Comprehensive Review of Solar-Blind Ultraviolet Photodetectors: Materials, Device Configuration, and Performance Metrics for Invisible-Flame Sensing.

Solar-blind deep-ultraviolet (DUV) sensing is crucial for early-warning systems, environmental monitoring, and secure optical communications. Invisible flames, although nearly transparent to the naked eye, emit ultraweak and intrinsically fluctuating DUV radiation (∼10 nW cm-2), posing a demanding detection challenge that requires solar-blind photodetectors (PDs) with the highest sensitivity, speed, and reliability under harsh environmental conditions. In this review, we first examine the physical origins of DUV emissions from invisible flames. We then discuss both established and emerging DUV-sensing materials, along with the structural design of DUV-selective windows and PDs, in relation to their underlying operating principles. Particular emphasis is placed on the interplay between material properties and device configuration, especially regarding how it determines the sensitivity, selectivity, and temporal response. Next, recognizing that performance metrics like shot-noise detectivity and internal gain under steady-state illumination are often reported inconsistently, we adopt recently proposed standardized evaluation guidelines as a critical framework to reassess the performance of reported devices. Finally, we employ two new application-oriented metrics, the responsivity-speed product and flame suitability index, to identify promising technological pathways for robust, real-time invisible-flame detection. We conclude with a brief outlook on the remaining challenges and future directions for the development of next-generation DUV-sensing systems.