Lead Halide Perovskite Thin Films Research Articles

Orthogonal Electrochemical Stability of Bulk and Surface in Lead Halide Perovskite Thin Films and Nanocrystals.

Lead halide perovskites have attracted significant attention for their wide-ranging applications in optoelectronic devices. A ubiquitous element in these applications is that charging of the perovskite is involved, which can trigger electrochemical degradation reactions. Understanding the underlying factors governing these degradation processes is crucial for improving the stability of perovskite-based devices. For bulk semiconductors, the electrochemical decomposition potentials depend on the stabilization of atoms in the lattice-a parameter linked to the material's solubility. For perovskite nanocrystals (NCs), electrochemical surface reactions are strongly influenced by the binding equilibrium of passivating ligands. Here, we report a spectro-electrochemical study on CsPbBr3 NCs and bulk thin films in contact with various electrolytes, aimed at understanding the factors that control cathodic degradation. These measurements reveal that the cathodic decomposition of NCs is primarily determined by the solubility of surface ligands, with diminished cathodic degradation for NCs in high-polarity electrolyte solvents where ligand solubilities are lower. However, the solubility of the surface ligands and bulk lattice of NCs are orthogonal, such that no electrolyte could be identified where both the surface and bulk are stabilized against cathodic decomposition. This poses inherent challenges for electrochemical applications: (i) The electrochemical stability window of CsPbBr3 NCs is constrained by the reduction potential of dissolved Pb2+ complexes, and (ii) cathodic decomposition occurs well before the conduction band can be populated with electrons. Our findings provide insights to enhance the electrochemical stability of perovskite thin films and NCs, emphasizing the importance of a combined selection of surface passivation and electrolyte.

The first proof-of-concept of straightforward and ambient-processed CsPbBr3 perovskite light-emitting electrochemical cell

All-inorganic cesium lead-halide perovskite thin films show great promise for optoelectronic applications. This study introduces a novel synthesis strategy for CsPbBr3 thin films, conducted on a preheated substrate. The synthesis process involves precise control and optimization of parameters under ambient conditions, eliminating the need for a glovebox. Therefore, a facile and low-cost way to fabricate CsPbBr3 perovskite thin films for potential optoelectronic applications has been provided by this method. The effects of various parameters, such as precursor concentration, annealing temperature, and polyethylene oxide (PEO) addition, on the structure, morphology, and optical properties of the perovskite films were systematically studied. The optimal conditions for the highest photoluminance and maximum surface coverage were found to be 0.26 M precursor concentration, 100 °C annealing temperature, and 100:65 CsPbBr3:PEO weight ratio. Uniform and bright perovskite films with over 99% surface coverage, reduced grain size, and suppressed trap density were obtained as a result of these conditions. The PL intensity of the optimized composite thin film exhibited a minor decrease of 6% from its initial value over six months in an ambient environment. A proof-of-concept perovskite light-emitting electrochemical cell (PeLEC) was constructed utilizing the optimized single-layer perovskite thin film. Consequently, green electroluminescence at 523 nm, featuring a full width at half maximum (FWHM) of 20 nm and a chromaticity coordinate of (0.121, 0.800), was successfully generated from the PeLEC. This showcases the viability of the mentioned methodology for the future fabrication of light-emitting devices.

Enhancing grain growth of caesium-formamidinium-based lead halide perovskite thin films through PbI2 precursor engineering in vapor-solid reaction

Vapor-solid reaction methods are highly regarded as potential solutions for large-scale production of perovskite thin films due to their scalability, compatibility with silicon tandem technology, and lack of solvents. However, the limited penetration of organic vapor through the solid inorganic film results in a slow growth rate of perovskite, leading to poor crystallinity and small grain size. This high defect density in the grain boundaries hinders the enhancement of device performance. In this study, we used 1,3-diaminoguanidine monohydrochloride as an additive in the PbI2 precursor films, which effectively improved perovskite grain growth in the vapor-solid reaction process. After optimization, we achieved high-quality perovskite thin films with a large grain size exceeding 5 μm. Notably, solar devices based on these large-grain perovskite thin films achieved a high power conversion efficiency up to 21.13%.

Pyrolytic fragmentation-induced defect formation in formamidinium lead halide perovskite thin films and photovoltaic performance limits

Defect formation through the fragmented formamidinium cation in lead iodide perovskite leads to the widening charge inversion layer and limited solar cell performance.

Optoelectronic Devices of Large-Scale Transferred All-Inorganic Lead Halide Perovskite Thin Films.

