Lead Iodide Perovskite Research Articles

Probing the photodegradation of MAPI perovskite with concentrated sunlight

The photodegradation of lead iodide (PbI2) scaffolds methylammonium lead iodide (CH3NH3PbI3, MAPI) perovskite under concentrated sunlight was investigated with absorbance, photoluminescence (PL), and conducting atomic force microscopy (CAFM) techniques. At the nanoscale, the CAFM technique provides direct evidence of change in the electrical properties of the materials. These measurements show direct evidence of photodegradation of MAPI perovskite. Direct measurements of photocurrent have been investigated at the nanoscale in the fresh and exposed area of the films. We also investigated the stability of MAPI cells under concentrated sunlight.

Role of A‐Site Composition in Charge Transport in Lead Iodide Perovskites

As the power conversion efficiency and stability of solar cells based on metal halide perovskites continue to improve, the community increasingly relies on compounds formed of mixed cations and mixed halides for the highest performing devices. The result is that device engineers now have a potentially infinite number of compositions to choose from. While this has provided a large scope for optimization, it has increased complexity of the field, and the rationale for choosing one composition over another remains somewhat empirical. Herein, the distribution of electronic properties for a range of lead iodide perovskite thin films is mapped. The relative percentages of methylammonium, formamidinium, and cesium are varied, and the electronic properties are measured with time‐resolved microwave conductivity, a contactless technique enabling extraction of electronic properties of isolated films of semiconductors. It is found a small amount of Cs leads to larger carrier mobilities and longer carrier lifetimes and that compositions with a tolerance factor close to 0.9 generally show lower performance that those closer to 0.8 or 1.0.

Photoluminescence Dynamics of MAPI Perovskite Films Induced by Lateral Electric Field

The time‐dependent behavior of perovskite devices is one of the limiting factors for wide application of these materials in sandwich‐type or lateral optoelectronic device architectures. Despite abundant research on the topic, the exact mechanisms underlying the dynamics of device performance remain elusive. Herein, photoluminescence (PL) and electric field‐induced second harmonic microscopy techniques are combined to image and identify voltage‐induced lateral dynamic processes in methylammonium lead iodide perovskite thin films. A sequence of voltage‐induced dynamic steps is observed: i) Appearance of a short‐lived bright PL next to the positive electrode; ii) formation of a moving dark PL front; iii) emergence and blinking of bright PL spots; iv) growth and subsequent decay of the bright PL spots after voltage turn off. This behavior is explained by proposing a model of varying spatial distribution of inter‐electrode electric field and the presence of two types of PL quenching defects.

Structural evolution, dielectric relaxation, and charge transport characteristics of formamidinium lead iodide (FAPbI3) perovskite

Recently, hybrid halide perovskite solar cells have drawn the researchers’ attention, owing to their high-power conversion efficiency. Among the different compounds, MAPbI3 (MA = Methylammonium CH3NH3) and FAPbI3 (FA = Formamidinium CH(NH2)2) are especially potential candidates since their bandgap suited the energy range of visible light. Here, the hybrid perovskite FAPbI3 is successfully synthesized via the inverse temperature crystallization method. Different characterization techniques such as variable-temperature structural analyses, differential scanning calorimetry (DSC), and impedance spectroscopic analysis show that FAPbI3 presents a phase transition at T = 300 K from the hexagonal to the cubic symmetry. Optical analysis of FAPbI3 shows the direct band-gap value of ∼1.4 eV. FAPbI3 is characterized by impedance spectroscopy technique performed in the 10−1–106 Hz frequency and 210–390 K temperature ranges. The alternating current (AC) conductivity is illustrated in terms of Jonncher's power law. The study on charge transportation within FAPbI3 suggests the presence of the NSPT (non-overlapping small polaron tunneling) and OLPT (overlapping large polaron tunneling) models in the hexagonal and cubic phases, respectively.

