Two-dimensional (2D) halide perovskites have emerged as outstanding semiconducting materials thanks to their superior stability and structural diversity. However, the ever-growing field of optoelectronic device research using 2D perovskites requires systematic understanding of the effects of the spacer on the structure, properties, and device performance. So far, many studies are based on trial-and-error tests of random spacers with limited ability to predict the resulting structure of these synthetic experiments, hindering the discovery of novel 2D materials to be incorporated into high-performance devices. In this review, we provide guidelines on successfully choosing spacers and incorporating them into crystalline materials and optoelectronic devices. We first provide a summary of various synthetic methods to act as a tutorial for groups interested in pursuing synthesis of novel 2D perovskites. Second, we provide our insights on what kind of spacer cations can stabilize 2D perovskites followed by an extensive review of the spacer cations, which have been shown to stabilize 2D perovskites with an emphasis on the effects of the spacer on the structure and optical properties. Next, we provide a similar explanation for the methods used to fabricate films and their desired properties. Like the synthesis section, we will then focus on various spacers that have been used in devices and how they influence the film structure and device performance. With a comprehensive understanding of these effects, a rational selection of novel spacers can be made, accelerating this already exciting field.

The 2D Halide Perovskite Rulebook: How the Spacer Influences Everything from the Structure to Optoelectronic Device Efficiency.

In several photovoltaic (PV) technologies, the presence of electronic defects within the semiconductor band gap limit the efficiency, reproducibility, as well as lifetime. Metal halide perovskites (MHPs) have drawn great attention because of their excellent photovoltaic properties that can be achieved even without a very strict film-growth control processing. Much has been done theoretically in describing the different point defects in MHPs. Herein, we discuss the experimental challenges in thoroughly characterizing the defects in MHPs such as, experimental assignment of the type of defects, defects densities, and the energy positions within the band gap induced by these defects. The second topic of this Review is passivation strategies. Based on a literature survey, the different types of defects that are important to consider and need to be minimized are examined. A complete fundamental understanding of defect nature in MHPs is needed to further improve their optoelectronic functionalities.

Reducing Detrimental Defects for High-Performance Metal Halide Perovskite Solar Cells.

Ultraviolet photodetectors (UV PDs) with “5S” (high sensitivity, high signal-tonoise ratio, excellent spectrum selectivity, fast speed, and great stability) have been proposed as promising optoelectronics in recent years. To realize highperformance UV PDs, heterojunctions are created to form a built-in electrical field for suppressing recombination of photogenerated carriers and promoting collection efficiency. In this progress report, the fundamental components of heterojunctions including UV response semiconductors and other materials functionalized with unique effects are discussed. Then, strategies of building PDs with lattice-matched heterojunctions, van der Waals heterostructures, and other heterojunctions are summarized. Finally, several applications based on heterojunction/heterostructure UV PDs are discussed, compromising flexible photodetectors, logic gates, and image sensors. This work draws an outline of diverse materials as well as basic assembly methods applied in heterojunction/heterostructure UV PDs, which will help to bring about new possibilities and call for more efforts to unleash the potential of heterojunctions.

Recent Progress of Heterojunction Ultraviolet Photodetectors: Materials, Integrations, and Applications

The organic-inorganic hybrid lead halide perovskites have emerged as a series of star materials for solar cells, lasers and detectors. However, the issues raised by the toxic lead element and marginal stability due to the volatile organic components have severely limited their potential applications. In this work, we develop a nucleation-controlled solution method to grow large size high-quality Cs3Bi2I9 perovskite single crystals (PSCs). Using the technique, we harvest some centimeter-sized single crystals and achieved high device performance. We find that X-ray detectors based on PSCs exhibit high sensitivity of 1652.3 μC Gyair−1 cm−2 and very low detectable dose rate of 130 nGyair s−1, both desired in medical diagnostics. In addition, its outstanding thermal stability inspires us to develop a high temperature X-ray detector with stable response at up to 100 °C. Furthermore, PSCs exhibit high X-ray imaging capability thanks to its negligible signal drifting and extremely high stability. Organic-inorganic hybrid halide perovskites have been emerged as promising candidates for X-ray detectors while toxicity and instability are known issues. Here, Zhang et al. grow large size lead-free single crystals for high performance X-ray detection and imaging devices that can operate at 100 °C.

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Nucleation-controlled growth of superior lead-free perovskite Cs3Bi2I9 single-crystals for high-performance X-ray detection.

Summary The two-dimensional (2D) layered organic-inorganic hybrid perovskites have demonstrated better performance in applications than their 3D counterparts due to their promising stability and excellent optoelectronic properties. In particular, the 2D perovskites show even better performance for certain optoelectronic devices, especially the ones fabricated on (001) plane. Unfortunately, these advantageous characteristics cannot be easily extracted from the randomly oriented microcrystalline films. Herein, we developed a surface-tension-controlled crystallization method to prepare large 2D (C6H5C2H4NH3)2PbI4 ((PEA)2PbI4) perovskite single crystals (PSCs). Using this technique, we have harvested some inch-sized 2D (PEA)2PbI4 PSCs, with the largest one reaching 36 mm in length, resulting in extraordinary device performance. More specifically, the photodetectors fabricated on the (001) plane exhibit responsivity as high as 139.6 A W−1, external quantum efficiency 37,719.6%, detectivity 1.89 × 1015 cm Hz1/2 W−1, and response speed as fast as τrise = 21 μs and τdecay = 37 μs.