We report the large-scale transfer process for monocrystalline CsPbBr3 thin films prepared by chemical vapor deposition (CVD) with excellent optical properties and stability. The transfer process is robust, simple, and effective, in which CsPbBr3 thin films could be transferred to several substrates and effectively avoid chemical or physical fabrication processes to damage the perovskite surface. Moreover, the transfer process endows CsPbBr3 and substrates with atomically clean and electronically flat interfaces. We utilize this transfer process to realize several optoelectronic devices, including a photonic laser with a threshold of 61 μJ/cm2, a photodetector with a responsivity of 2.4 A/W, and a transistor with a hole mobility of 11.47 cm2 V-1 s-1. High device performances mainly originate from low defects of high-quality single-crystal perovskite and seamless contact between CsPbBr3 and target substrates. The large-scale nondestructive transfer process provides promising opportunities for optoelectronic applications based on monocrystalline perovskites.

Two‐Step Vapor‐Solid Reaction for the Growth of High‐Quality CsFA‐Based Lead Halide Perovskite Thin Films

Thin‐film solar cells based on organic–inorganic hybrid perovskite light absorbers have attracted intense attention both from the scientific and industrial communities in recent years. Researchers worldwide have developed various strategies for depositing high‐quality perovskite thin films including composition optimization, surface modification, and solvent engineering. However, these strategies are mainly based on the perovskite thin films which are deposited via solution processes. Though vacuum and vapor‐based techniques have been widely used in the modern thin‐film industry, these technologies still need further developments for the deposition of high‐quality organic–inorganic hybrid perovskite thin films. Herein, a novel two‐step vapor–solid reaction procedure for the deposition of high‐quality methylamine‐free hybrid perovskite thin films is proposed. The first vapor–solid reaction step gives the perovskite a proper chemical stoichiometric ratio, and the second step can refine the crystallinity of the film. Therefore, by utilizing this method, it is possible to obtain high‐quality perovskite thin films with larger grain sizes and lower tap densities. Consequently, perovskite solar cells based on these films have achieved a high power conversion efficiency of 20.79%.

Binary Solvent System Used to Fabricate Fully Annealing‐Free Perovskite Solar Cells

AbstractHigh temperature post‐deposition annealing of hybrid lead halide perovskite thin films—typically lasting at least 10 min—dramatically limits the maximum roll‐to‐roll coating speed, which determines solar module manufacturing costs. While several approaches for “annealing‐free” perovskite solar cells (PSCs) have been demonstrated, many are of limited feasibility for scalable fabrication. Here, this work has solvent‐engineered a high vapor pressure solvent mixture of 2‐methoxy ethanol and tetrahydrofuran to deposit highly crystalline perovskite thin‐films at room temperature using gas‐quenching to remove the volatile solvents. Using this approach, this work demonstrates p‐i‐n devices with an annealing‐free MAPbI3 perovskite layer achieving stabilized power conversion efficiencies (PCEs) of up to 18.0%, compared to 18.4% for devices containing an annealed perovskite layer. This work then explores the deposition of self‐assembled molecules as the hole‐transporting layer without annealing. This work finally combines the methods to create fully annealing‐free devices having stabilized PCEs of up to 17.1%. This represents the state‐of‐the‐art for annealing‐free fabrication of PSCs with a process fully compatible with roll‐to‐roll manufacture.

Processing of Lead Halide Perovskite Thin Films Studied with In-Situ Real-Time X-ray Scattering.

Lead halide perovskites have been of paramount interest for solution-processable solar cells, reaching power conversion efficiencies larger than 25%. In this spotlight, we will provide a systematic overview of the influence of different solution-based processing routes of lead halide perovskites on their phase transformation and conversion as revealed through in-situ X-ray-scattering experiments. These experiments were performed in conditions closely mimicking thin film processing methods and conditions used for thin film solar cell device fabrication and therefore provide critical information about the mechanism of the phase transformation, its onset, the kinetics, as well as the emergence and disappearance of various (meso)phases along the way. The measurements capture the overall solidification and conversion process of lead halide perovskite inks into solid films via so-called one-step and two-step spin-coating processes as well as blade coating and hot casting. Processing routes are applied to films based on basic components as well as mixtures of different anions and cations, solvents, and antisolvents, all of which deeply affect the thin film microstructure and morphology of the light-absorbing semiconductor and associated solar cell devices.

