The increasing demand for near-infrared (NIR) covert illumination in mobile and wearable devices for functionalities such as facial recognition, eye-tracking, and motion/depth sensing presents a challenge due to limited space on these devices, often already occupied by color displays. Traditional NIR LEDs, typically used for such applications, would further reduce available space if integrated as large-area components. This research addresses this challenge by developing a transparent NIR LED that can be overlaid onto existing color displays. Perovskite light-emitting diodes (PeLEDs) are particularly attractive due to their potential for large-area fabrication and high efficiency. While progress in PeLED efficiency has been substantial, reaching over 20% in recent studies, the development of transparent, large-area NIR PeLEDs remains an area requiring significant advancement. This work aims to demonstrate the feasibility and advantages of a transparent, large-area NIR PeLED for integration with existing displays in wearable technology, allowing for the addition of advanced security and sensing functionalities without the need for additional space on the device. This could pave the way for innovative features in smartwatches, smartphones, gaming consoles, and augmented/virtual reality headsets.

Research on perovskite light emission has seen rapid advancements, with electroluminescent device efficiencies increasing from 0.8% in early studies to over 20% in recent reports. NIR PeLEDs have emerged as the most successful type, exhibiting high efficiency, reproducibility, and reasonable lifespan, with recent work demonstrating efficient large-area devices with excellent emission uniformity. The key advantage of PeLEDs over other semiconductor-based LEDs lies in their large-area fabrication capabilities on various substrates, making them suitable for display applications. However, large-area NIR LEDs, traditionally associated with covert functions, have limited applications in this form factor. This work directly addresses the need for a transparent large-area NIR-emitting PeLED to overcome this limitation.

The transparent PeLEDs were constructed using an ITO/AZO/PEIE/FAPbI3/poly-TPD/MoO3/Al/ITO/Ag/ITO architecture. The FAPbI3 perovskite layer was prepared by dissolving FAI, PbI2, and 5-AVA in N,N-dimethylformamide. The device fabrication involved sequential spin-coating of AZO nanoparticles, PEIE, the perovskite precursor, and poly-TPD onto pre-patterned ITO-glass substrates, followed by thermal evaporation of MoO3 and Al. Crucially, the top electrode was designed as a multilayer Al (10 nm)/ITO (40 nm)/Ag (10 nm)/ITO (40 nm) structure to achieve low sheet resistance, efficient charge injection, and high optical transparency. This multilayer electrode structure was developed to mitigate plasma damage from the ITO sputtering process, a significant challenge in creating transparent PeLEDs. The device's large area (120 mm²) facilitated uniform emission. Characterization included current density-voltage measurements using a Keithley 2450 source-measure unit, photon flux measurement with a calibrated silicon photodiode, EL spectra recording with an Ocean Optics Flame-T spectrometer, and EQE calculation assuming Lambertian emission. Optical transmittance was measured using an Ocean Optics HL-2000 broadband light source and a calibrated Ocean Optics Flame-T spectrometer and an Agilent CARY-7000 spectrophotometer. Comparative studies were performed using devices with different back electrode structures (500 nm ITO, ITO (40 nm)/Ag (10 nm)/ITO (40 nm), and 80 nm Al) to evaluate the impact of electrode design on device performance and plasma damage. The lifespan of the device was also tested.

The fabricated transparent PeLED exhibited characteristic NIR emission at 799 nm, consistent with the FAPbI3 photoluminescence spectrum. The device demonstrated a low turn-on voltage (~1.5 V), indicating efficient carrier injection. Front and back emission were measured separately, with the front emission being more intense (maximum radiance of 2.8 W sr⁻¹ m⁻²) due to higher substrate transmittance. The total EQE was calculated to be 5.7% at a current density of 5.3 mA cm⁻², with an average maximum EQE of 3.5% (front) and 1.2% (back) across 17 devices. However, the device exhibited a short lifespan (T50 of 4 min), likely due to plasma damage during ITO sputtering. A comparison of devices with different back electrode structures revealed that the Al/ITO/Ag/ITO electrode offered the optimal balance between low sheet resistance, high transparency, low damage, and efficient charge injection. The Al/ITO/Ag/ITO electrode and PeLED displayed a high average transmittance exceeding 55% in the 450-650 nm range. A proof-of-concept demonstration overlaid the transparent PeLED onto a smartwatch display, showing high optical transparency and bright NIR electroluminescence when viewed with a NIR camera, highlighting the potential for covert illumination and sensing applications. The multilayer structure of the top electrode, specifically the inclusion of the 10nm Al layer, was found to be crucial for mitigating plasma damage and improving device performance compared to other electrode structures tested.

The results demonstrate the successful fabrication of a transparent, large-area NIR PeLED with respectable optoelectronic performance. The high optical transparency and NIR emission efficiency enable novel functionalities in small wearable devices. The demonstration on a smartwatch showcases the potential for advanced security and sensing applications like facial recognition and eye-tracking. The findings address the limitations of traditional NIR LEDs in small form factors by offering a solution that integrates seamlessly with existing displays. The improved performance compared to other electrode structures emphasizes the significance of the Al/ITO/Ag/ITO design in mitigating plasma damage and enhancing device efficiency. This work opens avenues for integrating advanced functionalities into wearable technology without compromising display space.

This paper successfully demonstrates the fabrication of a transparent, large-area perovskite-based near-infrared light-emitting diode and illustrates its application in a wearable device. The high transparency and efficient NIR emission make it suitable for advanced security and sensing functionalities in space-constrained devices. Future research could focus on improving the device’s lifespan by exploring alternative electrode materials or deposition techniques to further reduce plasma damage. Investigating other perovskite compositions and optimizing the device architecture for even higher efficiency and broader spectral coverage are also promising avenues for future development.

The major limitation of the current work is the relatively short lifespan of the transparent PeLED (T50 of 4 min), which is inferior to opaque PeLEDs. This is attributed to plasma damage during ITO sputtering, highlighting the need for further research to improve the robustness of the device. While the demonstrated proof-of-concept is promising, further optimization and long-term stability testing are necessary before commercial applications are feasible. The study focuses on a specific perovskite material (FAPbI3); exploring other perovskite compositions to enhance performance could be beneficial.