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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - MatchForSolar (Mechanochemical Approach to Inorganic-Organic Hybrid Materials for Perovskite Solar Cells)

Teaser

Summary

The recent discovery of hybrid organic-inorganic metal halide perovskites led to a renaissance of thin film photovoltaics. The great diversity of hybrid perovskite compositions and preparation pathways makes them an excellent candidate for novel photovoltaic materials with unique combination of properties, the potential for low cost and easy processing along with relatively high power conversion efficiencies. In the last few years, perovskite solar cells have leapt from 10% to a certified value of 22.7%. This spectacular rise in cell performance attracted intense attention from the scientific community. The promise of perovskites in accessing these technologies is on account of their unique properties, which can be tuned on the nanoscale. Crystallinity, density of defects and impurities are key factors for optoelectronic properties, and are also highly dependent on the materials formation processes for most inorganic semiconductors. Understanding this behaviour and the structure/property relationship is crucial for fundamental understanding of perovskite materials, and for extending their properties to other process-tolerant systems.
The overall objectives of MatchForSolar project fit squarely into current trends in photovoltaic research. More specifically, the developed mechanochemical approach relying on simple grinding of the precursors has emerged as a straightforward and reliable method for preparing large quantities of perovskites. Mechanosynthesis of perovskites allows the introduction of inorganic (A-site cations, B-site metals, X-site anions) as well as organic (passivation agents) species in a controllable manner, both on molecular and nanoparticulate level. Developing a novel, solvent-free synthetic procedure based on grinding in the solid state provides a faster, cleaner and more robust alternative to the solution-based method. The mechanochemical approach also provides an efficient general method for incorporating poorly soluble salts into multi-component perovskite crystal lattices. These studies give an evident possibility for a more in-depth understanding of mechanochemical processes occurring in the solid state.
In addition, mechanochemistry allows the facile synthesis of large quantities of polycrystalline materials that is particularly well-suited for solid-state NMR studies, which can provide direct information about cation dynamics and atomic level phase compositions. For example, 133Cs, 87Rb, 39K, 13C, 2H and 14N solid-state MAS NMR was used to directly probe microscopic composition of Cs-, Rb-, K-, MA-, and FA-containing phases in double-, triple-, and quadruple-cation lead halides in bulk and in thin films. Notably, it has been shown that the structure of bulk mechanochemical perovskites is indistinguishable from that of thin films, making them a good benchmark for structural studies with high sensitivity. These studies highlight the essential need for atomic-level characterization of photovoltaic perovskite materials and provide fundamental understanding of photovoltaic parameters in these systems and their superior stability.
The subjects proposed in MatchForSolar project also include fabrication and photovoltaic characterization of perovskite solar cells. The obtained results demonstrate that the direct thin film crystallization from the lead-based mechanoperovskite is a promising method for achieving solar cells with less hysteresis. This study also opens up new possibilities for the efficient and sustainable synthesis of other lead and lead-free hybrid halide perovskites.

Work performed

MatchForSolar project proposes novel approaches for the development of hybrid inorganic-organic perovskite materials and their composites functionalized by various metal oxides nanoparticles and its further application in the fabrication of photovoltaic devices. In particular, mechanochemistry that refers to chemical processes (mostly of solid substances), induced by the input of mechanical energy such as grinding in ball mills was applied to the synthesis of variety hybrid organic-inorganic perovskites. Mechanochemistry as a powerful method for environmentally-friendly, clean and energy-efficient synthesis way was applied to synthesis of prominent hybrid organic-inorganic perovskite MAPbI3 with well-defined structure and composition. The use of MAPbI3 mechanoperovskite for the thin film formation provides a high degree of control of the stoichiometry and allows for the growth of relatively large crystalline grains. It was demonstrated that such approach applied for preparation of MAPbI3 perovskite material has advantage over a solution based synthetic routes in terms of device performance and VOC. This technique was further used as a powerful tool for searching for a new composition of perovskite materials with extraordinary properties.
A particular effort was also put to understand the physical properties of the resulted perovskite materials. In this context, solid-state NMR has emerged as a particularly useful tool for studying microscopic disorder, cation dynamics and phase segregation in perovskite systems. Multinuclear solid-state MAS NMR (14N, 2H, 13C, 1H, 133Cs, 87Rb and 39K) have been conducted to elucidate cation reorientation dynamics and microscopic phase composition in a variety of hybrid halide perovskites. Finally, these mechanochemically synthesized materials were utilized and thoroughly investigated as light-absorbing materials to fabricate solution-processed solid-state photovoltaic devices. It was demonstrated that the direct thin film crystallization from the lead-based mechanoperovskite is a promising method for achieving solar cells with higher performance and less hysteresis.

Final results

Organicâ€“inorganic lead halide perovskite solar cells are considered as a promising photovoltaic technology for low-cost solar energy conversion due to the rapid progress on the power conversion efficiencies made in the past few years, boosting from 3.8% to 22.7%. However, the use of very toxic Pb element can limits their broad applications and future commercialization. In this context, the development of the synthetis of lead-free perovskites and fabrication of efficient solar cells is highly desirable.
The study performed within MatchForSolar project also opens up new possibilities for the efficient and sustainable mechanosynthesis of lead-free hybrid perovskites. The progress in mechanosynthesis of pure tin Sn(II)/Sn(IV) and mixed tin/lead halide perovskites has been performed. The resulting materials were characterized by powder X-ray diffraction, UV-Vis-NIR and solid state NMR spectroscopy confirming the formation of pure phase compounds. These materials are highly sensitive toward air and moisture, and their processing requires strict condition under inert gas atmosphere. In addition, the mechanosynthesis of a family MA2CuX4-based 2D-perovskites having the general formula A2BX4 was also developed. The properties of these materials such as band gap and the crystal structure were tuned by an anion or a cation substitution. However, the initial performances for these materials were usually lower than those based on Pb based perovskites, and more systematic researches are required.
Overall, the lead free perovskites exhibit great potential for optoelectronic applications, while more efforts are still needed to optimize the properties in realizing stable and highly efficient environmentally friendly solar cells.