Perovskite photodetectors have emerged as a promising class of optoelectronic devices due to their exceptional light absorption, tunable bandgaps, and compatibility with low-cost solution processing. These devices leverage the unique properties of perovskite materials, such as high charge carrier mobility, long diffusion lengths, and strong light-matter interactions, making them suitable for applications ranging from visible-light imaging to X-ray detection. This article explores the key aspects of perovskite photodetectors, focusing on solution-processed films, bandgap engineering, and their applications, while addressing stability challenges and encapsulation strategies.

Solution-processed perovskite films are a cornerstone of perovskite photodetector fabrication, enabling scalable and cost-effective production. Techniques such as spin-coating, blade-coating, and inkjet printing allow for the deposition of uniform perovskite layers on various substrates. The quality of these films is critical for device performance, as defects and grain boundaries can act as recombination centers, reducing photodetector efficiency. To mitigate these issues, researchers have developed methods such as solvent engineering, additive incorporation, and post-deposition treatments. For example, the introduction of dimethyl sulfoxide (DMSO) as a solvent additive in methylammonium lead iodide (MAPbI3) precursors has been shown to improve film morphology and reduce pinhole density. Additionally, thermal annealing and antisolvent treatments enhance crystallinity and grain size, leading to better charge transport and higher responsivity.

Tunable bandgaps are another defining feature of perovskite photodetectors, allowing for customization across a broad spectral range. By varying the composition of the perovskite material, the bandgap can be adjusted to target specific wavelengths. For instance, formamidinium lead iodide (FAPbI3) exhibits a narrower bandgap than MAPbI3, making it suitable for near-infrared detection. Mixed halide perovskites, such as MAPb(I1-xBrx)3, enable continuous bandgap tuning from approximately 1.6 eV to 2.3 eV, covering the visible spectrum. This tunability is particularly advantageous for applications requiring multispectral or color-selective detection. The ability to tailor the bandgap also facilitates the design of heterostructures and tandem devices, where multiple perovskite layers with different bandgaps are stacked to achieve broadband photodetection.

Visible-light imaging is one of the most prominent applications of perovskite photodetectors. Their high sensitivity and fast response times make them ideal for use in cameras, optical sensors, and biomedical imaging systems. For example, perovskite-based photodetectors have demonstrated detectivities exceeding 10^12 Jones, rivaling those of commercial silicon photodiodes. Their thin-film form factor and compatibility with flexible substrates further enable integration into wearable and portable imaging devices. In addition, the solution-processability of perovskites allows for the fabrication of large-area detector arrays, which are essential for high-resolution imaging. Recent advancements have shown that perovskite photodetectors can achieve sub-millisecond response times, making them suitable for real-time imaging applications.

X-ray detection is another area where perovskite photodetectors show significant promise. The high atomic numbers of lead and iodine in lead halide perovskites result in strong X-ray absorption, enabling efficient detection at low doses. This property is particularly valuable for medical imaging and security screening, where reducing radiation exposure is critical. Perovskite-based X-ray detectors have achieved sensitivity values exceeding 10^4 µC Gy^-1 cm^-2, outperforming conventional amorphous selenium detectors. Furthermore, the solution-processed nature of perovskites allows for the fabrication of thick active layers, which are necessary for absorbing high-energy X-rays. Recent studies have also explored the use of bismuth-based perovskites as lead-free alternatives for X-ray detection, addressing toxicity concerns while maintaining high performance.

Despite their advantages, perovskite photodetectors face stability challenges that must be addressed for practical deployment. Perovskite materials are susceptible to degradation from moisture, oxygen, light, and heat, leading to performance deterioration over time. For example, MAPbI3 undergoes phase segregation and decomposition when exposed to humidity, resulting in the formation of PbI2 and loss of optoelectronic properties. To combat these issues, encapsulation strategies have been developed to isolate perovskite films from environmental factors. Thin-film barriers made of materials such as aluminum oxide (Al2O3) or silicon nitride (SiNx) can be deposited via atomic layer deposition (ALD) to provide hermetic sealing. Alternatively, polymer-based encapsulants like poly(methyl methacrylate) (PMMA) or ethylene-vinyl acetate (EVA) offer flexible and cost-effective protection. In addition to external encapsulation, compositional engineering has been employed to enhance intrinsic stability. For instance, the incorporation of cesium or rubidium into perovskite formulations has been shown to improve thermal and phase stability.

Another approach to improving stability involves the use of two-dimensional (2D) perovskites, which exhibit enhanced resistance to environmental degradation. These materials consist of alternating layers of perovskite and organic cations, forming a natural barrier against moisture and oxygen. While 2D perovskites typically exhibit lower charge carrier mobility compared to their three-dimensional counterparts, their stability makes them attractive for long-term applications. Hybrid 2D-3D perovskite structures have also been explored to balance stability and performance, combining the robustness of 2D perovskites with the high efficiency of 3D perovskites.

In conclusion, perovskite photodetectors represent a versatile and high-performance technology with applications spanning visible-light imaging and X-ray detection. Solution-processed films enable scalable fabrication, while tunable bandgaps allow for customization across the electromagnetic spectrum. Despite stability challenges, advancements in encapsulation and material engineering are paving the way for durable and reliable devices. As research continues to address these hurdles, perovskite photodetectors are poised to play a transformative role in optoelectronics, offering a compelling combination of performance, cost-effectiveness, and versatility.