Identification and Suppression of Point Defects in Bromide Perovskite Single Crystals Enabling Gamma-Ray Spectroscopy.

Methylammonium lead tribromide (MAPbBr3) stands out as the most easily grown wide-band-gap metal halide perovskite. It is a promising semiconductor for room-temperature gamma-ray (γ-ray) spectroscopic detectors, but no operational devices are realized. This can be largely attributed to a lack of understanding of point defects and their influence on detector performance. Here, through a combination of crystal growth design and defect characterization, including positron annihilation and impedance spectroscopy, the presence of specific point defects are identified and correlated to detector performance. Methylammonium (MA) vacancies, MA interstitials, and Pb vacancies are identified as the dominant charge-trapping defects in MAPbBr3 crystals, while Br vacancies caused doping. The addition of excess MABr reduces the MA and Br defects and so enables the detection of energy-resolved γ-ray spectra using a MAPbBr3 single-crystal device. Interestingly, the addition of formamidinium (FA) cations, which converted to methylformamidinium (MFA) cations by reaction with MA+ during crystal growth further reduced MA defects. This enabled an energy resolution of 3.9% for the 662 keV 137Cs line using a low bias of 100 V. The work provides direction toward enabling further improvements in wide-bandgap perovskite-based device performance by reducing detrimental defects.

All-silicon multidimensionally-encoded optical physical unclonable functions for integrated circuit anti-counterfeiting.

Integrated circuit anti-counterfeiting based on optical physical unclonable functions (PUFs) plays a crucial role in guaranteeing secure identification and authentication for Internet of Things (IoT) devices. While considerable efforts have been devoted to exploring optical PUFs, two critical challenges remain: incompatibility with the complementary metal-oxide-semiconductor (CMOS) technology and limited information entropy. Here, we demonstrate all-silicon multidimensionally-encoded optical PUFs fabricated by integrating silicon (Si) metasurface and erbium-doped Si quantum dots (Er-Si QDs) with a CMOS-compatible procedure. Five in-situ optical responses have been manifested within a single pixel, rendering an ultrahigh information entropy of 2.32 bits/pixel. The position-dependent optical responses originate from the position-dependent radiation field and Purcell effect. Our evaluation highlights their potential in IoT security through advanced metrics like bit uniformity, similarity, intra- and inter-Hamming distance, false-acceptance and rejection rates, and encoding capacity. We finally demonstrate the implementation of efficient lightweight mutual authentication protocols for IoT applications by using the all-Si multidimensionally-encoded optical PUFs.

Monolithic integration of perovskite heterojunction on TFT backplanes through vapor deposition for sensitive and stable x-ray imaging.

Metal halide perovskites exhibit substantial potential for advancing next-generation x-ray detection. However, fabricating high-performance pixelated imaging arrays remains challenging due to the substantial dark current density and stability issues associated with common organic-inorganic hybrid perovskites. Here, we develop a vapor deposition method to create the first all-inorganic perovskite heterojunction film. The heterojunction introduction effectively reduces the dark current density of detectors to about 0.8 nA·cm-2, satisfying thin-film transistor (TFT) integration standards, while also increases sensitivity to above 2.6 × 104 µC·Gyair-1·cm-2, thus giving rise to a record low detection limit of <1 nGyair·s-1 among all polycrystalline perovskite-based x-ray detectors. The devices also demonstrate remarkable stability across multifarious demanding working conditions. Last, through monolithic integration of the heterojunction film with a 64 × 64 pixelated TFT array, we have achieved high-resolution real-time x-ray imaging, which paves the way for the application of all-inorganic perovskite in low-dose flat-panel x-ray detection.

Iodide manipulation using zinc additives for efficient perovskite solar minimodules.

Interstitial iodides are the most critical type of defects in perovskite solar cells that limits efficiency and stability. They can be generated during solution, film, and device processing, further accelerating degradation. Herein, we find that introducing a small amount of a zinc salt- zinc trifluoromethane sulfonate (Zn(OOSCF3)2) in the perovskite solution can control the iodide defects in resultant perovskites ink and films. CF3SOO̶ vigorously suppresses molecular iodine formation in the perovskites by reducing it to iodide. At the same time, zinc cations can precipitate excess iodide by forming a Zn-Amine complex so that the iodide interstitials in the resultant perovskite films can be suppressed. The perovskite films using these additives show improved photoluminescence quantum efficiency and reduce deep trap density, despite zinc cations reducing the perovskite grain size and iodide interstitials. The zinc additives facilitate the formation of more uniform perovskite films on large-area substrates (78-108 cm2) in the blade-coating process. Fabricated minimodules show power conversion efficiencies of 19.60% and 19.21% with aperture areas of 84 and 108 cm2, respectively, as certified by National Renewable Energy Laboratory (NREL), the highest efficiency certified for minimodules of these sizes.