High-power nanogenerator of 2D-layered perovskite in a polymer matrix for self-charging battery-powered electronics

Recently two-dimensional layered halide perovskites (2D-layered HPs) with superior piezo-/ferroelectric properties and exceptional environmental stability have become potential alternatives to 3D-HPs. Herein, air-stable 2D-layered 1,3-propanediammonium lead iodide (DAPPbI4) perovskite is introduced as a promising filler material in a poly(vinylidene fluoride) (PVDF) polymer for the development of a high-performance and mechanically robust DAPPbI4–PVDF composite triboelectric nanogenerator (TENG). The triboelectric output performance is noticeably enhanced for 15 wt% DAPPbI4 owing to its improved electro-active β-phase PVDF (∼90%), high dielectric constant (∼47.4), and high piezoelectric coefficient (∼26.6 pm/V). The TENG achieved a high output voltage of ∼662 V, current density of ∼18.7 µA/cm2, unprecedented high power-density of ∼4.28 mW/cm2, and excellent pressure sensitivity of ∼13.31 V/kPa with outstanding mechanical durability and operational stability. The application of HP-based TENG as a sustainable power source for various portable electronics is demonstrated by charging a Li-ion battery (LIB) in a short time using the TENG output.

A Review of the Impact of Zinc Oxide Nanostructure Morphology on Perovskite Solar Cell Performance

Zinc oxide (ZnO) has been widely studied over the last decade for its remarkable properties in optoelectronic and photovoltaic devices because of its high electron mobility and excitonic properties. It has probably the broadest range of nanostructured forms that are also easy and cheap to synthesize using a wide variety of methods. The volume of recent work on ZnO nanostructures and their devices can potentially overshadow significant developments in the field. Therefore, there is a need for a concise description of the most recent advances in the field. In this review, we focus on the effect of ZnO nanostructure morphologies on the performance of ZnO-based solar cells sensitized using methylammonium lead iodide perovskite. We present an exhaustive discussion of the synthesis routes for different morphologies of the ZnO nanostructure, ways of controlling the morphology, and the impact of morphology on the photoconversion efficiency of a given perovskite solar cell (PSC). We find that although the ZnO nanostructures are empirically similar, one-dimensional structures appear to offer the most promise to increasing photoconversion efficiency (PCE) by their proclivity to align and form vertically stacked layers. This is thought to favor electron hopping, charge mobility, and conductivity by allowing multiple charge conduction pathways and increasing the effective junction cross-sectional area. The combined effect is a net increase in PCE due to the reduced surface reflection, and improved light absorption.

Efficient Semitransparent Perovskite Solar Cells Based on Thin Compact Vacuum Deposited CH3NH3PbI3 Films

AbstractLead halide perovskite materials are promising candidates for the application of semitransparent solar cells due to their bandgap tunability and high device efficiencies. The high absorption coefficient of these materials, however, makes it difficult to attain high average visible transmittance values without compromising the power conversion efficiencies (PCEs). In this work, a co‐evaporation process is employed to fabricate thin (50 and 100 nm) methylammonium lead iodide (MAPI) perovskite films and integrate them in semitransparent perovskite solar cells (ST‐PSCs). Due to the compact nature of the thin MAPI films, the resultant fully vacuum and room temperature‐processed devices demonstrate high fill factor values approaching 80% and open‐circuit voltage values close to 1100 mV. As a result of this, the ST‐PSCs exhibit PCE's as high as 9% with a photopic average visible transmittance of the full device of 23%.

Optimization of the Anti-Solvent Method for the Fabrication of Cu Electrode-Based High-Efficiency Perovskite Solar Cell

Organic–inorganic lead halide perovskite is promising photovoltaic energy harvesting material to fabricate high-efficiency solar cells. To enhance the power conversion efficiency (PCE) and stability of the fabricated device, a high-quality, pinhole-free, and larger grain size perovskite film is essential. Anti-solvent-assisted crystallization is a popular technique to deposit perovskite thin film. With the help of this technique, perovskite layer can be deposited with smooth surface morphology, low surface defect density, and excellent carrier transport. However, choosing a proper anti-solvent is essential for highly efficient perovskite solar cells (PvSCs). Here, we qualitatively evaluate the impacts of different anti-solvent on methylammonium lead iodide (MAPbI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) perovskite film. We used chlorobenzene (CB), diethyl ether (Eth), toluene (TL), and isopropyl alcohol (IPA) as an anti-solvent. Our result demonstrates that the anti-solvent with a relatively higher boiling point and lower polarity contributes to superior efficiency, low hysteresis, and better reproducibility of MAPbI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> active layer-based PvSCs. The device fabricated with CB anti-solvent exhibits the best PCE of 8.16% with a Cu-based electrode. This work provides a promising approach to controlling the growth and morphology of perovskite films and paves the way for further optimizing the fabrication process of large-area low-cost electrode-based high-efficiency devices.

