Chemical insights into perovskite ink stability

Reaction-Diffusion and Crystallization Kinetics Modulation of Two-Step Deposited Tin-Based Perovskite Film via Reducing Atmosphere

The two-step deposition method effectively mitigates the efficiency decline observed in tin-based perovskite solar cells (TPVSCs) with increasing cell area, stemming from film in-homogeneity. However, the high solubility of SnI2 in the conventionally used solvent isopropyl alcohol, coupled with the absence of effective modulation of reaction-diffusion process, results in inadequate film coverage and conversion. In this study, we introduce formic acid as the second-step solvent and introduce dithiothreitol (DTT) to regulate reaction-diffusion/crystallization kinetics meticulously. Moreover, this research underscores a fundamental principle that the suitable binding energy ranging from -1.38 to -10.10 kcal mol-1 between ligands and Sn2+ significantly enhances the effectiveness of two-step crystallization control. Notably, a uniform perovskite film is achieved on large-scale substrate, and TPVSCs processed with DTT exhibit the highest efficiencies of 12.68 % for 0.04 cm2 device and 11.30 % for 1 cm2 device among tin-based perovskite devices in two-step sequential deposition method, even in the absence of dimethyl sulfoxide. This study lays the groundwork for the potential scale-up development of lead-free perovskite solar cells.

Activated Corrosion and Recovery in Lead Mixed-Halide Perovskites Revealed by Dynamic Near-Ambient Pressure X-ray Photoelectron Spectroscopy

Herein, we quantify rates of O2-photoactivated corrosion and recovery processes within triple cation CsFAMAPb(IBr)3 perovskite active layers using dynamic near-ambient pressure X-ray photoemission spectroscopy (NAP-XPS). Activated corrosion is described as iodide oxidation and lead reduction, which occurs only in the presence of both O2 and light through photoinduced electron transfer. We observe electron density reorganization from the Pb-I bonds consistent with ligand exchange, evident from the nonstoichiometric redox change (i.e., <1 e-). Approximately half of the Pb centers are reduced to weakly coordinated Pb-higher oxidation number than metallic Pb-with a rate coefficient of ∼3 (±0.3) × 10-4 atomic percent/s. Hole capture by I- yields I3- and is accompanied by increased concentrations of near-surface bromides, hypothesized to be due to anion vacancies and/or oxidation of mobile iodide resulting from ion demixing. Activated corrosion is found to be quasi-reversible; initial perovskite stoichiometry slowly recovers when the O2/light catalyst is removed, postulated to be due to mobile halide species present within the film below XPS sampling depth. Small deviations in near-surface composition (<2%) of the perovskite are used to connect reaction rates to quantified, near-band edge donor and acceptor defect concentrations, demonstrating two energetically distinct sites are responsible for the redox process. Collectively, environmental flux and rate quantification are deemed critical for the future elucidation of chemical degradation processes in perovskites, where rate-dependent reaction pathways are expected to be very system dependent (environment and material).

Extending Shelf-Life of Two-Step Method Precursor Solutions through Targeted Regulation for Highly Efficient and Reproducible Perovskite Solar Cells

Precursor solution aging process can cause significant influence on the photovoltaic performance of perovskite solar cells (PVSCs). Notably, we first observe that the aging phenomenon is more severe in the precursor of two-step sequential method compared to that in one-step method due to that the protic solvent isopropanol facilitates amine-cation side reaction and iodide ions oxidation. Herein, we report a novel approach for selectively stabilizing both organic amine salt and lead iodide (PbI2) precursor solutions in two-step method. The introduction of benzene-1,3-dithiol into organic amine salt solution can mitigate amine-cation side reactions due to the formation of an acidic and reducing environment. Simultaneously, decamethylferrocene (FcMe10/FcMe+ 10) pair can act as a redox shuttle in PbI2 solution to concurrently oxidize Pb0 and reduce I2 in cyclic manner. Consequently, the PVSCs device fabricated from ameliorative precursor solutions demonstrates superior power conversion efficiency of 25.31 %, retaining 95 % of its efficiency after 21 days of solution aging. Moreover, the unencapsulated devices maintain 85 % of primitive efficiency for 1500 h at maximum power point tracking under continuous illumination. This work establishes a fundamental guidance and scientific direction for the stabilization of two-step perovskite precursor solutions.

Stabilization of Perovskite Solar Cells by a Universal Dilution Strategy: The Crystallization Control of Blade-Coating

Solvent engineering is one of the most effective strategies to control perovskite film quality, which directly affects the performance of perovskite solar cells (PSCs). Here, we introduce volatile acetonitrile (ACN) into the traditional solvent system (i.e., N,N-dimethylformamide and N-methyl-2-pyrrolidone) to dilute the perovskite precursors from 1.43 M to lower concentration (0.6-0.8 M). The dilution strategy can effectively improve the stability of the precursor solution and maintain similar film quality and device performance as those with high solution concentration (1.43 M). Notably, the devices with low-concentration precursors (0.6-0.8 M) show efficiency of 20.85% and improved long-term (>1000 h) storage stability compared to the device with high precursor concentration by blade-coating. Meanwhile, the material cost can be reduced by more than 50% when diluting to 0.6-0.8 M. These results demonstrate a universal dilution method which can provide guidance for the research and development of low-cost and high stability PSCs.

