perovskite single crystal photodetector

Day-Night imaging technology that obtains full-color and infrared images has great market demands for security monitoring and autonomous driving. The current mainstream solution relies on wide-spectrum silicon photodetectors combined with Infrared Cutfilter Removal, which increases complexity and failure rate. Here, we address these challenges by employing a perovskite photodetector based on Pb-Sn alloyed single crystal with a vertical bandgap-graded structure that presents variable-spectrum responses at different biases and extends the infrared detection range close to 1100 nm. Taking advantage of the Pb-Sn gradients in mobility and built-in field, the perovskite photodetector shows a large linear dynamic range of 177 dB. In addition, the optoelectronic characteristics feature long-term operational stability over a year. We further develop an imaging module prototype without Infrared Cutfilter Removal that exhibits excellent color fidelity with RGB color differences ranging from 0.48 to 2.46 under infrared interference and provides over 26-bit grayscale resolution in infrared imaging.

Red luminescent water stable lead-free 2D tin halide perovskite nanocrystals for photodetectors

In this report, we have synthesized an environmentally friendly hybrid organic-inorganic layered two-dimensional (2D) lead-free perovskite nanomaterial. The synthesized perovskites, namely (OleylAm)2SnI4 (MHP1), exhibit outstanding water stability and emit luminous red light. The photodetector constructed using our material showcases superior characteristics, including a faster response than comparable devices and improved rise and fall times compared to other 2D perovskite nanomaterials.

Perovskite Smart Windows: The Light Manipulator in Energy-Efficient Buildings

Uncontrolled sunlight entering through windows contributes to substantial heating and cooling demands in buildings, which leads to high energy consumption from the buildings. Recently, perovskite smart windows have emerged as innovative energy-saving technologies, offering the potential to adaptively control indoor solar heat gain through their impressive sunlight modulation capabilities. Moreover, harnessing the high-efficiency photovoltaic properties of perovskite materials, these windows have the potential to generate power, thereby realizing more advanced windows with combined light modulation and energy harvesting capabilities. This review summarizes the recent advancements in various chromic perovskite materials for achieving light modulation, focusing on both perovskite structures and underlying switching mechanisms. The discussion also encompasses device engineering strategies for smart windows, including the improvement of their optical and transition performance, durability, combination with electricity generation, and the evaluation of their energy-saving performance in building applications. Furthermore, the challenges and opportunities associated with perovskite smart windows are explicated, aimed at stimulating more academic research and advancing their pragmatic implementation for building energy efficiency and sustainability.

Performance Enhancement of Tin-Based Perovskite Photodetectors through Bifunctional Cesium Fluoride Engineering

Tin halide perovskites are rising as promising candidates for next-generation optoelectronic materials due to their good optoelectronic properties and relatively low toxicity. However, the high defect density and the easy oxidation of Sn2+ have limited their optoelectronic performance. Herein, we report the treatment of the FASnI3 (formamidinium tin, FA) perovskite film by a bifunctional cesium fluoride (CsF) additive, which improves the film quality and significantly enhances the photoelectric performance. The responsivity of the perovskite-based photodetector (PD) with an optimal CsF concentration of 15% is over 60 times larger than that of the PD without CsF. It indicates that both the Cs substitution and the fluoride anion additive from CsF inhibit the oxidation of Sn2+, optimize the crystal growth, and passivate the defects, demonstrating the dual roles of the CsF additive in improving the photoelectric performance. This work offers valuable insights into the additive selection for developing high-quality tin-based perovskite films and devices.

Volume-Confined Fabrication of Large-Scale Single-Crystalline Molecular Ferroelectric Thin Films and Their Applications in 2D Materials

With outstanding advantages of chemical synthesis, structural diversity, and mechanical flexibility, molecular ferroelectrics have attracted increasing attention, demonstrating themselves as promising candidates for next-generation wearable electronics and flexible devices in the film form. However, it remains a challenge to grow high-quality thin films of molecular ferroelectrics. To address the above issue, a volume-confined method is utilized to achieve ultrasmooth single-crystal molecular ferroelectric thin films at the sub-centimeter scale, with the thickness controlled in the range of 100-1000 nm. More importantly, the preparation method is applicable to most molecular ferroelectrics and has no dependency on substrates, showing excellent reproducibility and universality. To demonstrate the application potential, two-dimensional (2D) transitional metal dichalcogenide semiconductor/molecular ferroelectric heterostructures are prepared and investigated by optical spectroscopic method, proving the possibility of integrating molecular ferroelectrics with 2D layered materials. These results may unlock the potential for preparing and developing high-performance devices based on molecular ferroelectric thin films.

Ultrathin Self-Powered Heavy-Metal-Free Cu-In-Se Quantum Dot Photodetectors for Wearable Health Monitoring

Mechanically deformable photodetectors (PDs) are key device components for wearable health monitoring systems based on photoplethysmography (PPG). Achieving high detectivity, fast response time, and an ultrathin form factor in the PD is highly needed for next-generation wearable PPG systems. Self-powered operation without a bulky power-supply unit is also beneficial for point-of-care application. Here, we propose ultrathin self-powered PDs using heavy-metal-free Cu-In-Se quantum dots (QDs), which enable high-performance wearable PPG systems. Although the light-absorbing QD layer is extremely thin (∼40 nm), the developed PD exhibits excellent performance (specific detectivity: 2.10 × 1012 Jones, linear dynamic range: 102 dB, and spectral range: 250-1050 nm at zero bias), which is comparable to that of conventional rigid QD-PDs employing thick Pb-chalcogenide QD layers. This is attributed to material and device strategies─materials that include Cu-In-Se QDs, a MoS2-nanosheet-blended poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hole transport layer, a ZnO nanoparticle electron transport layer, Ag and ITO electrodes, and an ultrathin form factor (∼120 nm except the electrodes) that enable excellent mechanical deformability. These allow the successful application of QD-PDs to a wearable system for real-time PPG monitoring, expanding their potential in the field of mobile bioelectronics.

High-Speed Printing of Narrow-Band-Gap Sn-Pb Perovskite Layers toward Cost-Effective Manufacturing of Optoelectronic Devices

Narrow-band-gap Sn-Pb perovskites have emerged as one of the most promising solution-processed near-infrared (NIR) light-detection technologies, with the key figure-of-merit parameters already rivaling those of commercial inorganic devices, but maximizing the cost advantage of solution-processed optoelectronic devices depends on the ability to fast-speed production. However, weak surface wettability to perovskite inks and evaporation-induced dewetting dynamics have limited the solution printing of uniform and compact perovskite films at a high speed. Here, we report a universal and effective methodology for fast printing of high-quality Sn-Pb mixed perovskite films at an unprecedented speed of 90 m h^-1 by altering the wetting and dewetting dynamics of perovskite inks with the underlying substrate. A line-structured SU-8 pattern surface to trigger spontaneous ink spreading and fight ink shrinkage is designed to achieve complete wetting with a near-zero contact angle and a uniform dragged-out liquid film. The high-speed printed Sn-Pb perovskite films have both large perovskite grains (>100 μm) and excellent optoelectronic properties, yielding highly efficient self-driven NIR photodetectors with a large voltage responsivity over 4 orders of magnitude. Finally, the potential application of the self-driven NIR photodetector in health monitoring is demonstrated. The fast printing methodology provides a new possibility to extend the manufacturing of perovskite optoelectronic devices to industrial production lines.