Infrared (IR) photodetectors (PDs) are one of the PDs classified based on the wavelength range of light absorbed beyond the visible spectrum, comprising near-infrared (NIR, 0.7 µm to 1.4 µm), short-wave infrared (SWIR, 1.4 µm to 3 µm), and mid-wave infrared (MWIR, 3 µm to 5 µm). IR PDs play a crucial role in PDs technology with its extensive utilization as a key component in a wide range of applications including telecommunication, medical imaging, environmental monitoring, agriculture, astronomy, consumer electronics, automotive industry, defense, security, fiber optics, remote control, night vision, thermal imaging, wearable health sensors, IR spectroscopy, gas analysis, thermal radiation and heat detection [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. The seamless integration of IR PDs into various applications with utmost efficiency and effectiveness is a testament to continuous development in IR photodetection technology [11]. Innovation in IR photodetection technology is essential for harnessing its benefits for continuously developing its integration with various technologies. Moreover, integrating IR PDs with artificial intelligence (AI) advances intelligent data processing, enhancing performance and enabling the development of new applications. The integration of AI with IR photodetection eases data processing and its analysis by AI algorithms. The integration might facilitate the optimization of sensing systems, enhance imaging techniques, and support real-time decision-making in various domains. The rigorous development in IR photodetection displays widespread potential for PDs to address complex global challenges and improve our daily experiences. Fig. 1 shows the percentage contribution of total publications based on Scopus data on various potential semiconducting materials for IR PDs. The IR PDs based on emerging materials, including perovskites, two-dimensional (2D) materials, and colloidal quantum dots (CQDs) are being developed as next-generation IR absorbing materials [12], [13], [14], [15]. The low-cost and solution-processibility of CQDs and their compatibility with silicon-based technology make them more affordable and seamless as compared to the traditional PDs [16]. The wide-spectral tunability of CQDs has a great advantage in utilizing them according to the applications’ requirements. Until now, CQDs-based IR PDs rely on lead-chalcogenides (PbX), InGaAs, InSb, InAsSb, HgTe, and HgCdTe [2], [17], [18]. But the toxicity and environmental impact of Pb, Hg, and Cd based CQDs make the handling and their disposal quite challenging. The environmental persistence of these toxic materials leads to contamination and bioaccumulation in the food chain [19], [20]. Recent market analysis indicates that the global market for CQDs IR detector materials attained a valuation of approximately USD 412 million in 2024. Forecasts suggest a compound annual growth rate (CAGR) of 17.2 % from 2025 to 2033, with the market expected to reach USD 1482 million by the end of the forecast period [21]. This growth is driven by expanding applications across multiple sectors, including aerospace and defense, healthcare diagnostics, industrial monitoring, and consumer electronics, where CQDs-based IR PDs offer enhanced sensitivity, spectral tunability, and miniaturization potential [21].
In this frame of reference, binary (Ag2X; X = S, Se, and Te) and ternary silver chalcogenides (AgBiX2) are increasingly regarded as lead- and mercury-free alternatives to Pb/Hg-based infrared semiconductors, combining lower intrinsic toxicity with Restriction of Hazardous Substances (RoHS)-compliant compositions. Binary silver chalcogenides: Ag2S, Ag2Se, and Ag2Te, have emerged as promising materials for IR optoelectronic applications thanks to their narrow bandgap energies (∼0.15 eV to 1.1 eV). These bandgap values enable strong absorption and emission across NIR, SWIR, and MWIR spectral regions. In particular, Ag2S exhibits a bandgap of ∼0.9–1.1 eV, Ag2Se ∼0.15 eV, and Ag2Te ∼0.67 eV, positioning them as suitable candidates for a wide range of applications [22], [23], [24]. The Ag2S, Ag2Se, and Ag2Te CQDs possess optical band gaps of up to 1.5–2.2, 0.8–2.0, and 0.4–1.5 eV, depending on the quantum confinement effect [25], [26], [27]. The Ag2X CQDs are well-explored for bio-medical imaging, photocatalysis, and bio-photonic applications such as NIR imaging, biosensors, and therapeutics [28]. Similarly, among ternary silver chalcogenides, AgBiS2 CQDs with favorable charge-transport properties and low cost, offer a sustainable platform for next-generation optoelectronic and NIR detection devices. AgBiS2 CQDs possess exceptional opto-electronic performance, with high absorption coefficients (∼105 cm−1) spanning ultraviolet to near-IR wavelengths, an ideal bandgap for diverse applications, and robust stability during continuous operation. Recently, CQDs based on Ag2X and AgMX2, have garnered significant attention for IR PDs applications. Their exceptional bandgap tunability and strong light absorption characteristics present substantial potential for further development in high-performance IR photodetection technologies. Fig. 1 inset shows the number of publications on silver binary and ternary chalcogenide CQDs, demonstrating rapid development for the IR PDs. The development of binary and ternary silver chalcogenide CQDs-based IR PDs is shown in Fig. 2, indicating accelerated advancements in recent years.
The proposed review article focuses on the advancement of IR PDs utilizing environmentally benign silver chalcogenide CQDs. Given the pivotal role of IR PDs across a broad spectrum of applications, global interest in this topic is steadily increasing. This article presents a comprehensive review of recent advancements in IR PDs based on Ag2X CQDs, encompassing a range of device architectures, including photoconductors (PC), photodiodes, and phototransistors (PT). The discussion focuses on the influence of device structure and ligand exchange (LE) strategies on key performance metrics such as external quantum efficiency (EQE), responsivity, detectivity, and response time. Furthermore, the underlying photodetection mechanisms are examined in detail. The functional integration of Ag chalcogenide CQDs IR PDs for advanced practical applications such as infrared imaging, LiDAR, optical communication, and photoplethysmography has been discussed in depth. The review also highlights current challenges and future opportunities by comparing the performance characteristics of various Ag2X CQD-based PDs. The article is positioned to serve diverse and multidisciplinary understanding involved in the design, research, development, and commercialization of CQD-based IR PDs, offering a timely synthesis of current progress, existing challenges, and future directions in the field.