Thermodynamic origin of instability in hybrid halide perovskites

The dual nature of metal halide perovskites

Metal halide perovskites have brought about a disruptive shift in the field of third-generation photovoltaics. Their potential as remarkably efficient solar cell absorbers was first demonstrated in the beginning of the 2010s. However, right from their inception, persistent challenges have impeded the smooth adoption of this technology in the industry. These challenges encompass issues such as the lack of reproducibility in fabrication, limited mid- and long-term stability, and concerns over toxicity. Despite achieving record efficiencies that have outperformed even well-established technologies, such as polycrystalline silicon, these hurdles have hindered the seamless transition of this technology into industrial applications. In this Perspective, we discuss which of these challenges are rooted in the unique dual nature of metal halide perovskites, which simultaneously function as electronic and ionic semiconductors. This duality results in the intermingling of processes occurring at vastly different timescales, still complicating both their comprehensive investigation and the development of robust and dependable devices. Our discussion here undertakes a critical analysis of the field, addressing the current status of knowledge for devices based on halide perovskites in view of electronic and ionic conduction, the underlying models, and the challenges encountered when these devices are optoelectronically characterized. We place a distinct emphasis on the positive contributions that this area of research has not only made to the advancement of photovoltaics but also to the broader progress of solid-state physics and photoelectrochemistry.

Revealing the vertical structure of in-situ fabricated perovskite nanocrystals films toward efficient pure red light-emitting diodes

The development of efficient perovskite light-emitting diodes (PeLEDs) relies strongly on the fabrication of perovskite films with rationally designed structures (grain size, composition, surface, etc.). Therefore, an understanding of structure-performance relationships is of vital importance for developing high-performance perovskite devices, particularly for devices with in-situ fabricated perovskite nanocrystal films. In this study, we reveal the vertical structure of an in-situ fabricated quasi-two-dimensional perovskite film. By combining time-of-flight secondary ion mass spectrometry, energy dispersive spectroscopy, grazing incidence wide-angle X-ray scattering (GIWAXS), and low-temperature photoluminescence spectra, we illustrate that the resulting in-situ fabricated DPPA2Csn-1Pbn(Br0.3I0.7)3n+1 (DPPA+: 3,3-diphenylpropylammonium) film has a gradient structure with a very thin layer of ligands on the surface, predominantly small-n domains at the top, and predominantly large-n domains at the bottom owing to the solubility difference of the precursors. In addition, GIWAXS measurements show that the domain of n = 2 on the top layer has an ordered in-plane alignment. Based on the understanding of the film structure, we developed an in-situ fabrication process with ligand exchange to achieve efficient pure red PeLEDs at 638 nm with an average external quantum efficiency (EQE) of 7.4%. The optimized device had a maximum luminance of 623 cd/m2 with a peak EQE of 9.7%.

Stabilizing tetramethylammonium lead iodide perovskite and exploring its electronic and optical absorption for solar cell absorber application

Modeling perovskites as solar cell absorbers has become popular due to the breakthrough of methylammonium lead iodide (CH3NH3PbI3). In this study, we modeled a tetramethylammonium lead iodide (CH3)4NPbI3 structure. We further confirmed the stability of the structure by determining the phonon dispersion using density functional perturbation theory. We calculated the spin-orbit and non-spin-orbit coupling-based electronic structure using the Perdew-Burke-Ernzerhof exchange-correlation functional within the generalized gradient approximation of the density functional theory and the self-consistent GW quasiparticle methods. Similarly, the absorption spectra were calculated from the real and imaginary parts of the dielectric tensor obtained from solving the Bethe-Salpeter equation using the GW quasiparticle database. The solar cell absorber spectroscopic limited maximum efficiency was calculated at 293.15 K. The self-consistent GW method without spin-orbit coupling reported bandgaps of 2.63 eV and 2.89 eV for GW0 and GW methods, respectively, in agreement with experimental reports. The phonon dispersion showed positive phonon modes across the high symmetry point, which attest to its thermodynamic stability. The absorption coefficient on the order of 105 was reported along the ultraviolet region. The standard limited maximum efficiency between 7% and 12% was recorded at 293.15 K between 0.01 and 100 μm absorber thicknesses. The thermodynamic stability, high absorption coefficient, and low transmittance indicated exciting prospects for a non-transparent (CH3)4NPbI3 solar cell absorber.