Optimizing Black Phosphorus/Halide Perovskite Compositions by Scanning Photoelectrochemical Microscopy

The incorporation of black phosphorus (BP) into methylammonium lead iodide (MAPbI3) perovskites has been investigated and optimized by a high throughput screening method using scanning photoelectrochemical microscopy (SPECM) to determine how the addition of BP affects its photoelectrochemical and photovoltaic properties. An optimum ratio of 2.0 mole% BP/MAPbI3 perovskite composite generates an increased photocurrent response compared to pristine MAPbI3 for 2 mM benzoquinone (BQ) reduction at −0.6 V vs Ag/AgNO3 on a spot array electrode under illumination. Due to the relatively high quantum yield of MAPbI3, time-resolved photoluminescence measurements have been conducted to investigate photophysical behaviors of BP/MAPbI3 composites. The optimal 2.0 mole% BP/MAPbI3 exhibits an increased electron-hole diffusion lifetime compared to the pristine MAPbI3 perovskite. Finally, we demonstrate the enhanced efficiency and stability of 2.0% BP/MAPbI3-based perovskite solar cells arising from impeded Pb0-defect generation and suppressed charge-carrier recombination.

Influence of Formamidine Formate Doping on Performance and Stability of FAPbI3-Based Perovskite Solar Cells

Formamidine lead iodide (FAPbI3) perovskite material is very suitable for solar photovoltaic devices because of its ideal low band gap, theoretically high efficiency, and wide range of solar spectral absorption, coupled with its good thermal stability. A two-step spin coating method could control the crystallization process of formamidine lead iodide perovskite films better, resulting in more easily repeatable high-quality films. However, it is still difficult to avoid the formation of halide I-vacancy during the preparation of films, which will affect device performance and stability. In this paper, we added small molecular formamidine formate (FAHCOO) into the PbI2 precursor solution. Due to the high binding energy between HCOO− and I-vacancy, film defects caused by I-vacancies could be passivated. A molecular exchange process could be introduced in the two-step method with the addition of FAHCOO. The exchange process could delay the crystallization process in perovskite films and make them transform more fully; thus, ultimately improving the crystallization quality of the films. In addition, by adding FAHCOO to the PbI2 precursor solution, a small number of FAPbI3 can be pre-generated as templates. These templates could induce the growth of specific crystal planes of FAPbI3 in the second step reaction; thereby, improving the crystallinity of FAPbI3 films. The FAPbI3 of devices with optimized FAHCOO show a champion power conversion efficiency (PCE) of 19.04%, apparently higher than that of the controlled devices without FAHCOO (16.69%). For working stability tests under AM 1.5G illumination in an air environment, PSCs with FAHCOO showed nearly 100% of their initial efficiency after a 4100 s tracking test, while the original control device dropped to about 94%.

Textured MAPbI3 Thin Films Achieved via Solvent Engineering in the Solution‐Shearing Process

Solvent mixtures for making uniform, crystalline methylammonium lead iodide (MAPbI3) perovskite films are provided. The solvents dimethyl sulfoxide (DMSO), γ‐butyrolactone (GBL), and 1‐methyl‐2‐pyrrolidinone (NMP) are examined as well as their binary combinations to prepare solution‐sheared perovskite films at higher than 150 °C temperatures. Characterization methods such as imaging, X‐ray diffraction, grazing‐incidence wide‐angle X‐ray scattering, and photoluminescence (PL) to evaluate the crystal size, quality, and ordering that determine the viability of these films for device applications are explored and then a perovskite solar cell is made to test the stability of the most promising film. Excellent macroscopic crystals, over 1 mm in length, are achieved using a solvent mixture containing equal volumes of DMSO and NMP. These superior 3D MAPbI3 films, which retain solvent–perovskite intermediate phases and exhibit high crystalline ordering, are essential for high‐performing and stable optoelectronic devices.

