Lead Halide Perovskite Films Research Articles

Estimating the Optically Pumped Threshold Fluence of Thin‐Film Gain Media Using Arbitrary Excitation Beams

Optically pumped threshold fluences are a widely reported metric to benchmark the performance of thin‐film gain media and lasers. Estimating the threshold fluence for nonhomogeneous beams, such as a circular Gaussian excitation, is not trivial since the average fluence depends on the estimated spot diameter. Using an exemplary lead halide perovskite film, the inversion volume at different pump energies is mapped. It is shown that the peak fluence of an arbitrary spatial beam profile is more relevant at the threshold, as it provides an upper bound to the threshold fluence. Also, simple conversion factors to estimate the peak fluence using Gaussian excitation beams are provided and the methodology to arbitrary profiles is extrapolated. Furthermore, it is advocated for using flat‐top or uniform stripe excitations to unambiguously extract the threshold fluence, since these excitations display minor discrepancies between the average and peak fluence, and keep the inversion volume relatively constant during the measurement.

Removing Metallic Lead Defects in the Surface Layer of Lead Halide Perovskite Films Using a Pulsed UV Laser for Enhancing the Performance of Perovskite Solar Cells

Removing Metallic Lead Defects in the Surface Layer of Lead Halide Perovskite Films Using a Pulsed UV Laser for Enhancing the Performance of Perovskite Solar Cells

Characterization of Intrinsic Charge Carrier Dynamics in Lead Halide Perovskite Films Using Time-Resolved Terahertz Spectroscopy

Characterization of Intrinsic Charge Carrier Dynamics in Lead Halide Perovskite Films Using Time-Resolved Terahertz Spectroscopy

Solution Processed Hybrid Lead Perovskite Films for White Light Emission and Lasing Applications

Abstract2D organic–inorganic hybrid lead halide perovskite films are fascinating optoelectronic materials for white‐light emission and lasing applications. Here, it is demonstrated that nano‐meter‐thick films of perovskites can be formed at room temperature using Langmuir–Schaefer (LS) deposition technique. In situ X‐ray measurements reveal a self‐assembly process of the 2D perovskite formation at the air–water interface. This formation process requires the presence of stearic acid monolayer on the water subphase having a solution of lead‐perovskite in dimethylformamide. Microscopy and X‐ray measurements of these nanofilms transferred onto substrates show crystalline phase formation that deviates from the known orthorhombic structure of the bulk crystals. The nanofilms exhibit novel broad optical emission with sharp band‐edge transition in temperature‐dependent photoluminescence studies. Also, their band‐edge emission is distinctly blue‐shifted compared to that of thicker 2D perovskite flakes prepared by the slow‐cooling chemical synthesis process. 2D flakes exhibit lasing that cannot be obtained in the LS nanofilms as their thicknesses are lower than the lasing wavelength prohibiting wave‐guide formation. The results show that the 2D single‐crystalline nature of the LS films and flakes are suitable materials to develop broad‐band white light emission across the entire visible range and to obtain lasing without external cavities, respectively.

High-performance triboelectric nanogenerators based on blade-coating lead halide perovskite film and electrospinning PVDF/graphene nanofiber

Increasing the triboelectric charge density, and increasing the contact area are both important for improving the triboelectric output performance of triboelectric nanogenerators (TENGs). In this work, the triboelectric output performances of TENGs were significantly improved by changing the preparation method of positive friction layer material Cs0.05FA0.7MA0.25PbI3 perovskite film and negative friction layer material PVDF-graphene (PG) composite material. The TENG based on perovskite film prepared by blade-coating (P-B) and PG nanofiber prepared by electrospinning (PG-E) achieved champion triboelectric output performances, including an open-circuit voltage of 200 V, a short-circuit current of 16.3 μA, a maximum transfer charge of 88.2 nC and a power density of 11.32 W/m2 under 3 Hz and 50 N, mainly because of the smaller work function of the P-B film and the larger effective contact area of the PG-E nanofiber. Besides, the TENG shows good durability and stability under different humidity and temperature conditions, and it can quickly charge capacitors and light up 351 commercial LEDs. This work demonstrates that the P-B and PG-E based TENG can be applied to energy harvesting, low power electronic devices driving, and self-powered temperature and humidity sensors.