Thermal degradation of lead halide perovskite surfaces

The thermal degradation temperature of perovskite single crystal surfaces (MAPbI3, MAPbBr3, FAPbBr3) depends more on the halide than on the organic cation. However, different solid degradation products form depending on the cation.

Zn(O,S) Buffer Layer for in Situ Hydrothermal Sb2S3 Planar Solar Cells

Hydrothermal deposition is emerging as a highly potential route for antimony-based solar cells, in which the Sb₂(S,Se)₃ is typically in situ grown on a common toxic CdS buffer layer. The narrow band gap of CdS causes a considerable absorption in the short-wavelength region and then lowers the current density of the device. Herein, TiO₂ is first evaluated as an alternative Cd-free buffer layer for hydrothermally derived Sb₂S₃ solar cells. But it suffers from a severely inhomogeneous Sb₂S₃ coverage, which is effectively eliminated by inserting a Zn(O,S) layer. The surface atom of sulfur in Zn(O,S) uniquely provides a chemical bridge to enable the quasi-epitaxial deposition of Sb₂S₃ thin film, confirming by both morphology and binding energy analysis using DFT. Then the results of the first-principles calculations also show that Zn(O,S)/Sb₂S₃ has a more stable structure than TiO₂/Sb₂S₃. The resultant perfect Zn(O,S)/Sb₂S₃ junction, with a suitable band alignment and excellent interface contact, delivers a remarkably enhanced J_SC and V_OC for Sb₂S₃ solar cells. The device efficiency with the TiO₂/Zn(O,S) buffer layer boosts from 0.54% to 3.70% compared with the counterpart of TiO₂, which has a champion efficiency of Cd-free Sb₂S₃ solar cells with a structure of ITO/TiO₂/Zn(O,S)/Sb₂S₃/Carbon/Ag by in situ hydrothermal deposition. This work provides a guideline for the hydrothermal deposition of antimony-based films upon a nontoxic buffer layer.

Investigation of Opto-Electronic Properties and Stability of Mixed-Cation Mixed-Halide Perovskite Materials with Machine-Learning Implementation

The feasibility of mixed-cation mixed-halogen perovskites of formula AxA’1−xPbXyX’zX”3−y−z is analyzed from the perspective of structural stability, opto-electronic properties and possible degradation mechanisms. Using density functional theory (DFT) calculations aided by machine-learning (ML) methods, the structurally stable compositions are further evaluated for the highest absorption and optimal stability. Here, the role of the halogen mixtures is demonstrated in tuning the contrasting trends of optical absorption and stability. Similarly, binary organic cation mixtures are found to significantly influence the degradation, while they have a lesser, but still visible effect on the opto-electronic properties. The combined framework of high-throughput calculations and ML techniques such as the linear regression methods, random forests and artificial neural networks offers the necessary grounds for an efficient exploration of multi-dimensional compositional spaces.

II-VI Organic-Inorganic Hybrid Nanostructures with Greatly Enhanced Optoelectronic Properties, Perfectly Ordered Structures, and Shelf Stability of Over 15 Years

Organic-inorganic hybrids may offer material properties not available from their inorganic components. However, they are typically less stable and disordered. Long-term stability study of the hybrid materials, over the anticipated lifespan of a real-world electronic device, is practically nonexistent. Disordering, prevalent in most nanostructure assemblies, is a prominent adversary to quantum coherence. A family of perfectly ordered II-VI-based hybrid nanostructures has been shown to possess many unusual properties and potential applications. Here, using a prototype structure β-ZnTe(en)_0.5-a hybrid superlattice-and applying an array of optical, structural, surface, thermal, and electrical characterization techniques, in conjunction with density-functional theory calculations, we have performed a comprehensive and correlative study of the crystalline quality, structural degradation, electronic, optical, and transport properties on samples from over 15 years old to the recently synthesized. The findings show that not only do they exhibit an exceptionally high level of crystallinity in both macroscopic and microscopic scale, comparable to high-quality binary semiconductors; and greatly enhanced material properties, compared to those of the inorganic constituents; but also, some of them over 15 years old remain as good in structure and property as freshly made ones. This study reveals (1) what level of structural perfectness is achievable in a complex organic-inorganic hybrid structure or a man-made superlattice, suggesting a nontraditional strategy to make periodically stacked heterostructures with abrupt interfaces; and (2) how the stability of a hybrid material is affected differently by its intrinsic attributes, primarily formation energy, and extrinsic factors, such as surface and defects. By correlating the rarely found long-term stability with the calculated relatively large formation energy of β-ZnTe(en)_0.5 and contrasting with the case of hybrid perovskite, this work illustrates that formation energy can serve as an effective screening parameter for the long-term stability potential of hybrid materials. The results of the prototype II-VI hybrid structures will, on one hand, inspire directions for future exploration of the hybrid materials, and, on the other hand, provide metrics for assessing the structural perfectness and long-term stability of the hybrid materials.

