Structural and Optical Properties of Solvated PbI2 in γ-Butyrolactone: Insight into the Solution Chemistry of Lead Halide Perovskite Precursors

Electronic Structure and Optical Properties of Tin Iodide Solution Complexes

The emerging interest in tin halide perovskites demands a robust understanding of the fundamental properties of these materials starting from the earliest steps of their synthesis. In a first-principles work based on time dependent density functional theory, we investigate the structural, energetic, electronic, and optical properties of 14 tin iodide solution complexes formed by the SnI2 unit tetracoordinated with molecules of common solvents, which we classify according to their Gutmann's donor number. We find that all considered complexes are energetically stable and their formation energy expectedly increases with the donating ability of the solvent. The energies of the frontier states are affected by the choice of solvent, with their absolute values decreasing with the donor number. The occupied orbitals are predominantly localized on the tin iodide unit, while the unoccupied ones are distributed also on the solvent molecules. Owing to this partial wave function overlap, the first optical excitation is generally weak, although the spectral weight is red-shifted by the solvent molecules being coordinated to SnI2 in comparison to the reference obtained for this molecule alone. Comparisons with results obtained on the same level of theory on Pb-based counterparts corroborate our analysis. The outcomes of this study provide quantum-mechanical insight into the fundamental properties of tin iodide solution complexes. This knowledge is valuable in the research on lead-free halide perovskites and their precursors.

Solvent dependent iodide oxidation in metal-halide perovskite precursor solutions

Solar cell absorbing layers made of metal-halide perovskites (MHPs) are usually deposited from solution phase precursors, which is one of the reasons why these materials received huge research attention in the last few years. A detailed knowledge of the solution chemistry is critical to understand the formation of MHP thin films and thus to control their optoelectronic properties and the reproducibility issues that usually affect their synthesis. In this regard, the concentration of triiodide, I₃^-, is one factor known to have an influence on regulating important aspects such as the particle size in the solution and the defect concentration in the film. In this study, we highlight an underestimated source of I₃^-, namely the iodide salt solutions ubiquitously employed in MHP synthetic routes, which not only lead to the formation of I₃^- but also detracts available I^- for the MHP synthesis, thus establishing under-stoichiometric conditions. Particularly, we show how the oxidation of I^- to I₃^- changes in time with both the iodide salt counter-cation (K⁺, CH₃NH₃⁺) and the used solvent, meaning that variable quantities of I₃^- are found depending on the synthesis conditions, with enhanced oxidation found in the γ-butyrolactone (GBL) solvent. Though these differences are generally small, we shed light on a hidden and ever-present reaction which is likely to be related to the overall processing quality of MHP thin films.

Stability of Perovskite Light-Emitting Diodes: Existing Issues and Mitigation Strategies Related to Both Material and Device Aspects

Metal halide perovskites combine excellent electronic and optical properties, such as defect tolerance and high photoluminescence efficiency, with the benefits of low-cost, large-area, solution-based processing. Composition- and dimension-tunable properties of perovskites have already been utilized in bright and efficient light-emitting diodes (LEDs). At the same time, there are still great challenges ahead to achieving operational and spectral stability of these devices. In this review, the origins of instability of perovskite materials, and reasons for their degradation in LEDs are considered. Then, strategies for improving the stability of perovskite materials are reviewed, such as compositional engineering, dimensionality control, defect passivation, suitable encapsulation matrices, and fabrication of core/shell perovskite nanocrystals. For improvement of the operational stability of perovskite LEDs, the use of inorganic charge-transport layers, optimization of charge balance, and proper thermal management are considered. The review is concluded with a detailed account of the current challenges and a perspective on the key approaches and opportunities on how to reach the goal of stable, bright, and efficient perovskite LEDs.

Vacuum Quenching for Large-Area Perovskite Film Deposition

The removal of precursor solvents in perovskite wet films plays a vital role in controlling the quality of perovskite films and devices. The dripping antisolvent method (removing precursor solvents) has made great advances in small-area devices, but this method limits the preparation of large-area perovskite films. Vacuum quenching that evaporates solvents by dropping the pressure is a potential large-area manufacturing approach. Herein, we have conducted a systematic comparative study on these two methods of depositing perovskite films. It is found that vacuum quenching can obtain the same film quality and small-area device efficiency (∼22.5%) as the antisolvent method. However, on a large-area substrate, the fast vacuum quenching rate improves the solvent evaporation efficiency and nucleation density (i.e., forming a large number of crystal nuclei), thereby obtaining a more uniform and stable perovskite film. Notably, the manufacture window exceeds 10 min. As a result, the champion large-area (6 × 6 cm²) perovskite solar module exhibits an impressive efficiency (17.86%) and long-term operational stability. Furthermore, coupling slot-die coating, vacuum quenching can realize the industrial continuous deposition of large-area perovskite films, which is a potential route for large-scale production.

First-principles analysis of the optical properties of lead halide perovskite solution precursors

Lead halide perovskites (LHPs) are promising materials for opto-electronics and photovoltaics, thanks to favorable characteristics and low manufacturing costs enabled by solution processing. In light of this, it is crucial to assess the impact of solvent-solute interactions on the electronic and optical properties of LHPs and of their solution precursors. In a first-principles work based on time-dependent density-functional theory coupled with the polarizable continuum model, we investigate the electronic and optical properties of a set of charge-neutral compounds with chemical formula, PbX₂(Sol)₄, where X = Cl, Br, and I, and Sol are the six common solvents. We find that single-particle energies and optical gaps depend on the halogen species as well as on the solvent molecules, which also affect the energy and the spatial distribution of the molecular orbitals, thereby impacting on the excitations. We clarify that dark states at the absorption onset are promoted by electron-withdrawing solvents, and we show the correlation between oscillator strength and HOMO → LUMO contribution to the excitations. Our results provide microscopic insight into the electronic and optical properties of LHP solution precursors, complementing ongoing experimental research on these systems and on their evolution to photovoltaic thin films.

Using In Situ Optical Spectroscopy to Elucidate Film Formation of Metal Halide Perovskites

The research interest in halide perovskites has gained momentum enormously over the last recent years, also due to the demonstration of high-efficient perovskite-based optoelectronic devices. A prerequisite for such highly efficient devices is to realize high-quality perovskite layers, which requires a deep understanding about the perovskite formation and good process control. In that context, in situ optical spectroscopy during the processing of halide perovskites has become increasingly popular. Even though it is a relatively easily accessible yet powerful tool for studying perovskite formation, there exist some technical and analytical aspects that need to be considered to unfold its full potential. In this Perspective, we give an overview of the latest developments in the field of in situ optical spectroscopy to control and better understand the film processing of halide perovskites. We highlight possibilities and pitfalls regarding the analysis of measured optical data, discuss the development of technical concepts, and address future prospects of optical in situ spectroscopy.