Ayan A. Zhumekenov

Double Charged Surface Layers in Lead Halide Perovskite Crystals

Understanding defect chemistry, particularly ion migration, and its significant effect on the surfaces optical and electronic properties is one of the major challenges impeding the development of hybrid perovskite-based devices. Here, using both experimental and theoretical approaches, we demonstrated that the surface layers of the perovskite crystals may acquire a high concentration of positively charged vacancies with the complementary negatively charged halide ions pushed to the surface. This charge separation near the surface generates an electric field that can induce an increase of optical band gap in the surface layers relative to the bulk. We found that the charge separation, electric field, and the amplitude of shift in the bandgap strongly depend on the halides and organic moieties of perovskite crystals. Our findings reveal the peculiarity of surface effects that are currently limiting the applications of perovskite crystals and more importantly explain their origins, thus enabling viable surface passivation strategies to remediate them.

Ultralong Radiative States in Hybrid Perovskite Crystals: Compositions for Submillimeter Diffusion Lengths

Organic-inorganic hybrid perovskite materials have recently evolved into the leading candidate solution-processed semiconductor for solar cells due to their combination of desirable optical and charge transport properties. Chief among these properties is the long carrier diffusion length, which is essential to optimizing the device architecture and performance. Herein, we used time-resolved photoluminescence (at low excitation fluence, 10.59 μJ·cm-2 upon two-photon excitation), which is the most accurate and direct approach to measure the radiative charge carrier lifetime and diffusion lengths. Lifetimes of about 72 and 4.3 μs for FAPbBr3 and FAPbI3 perovskite single crystals have been recorded, presenting the longest radiative carrier lifetimes reported to date for perovskite materials. Subsequently, carrier diffusion lengths of 107.2 and 19.7 μm are obtained. In addition, we demonstrate the key role of the organic cation units in modulating the carrier lifetime and its diffusion lengths, in which the defect formation energies for FA cations are much higher than those with the MA ones.

All-inorganic perovskite nanocrystal scintillators

The rising demand for radiation detection materials in many applications has led to extensive research on scintillators1–3. The ability of a scintillator to absorb high-energy (kiloelectronvolt-scale) X-ray photons and convert the absorbed energy into low-energy visible photons is critical for applications in radiation exposure monitoring, security inspection, X-ray astronomy and medical radiography4,5. However, conventional scintillators are generally synthesized by crystallization at a high temperature and their radioluminescence is difficult to tune across the visible spectrum. Here we describe experimental investigations of a series of all-inorganic perovskite nanocrystals comprising caesium and lead atoms and their response to X-ray irradiation. These nanocrystal scintillators exhibit strong X-ray absorption and intense radioluminescence at visible wavelengths. Unlike bulk inorganic scintillators, these perovskite nanomaterials are solution-processable at a relatively low temperature and can generate X-ray-induced emissions that are easily tunable across the visible spectrum by tailoring the anionic component of colloidal precursors during their synthesis. These features allow the fabrication of flexible and highly sensitive X-ray detectors with a detection limit of 13 nanograys per second, which is about 400 times lower than typical medical imaging doses. We show that these colour-tunable perovskite nanocrystal scintillators can provide a convenient visualization tool for X-ray radiography, as the associated image can be directly recorded by standard digital cameras. We also demonstrate their direct integration with commercial flat-panel imagers and their utility in examining electronic circuit boards under low-dose X-ray illumination.All-inorganic perovskite nanocrystals containing caesium and lead provide low-cost, flexible and solution-processable scintillators that are highly sensitive to X-ray irradiation and emit radioluminescence that is colour-tunable across the visible spectrum.