Structural Transitions and Stability of FAPbI3 and MAPbI3: The Role of Interstitial Water

We studied the influence of water on the structural stability and transformations of MAPI and FAPI by anelastic and dielectric spectroscopies under various temperature and H2O partial pressure protocols. Before discussing the new results in terms of interstitial water in MAPI and FAPI, the literature is briefly reviewed, in search of other studies and evidences on interstitial water in hybrid halide perovskites. In hydrated MAPI, the elastic anomaly between the cubic α and tetragonal β phases may be depressed by more than 50%, demonstrating that there are H2O molecules dispersed in the perovskite lattice in interstitial form, that hinder the long range tilting of the PbI6 octahedra. Instead, in FAPI, interstitial water accelerates in both senses the reconstructive transformations between 3D α and 1D δ phases, which is useful during the crystallization of the α phase. On the other hand, the interstitial H2O molecules increase the effective size of the MA and FA cations to which are bonded, shifting the thermodynamic equilibrium from the compact perovskite structure to the open δ and hydrated phases of loosely bonded chains of PbI6 octahedra. For this reason, when fabricating devices based on hybrid metal-organic halide perovskites, it is important to reduce the content of interstitial water as much as possible before encapsulation.

Understanding liquefaction in halide perovskites upon methylamine gas exposure

Methylamine (CH₃NH₂, MA) gas-induced fabrication of organometal CH₃NH₃PbI₃ based perovskite thin films are promising photovoltaic materials that transform the energy from absorbed sunlight into electrical power. Unfortunately, the low stability of the perovskites poses a serious hindrance for further development, compared to conventional inorganic materials. The solid-state perovskites are liquefied and recrystallized from CH₃NH₂. However, the mechanism of this phase transformation is far from clear. Employing first principles calculations and ab initio molecular dynamics simulations, we investigated the formation energy of primary defects in perovskites and the liquefaction process in CH₃NH₂ vapor. The results indicated that defect-assisted surface dissolution leads to the liquefaction of perovskite thin films in CH₃NH₂ vapor. Two primary defects were studied: one is the Frenkel pair defect (including both negatively charged interstitial iodide ion (I_i⁻) and iodide vacancy (V_I⁺) at the PbI₂-termination surface, and the other is the Schottky defects (methylammonium vacancy, V_MA) at the MAI-termination surface. Moreover, the defect-induced disorder in the microstructure reduces the degeneration of energy levels, which leads to a blue shift and broader absorption band gap, as compared to the clean perovskite surface. The mechanism of how defects impact the surface dissolution could be applied for the further design of high-stability perovskite solar cells.

Methylamine (CH₃NH₂, MA) gas-induced fabrication of organometal CH₃NH₃PbI₃ based perovskite thin films are promising photovoltaic materials that transform the energy from absorbed sunlight into electrical power.

Opal-Like Photonic Structuring of Perovskite Solar Cells Using a Genetic Algorithm Approach

Light management is an important area of photovoltaic research, but little is known about it in perovskite solar cells. The present work numerically studies the positive effect of structuring the photo-active layer of perovskite material. This structuration consists of a hybrid absorbing layer made of an uniform part and an opal-like part. A genetic algorithm approach allows us to determine the optimal combination among more than 1.4 × 10 9 potential combinations. The optimal combination provides an internal quantum efficiency of 98.1%, nearly 2% higher than for an equivalent unstructured photo-active layer. The robustness of the optimum against potential experimental deviations, as well as the angular dependency of the proposed structure, are examined in the present study.