Multiple Stabilization Effects of Benzylhydrazine on Scalable Perovskite Precursor Inks for Improved Perovskite Solar Cell Production

Perovskite precursor inks suffer various forms of degradation, such as iodide anion oxidation and organic cation breakdown, hindering reliable perovskite solar cell manufacturing. Here we report that benzylhydrazine hydrochloride (BHC) not only retards the buildup of iodine as previously reported but also prevents the breakdown of organic cations. Through investigating BHC and iodine chemical reactions, we elucidate protonation and dehydration mechanisms, converting BHC to harmless volatile compounds, thus preserving perovskite film crystallization and solar cell performance. This inhibition effect lasts nearly a month with minimal BHC, contrasting control inks without BHC where organic cations fully react in less than a week. This enhanced understanding, from additive stabilization to end products, promises improved perovskite solar cell production reliability.

Current State and Future Perspectives of Printable Organic and Perovskite Solar Cells

Photovoltaic technology presents a sustainable solution to address the escalating global energy consumption and a reliable strategy for achieving net-zero carbon emissions by 2050. Emerging photovoltaic technologies, especially the printable organic and perovskite solar cells, have attracted extensive attention due to their rapidly transcending power conversion efficiencies and facile processability, providing great potential to revolutionize the global photovoltaic market. To accelerate these technologies to translate from the laboratory scale to the industrial level, it is critical to develop well-defined and scalable protocols to deposit high-quality thin films of photoactive and charge-transporting materials. Herein, the current state of printable organic and perovskite solar cells is summarized and the view regarding the challenges and prospects toward their commercialization is shared. Different printing techniques are first introduced to provide a correlation between material properties and printing mechanisms, and the optimization of ink formulation and film-formation during large-area deposition of different functional layers in devices are then discussed. Engineering perspectives are also discussed to analyze the criteria for module design. Finally, perspectives are provided regarding the future development of these solar cells toward practical commercialization. It is believed that this perspective will provide insight into the development of printable solar cells and other electronic devices.

Volatile Perovskite Precursor Ink Enables Window Printing of Phase-Pure FAPbI3 Perovskite Solar Cells and Modules in Ambient Atmosphere

Despite the great success of perovskite photovoltaics in terms of device efficiency and stability using laboratory-scale spin-coating methods, the demand for high-throughput and cost-effective solutions remains unresolved and rarely reported because of the complicated nature of perovskite crystallization. In this work, we propose a stable precursor ink design strategy to control the solvent volatilization and perovskite crystallization to enable the wide speed window printing (0.3 to 18.0 m/min) of phase-pure FAPbI3 perovskite solar cells (pero-SCs) in ambient atmosphere. The FAPbI3 perovskite precursor ink uses volatile acetonitrile (ACN) as the main solvent with DMF and DMSO as coordination additives is beneficial to improve the ink stability, inhibit the coffee rings, and the complicated intermediate FAPbI3 phases, delivering high-quality pin-hole free and phase-pure FAPbI3 perovskite films with large-scale uniformity. Ultimately, small-area FAPbI3 pero-SCs (0.062 cm2 ) and large-area modules (15.64 cm2 ) achieved remarkable efficiencies of 24.32 % and 21.90 %, respectively, whereas the PCE of the devices can be maintained at 23.76 % when the printing speed increases to 18.0 m/min. Specifically, the unencapsulated device exhibits superior operational stability with T90 >1350 h. This work represents a step towards the scalable, cost-effective manufacturing of perovskite photovoltaics with both high performance and high throughput.

Uncovering Aging Chemistry of Perovskite Precursor Solutions and Anti-aging Mechanism of Additives

The aging of precursor solutions is the major stumbling block for the commercialization of perovskite solar cells (PSCs). Herein, for the first time we used the state-of-the-art in situ liquid time-of-flight secondary ion mass spectrometry to molecularly explore the perovskite precursor solution chemistry. We identified that the methylammonium and formamidinium cations and the I^- anion are the motivators of the aging chemistry. Further, we introduced two kinds of Lewis bases, triethyl phosphate (TP) and ethyl ethanesulfonate (EE), as new additives in the solution and unraveled that both of them can protect the reactive cations from aging through weak interactions. Significantly, TP is superior to EE in enhancing long-term solution stability as it can well-maintain the internal interaction structures within the solution phase. The PSC derived from a fresh TP-doped solution delivered a high power conversion efficiency of 23.06 %, 92.23 % of which remained in that from a 21-day-old solution.

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.

Plasma-Deposited Fluorocarbon Coatings on Methylammonium Lead Iodide Perovskite Films

Metal halide perovskites are excellent semiconductors materials that can be exploited in many fields, from the vastly explored photovoltaics to the recent applications in photocatalysis. One of the material’s known limitations is the poor resistance to moisture which induces degradation, triggered by the soft and defective nature of its surface. In this study, we explore non-equilibrium plasmas, to deposit a fluorocarbon polymer on the surface of a MAPbI3 film. We found that the treatment generally enhances the film stability over time, and in certain conditions it improves the perovskite optical properties, demonstrating to be a good strategy aiming defects passivation. Thanks to the solvent-free and non-invasive nature of plasmas, this technique has the potential to be extensively applied to a wide range of perovskite materials targeting different applications.