Surface-Tension-Controlled Crystallization for High-Quality 2D Perovskite Single Crystals for Ultrahigh Photodetection

Single crystalline perovskites exhibit high optical absorption, long carrier lifetime, large carrier mobility, low trap-state-density and high defect tolerance. Unfortunately, all single crystalline perovskites attained so far are limited to bulk single crystals and small area wafers. As such, it is impossible to design highly demanded flexible single-crystalline electronics and wearable devices including displays, touch sensing devices, transistors, etc. Herein we report a method of induced peripheral crystallization to prepare large area flexible single-crystalline membrane (SCM) of phenylethylamine lead iodide (C6H5C2H4NH3)2PbI4 with area exceeding 2500 mm2 and thinness as little as 0.6 μm. The ultrathin flexible SCM exhibits ultralow defect density, superior uniformity and long-term stability. Using the superior ultrathin membrane, a series of flexible photosensors were designed and fabricated to exhibit very high external quantum efficiency of 26530%, responsivity of 98.17 A W−1 and detectivity as much as 1.62 × 1015 cm Hz1/2 W−1 (Jones). Hybrid halide perovskite single crystals show excellent optoelectronic properties but their small size and large thickness limit their application. Herein Liu et al. grow large area ultrathin flexible crystalline membrane of layered perovskite and demonstrate high detectivity in the flexible photosensors.

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Multi-inch single-crystalline perovskite membrane for high-detectivity flexible photosensors

Low-temperature-gradient crystallization for multi-inch high-quality perovskite single crystals for record performance photodetectors

Two-dimensional (2D) metal halide perovskites have shown great potential in high performance optoelectronic applications due to their intrinsic quantum well structure, enhanced moisture- and photo-stability. Unfortunately, there is still no effective method to produce large-size 2D hybrid perovskite single crystals, and consequently there is lack of research on its optoelectronic applications. Herein, we report the utilization of a controlled evaporation process to grow well-defined large-size (>200 mm2) 2D (PEA)2PbBr4 (C6H5CH2CH2NH3+, PEA+) single crystals. Upon careful examination of the crystal growth kinetics, it was found that the optimum temperature for the growth of (PEA)2PbBr4 single crystal wafer was 23 ± 0.5 °C. The wafers that were harvested showed high mu-tau (μτ) product, superior environmental stability and irradiation resistance. Furthermore, an array of planar-type UV photodetectors were designed and fabricated with extremely low dark current (∼10−13 A), large ON/OFF current ratio (∼105), very high specific detectivity (∼1013 cm Hz1/2 W−1) and fast response rate (∼0.4 ms). All the above performance data are among the best achieved compared to those of the state-of-the-art materials including ZnO, TiO2 and GaN in the field. It makes us believe that the availability of these materials may pave the way for ultrafast optical computing and optical communications.

Two-dimensional (PEA)2PbBr4 perovskite single crystals for a high performance UV-detector

Among the various developed perovskite materials, HC(NH2)2PbI3 (FAPbI3) is considered one of the best photovoltaic materials due to its high thermal stability and smaller band gap for light harvesting over a broader spectrum. However, the relatively large size of HC(NH2)2+ (FA) makes HC(NH2)2PbI3 prone to a phase transition from its black phase to the yellow phase at room temperature. Reduction of the effective ion size by doping with smaller ions is an effective way to stabilize black-phase FAPbI3. However, the effect of dopants on the optoelectronic properties of doped formamidinium lead halide perovskites has rarely been investigated. In this work, we prepared Cs-, Br- and Cs–Br co-doped FAPbI3 single-crystal materials with doping concentrations of 5%, 10% and 15%. The impacts of dopants on the phase stability, trap state density, carrier lifetime and carrier mobility are thoroughly investigated. It is found that Cs doping is not effective at stabilizing the black phase, while Br is a good stabilizer at concentrations exceeding 6.7%. Furthermore, the Cs and Br co-doped samples could be synergistically enhanced for high performance optoelectronics and show not only better stability but also much improved carrier lifetimes of 1.23 μs and carrier mobilities of 427.3 cm2 V−1 s−1, which resulted in a longer carrier diffusion length of approximately 30 μm as well as a photodetector with a high responsivity of 0.64 A/W.

Qingxian Li

Papers

Multi-inch single-crystalline perovskite membrane for high-detectivity flexible photosensors

Low-temperature-gradient crystallization for multi-inch high-quality perovskite single crystals for record performance photodetectors

Two-dimensional (PEA)2PbBr4 perovskite single crystals for a high performance UV-detector

Synergistic enhancement of Cs and Br doping in formamidinium lead halide perovskites for high performance optoelectronics