Halide perovskites, a class of semiconductors, offer attractive properties for optoelectronics and energy technologies due to their abundance, low cost, ease of fabrication, tunable properties, and ability to carry large charge currents. While primarily known for photovoltaic applications, they are showing promise in LEDs, scintillators, and radiation detectors. Efficient electron emission (photoelectric effect) is crucial for technologies like free-electron lasers and image intensifiers. Current state-of-the-art electron sources are based on metals, alkali antimonides/tellurides, and III-V semiconductors. Metals offer excellent lifetime but low quantum efficiency (QE), while semiconductors achieve high QE but suffer from degradation and require complex fabrication and ultra-high vacuum (UHV) operation. This research explores halide perovskites as a potential solution to create easily replaceable, efficient, durable, low-cost, scalable, and adaptable electron sources, aiming to reduce fabrication and operational costs, and to enable operation at higher vacuum pressures. The study investigates the photoelectric effect in halide perovskite thin films across the visible to ultraviolet spectrum, aiming to demonstrate a disruptive technology for low-cost, high-efficiency, spectrally tunable electron sources that can operate at higher vacuum pressures than traditional semiconductor-based counterparts.

The literature extensively covers the remarkable properties and applications of halide perovskites in solar cells and other optoelectronic devices. Record efficiencies in photovoltaics have been achieved, exceeding 25%. Research also showcases their potential in light-emitting diodes, scintillators, X-ray and gamma radiation detectors, demonstrating figures of merit comparable to state-of-the-art inorganic semiconductor devices. Studies on the charge carrier dynamics and transport properties of these materials reveal impressive electron-hole diffusion lengths and low trap-state densities. The photoelectric effect and its applications in various technologies, particularly using metals, alkali antimonides/tellurides, and III-V semiconductors as electron sources, are well-documented. The limitations of existing technologies, including the trade-off between quantum efficiency and lifetime, and the stringent requirements for material quality and UHV operation are discussed. The challenges of surface treatment and degradation mechanisms in semiconductor-based sources, particularly related to surface contamination and the role of alkali metal deposition (like Cs) in achieving negative electron affinity are also reviewed.

Halide perovskite thin films (CsPbX3, X = Br or I) were fabricated using solution processing techniques. Precursor powders were mixed and spin-coated onto doped Si wafers in an Ar-filled glovebox to minimize air contamination. The films' absorbance, photoluminescence, and crystal structure were characterized. Photoelectric effect characterization was conducted in an ultra-high vacuum (UHV) chamber (base pressure ~1 × 10−9 torr) after annealing at 350 °C to remove surface contaminants. Auger electron spectroscopy (AES) was used to analyze surface composition before and after annealing, confirming the removal of carbon and oxygen. Photoelectric effect measurements involved illuminating the samples with monochromatic light and measuring the photocurrent between the film and a counter anode under an applied voltage. Quantum efficiency (QE) was derived from the photocurrent response as a function of excitation light energy. In-situ deposition of an ultra-thin Cs layer was performed to activate the surface, and QE spectra were measured before and after Cs deposition. The same procedure was repeated for three distinct hybrid perovskites: the 3D perovskite FA0.7MA0.25Cs0.05PbI3, and the Ruddlesden-Popper layered hybrid perovskites BA2MAn−1PbnI3n+1 with n = 2 or 5. Hybrid perovskites were not annealed to prevent degradation. The internal quantum efficiency was calculated by dividing the QE by the absorbance to determine the rate-limiting step in the photoelectric effect. The effect of Cs deposition on the work function and electron emission onset was investigated. In-situ AES measurements were conducted to correlate Cs coverage with QE. Finally, the stability of the photoelectric effect and the degradation mechanisms were studied under continuous illumination, and the regeneration of QE through in-situ Cs deposition was demonstrated.

The study demonstrated a high photoelectric effect in halide perovskite thin films. CsPbBr3 films achieved a peak QE of 2.2% at 5 eV, while CsPbI3 showed a QE of 0.14% at 3.5 eV. QE values remained above 1% for CsPbBr3 in the UV range (energy >3.9 eV) and above 0.25% at 3.1 eV (400 nm). These values are comparable to state-of-the-art III-V semiconductors but achieved using a significantly simpler, low-cost fabrication process. The reproducibility of high QE (average >1.5%, maximum >2% at 5 eV) was confirmed across 17 CsPbBr3 thin films. The Cs surface activation process significantly improved the QE in all perovskite types tested (both inorganic and hybrid), including the 3D perovskite FA0.7MA0.25Cs0.05PbI3, and the 2D Ruddlesden-Popper layered perovskites BA2MAn−1PbnI3n+1 (n=2,5). Analysis of the internal quantum efficiency showed that photoexcitation is not the main limiting factor. Cs deposition lowered the electron emission onset to near the bandgap energy and increased QE dramatically, indicating that surface tunneling is the primary limiting factor. In 3D perovskites, the QE is limited by electron escape from the surface, influenced by surface recombination velocity and surface energy barriers. The Cs deposition creates a surface dipole layer, resulting in a negative electron affinity that facilitates electron emission. The QE continuously increased with Cs coverage until about 2.5 Cs/unit cell, after which it decreased, mirroring observations in GaAs photocathodes. The stability of CsPbBr3 films was tested under continuous illumination, observing QE degradation to 60% after 25 hours and 8% after 96 hours. The main degradation mechanism was attributed to oxygen contamination. However, QE could be fully regenerated by in-situ Cs deposition, suggesting that degradation primarily affects the Cs surface layer, similar to GaAs electron sources. In-situ AES measurements showed decreased Cs coverage and increased oxygen after degradation and restoration after adding fresh Cs.

The findings demonstrate that halide perovskites are highly promising materials for low-cost, high-efficiency, and spectrally tunable electron sources. The achieved QE values, while not yet surpassing state-of-the-art III-V semiconductors, are remarkably high considering the simplicity and low cost of the fabrication process. The regenerative nature of the electron sources through in-situ Cs deposition represents a significant advantage over existing semiconductor-based sources, which typically require extensive and costly replacement after degradation. The ability to operate at higher vacuum pressures further enhances the practical feasibility and reduces operational costs. While the current QE is limited by the efficiency of electron escape from the perovskite surface, optimization of the film quality and the Cs deposition process has the potential to achieve performance on par with or exceeding existing technologies.

This study successfully demonstrated efficient, regenerative, and low-cost electron sources based on solution-processed halide perovskite thin films, achieving quantum efficiencies comparable to state-of-the-art semiconductors but with significantly simpler and more cost-effective fabrication and operation. Future research should focus on improving the quality of the perovskite thin films, optimizing the Cs deposition process for homogeneity, and investigating new surface passivation strategies to minimize surface recombination and achieve higher QE values.

The current study shows some limitations. Although the QE has been improved substantially through Cs activation, it is still lower than the state-of-the-art. The inhomogeneous spatial distribution of QE and the different QE values observed in different 3D perovskite films highlight the current limited understanding and control of the Cs activation process. The stability of the devices could potentially be improved by further minimizing oxygen contamination in the UHV system. More investigation is needed to fully understand the long-term stability of these electron sources under continuous operation in more controlled environments. Finally, the scaling of the device and its integration into real-world systems need to be explored.