Periodic Reporting for period 1 - MatchForSolar (Mechanochemical Approach to Inorganic-Organic Hybrid Materials for Perovskite Solar Cells)
Reporting period: 2016-09-01 to 2018-02-28
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.
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.
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.