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.

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.

/pdf/nucleation-controlled-growth-of-superior-lead-free-3vffj0en4p.pdf

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

Metal halide perovskite semiconductors have sprung to the forefront of research into optoelectronic devices and materials, largely because of their remarkable photovoltaic efficiency records above ...

/pdf/toward-understanding-space-charge-limited-current-2crredt3oy.pdf

Toward understanding space-charge limited current measurements on metal halide perovskites

Summary Ion migration in perovskite materials accelerates its decomposition—deteriorating device performance, causing baseline drift, and lowering imaging resolution. In particular, in X-ray detectors, the effect of ion migration is more obvious under the necessary high working bias. Here, we report an effective strategy to grow superior inch-sized, high-quality, zero-dimensional (0D)-structured, lead-free (CH3NH3)3Bi2I9 perovskite single crystals. These crystals have significantly lower ion migration, reduced dark current, and better environmental stability compared with other perovskites, enabling us to fabricate a type of 0D-structured perovskite X-ray detector. The X-ray detector shows high sensitivity of 1,947 μC Gyair−1 cm−2, low detection limit of 83 nGyair s−1, short response time of 23.3 ms, the lowest baseline drift of 5.0 × 10−10 nA cm−1 s−1 V−1, and the best long-term stability among all perovskites reported. The combination of large crystal size and excellent X-ray response allows us to fabricate the first 0D-structured perovskite X-ray imaging system.

Inch-Size 0D-Structured Lead-Free Perovskite Single Crystals for Highly Sensitive Stable X-Ray Imaging

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.

/pdf/multi-inch-single-crystalline-perovskite-membrane-for-high-5343auqab7.pdf

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

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