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Ni, Zhenyi

Publications and source records attributed to Ni, Zhenyi.

14 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

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

Abstract Methylammonium lead tribromide (MAPbBr 3 ) 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 MAPbBr 3 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 MAPbBr 3 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 137 Cs 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.

Ni, Zhenyi↗
DOE OSTI2024

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.

14 SOLAR ENERGY↗
DOE OSTI2024

Surface-Defect-Passivation-Enabled Near-Unity Charge Collection Efficiency in Bromide-Based Perovskite Gamma-Ray Spectrum Devices

Hybrid lead halide perovskites have superior charge transport properties to all-inorganic perovskites, but high-resolution spectroscopic radiation detectors have not been realized. Here we show that surface deep traps severely limit charge collection in formamidinium lead bromide (FAPbBr3) single-crystal devices, despite having a good bulk transport property. Three types of defect on the crystal surface, namely, FA vacancies, uncoordinated lead and Pb-Pb dimers caused by bromide loss, are found to form deep traps, resulting in non-radiative charge recombinations at the metal/perovskite interface. By tailoring the passivation functional groups, we find that ammonium bromide can passivate all these three deep traps on FAPbBr3 surfaces, improving the charge collection efficiency to near unity. The comparable bulk and surface recombination lifetimes indicate that all the surface defects are effectively passivated. Surface passivation also reduces the dark current by 10 times and decreases the dark counts by ~60 times. The energy resolution of the 137Cs spectra acquired using the FAPbBr3 detectors is improved from 5.7% to 1.7% when all the surface defects are passivated without changing the bulk properties, which is the best among solution-grown semiconductor detectors. Surface passivation is stable for more than six months, and FAPbBr3 spectroscopic detectors can operate at unprecedented high temperatures of more than 130 degrees C.

gamma ray↗
DOE OSTI2023

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 (Pb 2+ ), resulting in a reduced amorphous region in perovskites near HTLs and a passivated perovskite bottom surface. Here, 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.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

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 fuels 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. Finally, this study opens a new application of photon counting for perovskites that uses their unique defect properties.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

Enhancing Photostability of Sn-Pb Perovskite Solar Cells by an Alkylammonium Pseudo-Halogen Additive

High-performance tin-lead perovskite solar cells (PSCs) are needed for all-perovskite-tandem solar cells. However, iodide related fast photodegradation severely limits the operational stability of Sn-Pb perovskites despite the demonstrated high efficiency and thermal stability. Herein, this work employs an alkylammonium pseudo-halogen additive to enhance the power conversion efficiency (PCE) and photostability of methylammonium (MA)-free, Sn-Pb PSCs. Density functional theory (DFT) calculations reveal that the pseudo-halogen tetrafluoroborate (BF 4 - ) has strong binding capacity with metal ions (Sn 2+ /Pb 2+ ) in the Sn-Pb perovskite lattice, which lowers iodine vacancy formation. Upon combining BF 4 - with an octylammonium (OA + ) cation, the PCE of the device with a built-in light-scattering layer is boosted to 23.7%, which represents a new record for Sn-Pb PSCs. The improved efficiency benefits from the suppressed defect density. Under continuous 1 sun illumination, the OABF 4 embodied PSCs show slower generation of interstitial iodides and iodine, which greatly improves the device photostability under open-circuit condition. Moreover, the device based on OABF 4 retains 88% of the initial PCE for 1000 h under the maximum-power-point tracking (MPPT) without cooling.

14 SOLAR ENERGY↗
DOE OSTI2022

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.

14 SOLAR ENERGY↗
DOE OSTI2022

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, we report a ternary Sn (II) alloy of SnOCl 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 which 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 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 1,200 h, and keep 85% efficiency under 85°C thermal stress for 1,500 h. Furthermore, the hybrid HTM further improve the stabilized efficiencies of single-function Sn-Pb PSCs and all-perovskite tandem solar cells to 23.2% and 25.9%, respectively. This discovery opens an avenue to the multi-component metal alloys as HTM in PSCs.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Carbazole-Based Hole Transport Polymer for Methylammonium-Free Tin-Lead Perovskite Solar Cells with Enhanced Efficiency and Stability

As the most commonly used hole transport material (HTM) in tin-lead (Sn-Pb) perovskite solar cells (PSCs), poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) limits the power conversion efficiency (PCE) and stability of the PSCs due to its acidic characteristics. Herein, an easily synthesized polymer HTM poly[(phenyl)imino[9-(2-ethylhexyl)carbazole]-2,7-diyl] (CzAn) with a shallow highest occupied molecular orbital (HOMO) level of -4.95 eV is used in a p-i-n structure, methylammonium-free, Sn-Pb PSC to replace PEDOT:PSS. Upon optimization using doping and surface engineering, high quality Sn-Pb PSCs could be successfully fabricated, boosting the PCE to 22.6% (stabilized PCE of 21.3%) compared with 21.2% for PEDOT:PSS. The perovskite films prepared on the modified CzAn HTM possess improved crystallinity, reduced trap-state density, and larger carrier mobility resulting in PSCs with greatly improved stability.

carrier mobility↗
DOE OSTI2022

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 (S GB ) 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 S GB 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 S GB by four times, with the lowest SGB of 410 cm/s, which is comparable to surface recombination velocities after passivation.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Efficient monolithic all-perovskite tandem solar modules with small cell-to-module derate

All-perovskite tandem solar modules are promising to reduce the cost of photovoltaic systems with their high efficiency and solution fabrication, but their sensitivity to air still imposes a great challenge. Here a hot gas-assisted blading method is developed to accelerate the perovskite solidification, forming compact and thick narrow bandgap (NBG) perovskite films. Adding a reduction agent into NBG films followed by a short period of air exposure and a post-fabrication storage surprisingly increases carrier recombination lifetime and enables laser scribing in ambient conditions without obvious loss of device performance. This combination suppresses tin and iodide oxidation and forms a thin SnO 2 layer on the NBG film surface. Monolithic all-perovskite tandem solar modules showed a champion efficiency of 21.6% with a 14.3 cm 2 aperture area, corresponding to an active area efficiency of 23.0%. Lastly, the very small cell-to-module derate of 6.5% demonstrates the advantage of a tandem monolithic structure for solar modules.

14 SOLAR ENERGY↗
DOE OSTI2022

Defect engineering in wide-bandgap perovskites for efficient perovskite–silicon tandem solar cells

Wide-bandgap (WBG) mixed-halide perovskites show promise of realizing efficient tandem solar cells but at present suffer from large open-circuit voltage loss and the mechanism is still unclear. Here we show that WBG perovskites with iodide–bromide compositions have an increased concentration of deep traps induced by iodide interstitials, which limits performance of WBG perovskite cells. We employ tribromide ions to suppress the iodide interstitial formation and thus reduce charge recombination in bladed WBG perovskite films of Cs 0.1 FA 0.2 MA 0.7 Pb(I 0.85 Br 0.15 )3. The 1-µm-thick opaque WBG perovskite solar cells have an efficiency of 21.9%, a small open-circuit voltage deficit of 0.40 V and a large fill factor of 83%. The efficiency of the best-performing monolithic perovskite–silicon tandem cell using this perovskite reaches 28.6%. Furthermore, the tribromide addition also suppresses light-induced phase segregation in WBG perovskites and thus enhance device stability. Encapsulated tandem cells maintain 93% of their initial efficiency after operation for 550 h.

14 SOLAR ENERGY↗