Effects of Alkaline Earth Metal Additives on Methylammonium‐Free Lead Halide Perovskite Thin Films and Solar Cells

Organic–inorganic lead halide perovskite solar cells are regarded as one of the most promising technologies for the next generation of photovoltaics due to their high power conversion efficiency (PCE) and simple solution manufacturing. Among the different compositions, the formamidinium lead iodide (FAPbI3) photoactive phase has a bandgap of 1.4 eV, which enables the corresponding higher PCEs according to the Shockley–Queisser limit. However, the photoactive crystal phase of FAPbI3 is not stable at room temperature. The most high‐performing compositions to date have reduced this problem by incorporating the methylammonium (MA) cation into the FAPbI3 composition, although MA has poor stability at high temperatures and in humid environments, which can limit the lifetime of FAxMA1−xPbI3 films. CsxFA1−xPbI3 perovskites are also explored, but despite better stability they still lag in performance. Herein, the additive engineering of MA‐free organic−inorganic lead halide perovskites using divalent cations Sr2+ and Ca2+to enhance the performances of CsxFA1−xPbI3 perovskite compositions is explored. It is revealed that the addition of up to 0.5% of Sr2+ and Ca2+ leads to improvements in morphology and reduction in microstrain. The structural improvements observed correlate with improved solar cell performances at low additive concentrations.

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.

Nanoscale Photoexcited Carrier Dynamics in Perovskites.

The optoelectronic properties of lead halide perovskite thin films can be tuned through compositional variations and strain, but the associated nanocrystalline structure makes it difficult to untangle the link between composition, processing conditions, and ultimately material properties and degradation. Here, we study the effect of processing conditions and degradation on the local photoconductivity dynamics in [(CsPbI3)0.05(FAPbI3)0.85(MAPbBr3)0.15] and (FA0.7Cs0.3PbI3) perovskite thin films using temporally and spectrally resolved microwave near-field microscopy with a temporal resolution as high as 5 ns and a spatial resolution better than 50 nm. For the latter FACs formulation, we find a clear effect of the process annealing temperature on film morphology, stability, and spatial photoconductivity distribution. After exposure of samples to ambient conditions and illumination, we find spectral evidence of halide segregation-induced degradation below the instrument resolution limit for the mixed halide formulation, while we find a clear spatially inhomogeneous increase in the carrier lifetime for the FACs formulation annealed at 180 °C.

Quantitative comparison between different methods for the determination of the amplified spontaneous emission threshold in dye-polymer blends and perovskite thin films

Amplified Spontaneous Emission (ASE) properties and ASE threshold are usually investigated for the characterization of a candidate active material for laser applications. However, the comparison among different materials is often hampered by the use in literature of several different methods to estimate the ASE threshold. In this work we quantitatively compare the ASE threshold values obtained by using the most employed methods in dye-doped polymer and lead halide perovskite thin films, highlighting the dependence of the value obtained on the applied method.

In-situ observation of trapped carriers in organic metal halide perovskite films with ultra-fast temporal and ultra-high energetic resolutions

We in-situ observe the ultrafast dynamics of trapped carriers in organic methyl ammonium lead halide perovskite thin films by ultrafast photocurrent spectroscopy with a sub-25 picosecond time resolution. Upon ultrafast laser excitation, trapped carriers follow a phonon assisted tunneling mechanism and a hopping transport mechanism along ultra-shallow to shallow trap states ranging from 1.72–11.51 millielectronvolts and is demonstrated by time-dependent and independent activation energies. Using temperature as an energetic ruler, we map trap states with ultra-high energy resolution down to < 0.01 millielectronvolt. In addition to carrier mobility of ~4 cm2V−1s−1 and lifetime of ~1 nanosecond, we validate the above transport mechanisms by highlighting trap state dynamics, including trapping rates, de-trapping rates and trap properties, such as trap density, trap levels, and capture-cross sections. In this work we establish a foundation for trap dynamics in high defect-tolerant perovskites with ultra-fast temporal and ultra-high energetic resolution.

Large-Area Synthesis and Patterning of All-Inorganic Lead Halide Perovskite Thin Films and Heterostructures.

All-inorganic lead halide perovskites have attracted tremendous interest for their excellent stability when compared with hybrid perovskites. Here we report a large-area growth of monocrystalline all-inorganic perovskite thin films and further patterning them into heterostructure arrays. We show that highly oriented CsPbBr3 microcrystal domains can be readily grown on muscovite mica substrates with a well-defined epitaxial relationship, which can further expand and eventually merge into large-area monocrystalline CsPbBr3 thin films with an excellent optical quality. Taking a step further, we show the large-area CsPbBr3 thin film can be further patterned and selectively transformed into CsPbI3 using a selective anion-exchange process to produce CsPbBr3-CsPbI3 lateral heterostructure arrays with spatially modulated photoluminescence emission and an apparent current rectification behavior. The capability to grow large-area CsPbBr3 monocrystalline thin films and heterostructure arrays defines a robust material platform for both the fundamental investigations and potential applications in optoelectronics.

Electron-Beam Irradiation Induced Regulation of Surface Defects in Lead Halide Perovskite Thin Films.

