Dissociation of Methylammonium Cations in Hybrid Organic-Inorganic Perovskite Solar Cells

Surface chemical heterogeneous distribution in over-lithiated Li1+xCoO2 electrodes

In commercial Li-ion batteries, the internal short circuits or over-lithiation often cause structural transformation in electrodes and may lead to safety risks. Herein, we investigate the over-discharged mechanism of LiCoO₂/graphite pouch cells, especially spatially resolving the morphological, surface phase, and local electronic structure of LiCoO₂ electrode. With synchrotron-based X-ray techniques and Raman mapping, together with spectroscopy simulations, we demonstrate that over-lithiation reaction is a surface effect, accompanied by Co reduction and surface structure transformation to Li₂CoO₂/Co₃O₄/CoO/Li₂O-like phases. This surface chemical distribution variation is relevant to the depth and exposed crystalline planes of LiCoO₂ particles, and the distribution of binder/conductive additives. Theoretical calculations confirm that Li₂CoO₂-phase has lower electronic/ionic conductivity than LiCoO₂-phase, further revealing the critical effect of distribution of conductive additives on the surface chemical heterogeneity evolution. Our findings on such surface phenomena are non-trivial and highlight the capability of synchrotron-based X-ray techniques for studying the spatial chemical phase heterogeneity.

Over-lithiation often causes structural transformation in electrodes and may lead to safety issues in Li-ion batteries. Here, authors investigate the over-discharged mechanism of LiCoO2/graphite pouch cells, and spatially resolve the morphological, surface phase, and local electronic structure of LiCoO2 electrode.

Experimental and Theoretical Core Level and Valence Band Analysis of Clean Perovskite Single Crystal Surfaces

A detailed understanding of the surface and interface properties of lead halide perovskites is of interest for several applications, in which these materials may be used. To develop this understanding, the study of clean crystalline surfaces can be an important stepping stone. In this work, the surface properties and electronic structure of two different perovskite single crystal compositions (MAPbI₃ and Cs_x FA_{1- x} PbI₃ ) are investigated using synchrotron-based soft X-ray photoelectron spectroscopy (PES), molecular dynamics simulations, and density functional theory. The use of synchrotron-based soft X-ray PES enables high surface sensitivity and nondestructive depth-profiling. Core level and valence band spectra of the single crystals are presented. The authors find two carbon 1s contributions at the surface of MAPbI₃ and assign these to MA⁺ ions in an MAI-terminated surface and to MA⁺ ions below the surface. It is estimated that the surface is predominantly MAI-terminated but up to 30% of the surface can be PbI₂ -terminated. The results presented here can serve as reference spectra for photoelectron spectroscopy investigations of technologically relevant polycrystalline thin films, and the findings can be utilized to further optimize the design of device interfaces.

Synthesis, Characterization, and Stability Studies of Ge-Based Perovskites of Controllable Mixed Cation Composition, Produced with an Ambient Surfactant-Free Approach

In this report, we have applied a facile, ligand-free, ambient synthesis protocol toward the fabrication of not only a series of lead-free Ge-based perovskites with the general formulation of MA1-x FA x GeI3 (where x was changed from 0, 0.25, 0.5, 0.75, to 1) but also CsGeI3. Specifically, our methodology for producing ABX3 systems is generalizable, regardless of the identity of either the A site cation or the X site halide ion. Moreover, it incorporates many advantages, including (i) the possibility of efficiently generating pure Ge-based perovskite particles of any desired chemical composition, (ii) the use of readily available, commercial precursors and comparatively lower toxicity solvents, (iii) the practicality of scale up, and (iv) the elimination of the need for any superfluous organic surface ligands or surfactants. In addition to providing mechanistic insights into their formation, we have examined the chemical composition, crystallite size, morphology, surface attributes, oxidation states, and optical properties of our as-prepared perovskites using a combination of diffraction, microscopy, and spectroscopy techniques. Specifically, we noted that the optical band gap could be reliably tuned as a function of chemical composition, via the identity of the A site cation. Moreover, we have probed their stability, not only under standard storage conditions but also, for the first time, when subjected to both e-beam- and X-ray-induced degradation, using cumulative data from sources such as synchrotron-based scanning hard X-ray microscopy. Importantly, of relevance for the potential practical incorporation of these Pb-free perovskites, our work has emphasized the possibility of controlling the chemical composition within Ge-based perovskites as a means of rationally tuning their observed band gaps and optical behavior.

