Lead Iodide Perovskite Research Articles

Combined Precursor Engineering and Grain Anchoring Leading to MA‐Free, Phase‐Pure, and Stable α‐Formamidinium Lead Iodide Perovskites for Efficient Solar Cells

Abstractα‐Formamidinium lead iodide (α‐FAPbI3) is one of the most promising candidate materials for high‐efficiency and thermally stable perovskite solar cells (PSCs) owing to its outstanding optoelectrical properties and high thermal stability. However, achieving a stable form of α‐FAPbI3 where both the composition and the phase are pure is very challenging. Herein, we report on a combined strategy of precursor engineering and grain anchoring to successfully prepare methylammonium (MA)‐free and phase‐pure stable α‐FAPbI3 films. The incorporation of volatile FA‐based additives in the precursor solutions completely suppresses the formation of non‐perovskite δ‐FAPbI3 during film crystallization. Grains of the desired α‐phase are anchored together and stabilized when 4‐tert‐butylbenzylammonium iodide is permeated into the α‐FAPbI3 film interior via grain boundaries. This cooperative scheme leads to a significantly increased efficiency close to 21 % for FAPbI3 perovskite solar cells. Moreover, the stabilized PSCs exhibit improved thermal stability and maintained ≈90 % of their initial efficiency after storage at 50 °C for over 1600 hours.

Nanoporous anodic alumina with ohmic contact between substrate and infill: Application to perovskite solar cells

AbstractThe use of a nanostructured aluminum substrate as the cathode for a perovskite photovoltaic device is described. This cathode consists of an aluminum substrate onto which a highly ordered array of aluminum oxide tubular pores was grown via anodization. The 1‐μm thick pores – arranged in a closed‐packed hexagonal pattern – were subsequently selectively etched at the bottom to remove the so‐called aluminum oxide barrier layer. The subsequent infiltration of the pores with the components of a methylammonium lead iodide perovskite solar cells, and the completion with a semi‐transparent anode, have shown to allow the establishment of an ohmic contact between the substrate itself and the components infiltrated into the Al2O3 pores. Indeed, a clear rectifying behavior was observed on the full devices, as well as modest photovoltaic conversion efficiencies. This paper demonstrates that an ohmic contact can be established between the aluminum substrate from which nanoporous anodic alumina was grown, and that the pores can be used to compartmentalize the infill material down to the nanoscopic level.

Enhanced Stability of MAPbI3 Perovskite Films with Zirconium Phosphate‐Phosphonomethylglycine Nanosheets as Additive

AbstractMethylammonium lead iodide perovskite (MAPbI3) is today considered the most promising component for highly efficient third generation solar cells. However, the lifetime of the solar devices is strongly affected by the stability of the MAPbI3 films toward humidity, UV irradiation, and temperature. The search for efficient protective additives to be used for building up composite perovskite films with enhanced stability is a topic of great interest in the scientific community. In the present paper, a layered zirconium phosphate‐phosphonate based on N,N‐phosphonomethylglycine, exfoliated in thin nanosheets (NS), as additive for the stabilization of MAPbI3 crystalline films toward humidity, UV‐irradiation, and temperature changes is applied. Notably, the additive is extremely efficient in preventing degradation of the perovskite film, preserving the optical and structural properties, and avoiding the phase transitions normally observed due to temperature increase.

Improved crystallinity and self-healing effects in perovskite solar cells via functional incorporation of polyvinylpyrrolidone

Air moisture is the key issue for perovskites which invades the films and accelerates the damage of devices. Here, polyvinylpyrrolidone (PVP) is introduced to the methylammonium lead iodide (MAPbI3) perovskite precursor to control crystal growth and endow the devices with self-healing ability in a moisture environment. The strong CΟ⋯ΗΝ hydrogen bonding interactions between PVP and MAPbI3 was confirmed by nuclear magnetic resonance measurements. By introducing hydrogen bonding in the MAPbI3-based PSCs, we form a compact perovskite film of excellent electronic quality with a power conversion efficiency (PCE) of up to 20.32%. Furthermore, the Ο⋯ΗΝ hydrogen bonding interactions at the grain boundaries suppress the decomposition of methylammonium cations and improve the recyclable dissolution–recrystallization of perovskite. As a result, the MAPbI3-PVP based cells exhibited striking moisture stability and self-healing behavior, with negligible decay in efficiency after 500 h of operation in high humidity (65% ± 5% relative humidity) and rapid recovering ability after their removal from the humid environment.