Four‐Dimensional Design Space of High‐Q Second‐Order Distributed Feedback Perovskite Lasers

AbstractSecond‐order distributed feedback (DFB) resonators are widely used for thin‐film lasers as they combine low lasing thresholds with ease of manufacturing. Here, the grating design parameters are varied to map the lasing characteristics of lead halide perovskite films deposited on a single substrate that contains hundreds of high‐quality silicon nitride surface gratings. The lowest lasing threshold (≈130 µJ cm−2) is identified through the aid of near‐field and far‐field imaging spectroscopy in a four‐dimensional design space composed of grating period, duty cycle, active layer thickness, and cavity length. Moreover, it is shown that antisymmetric modes support high‐quality lasing (Q up to ≈10800) in the second‐order DFBs. The multi‐dimensional experimental analysis is accompanied by a thorough theoretical study with a semi‐empirical model based on coupled wave equations, which is used to investigate the lasing characteristics beyond the manufacturing range. The results can be applied to a broad range of thin‐film DFB resonators, enabling the design of more complex laser stack configurations including light‐emitting devices for current‐injection lasing.

Long-term air-stable amplified spontaneous emission in quasi-2D perovskite films through ligand engineering

High-performance amplified spontaneous emission with a low threshold and long-term air stability during 8 months in ambient air is demonstrated in OA-CsPbBr3 films.

Stability of Mixed Lead Halide Perovskite Films Encapsulated in Cyclic Olefin Copolymer at Room and Cryogenic Temperatures.

Lead Mixed Halide Perovskites (LMHPs), CsPbBrI2, have attracted significant interest as promising candidates for wide bandgap absorber layers in tandem solar cells due to their relative stability and red-light emission with a bandgap ∼1.7 eV. However, these materials segregate into Br-rich and I-rich domains upon continuous illumination, affecting their optical properties and compromising the operational stability of devices. Herein, we track the microscopic processes occurring during halide segregation by using combined spectroscopic measurements at room and cryogenic temperatures. We also evaluate a passivation strategy to mitigate the halide migration of Br/I ions in the films by overcoating with cyclic olefin copolymer (COC). Our results explain the correlation between grain size, intensity dependencies, phase segregation, activation energy barrier, and their influence on photoinduced carrier lifetimes. Importantly, COC treatment increases the lifetime charge carriers in mixed halide thin films, improving efficient charge transport in perovskite solar cell applications.

Eradicating impurities in zero-dimensional metal lead halide perovskite films for stable, low dark current X-ray detectors

Eradicating impurities in zero-dimensional metal lead halide perovskite films for stable, low dark current X-ray detectors

Cation Migration in Physically Paired 2D and 3D Lead Halide Perovskite Films

Abstract2D/3D interfaces provide long‐term stability for the operation of metal halide perovskite solar cells. However, the cation migration under heat and light can disturb the interface and create a graded cation interface. This study has now probed the cation migration by physically pairing X2PbI4 (X = butylammonium BA, oleylammonium OA, or phenethylammonium PEA) 2D film and (CH3NH3, MA)PbI3 3D film at different temperatures and recording changes in the absorption and emission spectra. The migration of the methyl ammonium cation toward the 2D film slowly transforms the n = 1 layered phase into n = 2 and 3 layered phases. The 3D film, on the other hand, exhibits relatively small changes, as the inclusion of spacer cation has little effect on its phase. The apparent activation energy determined from the temperature‐dependent cation migration kinetics (Ea = 29– 50 kJ mol−1) indicates alkyl ammonium cations such as butyl ammonium migrate more readily into the 3D layer than aromatic cations such as PEA. The ease of migration of spacer cations between physically paired films suggests that the varied composition at the interface should be considered while evaluating the effectiveness of the 2D/3D design strategy for improving the performance of perovskite solar cells.