Unraveling radiation damage and healing mechanisms in halide perovskites using energy-tuned dual irradiation dosing.

Perovskite photovoltaics have been shown to recover, or heal, after radiation damage. Here, we deconvolve the effects of radiation based on different energy loss mechanisms from incident protons which induce defects or can promote efficiency recovery. We design a dual dose experiment first exposing devices to low-energy protons efficient in creating atomic displacements. Devices are then irradiated with high-energy protons that interact differently. Correlated with modeling, high-energy protons (with increased ionizing energy loss component) effectively anneal the initial radiation damage, and recover the device efficiency, thus directly detailing the different interactions of irradiation. We relate these differences to the energy loss (ionization or non-ionization) using simulation. Dual dose experiments provide insight into understanding the radiation response of perovskite solar cells and highlight that radiation-matter interactions in soft lattice materials are distinct from conventional semiconductors. These results present electronic ionization as a unique handle to remedying defects and trap states in perovskites.

Lead-chelating hole-transport layers for efficient and stable perovskite minimodules.

The defective bottom interfaces of perovskites and hole-transport layers (HTLs) limit the performance of p-i-n structure perovskite solar cells. We report that the addition of lead chelation molecules into HTLs can strongly interact with lead(II) ion (Pb2+), resulting in a reduced amorphous region in perovskites near HTLs and a passivated perovskite bottom surface. The minimodule with an aperture area of 26.9 square centimeters has a power conversion efficiency (PCE) of 21.8% (stabilized at 21.1%) that is certified by the National Renewable Energy Laboratory (NREL), which corresponds to a minimal small-cell efficiency of 24.6% (stabilized 24.1%) throughout the module area. Small-area cells and large-area minimodules with lead chelation molecules in HTLs had a light soaking stability of 3010 and 2130 hours, respectively, at an efficiency loss of 10% from the initial value under 1-sun illumination and open-circuit voltage conditions.

Self-powered perovskite photon-counting detectors.

Metal-halide perovskites (MHPs) have been successfully exploited for converting photons to charges or vice versa in applications of solar cells, light-emitting diodes and solar fuels1-3, for which all these applications involve strong light. Here we show that self-powered polycrystalline perovskite photodetectors can rival the commercial silicon photomultipliers (SiPMs) for photon counting. The photon-counting capability of perovskite photon-counting detectors (PCDs) is mainly determined by shallow traps, despite that deep traps also limit charge-collection efficiency. Two shallow traps with energy depth of 5.8 ± 0.8 millielectronvolts (meV) and 57.2 ± 0.1 meV are identified in polycrystalline methylammonium lead triiodide, which mainly stay at grain boundaries and the surface, respectively. We show that these shallow traps can be reduced by grain-size enhancement and surface passivation using diphenyl sulfide, respectively. It greatly suppresses dark count rate (DCR) from >20,000 counts per second per square millimetre (cps mm-2) to 2 cps mm-2 at room temperature, enabling much better response to weak light than SiPMs. The perovskite PCDs can collect γ-ray spectra with better energy resolution than SiPMs and maintain performance at high temperatures up to 85 °C. The zero-bias operation of perovskite detectors enables no drift of noise and detection property. This study opens a new application of photon counting for perovskites that uses their unique defect properties.

Perovskite grain wrapping by converting interfaces and grain boundaries into robust and water-insoluble low-dimensional perovskites.

Stabilizing perovskite solar cells requires consideration of all defective sites in the devices. Substantial efforts have been devoted to interfaces, while stabilization of grain boundaries received less attention. Here, we report on a molecule tributyl(methyl)phosphonium iodide (TPI), which can convert perovskite into a wide bandgap one-dimensional (1D) perovskite that is mechanically robust and water insoluble. Mixing TPI with perovskite precursor results in a wrapping of perovskite grains with both grain surfaces and grain boundaries converted into several nanometer-thick 1D perovskites during the grain formation process as observed by direct mapping. The grain wrapping passivates the grain boundaries, enhances their resistance to moisture, and reduces the iodine released during light soaking. The perovskite films with wrapped grains are more stable under heat and light. The best device with wrapped grains maintained 92.2% of its highest efficiency after light soaking under 1-sun illumination for 1900 hours at 55°C open-circuit condition.