Tuning Spin-Polarized Lifetime in Two-Dimensional Metal-Halide Perovskite through Exciton Binding Energy.

Metal-halide perovskite semiconductors have attracted attention for opto-spintronic applications where the manipulation of charge and spin degrees of freedom have the potential to lower power consumption and achieve faster switching times for electronic devices. Lower-dimensional perovskites are of particular interest since the lower degree of symmetry of the metal-halide connected octahedra and the large spin-orbit coupling can potentially lift the spin degeneracy. To achieve their full application potential, long spin-polarized lifetimes and an understanding of spin-relaxation in these systems are needed. Here, we report an intriguing spin-selective excitation of excitons in a series of 2D lead iodide perovskite (n = 1) single crystals by using time- and polarization-resolved transient reflection spectroscopy. Exciton spin relaxation times as long as ∼26 ps at low excitation densities and at room temperature were achieved for a system with small binding energy, 2D EOA2PbI4 (EOA = ethanolamine). By tuning the excitation density and the exciton binding energy, we identify the dominant mechanism as the D'yakonov-Perel (DP) mechanism at low exciton densities and the Bir-Aronov-Pikus (BAP) mechanism at high excitation densities. Together, these results provide new design principles to achieve long spin lifetimes in metal-halide perovskite semiconductors.

Simple Additive to MAPbI3 Solution that Enhances Film Quality of Mini‐Module Perovskite Solar Cells Fabricated under Moderate Humidity

An additive to perovskite material can play a crucial role in the fabrication of perovskite solar cells (PSCs); the additive treatment investigated herein produces promising overall power conversion efficiency (PCE) enhancements. Currently, the fabrication of PSCs is mainly carried out under the dry air of a glove box to avoid moisture adsorption by methylammonium lead iodide perovskites; these tight restrictions in how PSCs are fabricated hinder the possibility of their commercialization. In this study, a technique that uses C12F4N4 as an additive to the MAPbI3 for the one‐step deposition of perovskite layers under moderate humidity is presented; this approach is able to achieve high‐quality perovskite films despite the adverse condition. The added C12F4N4 bridges the gaps between the MAPbI3 grain boundaries and significantly enhances all the photovoltaic parameters compared to using pristine methylammonium lead iodide; this method even eventually enhances the overall PCE. Both, single‐cell and optimized five‐cell integrated mini‐module devices in moderately humid conditions are fabricated. This facile method presents new possibilities for scaling up PSCs production in humid environments.

Improving performance of hybrid perovskite/graphene-based photodetector via hot carriers injection

Thermal photodetectors, such as bolometers, are the most promising option for detecting infrared (IR) to terahertz (THz) radiation at room temperature. Previous studies have shown that graphene has great potential for broadband thermal photodetection because of its distinctive optical and electrical properties. However, the responsivity of graphene photodetector is limited by the weak absorption and the finite temperature coefficient of resistance (TCR). Here, we synthesize the methylamine lead iodide perovskite (MAPbI3)/graphene oxide (RGO) composite material, exhibiting high TCR of −0.89%K−1 and low heat capacity at room temperature. The composite is designed to construct a sensitive and fast IR to THz bolometer. The MAPbI3/RGO detector shows significant performance breakthroughs, with the responsivity as high as 805 mA/W, the response time of about 700 μs, at 220 GHz, which are about an order of magnitude higher than those of pure RGO detector. These results suggest that compositing engineering gives a new approach to the technological innovation of THz detectors.