Scattered Emission Profile Technique for Accurate and Fast Assessment of Optical Gain in Thin Film Lasing Materials

Accurate measurement of optical gain is essential to screen materials as viable active media for thin-film laser applications. The net modal gain is typically measured using the variable stripe length (VSL) method, which has been extensively studied for the last few decades. In this work, we propose an alternative method, which we name scattered emission profile (SEP) method, to measure the net modal gain. It relies on the collection of amplified spontaneous emission (ASE) scattered from the surface of the film illuminated by a pump stripe. By using an appropriate setup, the new method results in a significantly faster measurement of net modal gain, while simultaneously providing a more accurate gain value. The setup and algorithm to extract the net modal gain are detailed in this paper and are demonstrated on lead halide perovskite films. The influence of the stripe length on the measured gain value is shown. Gain measurements performed over two different perovskite films, fabricated either via spin-coating or thermal evaporation, confirm the broad applicability of the SEP method. Finally, we show a quantitative comparison of the SEP method with VSL measurements and highlight the advantages and shortcomings of each method.

Realization of 1.54-μm Light-Emitting Diodes Based on Er3+ /Yb3+ co-Doped CsPbCl3 Films.

1.54-μm erbium ions (Er3+ ) electric pumped light sources with excellent optical property and simple fabrication process are urgently desired to satisfy the development of silicon-based integration photonics. The previous Er-based electroluminescence (EL) devices are mainly based on Er-complexes or Er-doped oxide compounds, which usually suffer from low external quantum efficiency(EQE)or high applied voltage etc. In this work, a novel type of Er3+ /Yb3+ co-doped lead-halide perovskite films (Er3+ /Yb3+ :CsPbCl3 ) with the maximum photoluminescence quantum yield (PLQY) of 30.12% are prepared by a simple two-step solution-coating method and the corresponding light emitting diodes (Er-PeLEDs) are fabricated, which demonstrate a almost pure 1.54-μm emission and a peak EQE up to 0.366% at a low applied voltage of 1.4V. Strong negative thermal quenching effect may help Er-PeLEDs suppress Joule heating quenching. These excellent LED properties benefit mainly from outstanding regulatory performance of acetate to perovskite films, the excellent semiconductor behavior and strong ionic property of the perovskite, and the involving of Yb3+ ions, which can directly and efficiently transfer the exciton energy to Er3+ through a quantum cutting process. Overall, the realization of 1.54-μm Er-PeLEDs offers new opportunities for silicon-based integrated light sources. This article is protected by copyright. All rights reserved.

Chemical Reaction Kinetics of the Decomposition of Low-Bandgap Tin–Lead Halide Perovskite Films and the Effect on the Ambipolar Diffusion Length

Mixed tin–lead halide perovskite materials, with a bandgap of ∼1.2–1.3 eV, are promising absorber materials for solar cells, with their bandgap ideally matched to the solar spectrum, and for the low-bandgap part of all-perovskite tandem solar cells. However, tin(II) is not very stable and can be oxidized to tin(IV), resulting in the decomposition of the perovskite. Here, we evaluate the reaction products and reaction kinetics of the decomposition of FA0.75Cs0.25Pb0.5Sn0.5I3 thin films in response to exposure to oxygen, moisture, and illumination using in situ measurements of optical transmittance, X-ray diffraction, and UV-vis-NIR spectroscopy. We found the decomposition occurs by a dry oxidation pathway (1 × 10–9 mol/m2·s at 25 °C in air) and a water-accelerated oxidation pathway (3 × 10–9 mol/m2·s at 25 °C in 50% RH air). An analytical kinetic rate expression for the decomposition is derived and validated with a mean test error of 18%. Further, we develop a predictive model of the decay of the ambipolar diffusion length, in which the chemical decomposition rate expression is the most dominant feature. The results highlight the importance and utility of quantitative measurements of perovskite degradation.

