X-ray detection is crucial for various applications, including medical imaging and industrial inspections. Metal halide perovskites, particularly 3D perovskites, have shown exceptional promise due to their high sensitivity and low detection limits. However, ion migration in 3D perovskites limits their long-term stability, hindering commercialization. Low-dimensional perovskites, especially Q-2D perovskites, offer improved stability by suppressing ion migration through quantum well effects and the use of long-chain spacer cations. However, producing sufficiently thick films for effective X-ray absorption remains a challenge. Conventional methods like spin-coating and blade-coating yield films too thin for optimal X-ray absorption. This paper addresses this challenge by developing a novel bottom-up crystal growth method to create thick, high-quality Q-2D perovskite films suitable for high-performance X-ray detection and imaging.

Existing literature highlights the superior photoelectric properties and stability of Q-2D perovskites compared to their 3D counterparts, making them attractive candidates for X-ray detection. Studies have demonstrated that the careful selection of spacer cations and precise control over the number of [BX]₂ layers in Q-2D perovskites significantly improve stability and suppress ion migration. However, a major limitation is the difficulty in achieving the hundreds of micrometer thickness needed for efficient X-ray absorption using conventional thin-film deposition techniques. The authors' review of existing fabrication techniques like spin-coating and blade-coating reveal limitations in achieving the desired thickness and quality of perovskite films for efficient X-ray detection, motivating the need for a novel approach.

The researchers developed a novel atmosphere-assisted method to control the crystallization of Q-2D perovskite (BA₂MA₉Pb₁₀I₁₃·xCH₃NH₂). A high solid content liquid perovskite was prepared using a solid-liquid conversion method with CH₃NH₂ intercalation. This liquid precursor provided advantages in terms of high solid content, fluidity, and easy processing. The key innovation lies in using a mixed atmosphere of CH₃NH₂ and NH₃ during annealing. This controlled atmosphere regulates the crystallization process, preventing rapid surface crystallization and allowing for bottom-up growth from the substrate. The process promotes the formation of a dense film with fewer defects and controlled thickness (tens to hundreds of micrometers) by adjusting precursor volume and atmosphere composition. A TiO₂ layer was subsequently integrated to form a heterojunction with the perovskite film. Characterization techniques included scanning electron microscopy (SEM), X-ray diffraction (XRD), steady-state photoluminescence (PL), resistivity measurements, temperature-dependent conductivity measurements for activation energy determination, photoconductivity measurements for μτ product calculation, time of flight (TOF) method for mobility measurement, and X-ray absorption spectroscopy.

The atmosphere-assisted method resulted in high-quality Q-2D perovskite films with thicknesses exceeding 100 µm, significantly thicker than films produced by conventional methods. SEM images revealed a uniform and dense morphology with fewer cracks and holes compared to films prepared by direct annealing. XRD analysis confirmed the high crystallinity of the films. The mixed CH₃NH₂/NH₃ atmosphere led to a higher resistivity (2.48 × 10⁸ Ω cm) compared to films prepared in other atmospheres, reducing leakage current and noise. The activation energy for ion migration (Eₐ) was also enhanced (0.632 eV), effectively suppressing ion migration and improving device stability. The Q-2D perovskite exhibited a high μτ product (7.81 × 10⁻⁵ cm² V⁻¹), exceeding that of commercial α-Se. TOF measurements showed a high carrier mobility (19.3 cm² V⁻¹ s⁻¹). The TiO₂-perovskite heterojunction further reduced dark current density and improved charge carrier extraction efficiency. The resulting X-ray detector exhibited an ultrahigh sensitivity (29721.4 µC Gyair⁻¹ cm⁻²) and a low detection limit (20.9 nGyair s⁻¹). The fabricated FPXI demonstrated high spatial resolution (3.6 lp mm⁻¹) and clear X-ray images at low doses.

The significantly improved performance of the Q-2D perovskite X-ray detector is attributed to the synergistic effects of the bottom-up film growth method and the TiO₂ heterojunction. The controlled atmosphere approach allows for the fabrication of thick, high-quality perovskite films with fewer defects, leading to higher resistivity and reduced noise. The increased Eₐ effectively suppresses ion migration, enhancing long-term stability. The TiO₂ layer facilitates efficient charge carrier extraction, further contributing to the high sensitivity and low detection limit. These results demonstrate a significant advance in low-dimensional perovskite X-ray detectors, surpassing the performance of many previously reported devices. The high spatial resolution of the FPXI highlights its potential for applications requiring high-resolution imaging.

This work presents a novel and effective methodology for fabricating high-performance Q-2D perovskite X-ray detectors and FPXIs. The bottom-up growth method, coupled with the TiO₂ heterojunction, yields devices with ultrahigh sensitivity, low detection limit, and high spatial resolution. This approach opens new avenues for developing advanced X-ray imaging technologies with enhanced performance and stability. Future research could explore the optimization of the mixed atmosphere composition, the integration of other charge transport layers, and the fabrication of larger-area FPXIs for practical applications.

While the study demonstrates excellent results, there are some limitations. The long-term stability of the devices under continuous operation and under various environmental conditions requires further investigation. The scalability of the bottom-up growth method to larger areas needs to be explored for practical applications. Further investigation into the impact of different TiO2 deposition methods and other charge transport layers on device performance could also be beneficial. The current study primarily focuses on laboratory-scale devices and further optimization may be required for real-world implementation.