Photo tuned electron field emission from vertically aligned CH3NH3PbI3 nanorods grown in AAO template

Among the several untapped aspects of CH3NH3PbI3 i.e. Methylammonium lead iodide perovskite, electron field emission is an important one. Motivated by the scarcity of reports available in this segment, this work carries out a detail investigation of the field emission properties of vertically aligned CH3NH3PbI3 nanorods (NRs) grown within AAO (Anodic aluminum oxide) template. The emission produced considerable current density of 187 μA/cm2 with turn on field 3.4 V/μm and enhancement factor 5500. Accompanied by the simulation results of local electric field distribution, examination of incident photons impact upon emission behaviour under varying light intensities revealed enhanced electron emission under increased light intensities due to improved number of photo-excited carriers.

(Invited) Halide Perovskite Thin Films via Alternate Laser Deposition: Change in the Physical Properties with the Layered Structures

We have formed methylammonium lead iodide (MAPbI3) perovskite by alternating irradiation of PbI2 and CH3NH3I (MAI) with an infrared laser in an ultra-high vacuum to elucidate the growth mechanism and realize structural control of MAPbI3 [1]. In this study, we found that MAPbI3 contains both tetragonal and orthorhombic phases, and the ratio of these phases systematically changes depending on the thickness of alternating layers of PbI2 film.In the sample preparation, the PbI2 and MAI powder targets were alternately irradiated with an infrared laser (λ = 808 nm) to convert them into molecular beams, and then alternately deposited on a synthetic fused silica substrate at room temperature. The deposition rate of PbI2 was controlled to be about 0.1 nm/s using a quartz crystal microbalance thickness monitor. The deposition amount of MAI was adjusted by changing the deposition time while the evaporation pressure was fixed at 4×10-4 Pa. Four different multilayer samples consisting of [PbI2/MAI] × n (n = 1, 2, 5, 10) were prepared under the condition that the total thickness of PbI2 was 300 nm.By analyzing the crystal structures by X-ray diffraction measurements, the prepared samples were found to have the following two unique features: (1) In the samples with n = 1, unreacted PbI2 was present in addition to MAPbI3, while in the samples with n = 2, 5, and 10, only MAPbI3 was observed, indicating a complete solid-phase reaction. (2) The full width at half maximum and peak position of the diffraction peak for MAPbI3 at 2θ = 14.0° ~ 14.1° were different among the samples, and thus we performed the analysis by peak separation. As a result, it was determined that the tetragonal (002) and orthorhombic (020) diffraction peaks were included.Defining r tetra and r ortho as the ratio of the tetragonal and orthorhombic diffraction peak areas to the total peak area, it was found that r tetra : r ortho changes significantly with the thickness of the PbI2 layer. Specifically, r tetra : r ortho =[0.64 : 0.36], [0.39 : 0.61], [0.20 : 0.80], and [0.25 : 0.75] for n =1, 2, 5, and 10, respectively. It is noteworthy that the orthorhombic MAPbI3, which usually appears below 161 K due to the phase transition [2], is contained in the MAPbI3 film formed at room temperature, and that the mixing ratio of tetragonal and orthorhombic phases varies systematically with the thickness of the alternating deposited PbI2 films.[1] K. Kawashima et al., Sci. and Tech. of Adv. Mater. 18, 307 (2017).[2] N. Onoda-Yamamuro et al., J. Phys. Chem. Solids 51, 1383 (1990).

(Invited) Surface Ligands for Methylammonium Lead Iodide Perovskites and Their Impact on Interfacial Energetics, Material Stability, and Photovoltaic Performance

Surface ligand treatment of organic metal halide perovskites can improve material and device stability, alter interfacial energetics, and increase perovskite solar cell (PSC) performance. Unlike in traditional inorganic semiconductors such as silicon, surface ligands often penetrate into the perovskite crystal and result in the formation of reduced dimensionality structures (where the 3D perovskite framework is broken into 2D sheets of varying thickness). Such ligand penetration can have a large impact on the surface energetics and material stability; thus, it is important to understand how surface ligand structure impacts ligand penetration. Using angle-dependent x-ray photoelectron spectroscopy (XPS) and x-ray diffraction we show that surface ligands with ammonium and phosphonic acid binding groups often penetrate into MAPbI3. This ligand penetration can significantly influence surface energetics, as probed with ultraviolet and inverse photoelectron spectroscopy measurements. The observed changes in surface energetics upon surface ligand treatment have a large impact on the PSC performance, with better photovoltaic performance arising for ligands that result in more favorable energy landscapes for electron transfer from MAPbI3 to the electron transport layer, C60. For a series of ammonium containing ligands, we find that both the cross-sectional area and extent of ligand fluorination impact the penetration of the surface ligand into the perovskite. Furthermore, surface ligands with larger cross-sectional areas result in improved MAPbI3 stability.