Exploring the Limits: Degradation Behavior of Lead Halide Perovskite Films under Exposure to Ultrahigh Doses of γ Rays of Up to 10 MGy

Herein, we show that thin films of MAPbI3, FAPbI3, (CsMA)PbI3, and (CsMAFA)PbI3, where MA and FA are methylammonium and formamidinium cations, respectively, tolerate ultrahigh doses of γ rays approaching 10 MGy without significant changes in their absorption spectra. However, among the studied materials, FAPbI3 was the only one that did not form metallic lead due to its extreme radiation hardness. Infrared near-field optical microscopy revealed the radiation-induced depletion of organic cations from the grains of MAPbI3 and their accumulation at the grain boundaries, whereas FAPbI3 on the contrary lost FA cations from the grain boundaries. The multication (CsMAFA)PbI3 perovskite underwent a facile phase segregation to domains enriched with MA and FA cations, which is a principally new radiation-induced degradation pathway. Our findings suggest that the radiation hardness of the rationally designed perovskite semiconductors could go far beyond the impressive threshold of 10 MGy we set herein for FAPbI3 films, which opens many exciting opportunities for practical implementation of these materials.

Key role of residual lead iodide in two-step processed perovskite layer for high performance perovskite solar cells

The delicate control of the crystallization behavior of the perovskite layer is critical for achieving high power conversion efficiency (PCE) and longer-term stability of perovskite solar cells (PSCs). Residual lead iodide (PbI2) was usually evitable in the two-step processed perovskite film due to incomplete reaction; however, its exact effects on the perovskite layer and resulting PSCs were not well elucidated so far. Here, the lead halide perovskite films were grown by the two-step process, where the residual PbI2 can be fine controlled by the concentration of organic salt solution. Results indicated that the properties of the perovskite layer may be varied greatly depending on the amount of residual PbI2, and a series of beneficial effects for PSCs can be achieved via suitable management of residue PbI2, i.e., prolonged carrier diffusion capacity, reduced defect density, inhibited ion migration, and interfacial charge recombination. Accordingly, an impressive PCE of 22.4% was achieved from the PSCs with optimized PbI2. Furthermore, the PSCs also displayed fairly good operational stability and retained 68% of its initial PCE after 400 h (at continuous irradiation under 1-sun illumination). This work may inspire solutions for further enhancing the performance of PSCs via fine controlled residual PbI2 in the perovskite layer.

Air-stable mixed cation lead halide perovskite films and microscopic study of their degradation process.

We report the preparation and nanoscale photophysical characterization of mixed cation perovskite films of the composition MA1-xFAxPbI3, with x = 0, 0.3 and 0.5. Films with x = 0.5 and 0.3 prepared in air using ethyl acetate as an antisolvent in a one-step spin-coating process are compositionally stable in ambient air for more than a year, in contrast to films prepared using a chlorobenzene antisolvent. The onset of degradation of the films near the film edges was monitored using in situ photoluminescence (PL) spectroscopy. The PL spectra of the degradation products are consistent with the PL spectra of 2D perovskite sheets of varying thicknesses. Morphologically, aging of the films brings about coalescing of the film grain structure into larger crystal grains. Furthermore, monitoring of the time traces of PL from individual nanoscale locations in the films (PL blinking) reveals that aging of the films does not change the extent of dynamic PL quenching or affect the observed long-range charge diffusion on the order of micrometers.

Improving inorganic perovskite photovoltaic performance via organic cation addition for efficient solar energy utilization

Organic cations help in executing better crystal growth of halide perovskites. Herein, we have realized improved all-inorganic cesium lead halide perovskite film quality with reduced pinholes and high crystallinity in phenylethylammonium (PEA+) assisted perovskite. Additionally, PEA+ cation acts as an additive to control intermediate perovskite phase and executes pure phase perovskite crystal growth as compared to MA+ and FA+ cations assisted perovskite films. X-ray photoelectron and UV–vis spectroscopy results revealed that adding PEA+ in the perovskite helps in almost uniform distribution of Br− ion after post-annealing process. Furthermore, depth profiling analysis reveals that PEA+ in the intermediate phase perovskite interacts more with the ZnO layer beneath perovskite, which helps in the fabrication of compact film. PEA+ sublimates after post-annealing treatment of 300 °C, as no evidence of PEA+ existence was found in characterizations for post-annealed perovskite films. This brings multiple benefits to inorganic mixed halide perovskite film formation. The best performing perovskite solar cell (PSC) exhibits a high conversion efficiency of 14.75% as well as enhanced stability of maintaining almost 83% of its initial efficiency for 400 h. Electrochemical impedance spectroscopy and space-charge-limited current results suggests reduced recombination loss and defect densities in PEA+ assisted PSC.