Microwave-Assisted Preparation of Organo-Lead Halide Perovskite Single Crystals

The efficiency of organo-lead halide perovskite-based optoelectronic devices is dramatically lower for amorphous materials compared to highly crystalline ones. Therefore, it is challenging to optimize and scale up the production of large-sized single crystals of perovskite materials. Here, we describe a novel and original approach to preparing lead halide perovskite single crystals by applying microwave radiation during the crystallization. The microwave radiation primarily causes precise heating control in the whole volume and avoids temperature fluctuations. Moreover, this facile microwave-assisted method of preparation is highly reproducible and fully automated, it and can be applied for various different perovskite structures. In addition, this cost-effective method is expected to be easily scalable because of its versatility and low energy consumption. The crystallization process has low heat losses; therefore, only a low microwave reactor power of 8-15 W during the temperature changes and of less than 1 W during the temperature holding is needed.

XPS evidence of degradation mechanism in CH3NH3PbI3 hybrid perovskite

In this study, we investigate the photo-/thermal degradation mechanism of hybrid perovskites by using x-ray photoelectron (XPS) valence band (VB) spectra coupling with density functional theory (DFT) calculations. Herein, CH₃NH₃PbI₃ is respectively subjected to irradiation with visible light and annealing at an exposure of 0-1000 h. It is found from XPS survey spectra that, in both cases (irradiation and annealing), a decrease in the I:Pb ratio is observed with aging time, which unambiguously indicates the formation of PbI₂ as the product of photo/thermal degradation. The comparison of the XPS VB spectra of irradiated and annealed perovskites with the DFT calculations of CH₃NH₃PbI₃ and PbI₂ compounds have showed a systematic decrease in the contribution of I-5p states, which allows us to determine the respective threshold for degradation, which is 500 h for light irradiation and 200 h for annealing. This discrepancy might be due to the fact that the relaxation of thermal excitations of the system is carried out only by the phonons (which are non-radiative physical processes) while the radiative processes occurred during the photoexcitation will elastically or inelastically divert part of the external energy from the system to reduce its impact on perovskite degradation.

Composition-Controlled Synthesis of Hybrid Perovskite Nanoparticles by Ionic Metathesis: Bandgap Engineering Studies from Experiments and Theoretical Calculations

Herein a newly discovered non-polar solvent based synthesis of MAPbX₃ hybrid perovskite nanoparticles (NPs) is presented, where MA=Methylammonium and X=I, Br and Cl, as well as their mixed halide counterparts. The methodology proposed is simple and uses low-cost commercial precursors. The conventional method of hybrid perovskite preparation requires methylammonium halide precursors and highly polar solvents. Mandatory use of polar solvents and a particular perovskite precursor makes an intermediate compound which then requires a non-polar solvent to recover the NPs. In contrast here, a whole range of mixed halide perovskite NPs is fabricated without using a methylammonium halide precursor and a polar solvent. In this method, a non-polar solvent is used, which provides a better platform for the particle recovery. Organic cations on the nanoparticle surface prevent degradation from water, due to their hydrophobic nature, and hence offer a stable colloidal suspension in toluene for more than three months. Ab-initio calculations within density functional theory (DFT) predict lower formation energies compared to previously reported values, confirming better chemical stability for this synthesis pathway. Through the halide compositional tuning, these NPs exhibit a variety of emission and absorption starting from ultraviolet to near infrared (IR). The absorption spectra of various halide perovskite show a sharp band edge over the visible wavelength with high absorption coefficient. High oscillator strengths due to high excitonic binding energies combined with the simulated finite dipole transition probabilities point towards the observed high absorption. The emission spectra of mixed halide perovskites vary from 400 to 750 nm, which covers the whole range of visible spectra with sharp full-width at half maxima. Different halide perovskite exhibit average recombination lifetime from 5 to 227 ns. Ambient synthesis, chemical robustness and tunability of emission with varying halide compositions make MAPbX₃ (X=I, Br and Cl) NPs appealing for the optoelectronic applications.