Achieving Efficient and Stable Perovskite Solar Cells in Ambient Air Through Non‐Halide Engineering

AbstractThe realization of highly efficient perovskite solar cells (PSCs) in ambient air is considered to be advantageous for low‐cost commercial manufacturing. However, it is fundamentally difficult to achieve comparable device performance to that obtained in an inert atmosphere, especially when the ambient humidity is high. Here, an effective precursor engineering that simultaneously employs non‐halide lead acetate and lead thiocyanate lead sources for fabricating high‐quality methylammonium lead iodide perovskite films in ambient air with enhanced moisture tolerance, is reported. The presence of Ac– and SCN– ions not only enables the facile formation of homogeneous and highly crystalized perovskite films, but also directs the uniform growth of the crystals along the (110) direction. Accordingly, a 20.55% efficiency is demonstrated, one of the best results for air‐processed MAPbI3 PSCs, which is also the highest value achieved with non‐halide lead sources. Furthermore, the unencapsulated device shows fivefold prolonged air stability (3600 h) compared to the conventional PbI2‐based PSC. Together with the use of non‐toxic antisolvent, this strategy is fully compatible with ambient air operation and thus of great potential for practical applications.

Managing Growth and Dimensionality of Quasi 2D Perovskite Single-Crystalline Flakes for Tunable Excitons Orientation.

Hybrid perovskites are among the most promising materials for optoelectronic applications. Their 2D crystalline form is even more interesting since the alternating inorganic and organic layers naturally forge a multiple quantum-well structure, leading to the formation of stable excitonic resonances. Nevertheless, a controlled modulation of the quantum well width, which is defined by the number of inorganic layers (n) between two organic ones, is not trivial and represents the main synthetic challenge in the field. Here, a conceptually innovative approach to easily tune n in lead iodide perovskite single-crystalline flakes is presented. The judicious use of potassium iodide is found to modulate the supersaturation levels of the precursors solution without being part of the final products. This allows to obtain a fine tuning of the n value. The excellent optical quality of the as synthesized flakes guarantees an in-depth analysis by Fourier-space microscopy, revealing that the excitons orientation can be manipulated by modifying the number of inorganic layers. Excitonic out-of-plane component, indeed, is enhanced when "n" is increased. The combined advances in the synthesis and optical characterization fill in the picture of the exciton behavior in low-dimensional perovskite, paving the way to the design of materials with improved optoelectronic characteristics.

Impermeable inorganic “walls” sandwiching perovskite layer toward inverted and indoor photovoltaic devices

Interfaces between the perovskite active layer and the charge-transport layers (CTLs) play a critical role in both efficiency and stability of halide-perovskite photovoltaics. One of the major concerns is that surface defects of perovskite could cause detrimental nonradiative recombination and material degradation. In this work, we addressed this challenging problem by inserting ultrathin alkali-fluoride (AF) films between the tri-cation lead-iodide perovskite layer and both CTLs. This bilateral inorganic “walls” strategy makes use of both physical-blocking and chemical-anchoring functionalities of the continuous, uniform and compact AF framework: on the one hand, the uniformly distributed alkali-iodine coordination at the perovskite-AF interfaces effectively suppresses the formation of iodine-vacancy defects at the surfaces, thus reducing the trap-assisted recombination at the perovskite-CTL interfaces and therewith the open-voltage loss; on the other hand, the impermeable AF buffer layers effectively prevent the bidirectional ion migration at the perovskite-CTLs interfaces even under harsh working conditions. As a result, a power-conversion efficiency (PCE) of 22.02% (certified efficiency 20.4%) with low open-voltage deficit (<0.4 V) was achieved for the low-temperature processed inverted planar perovskite solar cells. Exceptional operational stability (500 h, ISOS-L-2) and thermal stability (1000 h, ISOS-D-2) were obtained. Meanwhile, a 35.7% PCE was obtained under dim-light source (1000 lux white LED light) with the optimized device, which is among the best records in perovskite indoor photovoltaics.