Time‐Resolved Orientation and Phase Analysis of Lead Halide Perovskite Film Annealing Probed by In Situ GIWAXS

AbstractScalable thin‐film deposition methods are increasingly important for hybrid lead halide perovskite thin films. Understanding the structure evolution during non‐equilibrium processes helps to find suitable materials and processing parameters to produce films with well‐performing optoelectronic properties. Here, spin‐cast and slot‐die coated bilayers of lead iodide (PbI2) and methylammonium iodide (MAI) are investigated by in situ grazing‐incidence wide‐angle X‐ray scattering during the thermal annealing process, which converts the bilayer into methylammonium lead iodide (MAPI). Photoluminescence (PL) and UV/Vis measurements show increasing crystallinity during the annealing process and a slight PL red‐shift of the spin‐cast film, attributed to crystallite coalescence that is not prominent for the slot‐die coated film. The disintegration of the solvent‐precursor complex (MA)2(Pb3I8) ⋅ 2 DMSO and conversion into perovskite are followed in situ and differences in the morphology and time evolution are observed. In both, spin‐cast and slot‐die coated thin‐films, the isotropic orientation is dominant, however, in the slot‐die coated films, the perovskite crystallites have an additional face‐on orientation ((110) parallel to substrate) that is not detected in spin‐cast films. An Avrami model is applied for the perovskite crystal growth that indicates reduced dimensionality of the growth for the printed thin films.

Quadruple-Cation Wide-Bandgap Perovskite Solar Cells with Enhanced Thermal Stability Enabled by Vacuum Deposition.

Vacuum processing of multicomponent perovskites is not straightforward, because the number of precursors is in principle limited by the number of available thermal sources. Herein, we present a process which allows increasing the complexity of the formulation of vacuum-deposited lead halide perovskite films by multisource deposition and premixing both inorganic and organic components. We apply it to the preparation of wide-bandgap CsMAFA triple-cation perovskite solar cells, which are found to be efficient but not thermally stable. With the aim of stabilizing the perovskite phase, we add guanidinium (GA+) to the material formulation and obtained CsMAFAGA quadruple-cation perovskite films with enhanced thermal stability, as observed by X-ray diffraction and rationalized by microstructural analysis. The corresponding solar cells showed similar performance with improved thermal stability. This work paves the way toward the vacuum processing of complex perovskite formulations, with important implications not only for photovoltaics but also for other fields of application.

Inorganic CsPbBr3 Perovskite Nanocrystals as Interfacial Ion Reservoirs to Stabilize FAPbI3 Perovskite for Efficient Photovoltaics

AbstractBulk lead halide perovskite films with unique optoelectronic properties and facile low‐cost processing methods are promising for various optoelectronic applications including photovoltaics. Solution‐processed colloidal inorganic perovskite nanocrystals with higher crystallinity hold good potential for devices with better stability and optoelectronic properties. To address phase stability and defect passivation issues in black phase FAPbI3 perovskite, all‐inorganic perovskite CsPbBr3 nanocrystals (CPB NCs) are adapted as an effective modifier to form CPB NC‐treated FAPbI3 perovskite films (noted as CPB NC‐FAPbI3). It is identified in this study that, the incorporated CsPbBr3 NCs not only act as surface additives for the bulk perovskite films, but can also serve as ion reservoirs to supply both cation and halide exchanges at the interface. This two‐way ion exchange in CPB NC‐FAPbI3 samples leads to the formation of a compositional gradient as FA1−xCsxPbI3−yBry layers on the bulk perovskite film surface with vertically varied x and y values. Defects at interfaces and grain boundaries are therefore effectively passivated, leading to improved stability and charge carrier dynamics. As a result, the CPB NC‐FAPbI3 perovskite based solar cells exhibit overall enhancements in device efficiency and operational stability.