Highly luminescent red emissive perovskite quantum dots-embedded composite films: ligands capping and caesium doping-controlled crystallization process

Perovskite quantum dots (PQDs) are emerging as functional luminescence down-shifting materials for light conversion applications. The incorporation of PQDs into a polymeric matrix is a key step to improving their stability, thus facilitating device integration. Compared to the conventional way of mixing the pre-synthesized PQDs into a polymer, the in situ fabrication of perovskite quantum dots-embedded composite films (PQDCFs) is an efficient and cost-effective method, which yields enhanced photoluminescence properties. This method has been successfully developed for green emissive CH3NH3PbBr3 PQDCFs, whereas the red CH3NH3PbI3 PQDCFs only show the photoluminescence quantum yields (PLQYs) less than 15%. By means of combining transmittance electron microscopy (TEM) and absorption spectrum analysis, we showed that the "perovskite red wall" in PQDCFs was mainly related to the phase separation, formation of large-sized particles and incomplete chemical conversion of precursors. These problems are caused by the solubility variance of perovskite precursors in the solvent as well as the solvation compatibility between perovskite precursors and the polymer during the crystallization process. Based on these findings, we introduced Cs+ as a dopant and 3,3-diphenylpropyamine (DPPA) as capping ligands, respectively, to decrease the solubility variance of the precursors and improve the compatibility between PQDs and the polymer. Consequently, highly luminescent red emissive PQDCFs with a PLQY of 91% were achieved with this strategy.

The Impact of Nano- and Microstructure on the Stability of Perovskite Solar Cells

Halide perovskites have emerged recently as a promising candidate for the next generation of photovoltaics. Power conversion efficiencies for laboratory-scale devices surpass those of established technologies, such as multicrystalline silicon. However, perovskite solar cells lose their initial efficiency rapidly due to the convolution of several degradation factors, which hinder the process of industrialization. In this review, the important role of the nano- and microstructure of the perovskite layer in the performance and stability of the device are discussed. The defects located predominantly at the grain boundaries within the perovskite film and at the interface of the perovskite with the other materials can compromise the devices' stability. Thus, lowering the surface and interface concentration of defects is a key approach toward long-term stable perovskite solar cells.

Toward Perovskite Solar Cell Commercialization: A Perspective and Research Roadmap Based on Interfacial Engineering

High-efficiency and low-cost perovskite solar cells (PVKSCs) are an ideal candidate for addressing the scalability challenge of solar-based renewable energy. The dynamically evolving research field of PVKSCs has made immense progress in solving inherent challenges and capitalizing on their unique structure-property-processing-performance traits. This review offers a unique outlook on the paths toward commercialization of PVKSCs from the interfacial engineering perspective, relevant to both specialists and nonspecialists in the field through a brief introduction of the background of the field, current state-of-the-art evolution, and future research prospects. The multifaceted role of interfaces in facilitating PVKSC development is explained. Beneficial impacts of diverse charge-transporting materials and interfacial modifications are summarized. In addition, the role of interfaces in improving efficiency and stability for all emerging areas of PVKSC design are also evaluated. The authors' integral contributions in this area are highlighted on all fronts. Finally, future research opportunities for interfacial material development and applications along with scalability-durability-sustainability considerations pivotal for facilitating laboratory to industry translation are presented.

Thermodynamics and the Intrinsic Stability of Lead Halide Perovskites CH3NH3PbX3

The role of thermodynamics in assessing the intrinsic instability of the CH₃NH₃PbX₃ perovskites (X = Cl,Br,I) is outlined on the basis of the available experimental information. Possible decomposition/degradation pathways driven by the inherent instability of the material are considered. The decomposition to precursors CH₃NH₃X(s) and PbX₂( s) is first analyzed, pointing out the importance of both the enthalpic and the entropic factor, the latter playing a stabilizing role making the stability higher than often asserted. For CH₃NH₃PbI₃, the disagreement between the available calorimetric results makes the stability prediction uncertain. Subsequently, the gas-releasing decomposition paths are discussed, with emphasis on the discrepant results presently available, probably reflecting the predominance of thermodynamic or kinetic control. The competition between the formation of NH₃(g) + CH₃X(g), CH₃NH₂(g) + HX(g) or CH₃NH₃X(g) is analyzed, in comparison with the thermal decomposition of methylammonium halides. In view of the scarce and inconclusive thermodynamic studies to-date available, the need for further experimental data is emphasized.