Low-field onset of Wannier-Stark localization in a polycrystalline hybrid organic inorganic perovskite

Methylammonium lead iodide perovskite (MAPbI3) is renowned for an impressive power conversion efficiency rise and cost-effective fabrication for photovoltaics. In this work, we demonstrate that polycrystalline MAPbI3s undergo drastic changes in optical properties at moderate field strengths with an ultrafast response time, via transient Wannier Stark localization. The distinct band structure of this material - the large lattice periodicity, the narrow electronic energy bandwidths, and the coincidence of these two along the same high-symmetry direction – enables relatively weak fields to bring this material into the Wannier Stark regime. Its polycrystalline nature is not detrimental to the optical switching performance of the material, since the least dispersive direction of the band structure dominates the contribution to the optical response, which favors low-cost fabrication. Together with the outstanding photophysical properties of MAPbI3, this finding highlights the great potential of this material in ultrafast light modulation and novel photonic applications.

Additive effects of methyl ammonium bromide or formamidinium bromide in methylammonium lead iodide perovskite solar cells using decaphenylcyclopentasilane

Additive effects of methyl ammonium bromide or formamidinium bromide in methylammonium lead iodide perovskite solar cells using decaphenylcyclopentasilane

Iodide vs Chloride: The Impact of Different Lead Halides on the Solution Chemistry of Perovskite Precursors

Controlled perovskite growth from solution is crucial for efficient optoelectronic applications and requires a deep understanding of the perovskite precursor chemistry. The so-called “chlorine route” to lead–iodide perovskite, using PbCl2 or MACl additive as a precursor, is frequently employed to form homogeneous perovskite layers by retarding perovskite crystallization. To understand the role of chlorine-containing lead precursors in solution, we analyze the chemical interaction of PbCl2 and PbI2 precursors with commonly employed solvents (γ-butyrolactone (GBL), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO)) by combining first-principles simulations and experimental UV–vis spectroscopy in diluted precursor solutions. Ab initio molecular dynamics simulations reveal reduced solvation and an increased free energy barrier of lead-halide bond dissociation of PbCl2 compared to PbI2 with chlorine acting as a stronger ligand, which, in turn, limits the solvent coordination. In contrast to PbI2, PbCl2 absorption spectra lack signatures of high-valent [PbCln]2–n complexes and show low sensitivity on the employed solvent, as confirmed by combined UV–vis and excited-state time-dependent density functional theory (TD-DFT) analysis. Altogether, our data suggest the presence of residual chlorine coordinated to Pb even in the presence of high iodine excess, which may retard the perovskite growth and could also lead to chlorine incorporation within the lead–iodide perovskite crystal.

Optimization of optoelectrical properties during synthesizing methylammonium lead iodide perovskites via a two-step dry process

We investigated the defect states and optoelectrical properties of methylammonium lead iodide (MAPbI3) perovskite films synthesized via a two-step dry process. To synthesize MAPbI3 thin films, PbI2 thin films were deposited via rf magnetron sputtering and were subsequently exposed to methylammonium iodide (MAI) vapor to synthesize MAPbI3 thin films. By using deep level transient spectroscopy and space-charge-limited current methods, the defect density of the perovskite films was proven to be reduced by about 6 times as the reaction temperature increased from 90 to 110 °C. At a high reaction temperature of 130 °C, the deformation process could occur, resulting in a rising trap density within the perovskite film. Along with a significant decline in defect density, the photoresponsivity of films fabricated at 110 °C were higher by about 2.05 and 18.76 times more than those of samples prepared at 100 and 130 °C, respectively. An excellent detectivity of 1.17 × 1013 Jones was observed from the sample synthesized at 110 °C, demonstrating the superiority of a two-step dry process.

