Rounak Naphade

The 5 μm intraband photoluminescence (PL) from 200 nm thin films of n-doped mercury selenide/cadmium sulfide (HgSe/CdS) core/shell colloidal quantum dots reaches 0.2% apparent external efficiency, after solid-state ligand exchange with a solution of lead iodide (PbI₂) and annealing at 140 °C. The PbI₂ treatment removes 95% of the C-H vibration absorption, but the PL increase is correlated with improved n-doping and not with organic ligand removal. The integrated emission intensity increases only by 20% upon cooling from 423 to 85 K. This suggests that multiphonon relaxation is not limiting the PL and that brighter emission should still be possible. PbI₂ treatment gave the brightest films, but PbBr₂ and PbCl₂ have similar effects.

A major thrust of medical X-ray imaging is to minimize the X-ray dose acquired by the patient, down to single-photon sensitivity. Such characteristics have been demonstrated with only a few direct-detection semiconductor materials such as CdTe and Si; nonetheless, their industrial deployment in medical diagnostics is still impeded by elaborate and costly fabrication processes. Hybrid lead halide perovskites can be a viable alternative owing to their facile solution growth. However, hybrid perovskites are unstable under high-field biasing in X-ray detectors, owing to structural lability and mixed electronic–ionic conductivity. Here we show that both single-photon-counting and long-term stable performance of perovskite X-ray detectors are attained in the photovoltaic mode of operation at zero-voltage bias, employing thick and uniform methylammonium lead iodide single-crystal films (up to 300 µm) and solution directly grown on hole-transporting electrodes. The operational device stability exceeded one year. Detection efficiency of 88% and noise-equivalent dose of 90 pGy_air are obtained with 18 keV X-rays, allowing single-photon-sensitive, low-dose and energy-resolved X-ray imaging. Array detectors demonstrate high spatial resolution up to 11 lp mm⁻¹. These findings pave the path for the implementation of hybrid perovskites in low-cost, low-dose commercial detector arrays for X-ray imaging.

Lead halide perovskites are widely used in optoelectronic applications owing to their promising photophysical properties, but developing components of the order of nanoscale remains challenging. In this work, the authors fabricated cesium lead bromide (CsPbBr3) thin films of varying thicknesses and investigated their visible-light communication (VLC) performance. The thickness of the CsPbBr3 thin films was precisely controlled by using a single-beam thermal evaporation technique, and their morphology was analyzed through scanning electron microscopy and x-ray diffraction. Thicker films were found to have a homogeneous surface, with gain boundaries of increasing size and fewer surface trap states than the thinner films. Furthermore, we identified the thickness-dependent photoluminescence (PL) property of the CsPbBr3 thin films based on steady-state PL measurements and verified it by using time-correlated single-photon counting as well as femtosecond upconversion measurements. Films with thicknesses of 5, 10, and 20 nm, with enhanced surface homogeneity and purity, were used in a VLC link as color-converting fluorescent components. The 20-nm-thick CsPbBr3 film delivered the best performance because it had the highest PL intensity and the most suitable morphology, with a –3-dB bandwidth of 30.7 MHz and a net data rate of 330 Mb/s. These results reflect a facile and well-controlled approach to fabricating such films that can be used for high-power, high-speed, and large-area transmission and detection of visible-light signals.

Two-dimensional (2D) lead halide perovskites (LHPs) have garnered incredible attention thanks to their exciting optoelectronic properties and intrinsic strong quantum confinement effect. Herein, we carefully investigate and decipher the charge carrier dynamics at the interface between CsPbBr₃ multiple quantum wells (MQWs) as the photoactive layer and TiO₂ and Spiro-OMeTAD as electron and hole transporting materials, respectively. The fabricated MQWs comprise three monolayers of CsPbBr₃ separated by 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as barriers. By varying the BCP thickness, we show that charge carrier extraction from MQWs to the corresponding extracting layer occurs through a quantum tunneling effect, as elaborated by steady-state and time-resolved photoluminescence measurements and further verified by femtosecond transient absorption experiments. Ultimately, we have investigated the impact of the barrier-thickness-dependent quantum tunneling effect on the photoelectric behavior of the synthesized QW photodetector devices. Our findings shed light on one of the most promising approaches for efficient carrier extraction in quantum-confined systems.