Chemical Strain Engineering of MAPbI3 Perovskite Films

AbstractThis study introduces a new chemical method for controlling the strain in methylammonium lead iodide (MAPbI3) perovskite crystals by varying the ratio of Pb(Ac)2 and PbCl2 in the precursor solution. To observe the effect on crystal strain, a combination of piezoresponse force microscopy (PFM) and X‐ray diffraction (XRD) is used. The PFM images show an increase in the average size of ferroelastic twin domains upon increasing the PbCl2 content, indicating an increase in crystal strain. The XRD spectra support this observation with strong crystal twinning features that appear in the spectra. This behavior is caused by a strain gradient during the crystallization due to different evaporation rates of methylammonium acetate and methylammonium chloride as revealed by time‐of‐flight secondary ion mass spectroscopy and grazing incidince X‐ray diffraction measurements. Additional time‐resolved photoluminescence shows an increased carrier lifetime in the MAPbI3 films prepared with higher PbCl2 content, suggesting a decreased trap density in films with larger twin domain structures. The results demonstrate the potential of chemical strain engineering as a simple method for controlling strain‐related effects in lead halide perovskites.

Cesium Lead Iodide Perovskites: Optically Active Crystal Phase Stability to Surface Engineering.

Among perovskites, the research on cesium lead iodides (CsPbI3) has attracted a large research community, owing to their all-inorganic nature and promising solar cell performance. Typically, the CsPbI3 solar cell devices are prepared at various heterojunctions, and working at fluctuating temperatures raises questions on the material stability-related performance of such devices. The fundamental studies reveal that their poor stability is due to a lower side deviation from Goldschmidt’s tolerance factor, causing weak chemical interactions within the crystal lattice. In the case of organic–inorganic hybrid perovskites, where their stability is related to the inherent chemical nature of the organic cations, which cannot be manipulated to improve the stability drastically whereas the stability of CsPbI3 is related to surface and lattice engineering. Thus, the challenges posed by CsPbI3 could be overcome by engineering the surface and inside the CsPbI3 crystal lattice. A few solutions have been proposed, including controlled crystal sizes, surface modifications, and lattice engineering. Various research groups have been working on these aspects and had accumulated a rich understanding of these materials. In this review, at first, we survey the fundamental aspects of CsPbI3 polymorphs structure, highlighting the superiority of CsPbI3 over other halide systems, stability, the factors (temperature, polarity, and size influence) leading to their phase transformations, and electronic band structure along with the important property of the defect tolerance nature. Fortunately, the factors stabilizing the most effective phases are achieved through a size reduction and the efficient surface passivation on the delicate CsPbI3 nanocrystal surfaces. In the following section, we have provided the up-to-date surface passivating methods to suppress the non-radiative process for near-unity photoluminescence quantum yield, while maintaining their optically active phases, especially through molecular links (ligands, polymers, zwitterions, polymers) and inorganic halides. We have also provided recent advances to the efficient synthetic protocols for optically active CsPbI3 NC phases to use readily for solar cell applications. The nanocrystal purification techniques are challenging and had a significant effect on the device performances. In part, we summarized the CsPbI3-related solar cell device performances with respect to the device fabrication methods. At the end, we provide a brief outlook on the view of surface and lattice engineering in CsPbI3 NCs for advancing the enhanced stability which is crucial for superior optical and light applications.

Principled Exploration of Bipyridine and Terpyridine Additives to Promote Methylammonium Lead Iodide Perovskite Crystallization

Additives in the precursor solution can promote lead-halide perovskite (LHP) crystallization. We present a systematic exploration of nine (9) bipyridine- and terpyridine-based additives selected from 29 candidates using high-throughput single-crystal growth. To combat selection bias and generate hypotheses for future experimental cycles of learning, we featurize candidate additives using Mordred descriptors and compare similarity metrics. A previously unreported additive, 6,6′-dimethyl-2,2′-dipyridyl, is shown to work particularly well (the highest top 10th percentile is ∼3.8 mm, in comparison to ∼1.9 mm without additive) in improving the crystallization of prototypical methylammonium lead iodide (MAPbI3). Our strategy of machine-learning-guided high-throughput experimentation is generally applicable to other crystal growth problems.