Effect of chlorobenzene on the optical and structural properties of CH3NH3PbI3:DMF perovskite films

Methylammonium lead iodide perovskites (MAPbI3) thin films were prepared by changing the molarity concentration of methylammonium iodide and lead iodide in the N,N- dimethylformamide solvent, with and without chlorobenzene (CB). Rietveld refinement analysis revealed that all prepared samples belong to tetragonal CH3NH3PbI3 phase (sp. gr. I4/mcm). The effect of MAPbI3 concentration on the lattice parameters, crystallite size, lattice microstrain and dislocation density of the prepared films with and without CB was investigated in detail. The FESEM images of perovskite films indicated the presence of pinholes in each sample, and the number of pinholes reduced as CB added to MAPbI3. Generally, the transmission and reflection spectra decreased as the amount of MAPbI3 increased in the films; also, both are reduced as CB added to films. The value of the reflectivity over measured wavelengths changed from about 1% to about 18% and 12% in the case of films without and with CB, respectively. Without CB, the highest absorbance intensity was obtained for 1.3 M films, while with CB the 1.5 M film has the highest absorbance. The number of bandgaps appeared in different samples depending on the concentration of MAPbI3 and CB. The refractive index at 500 nm for (1, 1.3 and 1.5 M) MAPbI3 films without CB are 1.6, 1.44, 1.43 and 1.31 respectively, where its value in the corresponding films with CB are 1.34, 1.3, 1.3 and 1.28, respectively. The effect of MAPbI3 concentration and the existence of CB during the film preparation on the extinction coefficient, refractive index, dielectric constant, the photoluminescence peak position, and PL intensity for the different films were explored.

Investigating the effect of applied bias on methylammonium lead iodide perovskite by electrical and positron annihilation spectroscopic studies

The formation of point defects in the crystal lattice of organic–inorganic hybrid perovskites is detrimental to the long-term stability of the perovskite-based solar cell. The observed hysteresis in organolead halide perovskite solar cell during the photocurrent measurement is believed to be the consequence of such defect sites. In the present work, we are reporting the formation of reversible and/or irreversible lattice defects in methylammonium lead iodide perovskite depending on the magnitude of bias voltage studied by electrical measurements and positron annihilation spectroscopy (PAS). Current vs voltage (I–V) measurements support the formation of p/n junction at the metal/semiconductor interfaces, suggesting the defects-driven ion migration under the applied bias. In-situ PAS has elucidated the nature of vacancy sites formed due to the application of bias voltage. In the lower biased voltage (<1 V), the formation of lattice defects is reversible, while beyond a threshold voltage of 1.2 V, the defects become permanent as observed by the PAS.

Methylammonium- and bromide-free perovskites enable efficient and stable photovoltaics

Hybrid perovskite solar cell (PSC) has attracted extensive research interest due to its rapid increase in efficiency, regarding as one of the most promising candidates for the next-generation photovoltaic technology. The certified power conversion efficiency of the devices based on formamidinium lead iodide (FAPbI3) perovskite has reached 25.5%, approaching the record of monocrystalline silicon solar cells. Unfortunately, the black α-phase FAPbI3 materials can spontaneously transform to non-optically active δ-phase at room temperature, which greatly hinder their photovoltaic application. In order to overcome this problem, various strategies, especially introducing methylammonium (MA+), caesium (Cs+) and bromide (Br-) ions into the materials, have been widely adopted. However, MA+ can largely reduce the thermal stability of the materials. Furthermore, the introduction of Br- can enlarge the materials’ bandgap, resulting in a reduced theoretical efficiency. Keeping these in mind, developing the strategies which without using MA+ and Br- is the inevitable trend. Here, we focus on the recent progresses of stabilizing α FAPbI3 without employing MA+ and Br-, and discuss the advantages of inorganic ions doping and dimensionality engineering to stabilized α FAPbI3. Meanwhile, in order to deeply understand the relationship between the semiconducting properties and device performance of the corresponding materials, we then summarize several significant strategies to suppress the non-radiation recombination, such as interface modification and trap passivation. Finally, we propose to develop more effective ‘A-site’ alternatives to stabilize α FAPbI3, which is expected to achieve high-efficient PSCs with long-term stability, facilitating its commercialization process.