Bright-red light-emitting diodes (LEDs) with a narrow emission line width that emit between 620 and 635 nm are needed to meet the latest industry color standard for wide color gamut displays, Rec. 2020. CsPbI₃ perovskite quantum dots (QDs) are one of the few known materials that are ideally suited to meet these criteria. Unfortunately, CsPbI₃ perovskite QDs are prone to transform into a non-red-emitting phase and are subject to further degradation mechanisms when their luminescence wavelength is tuned to match that of the Rec. 2020 standard. Here, we show that zwitterionic lecithin ligands can stabilize the perovskite phase of CsPbI₃ QDs for long periods in air for at least 6 months compared to a few days for control samples. LEDs fabricated with our ultrastable lecithin-capped CsPbI₃ QDs exhibit an external quantum efficiency (EQE) of 7.1% for electroluminescence centered at 634 nm─a record for all-inorganic perovskite nanocrystals in Rec. 2020 red. Our devices achieve a maximum luminance of 1391 cd/m² at 7.5 V, and their operational half-life is 33 min (T₅₀) at 200 cd/m²─a 10-fold enhancement compared to control samples. Density functional theory results suggest that the surface strain in CsPbI₃ QDs capped with the conventional ligands, oleic acid and oleylamine, contributes to the instability of the perovskite structural phase. On the other hand, lecithin binding induces virtually no surface strain and shows a stronger binding tendency for the CsPbI₃ surface. Our study highlights the tremendous potential of zwitterionic ligands in stabilizing the perovskite phase and particle size of CsPbI₃ QDs for various optoelectronic applications.

Layered quasi-two-dimensional perovskites are promising candidates for optoelectronic applications exhibiting excitons with high emission quantum yields, high stability, and ease of bandgap tunability. Here, we demonstrate a long-range (∼100 μm) exciton transfer in a layered perovskite structure (en)₄Pb₂Br₉·3Br, with the ethylene diammonium (en) as a spacer that takes place via the reabsorption of emitted photons. Using the two-objectives setup, we directly map the spatiotemporal dynamics of photoluminescence (PL) from perovskite microwires that reveal a clear spectroscopic signature of photon recycling: the appearance of PL emission rise times and the corresponding elongation of the PL decay as a function of separation distance between the excitation and emission locations. We further show that a kinetic model based on the photon-mediated mechanism of the lateral exciton propagation indeed successfully describes all the salient features of the experimental data and gives an independent assessment of the radiative efficiency of the exciton recombination. Our demonstration points out the possibility of judiciously exploiting light management strategies for future high-performance optoelectronic devices with layered perovskite structures.

Two-dimensional (2D) Ruddlesden-Popper (RP) perovskites are emerging materials for light emitting applications. Unfortunately, their desirable narrowband emission coexists with broadband emissions, which limits the color quality and performance of the light source. However, the origin of such broadband emission in -oriented perovskites is still under debate. Here, we experimentally and theoretically demonstrate that unlike -oriented RP perovskites, the broadband emission of the 2D -oriented RP (PEA)2PbI4 (PEA = C6H5C2H4NH3 + ) perovskites originates from defect-related luminescence centers. We find that the broadband emission of this prototype 2D structure can be largely suppressed by using excess PEAI treatment. Density functional theory (DFT) calculations indicate that iodine (I) vacancies both in the bulk and on the surface are responsible for the broadband emission. We attribute the decreased broadband emission after PEAI treatment to the passivation of both undercoordinated Pb2+ ions on the surface and I vacancies in the bulk through I– ion migration.