Mechanism and effect of γ-butyrolactone solvent vapor post-annealing on the performance of a mesoporous perovskite solar cell

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.

Sulfonium-Cations-Assisted Intermediate Engineering for Quasi-2D Perovskite Solar Cells

Quasi-2D Ruddlesden-Popper (RP) perovskites with superior stability are admirable candidates for perovskite solar cells (PSCs) toward commercialization. However, the device performance remains unsatisfactory due to the disordered crystallization of perovskites. In this work, the effects of sulfonium cations on the evolution of intermediates and photovoltaic properties of 2D RP perovskites are investigated. The introduction of sulfonium cations leads to preferred intermediate transformation and improved film quality of perovskites. The resulting devices deliver a champion efficiency of 19.08% at room temperature and 20.52% at 180 K, due to reduced recombination and enhanced charge transport. More importantly, the unencapsulated device maintains 84% of the initial efficiency under maximum power point (MPP) tracking at 40 °C for 1000 h. This work helps to gain a comprehensive understanding of the crystallization process of quasi-2D perovskites and provides a simple strategy to modulate the intermediates of perovskites.

Moisture Stability of Perovskite Solar Cells Processed in Supercritical Carbon Dioxide

Performance degradation under environmental conditions currently limits the practical utility of perovskite-based solar cells. The moisture stability of CH3NH3PbI3 perovskite films and solar cells was measured during exposure to three different levels of relative humidity. The films were crystallized at two different temperatures with and without simultaneous exposure to supercritical carbon dioxide. The film crystallinity, optical absorption, and device photoconversion efficiency was measured over time for three relative humidity levels and both crystallization methods. It was determined that film crystallization in supercritical CO2 resulted in significant improvement in moisture stability for films processed at 50 °C, but negligible improvement in stability for films processed at 100 °C.

The Impact of Solvent Vapor on the Film Morphology and Crystallization Kinetics of Lead Halide Perovskites during Annealing

One of the key factors for the remarkable improvements of halide perovskite solar cells over the last few years is the increased control over perovskite crystallinity and its thin film morphology. Among various processing methods, solvent vapor-assisted annealing (SVAA) has proven to be promising in achieving high-quality perovskite films. However, a comprehensive understanding of the perovskite crystallization process during SVAA is still lacking. In this work, we use a home-built setup to precisely control the SVAA conditions to investigate in detail the perovskite crystallization kinetics. By changing the solvent vapor concentration during annealing, the perovskite grain size can be tuned from 200 nm to several micrometers. We monitor the crystallization kinetics during solvent-free annealing and SVAA using in situ grazing incidence wide-angle X-ray scattering, where we find a diminished perovskite growth rate and the formation of low dimensional perovskite at the top of the perovskite layer during SVAA. Scanning electron microscopy images of the final films further suggest that the perovskite growth follows an Ostwald ripening process at higher solvent concentrations. Thus, our results will contribute to achieve a more targeted processing of perovskite films.

Investigating the effect of solvent vapours on crystallinity, phase, and optical, morphological and structural properties of organolead halide perovskite films

A comprehensive study regarding the effect of different solvent vapours on organolead halide perovskite properties is lacking. In the present work, the impact of exposing CH₃NH₃PbI₃ films to the vapours of commonly available solvents has been studied. The interaction with perovskite has been correlated to solvent properties like dielectric constant, molecular dipole moment, Gutmann donor number and boiling point. Changes in the crystallinity, phase, optical absorption, morphologies at both nanometer and micrometer scale, functional groups and structures were studied using X-ray diffraction, UV-visible absorption, FE-SEM, FTIR and Raman spectroscopies. Among the aprotic solvents DMSO and DMF vapours deteriorate the crystallinity, phase, and optical, morphological and structural properties of the perovskite films in a very short time, but due to the difference in solvent property values acetone affects the perovskite properties differently. Polar protic 2-propanol and water vapours moderately affect the perovskite properties. However 2-propanol can solvate the organic cation CH₃NH₃⁺ more efficiently as compared to water and a considerable difference was found in the film properties especially the morphology at the nanoscale. Nonpolar chlorobenzene vapour minutely affects the perovskite morphology but toluene was found to enhance perovskite crystallinity. Solvent properties can be effectively used to interpret the coordination ability of a solvent. The present study can be immensely useful in understanding the effects of different solvent vapours and also their use for post-deposition processing (like solvent vapour annealing) to improve their properties.

Effect of solvent vapours on the micrometer and nanometre scale morphology of CH₃NH₃PbI₃.

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

We employ a fine-tuned theoretical framework, combining ab initio molecular dynamics (AIMD), density functional theory (DFT), and time-dependent (TD) DFT methods, to investigate the interactions and optical properties of the iodoplumbates within the low coordinative γ-butyrolactone (GBL) solvent environment, widely employed in the perovskite synthesis. We uncover the extent of GBL coordination to PbI2 investigating its relation to the solvated PbI2 optical properties. The employed approach has been further validated by comparison with the experimental UV-vis absorption spectrum of PbI2 in GBL solvent. A comparison with other solvents, commonly employed in the perovskite synthesis, such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) is also reported. The methodology developed in this work can be reasonably extended to the investigation of similar systems.