Interstitial Occupancy by Extrinsic Alkali Cations in Perovskites and Its Impact on Ion Migration

Recent success in achieving highly stable Rb-containing organolead halide perovskites has indicated the possibility of incorporating small monovalent cations, which cannot fit in the lead-halide cage with an appropriate tolerance factor, into the perovskite lattice while maintaining a pure stable "black" phase. In this study, through a combined experimental and theoretical investigation by density functional theory (DFT) calculations on the incorporation of extrinsic alkali cations (Rb⁺ , K⁺ , Na⁺ , and Li⁺ ) in perovskite materials, the size-dependent interstitial occupancy of these cations in the perovskite lattice is unambiguously revealed. Interestingly, DFT calculations predict the increased ion migration barriers in the lattice after the interstitial occupancy. To verify this prediction, ion migration behavior is characterized through hysteresis analysis of solar cells, electrical poling, temperature-dependent conductivity, and time-dependent photoluminescence measurements. The results collectively point to the suppression of ion migration after lattice interstitial occupancy by extrinsic alkali cations. The findings of this study provide new material design principles to manipulate the structural and ionic properties of multication perovskite materials.

Ligand removal and photo-activation of CsPbBr3 quantum dots for enhanced optoelectronic devices

Perovskite quantum dots have recently emerged as a promising light source for optoelectronic applications. However, integrating them into devices while preserving their outstanding optical properties remains challenging. Due to their ionic nature, perovskite quantum dots are extremely sensitive and degrade on applying the simplest processes. To maintain their colloidal stability, they are surrounded by organic ligands; these prevent efficient charge carrier injection in devices and have to be removed. Here we report on a simple method, where a moderate thermal process followed by exposure to UV in air can efficiently remove ligands and increase the photo-luminescence of the room temperature synthesized perovskite quantum dot thin films. Annealing is accompanied by a red shift of the emission wavelength, usually attributed to the coalescence and irreversible degradation of the quantum dots. We show that it is actually related to the relaxation of the quantum dots upon the ligand removal, without the creation of non-radiative recombining defects. The quantum dot surface, as devoid of ligands, is subsequently photo-oxidized and smoothened upon exposure to UV in air, which drastically enhances their photo-luminescence. This adequate combination of treatments improves by more than an order of magnitude the performances of perovskite quantum dot light emitting diodes.

Self-Healing Inside APbBr3 Halide Perovskite Crystals

Self-healing, where a modification in some parameter is reversed with time without any external intervention, is one of the particularly interesting properties of halide perovskites. While there are a number of studies showing such self-healing in perovskites, they all are carried out on thin films, where the interface between the perovskite and another phase (including the ambient) is often a dominating and interfering factor in the process. Here, self-healing in perovskite (methylammonium, formamidinium, and cesium lead bromide (MAPbBr₃ , FAPbBr₃ , and CsPbBr₃ )) single crystals is reported, using two-photon microscopy to create damage (photobleaching) ≈110 µm inside the crystals and to monitor the recovery of photoluminescence after the damage. Self-healing occurs in all three perovskites with FAPbBr₃ the fastest (≈1 h) and CsPbBr₃ the slowest (tens of hours) to recover. This behavior, different from surface-dominated stability trends, is typical of the bulk and is strongly dependent on the localization of degradation products not far from the site of the damage. The mechanism of self-healing is discussed with the possible participation of polybromide species. It provides a closed chemical cycle and does not necessarily involve defect or ion migration phenomena that are often proposed to explain reversible phenomena in halide perovskites.

Aziridinium Lead Iodide: A Stable, Low-Band-Gap Hybrid Halide Perovskite for Photovoltaics

The low ionization energy of an A site molecule is a very important factor, which determines the thermodynamical stability of APbI₃ hybrid halide perovskites, while the size of the molecule governs the stable phase at room temperature and, eventually, the band gap. It is challenging to achieve both a low ionization energy and the reasonable size for the PbI₃ cage to circumvent the stability issue inherent to hybrid halide perovskites. Here we propose a new three-membered charged ring radical, which demonstrates a low ionization energy that renders a good stability for its corresponding perovskite and a reasonable cation size that translates into a suitable band gap for the photovoltaic application. We use ab initio calculations to evaluate a polymorphism of the crystal structure of the proposed hybrid halide perovskite, its stability, and electronic properties in comparison with the mainstream perovskites, such as the methylammonium and formamidinium lead iodide.