Effects of Different Anti-Solvents and Annealing Temperatures on Perovskite Thin Films

Since perovskite materials are currently mostly used in the active layer of solar cells, how to maximize the conversion efficiency of the active layer is the most urgent problem at present. In this regard, the extremely low voltage loss and tunable energy gap of methyl lead iodide (MAPbI3) perovskites make them very suitable for all perovskite solar cell applications, and are also compatible with silicon crystalline systems. Therefore, the future development of MAPbI3 perovskite will be very important. The key point of film formation in MAPbI3 is the addition of anti-solvent, which will affect the overall quality of the film. Whether it can be used as an excellent active layer to improve the application value will be very important. Therefore, the research purpose of this topic “Effects of different anti-solvents and annealing temperatures on perovskites” is to complete the basic research and development of a light-absorbing layer of a solar cell element, in which three different anti-solvents need to be matched with each other as the active light-absorbing layer of a solar cell. Through optimization, using the chemical properties of different anti-solvents and different annealing temperatures, combined with the low-process-cost characteristics of organic materials and many other advantages, we researched the optimized process methods and parameters to improve the absorption efficiency of the active light-absorbing layer.

Solvent‐Free Preparation and Moderate Congruent Melting Temperature of Layered Lead Iodide Perovskites for Thin‐Film Formation

AbstractLead halide 2D hybrid perovskites (HP) have emerged as promising materials for photovoltaic and lighting applications. Solvent‐free preparations offer greener route, but require congruent melting of halide perovskite for thin films, which has been demonstrated only for monolayered HP (n=1 of the (A)2(MA)n−1PbnI3n+1 series) at the quite high temperature Tm (Tmelting) of 171 °C. Here, we report on the solvent‐free preparation (n=2, 4), thermal behaviour and melt‐processed thin films of a series of HP (GABA)2(MA)n−1PbnI3n+1 (n=1, 2, 4; GABA+: 4‐ammoniumbutyric acid cation). The n=1 and n=2 HP have an exceptional low congruent Tm of 126 °C and 136 °C, respectively, and, for n=1, a very good stability in the molten state. The liquid–solid reaction of molten (GABA)2PbI4 or (GABA)2(MA)Pb2I7 with MAPbI3 in 1/1 (T=130 °C) or 1/2 (T=135 °C) ratio leads to pure phases of the n=2 HP and (GABA)2(MA)3Pb4I11 (n=4, Tm=185 °C), respectively. Melt‐processed thin films of n=1, n=2 and n=4 HP have also been prepared.

Solvent-Free Preparation and Moderate Congruent Melting Temperature of Layered Lead Iodide Perovskites for Thin-Film Formation.

Lead halide 2D hybrid perovskites (HP) have emerged as promising materials for photovoltaic and lighting applications. Solvent-free preparations offer greener route, but require congruent melting of halide perovskite for thin films, which has been demonstrated only for monolayered HP (n=1 of the (A)2 (MA)n-1 Pbn I3n+1 series) at the quite high temperature Tm (Tmelting ) of 171 °C. Here, we report on the solvent-free preparation (n=2, 4), thermal behaviour and melt-processed thin films of a series of HP (GABA)2 (MA)n-1 Pbn I3n+1 (n=1, 2, 4; GABA+ : 4-ammoniumbutyric acid cation). The n=1 and n=2 HP have an exceptional low congruent Tm of 126 °C and 136 °C, respectively, and, for n=1, a very good stability in the molten state. The liquid-solid reaction of molten (GABA)2 PbI4 or (GABA)2 (MA)Pb2 I7 with MAPbI3 in 1/1 (T=130 °C) or 1/2 (T=135 °C) ratio leads to pure phases of the n=2 HP and (GABA)2 (MA)3 Pb4 I11 (n=4, Tm =185 °C), respectively. Melt-processed thin films of n=1, n=2 and n=4 HP have also been prepared.