Solution-Processed Ternary Tin (II) Alloy as Hole-Transport Layer of Sn-Pb Perovskite Solar Cells for Enhanced Efficiency and Stability.

Tin-lead (Sn-Pb) narrow-bandgap (NBG) perovskites show great potential in both single-junction and all-perovskite tandem solar cells. Sn-Pb perovskite solar cells (PSCs) are still limited by low charge collection efficiency and poor stability. Here, a ternary Sn (II) alloy of SnOCl is reported as the hole-transport material (HTM) with a work function of 4.95 eV for Sn-Pb PSCs. The solution-processed SnOCl layer has a texture structure that not only reduces the optical loss of the devices, but also changes grain growth of Sn-Pb perovskites and boosts the carrier diffusion length to 3.63 µm. The formation of small perovskite grains at the HTM/perovskite interface is suppressed. These result in an almost constant internal quantum efficiency (IQE) of 96 ± 2% across the absorption spectrum of Sn-Pb perovskites. The SnOCl HTM significantly enhances the stability of Sn-Pb PSCs with 87% of its initial efficiency retained after 1-sun illumination for 1200 h, and keeps 85% efficiency under 85 °C thermal stress for 1500 h. The hybrid HTM further improves the stabilized efficiencies of single-junction Sn-Pb PSCs and all-perovskite tandem solar cells to 23.2% and 25.9%, respectively. This discovery opens an avenue to the multicomponent metal alloys as HTM in PSCs.

High grain boundary recombination velocity in polycrystalline metal halide perovskites.

Understanding carrier recombination processes in metal halide perovskites is fundamentally important to further improving the efficiency of perovskite solar cells, yet the accurate recombination velocity at grain boundaries (GBs) has not been determined. Here, we report the determination of carrier recombination velocities at GBs (SGB) of polycrystalline perovskites by mapping the transient photoluminescence pattern change induced by the nonradiative recombination of carriers at GBs. Charge recombination at GBs is revealed to be even stronger than at surfaces of unpassivated films, with average SGB reaching 2200 to 3300 cm/s. Regular surface treatments do not passivate GBs because of the absence of contact at GBs. We find a surface treatment using tributyl(methyl)phosphonium dimethyl phosphate that can penetrate into GBs by partially dissolving GBs and converting it into one-dimensional perovskites. It reduces the average SGB by four times, with the lowest SGB of 410 cm/s, which is comparable to surface recombination velocities after passivation.

Blading of Conformal Electron-Transport Layers in p-i-n Perovskite Solar Cells.

Perovskite solar cells (PSCs) are promising to reduce the cost of photovoltaic system due to their low-cost raw materials and high-throughput solution process; however, fabrication of all the active layers in perovskite modules using a scalable solution process has not yet been demonstrated. Herein, the fabrication of highly efficient PSCs and modules in ambient conditions is reported, with all layers bladed except the metal electrode, by blading a 36 ± 9 nm-thick electron-transport layer (ETL) on perovskite films with a roughness of ≈80 nm. A combination of additives in phenyl-C61 -butyric acid methyl ester (PCBM) allows the PCBM to conformally cover the perovskites and still have a good electrical conductivity. Amine-functionalized molecules are added to enhance both the dispersity of PCBM and the affinity to perovskites. A PCBM dopant of 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI) recovers the conductivity loss induced by the small amine molecules. PSCs (0.08 cm2 ) fabricated by the all-blading process reache an average efficiency of 22.4 ± 0.5% and a champion efficiency of 23.1% for perovskites with a bandgap of 1.51 eV, with much better stability compared to evaporated ETL PSCs. The all-bladed minimodule (25.03 cm2 ) shows an aperture efficiency of ≈19.3%, showing the good uniformity of the bladed ETLs.

Gradient Doping in Sn-Pb Perovskites by Barium Ions for Efficient Single-Junction and Tandem Solar Cells.