Reaction Temperature and Partial Pressure Induced Etching of Methylammonium Lead Iodide Perovskite by Trimethylaluminum

Al₂O₃ atomic layer deposition (ALD), which uses trimethylaluminum (TMA) as the metal precursor, shows promise in improving the environmental stability of hybrid halide perovskites. However, it is not yet entirely clear how TMA, a strong Lewis acid, reacts with fresh perovskites and how the reaction affects the nucleation of ALD Al₂O₃. Here, the effects of reaction temperature and partial pressure of TMA on the mechanisms of TMA/CH₃NH₃PbI₃ reactions are investigated. Our real time mass gain data and in situ mass spectrometry data show that the TMA/CH₃NH₃PbI₃ reaction can either remove mass or accumulate mass onto CH₃NH₃PbI₃ substrates, depending strongly on the reaction temperature and partial pressure of TMA. The TMA/CH₃NH₃PbI₃ reaction probably generates TMA-CH₃NHx adduct compounds, which protects CH₃NH₃PbI₃ from TMA by forming a shell at 25 °C in the vacuum process. However, these adduct compounds decompose at higher temperatures (e.g., 75 °C). This product layer is much thicker than a monolayer, suggesting the interface formed between Al₂O₃ coating and CH₃NH₃PbI₃ is blurring and messy. These results have not yet, but should be, carefully considered to correctly interpret the effect of ALD Al₂O₃ treatment on optoelectronic properties of CH₃NH₃PbI₃.

Degradation Mechanism and Relative Stability of Methylammonium Halide Based Perovskites Analyzed on the Basis of Acid-Base Theory

The correct identification of all gases released during hybrid perovskite degradation is of great significance to develop strategies to extend the lifespan of any device based on this semiconductor. CH₃X (X = Br/I) is a released degradation gas/low boiling point liquid arising from methylammonium (MA⁺) based perovskites, which has been largely overlooked in the literature focusing on stability of perovskite solar cells. Herein, we present an unambiguous identification of CH₃I release using microwave (rotational) spectroscopy. An experimental back-reaction test demonstrates that the well-known CH₃NH₂/HX degradation route may not be the ultimate degradation pathway of MAPbX₃ in thermodynamic closed systems. Meanwhile, the CH₃X/NH₃ route cannot back-react selectively to MAX formation as occurred for the former back-reaction. Metadynamics calculations uncover the X halide effect on energy barriers for both degradation reactions showing a better stability of Br based perovskite ascribed to two aspects: (i) lower Brönsted-Lowry acidity of HBr compared to HI and (ii) higher nucleophilic character of CH₃NH₂ compared to NH₃. The latter property makes CH₃NH₂ molecules stay preferentially attached on the electrophilic perovskite surface (Pb²⁺) during the dynamic simulation instead of being detached as observed for the NH₃ molecule.

Prospects for Mitigating Intrinsic Organic Decomposition in Methylammonium Lead Triiodide Perovskite

Organometallic lead halide perovskites seem to be on the threshold of becoming viable commercial photovoltaics; however, further improvements to the stability of these materials must be made before they can compete with existing photovoltaic technologies. Of the organometallic lead halide perovskites used in photovoltaics, methylammonium lead triiodide perovskite (MAPI) is perhaps the most studied, and understanding how MAPI degrades is crucial for developing strategies to improve stability. We discuss the experimental evidence behind several possible routes for MAPI to degrade into PbI₂ and various organics, and how the decomposition path of MAPI may strongly depend on substrate, precursors, intrinsic organic defects, and morphology. Exploring the conditions required for MAPI to degrade according to a particular pathway is important not only from a fundamental materials chemistry perspective, but also for understanding intrinsic instability in MAPI-based photovoltaics and to develop strategies to improve stability.