Organic-inorganic hybrid perovskites (OIHPs) have been intensively studied due to their fascinating optoelectronic performance. Electron microscopy and related characterization techniques are powerful to figure out their structure-property relationships at the nanoscale. However, electron beam irradiation usually causes damage to these beam-sensitive materials and thus deteriorates the associated devices. Taking a widely used CH3NH3PbI3 film as an example, here, we carry out a comprehensive study on how electron beam irradiation affects its properties. Interestingly, our results reveal that photoluminescence (PL) intensity of the film can be significantly improved along with blue-shift of emission peak at a specific electron beam dose interval. This improvement stems from the reduction of trap density at the CH3NH3PbI3 surface. The knock-on effect helps expose a fresh surface assisted by the surface defect-induced lowering of displacement threshold energy. Meanwhile, the radiolysis process consistently degrades the crystal structure and weaken the PL emission with the increase of electron beam dose. Consequently, the final PL emission comes from a balance between knock-on and radiolysis effects. Taking advantage of the defect regulation, we successfully demonstrate a patterned CH3NH3PbI3 film with controllable PL emission and a photodetector with enhanced photocurrent. This work will trigger the application of electron beam irradiation as a powerful tool for perovskite materials processing in micro-LEDs and other optoelectronic applications.

Rare-earth quantum cutting in metal halide perovskites – a review

Ytterbium-doped lead halide perovskite (Yb3+:CsPbX3 with x = Cl or Cl/Br) nanocrystals and thin films have shown surprisingly efficient downconversion by quantum cutting with PLQYs up to 193%. After excitation of the perovskite host with high-energy photons, the excited states of two Yb ions are rapidly populated, subsequently emitting lower-energy photons. Several synthesis routes lead to highly efficient materials, and we review the progress on both the synthesis, material quality and applicability of these downconversion layers. For solar cells they could be used to increase the power converted from high-energy photons, and first applications have already shown an increase in the power conversion efficiency of silicon and CIGS solar cells. Applications such as luminescent solar concentrators an LEDs are also explored. With further research to overcome challenges regarding power saturation and stability, this material has great potential for a simple route to enhance solar cells.

A comparative study of two-step and three-step methods for coating organometallic lead halide perovskite thin films

Two-step sequential spin coating deposition technique provides a cost-effective and high performance route for the synthesis of the organometallic lead halide perovskite materials. In this method, initially the metal halide film is coated, over which the MAI with a particular concentration is again spin-coated. However, it is difficult to obtain complete conversion of PbI2 in CH3NH3PbI3 perovskite. In this work, we show that good quality perovskite films can be coated by a three-step method to obtain improved conversion to perovskite and with improved materials properties. We have coated the perovskite films by two-step as well as three-step methods and presented a comparison of obtained film properties. Our results show that at lower concentration of MAI, complete conversion of PbI2 does not take place. With increase in MAI concentration, in addition to the perovskite phase an intercalated MAI into PbI2 phase is formed. In three-step coating, good phase of perovskite and improved morphology is obtained at comparatively lower MAI concentration. At higher concentrations of MAI in three-step method, leads to removal of already existing perovskite material and therefore leads to lower absorption.

Is Disorder Beneficial in Perovskite-Sensitized Solid-State Upconversion? The Role of DBP Doping in Rubrene

Solid-state bulk lead halide perovskite thin films have recently shown progress as triplet sensitizers in infrared-to-visible photon upconversion (UC) schemes. Common systems pair lead halide perov...

Simple Method for Efficient Slot-Die Coating of MAPbI3 Perovskite Thin Films in Ambient Air Conditions.

Scalable methods for deposition of lead halide perovskite thin films are required to enable commercialization of the highly promising perovskite photovoltaics. Here, we have developed a slot-die coating process under ambient conditions for methylammonium lead iodide (MAPbI3) perovskite on heated substrates (about 90 °C on the substrate surface). Dense, highly crystalline perovskite films with large grains (100–200 μm) were obtained by careful adjustment of the deposition parameters, using solutions that are similar but more dilute than those used in typical spin-coating procedures. Without any further after treatments, such as antisolvent treatment or vapor annealing, we achieved power conversion efficiencies up of 14.5% for devices with the following structure: conducting tin oxide glass (FTO)/TiO2/MAPbI3/spiro-MeOTAD/Au. The performance was limited by the significant roughness of the deposited films, resulting from the hot-casting method, and the relatively high deposition temperature, which led to a defect-rich surface due to loss of MAI.

Photoinduced Degradation of Lead Halide Perovskite Thin Films in Air

Photoinduced Degradation of Lead Halide Perovskite Thin Films in Air