Narrow-bandgap (NBG) tin (Sn)-lead (Pb) perovskites generally have a high density of unintentional p-type self-doping, which reduces the charge-carrier lifetimes, diffusion lengths, and device efficiencies. Here, a p-n homojunction across the Sn-Pb perovskite is demonstrated, which results from a gradient doping by barium ions (Ba2+ ). It is reported that 0.1 mol% Ba2+ can effectively compensate the p-doping of Sn-Pb perovskites or even turns it to n-type without changing its bandgap. Ba2+ cations are found to stay at the interstitial sites and work as shallow electron donor. In addition, Ba2+ cations show a unique heterogeneous distribution in perovskite film. Most of the barium ions stay in the top 600 nm region of the perovskite films and turn it into weakly n-type, while the bottom portion of the film remains as p-type. The gradient doping forms a homojunction from top to bottom of the perovskite films with a built-in field that facilitates extraction of photogenerated carriers, resulting in an increased carrier extraction length. This strategy enhances the efficiency of Sn-Pb perovskite single-junction solar cells to over 21.0% and boosts the efficiencies of monolithic perovskite-perovskite tandem solar cells to 25.3% and 24.1%, for active areas of 5.9 mm2  and 0.94 cm2 , respectively.

Heterojunction structures for reduced noise in large-area and sensitive perovskite x-ray detectors.

Polycrystalline perovskites can be readily fabricated into large areas using solution depositions; however, they suffer from large dark currents that are tens to hundreds times higher than industrially relevant values, limiting their application in low-dose x-ray detection. Here, we show that the application of a heterojunction structure into polycrystalline films significantly reduces the dark current density by more than 200 times to subnanoampere per square centimeter without reducing the sensitivity of the detectors. The heterojunction perovskite films are formed by laminating several membrane films filled with perovskites of different bandgaps. A gradient bandgap is formed during annealing. The detectors have a lowest detectable dose rate of 13.8 ± 0.29 nGyair s−1 for 40-keV x-ray and can conduct dynamic x-ray imaging at a low-dose rate of 32.2 nGyair s−1. Simulation and experimental analysis show that the heterojunction is tolerant of halide diffusion and can be stable for over 15 years.

Highly Efficient Pure-Blue Light-Emitting Diodes Based on Rubidium and Chlorine Alloyed Metal Halide Perovskite.

Perovskite light-emitting diodes (PeLEDs) are promising candidates for display and solid-state lighting, due to their tunable colors, high conversion efficiencies, and low cost. However, the performance of blue PeLEDs is far inferior to that of the near-infrared, red, and green counterparts. Here, the fabrication of pure-blue PeLEDs with an emission peak at 475 nm, a peak external quantum efficiency of 10.1%, and a maximum luminance of 14 000 cd m-2 is demonstrated by tailoring the compositions of perovskites. The pure-blue electroluminescence is achieved by simultaneous addition of rubidium and chlorine ions into CsPbBr3 and incorporation of phenylethylammonium chloride forms quasi-2D hybrid perovskites. The combination of these composition engineering results in blueshifted emissions without reducing the quantum yield. The judicious alloying is shown to be critical to result in the better morphology with suppressed current leakage and enhanced light outcoupling.

Ligand assisted growth of perovskite single crystals with low defect density.

A low defect density in metal halide perovskite single crystals is critical to achieve high performance optoelectronic devices. Here we show the reduction of defect density in perovskite single crystals grown by a ligand-assisted solution process with 3-(decyldimethylammonio)-propane-sulfonate inner salt (DPSI) as an additive. DPSI ligands anchoring with lead ions on perovskite crystal surfaces not only suppress nucleation in solution, but also regulate the addition of proper ions to the growing surface, which greatly enhances the crystal quality. The grown CH3NH3PbI3 crystals show better crystallinity and a 23-fold smaller trap density of 7 × 1010 cm-3 than the optimized control crystals. The enhanced material properties result in significantly suppressed ion migration and superior X-ray detection sensitivity of CH3NH3PbI3 detectors of (2.6 ± 0.4) × 106 µC Gy-1air cm-2 for 60 kVp X-ray and the lowest detectable dose rate reaches (5.0 ± 0.7) nGy s-1, which enables reduced radiation dose to patients in medical X-ray diagnostics.

Response to Comment on "Resolving spatial and energetic distributions of trap states in metal halide perovskite solar cells".

Ravishankar et al claimed that drive-level capacitance profiling (DLCP) cannot resolve trap density in perovskites of given thickness. We point out that the trap densities derived by DLCP are from the differential capacitance at different frequencies; thus, the background charges caused by diffusion and geometry capacitance have been subtracted. Even for the nondifferential doping analysis, the contribution from diffusion capacitance is negligible and that from geometry capacitance is excluded.

Ultrafast Exciton Transport with a Long Diffusion Length in Layered Perovskites with Organic Cation Functionalization.

