Phase-regulated FACsPbI3 films via triple-source thermal co-evaporation enable high-efficiency near-infrared perovskite LEDs

A team led by Zhengzheng Liu, Jiajun Luo, and Juan Du has introduced a novel approach for fabricating high-performance near-infrared perovskite light-emitting diodes (NIR-PeLEDs) using a triple-source thermal co-evaporation method. This strategy enables the direct formation of the black-phase α-FACsPbI₃ perovskite and significantly enhances device performance by overcoming the common issues of phase instability, surface roughness, and trap-induced non-radiative recombination in thermally evaporated perovskite films.

Why This Research Matters

• Phase Stabilization: Traditional FAPbI₃ films tend to form a mixture of photoactive α-phase and photoinactive δ-phase under vacuum processing. The introduction of Cs⁺ ions via co-evaporation converts yellow δ-phase to stable black α-phase, which improves crystallinity and eliminates metallic Pb defects.
• Enhanced Efficiency: Devices based on FACsPbI₃ achieved a maximum external quantum efficiency (EQE) of 10.25%, over six times higher than the 1.58% of FAPbI₃-based devices.
• Vacuum Compatibility: The method is fully compatible with industrial OLED processes and offers a solvent-free, scalable route for large-area optoelectronics.

Key Design Innovations

• Triple-Source Co-Evaporation: Simultaneous evaporation of FAI, PbI₂, and CsI under controlled rates enables homogeneous incorporation of Cs into the perovskite lattice, reducing nanocrystal size (~61 nm), surface roughness (RMS ~5.35 nm), and trap density.
• Morphology & Energy Landscape Engineering: Cs incorporation leads to denser films with reduced redshift in ground-state bleaching and prolonged carrier lifetimes, as confirmed by time-resolved photoluminescence (TRPL) and transient absorption (TA) measurements.
• Trap Density Reduction: Space-charge-limited current (SCLC) analysis revealed that FACsPbI₃ films exhibit lower trap densities (~4.59 × 10¹⁸ cm^-3 for holes), contributing to better charge transport and radiative recombination.

Device Performance

FAPbI₃- and FACsPbI₃-based NIR-PeLEDs were constructed. Compared to FAPbI₃ devices, FACsPbI₃-based LEDs showed:

• More stable and intense electroluminescence centered at ~770 nm.
• Uniform EL emission mapping with reduced dark zones.
• Comparable radiance (~2.64 W Sr^-1 m^-2) and improved operational reliability.

Future Outlook

This work establishes a robust vacuum deposition protocol for preparing high-quality FACsPbI₃ films with tailored phase composition and optoelectronic properties. It serves as a foundation for further integration of perovskite emitters into commercial LED manufacturing lines.

Stay tuned for more innovations from this collaborative team as they advance the scalable fabrication of stable, high-efficiency perovskite-based optoelectronic devices.