Investigation of Thermally Induced Degradation in CH3NH3PbI3 Perovskite Solar Cells using In-situ Synchrotron Radiation Analysis

In this study, we employ a combination of various in-situ surface analysis techniques to investigate the thermally induced degradation processes in MAPbI₃ perovskite solar cells (PeSCs) as a function of temperature under air-free conditions (no moisture and oxygen). Through a comprehensive approach that combines in-situ grazing-incidence wide-angle X-ray diffraction (GIWAXD) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) measurements, we confirm that the surface structure of MAPbI₃ perovskite film changes to an intermediate phase and decomposes to CH₃I, NH₃, and PbI₂ after both a short (20 min) exposure to heat stress at 100 °C and a long exposure (>1 hour) at 80 °C. Moreover, we observe clearly the changes in the orientation of CH₃NH₃⁺ organic cations with respect to the substrate in the intermediate phase, which might be linked directly to the thermal degradation processes in MAPbI₃ perovskites. These results provide important progress towards improved understanding of the thermal degradation mechanisms in perovskite materials and will facilitate improvements in the design and fabrication of perovskite solar cells with better thermal stability.

Double Charged Surface Layers in Lead Halide Perovskite Crystals

Understanding defect chemistry, particularly ion migration, and its significant effect on the surface's optical and electronic properties is one of the major challenges impeding the development of hybrid perovskite-based devices. Here, using both experimental and theoretical approaches, we demonstrated that the surface layers of the perovskite crystals may acquire a high concentration of positively charged vacancies with the complementary negatively charged halide ions pushed to the surface. This charge separation near the surface generates an electric field that can induce an increase of optical band gap in the surface layers relative to the bulk. We found that the charge separation, electric field, and the amplitude of shift in the bandgap strongly depend on the halides and organic moieties of perovskite crystals. Our findings reveal the peculiarity of surface effects that are currently limiting the applications of perovskite crystals and more importantly explain their origins, thus enabling viable surface passivation strategies to remediate them.

CH3NH3PbI3-xClx under Different Fabrication Strategies: Electronic Structures and Energy-Level Alignment with an Organic Hole Transport Material

We report a photoelectron spectroscopy study on the electronic structure of CH₃NH₃PbI_3-xCl_x thin films fabricated by physical evaporation from CH₃NH₃I and PbCl₂ precursors, including (1) simultaneously evaporation and (2) sequential evaporation. The results are compared with CH₃NH₃PbI_3-xCl_x made using conventional solution chemistry (i.e., spin-coating). Depending on the fabrication method, CH₃NH₃PbI_3-xCl_x films show different chemical constituents in the near-surface region, leading to disparities in their energetic levels. The chemical identities of the surface species are revealed by an in situ study on the sequentially evaporated film. Moreover, air-exposure treatment also greatly alters the energetic levels of the film. Using hole transport layer of N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (NPB) as a model system, we find that the energy-level alignment with the spin-coated film after air exposure is most suitable for efficient hole transport.

Tuning the gap of lead-based halide perovskites by introducing superalkali species at the cationic sites of ABX3-type structure

New perovskites are formed by introducing polyatomic yet aprotic superalkali species at the cationic A-sites of the ABX₃ structure.

Surface and Interface Aspects of Organometal Halide Perovskite Materials and Solar Cells

The current challenges (e.g., stability, hysteresis, etc.) in organometal halide perovskite solar cell research are closely correlated with surfaces and interfaces. For instance, efficient generation of charges, extraction, and transport with minimum recombination through interlayer interfaces is crucial to attain high-efficiency solar cell devices. Furthermore, intralayer interfaces may be present in the form of grain boundaries within a film composed of the same material, for example, a polycrystalline perovskite layer. The adjacent grains may assume different crystal orientations and/or have different chemical compositions, which impacts charge excitation and dynamics and thereby the overall solar cell performance. In this Perspective, we present case studies to demonstrate (1) how surfaces and interfaces can impact material properties and device performance and (2) how these issues can be investigated by surface science techniques, such as scanning probe microscopy, photoelectron spectroscopy, and so forth. We end this Perspective by outlining the future research directions based on the reported results as well as the new trends in the field.