Layered perovskites have been employed for various optoelectronic devices including solar cells and light-emitting diodes for improved stability, which need exciton transport along both the in-plane and the out-of-plane directions. However, it is not clear yet what determines the exciton transport along the in-plane direction, which is important to understand its impact toward electronic devices. Here, by employing both steady-state and transient photoluminescence mapping, it is found that in-plane exciton diffusivities in layered perovskites are sensitive to both the number of layers and organic cations. Apart from exciton-phonon coupling, the octahedral distortion is revealed to significantly affect the exciton diffusion process, determined by temperature-dependent photoluminescence, light-intensity-dependent time-resolved photoluminescence, and density function theory calculations. A simple fluorine substitution to phenethylammonium for the organic cations to tune the structural rigidity and octahedral distortion yields a record exciton diffusivity of 1.91 cm2 s-1 and a diffusion length of 405 nm along the in-plane direction. This study provides guidance to manipulate exciton diffusion by modifying organic cations in layered perovskites.

Interfacial Molecular Doping of Metal Halide Perovskites for Highly Efficient Solar Cells.

Tailoring the doping of semiconductors in heterojunction solar cells shows tremendous success in enhancing the performance of many types of inorganic solar cells, while it is found challenging in perovskite solar cells because of the difficulty in doping perovskites in a controllable way. Here, a small molecule of 4,4',4″,4″'-(pyrazine-2,3,5,6-tetrayl) tetrakis (N,N-bis(4-methoxyphenyl) aniline) (PT-TPA) which can effectively p-dope the surface of FAx MA1- x PbI3 (FA: HC(NH2 )2 ; MA: CH3 NH3 ) perovskite films is reported. The intermolecular charge transfer property of PT-TPA forms a stabilized resonance structure to accept electrons from perovskites. The doping effect increases perovskite dark conductivity and carrier concentration by up to 4737 times. Computation shows that electrons in the first two layers of octahedral cages in perovskites are transferred to PT-TPA. After applying PT-TPA into perovskite solar cells, the doping-induced band bending in perovskite effectively facilitates hole extraction to hole transport layer and expels electrons toward cathode side, which reduces the charge recombination there. The optimized devices demonstrate an increased photovoltage from 1.12 to 1.17 V and an efficiency of 23.4% from photocurrent scanning with a stabilized efficiency of 22.9%. The findings demonstrate that molecular doping is an effective route to control the interfacial charge recombination in perovskite solar cells which is in complimentary to broadly applied defect passivation techniques.

Resolving spatial and energetic distributions of trap states in metal halide perovskite solar cells.

We report the profiling of spatial and energetic distributions of trap states in metal halide perovskite single-crystalline and polycrystalline solar cells. The trap densities in single crystals varied by five orders of magnitude, with a lowest value of 2 × 1011 per cubic centimeter and most of the deep traps located at crystal surfaces. The charge trap densities of all depths of the interfaces of the polycrystalline films were one to two orders of magnitude greater than that of the film interior, and the trap density at the film interior was still two to three orders of magnitude greater than that in high-quality single crystals. Suprisingly, after surface passivation, most deep traps were detected near the interface of perovskites and hole transport layers, where a large density of nanocrystals were embedded, limiting the efficiency of solar cells.

Reducing Surface Halide Deficiency for Efficient and Stable Iodide-Based Perovskite Solar Cells.

State-of-the-art, high-performance perovskite solar cells (PSCs) contain a large amount of iodine to realize smaller bandgaps. However, the presence of numerous iodine vacancies at the surface of the film formed by their evaporation during the thermal annealing process has been broadly shown to induce deep-level defects, incur nonradiative charge recombination, and induce photocurrent hysteresis, all of which limit the efficiency and stability of PSCs. In this work, modifying the defective surface of perovskite films with cadmium iodide (CdI2) effectively reduces the degree of surface iodine deficiency and stabilizes iodine ions via the formation of strong Cd-I ionic bonds. This largely reduces the interfacial charge recombination loss, yielding a high efficiency of 21.9% for blade-coated PSCs with an open-circuit voltage of 1.20 V, corresponding to a record small voltage deficit of 0.31 V. The CdI2 surface treatment also improves the operational stability of the PSCs, retaining 92% efficiency after constant illumination at 1 sun intensity for 1000 h. This work provides a promising strategy to optimize the surface/interface optoelectronic properties of perovskites for more efficient and stable solar cells and other optoelectronic devices.