Influence of spin–orbit coupling and biaxial strain on the inorganic lead iodide perovskites, APbI3 (A = K, Rb, and Cs)

Recently, inorganic metal halide perovskites, APbI3 (A = K, Rb, and Cs), have attracted increased attention in the research community of photovoltaic technology due to their superior stability and exceptional structural, electronic, and optical properties. In this paper, the influence of biaxial compressive and tensile strains (−6% to +6%) on the optoelectronic properties of APbI3 perovskites is thoroughly investigated using density functional theory, based on first-principles calculations. Additionally, this study identifies the function of the A-cation in the optoelectronic properties of APbI3. We find that the biaxial strain in APbI3 can significantly increase its absorbance, both in the visible and ultraviolet light energy range. Interestingly, the absorption coefficient, dielectric function, and electron loss function are tuned into visible ranges when a compressive strain is applied, whereas these functions move into the ultraviolet region for tensile strain. Furthermore, the electronic band structure shows that the APbI3 compounds are all semiconductors with direct bandgap ranges of 0.92–1.09 eV and 1.9 eV at the R-point. The value of the bandgap decreases to 0.17, 0.25, and 0.32 eV for KPbI3, RbPbI3, and CsPbI3, respectively, when the spin–orbit coupling effect is considered. Moreover, the bandgap of APbI3 shows a decreasing trend and tends towards the metallic condition when compressive strain is applied. In addition, when tensile strain is increased, the bandgap of APbI3 exhibits an increasing trend. Therefore, based on the optical and electronic properties, applying a biaxial strain should make APbI3 compounds suitable for optoelectronic device applications.

In Situ Growth of Strained Matrix on CsPbI3 Perovskite Quantum Dots for Balanced Conductivity and Stability.

Their nanoscale size endows perovskite quantum dots (QDs) with processing flexibility and high tunability of optoelectronic properties. The vast surface area also provides an opportunity for ligand engineering to offer QDs extra protection, which however, will impede charge transport in the QD array. Currently, the surface treatments that can balance both stability and conductivity of the perovskite QD array remain a huge challenge. Here, we report in situ growth of an atomic guanidinium lead iodide perovskite matrix on CsPbI3 QDs. In addition to the effect of trap passivation, the matrix can also provide substantial surface strain to improve the QD phase stability. Meanwhile, the ultrathin matrix allows efficient coupling and charge transport in the QD solids. As a result, the CsPbI3 QD solar cells can achieve both superior device stability and performance. We believe the development of a multifunctional surface matrix will become one of the future research focuses in perovskite QD-based devices.

(Invited) Simulating Dynamic Excitons Via Quantum Molecular Dynamics: A Case Study in Lead Halide Perovskites

The ultrafast electronic and structural dynamics invoked by photoexcitation, i.e., “dynamic exciton” phenomena, not only are important in the context of practical applications such as solar cells, but also raise many questions from the viewpoint of fundamental science. Experimental measurement, computational simulations, and theoretical interpretation will be the three pillars for deciphering the dynamic exciton phenomena.From the viewpoint of computational simulations, molecular dynamics (MD) techniques combined with quantum chemical calculations, i.e., quantum molecular dynamics (QMD), has been the popular tool to simulate the dynamic exciton phenomena. The quantum chemical calculations, which are typically conducted on the basis of the density-functional theory (DFT), require the large computational resources and time. These have been the limiting factors for the accessible spatial and time scales by the QMD simulations.To extend the coverage of simulations to more complex, large-scale systems, we have developed efficient excited-state QMD methods that can include nonadiabatic effects. Our method combines the density-functional tight binding (DFTB) method, which is an approximate DFT, and the surface hopping method, which is a theoretical framework to incorporate the nonadiabatic effects into the QMD simulations. The method was further improved to be suitable for condensed-phase simulations explicitly including the environment, i.e., solvent, by using a “divide-and-conquer” style quantum chemical calculation technique. These theoretical framework enables us to simulate the coupled electronic–structural dynamics in excited states of systems consisting of 102–103 atoms[1,2,3].In addition, using the developed method, we conducted the real-time simulations of the ultrafast processes invoked by photoexcitation of lead iodide perovskites, which are known as the key materials for perovskite solar cells. The dissociation of the exciton into the positive and negative charge carriers was observed. Moreover, the hot carrier cooling, where the charge carriers dissipate excess energy via the electron–phonon coupling and relax to the band edges, was also tracked. Finally, the direct evidence of the polaron formation, where the structural deformation is induced by the presence of charge carriers, was observed. These results highlight the importance of the coupling between electronic and structural degrees of freedom.In the talk, recent improvements in the methodology and future perspectives will also be presented[5].