Significance Semiconductor infrared upconversion technology represents a transformative approach in the field of optoelectronics, enabling the direct conversion of low-energy infrared photons into high-energy visible or near-infrared signals. This capability is fundamental to the realization of pixelless imaging, a technology that bypasses the limitations of traditional focal plane arrays by integrating a photodetector (PD) and a light-emitting diode (LED) into a single, compact device. This integration eliminates the need for complex and costly readout integrated circuits (ROIC), offering unparalleled advantages such as large-area detection, simplified device architecture, and on-demand wavelength tuning. Consequently, this technology has become indispensable for a wide range of critical applications, including non-invasive biomedical imaging, high-resolution military night vision, astronomical observation, and secure laser communication. The performance of the entire upconversion device is intrinsically linked to the characteristics of the LED component, which dictates the final imaging resolution, speed, and efficiency. Despite significant progress, a major bottleneck persists: the stark disparity between the high internal quantum efficiency (IQE) of LEDs, which can exceed 90%, and their low external quantum efficiency (EQE), often remaining below 2% due to total internal reflection at the semiconductor-air interface. Furthermore, challenges related to material stability, spectral purity, and the complexity of device fabrication continue to hinder widespread adoption.

Progress This paper provides a comprehensive and systematic review of the developmental history, performance metrics, and optimization strategies for LEDs used in upconversion devices, categorized by three distinct material systems: traditional inorganic, organic, and emerging novel materials.

Inorganic LEDs, primarily based on III-V compound semiconductors like GaAs/AlGaAs and InP, constitute the foundational technology for this field. These devices offer mature fabrication processes, excellent thermal and chemical stability, and the ability to achieve a broadband spectral response from the near-infrared to the terahertz regime through advanced techniques like wafer bonding and quantum well engineering. The review details the evolution from early quantum well infrared photodetector (QWIP) and LED integrations to more recent heterojunction internal work function photoemission (HEIWIP) detectors, which have achieved ultra-broadband responses up to 200 THz. A significant portion of the progress in inorganic systems has focused on overcoming low light extraction efficiency. We discuss recent advances in surface engineering, where microstructures such as diffraction gratings, microspheres, and photonic crystals have been successfully implemented to redirect trapped photons, boosting the light extraction efficiency by over 55% relative to planar devices, particularly at cryogenic temperatures.

Organic LEDs (OLEDs) have introduced a new paradigm with their exceptional molecular design flexibility and compatibility with low-cost, solution-based processing on flexible substrates. The field has witnessed a remarkable evolution in发光 mechanisms. Early devices based on fluorescent materials were constrained by spin statistics to a maximum internal quantum efficiency of 25%. This limitation was overcome with the advent of phosphorescent materials incorporating heavy-metal complexes, which harvest both singlet and triplet excitons to achieve near-unity internal efficiency. More recently, the development of thermally activated delayed fluorescence (TADF) and triplet-triplet annihilation (TTA) systems has eliminated the need for expensive noble metals while maintaining high efficiency. These advancements have enabled organic upconversion devices to achieve impressive performance, including high-resolution imaging exceeding 3600 dpi and photon-to-photon conversion efficiencies as high as 29.6%.

Emerging materials, including colloidal quantum dots (QDs), metal halide perovskites, and plasmonic nanostructures, represent the most promising frontier for next-generation devices. These materials aim to synergize the advantages of their inorganic and organic counterparts. Colloidal QDs enable precise spectral tuning across the visible to mid-infrared spectrum via the quantum confinement effect, with narrow emission linewidths. Perovskites offer exceptionally high carrier mobilities and excellent color purity. Plasmonic nanostructures enhance light-matter interactions through localized surface plasmon resonance, leading to dramatic improvements in upconversion luminescence. The review highlights how these novel materials are paving the way for high-performance, flexible, and spectrally versatile upconversion devices.

Conclusion and Prospect In conclusion, the three material systems—inorganic, organic, and emerging—exhibit complementary strengths and are suited for different application niches. Inorganic LEDs remain dominant in high-performance, cryogenic, and terahertz applications where stability and broadband response are critical. OLEDs excel in low-cost, flexible electronics and high-resolution imaging for consumer applications. Emerging materials offer a pathway to combine high performance with processing flexibility. However, significant challenges persist that must be addressed to unlock the full potential of this technology. Future research should prioritize three key directions. First, the engineering of advanced surface microstructures and optical cavities is essential to further improve the light extraction efficiency of inorganic LEDs. Second, the development of robust encapsulation technologies and molecular engineering strategies is crucial to enhance the long-term stability of organic and perovskite materials against environmental factors like oxygen and moisture. Third, investigating the monolithic integration of these heterogeneous material systems will be key to achieving synergistic performance enhancements. Successfully addressing these challenges will facilitate the transition of upconversion technology from specialized military and scientific instruments to widespread use in consumer electronics and biomedical devices, driving transformative advances across the entire optoelectronic landscape.