Context of the project

- The issue to be addressed

 Recently, the emerging photovoltaic technology based on organic metal halide (OMH) perovskite has attracted a lot of interest due to its significant increase in power conversion efficiency up to 22%. Metal halide perovskites are crystalline materials with the chemical formula ABX3, where A and B are cations and X is an anion, and the overall charge of the resulting crystal is zero. The cation A can be either organic or inorganic, for example, methylammonium (MA+) for an organic cation and Cs+ for an inorganic cation. B is a bivalent metal cation, Pb+2, and X is a halide, usually Cl-, Br-, or I-. However, under the scene of prosperity, many problems still remain unsolved, especially the instability of OMH-perovskite. The extreme sensitivity to oxygen and moisture incurs the constraint of a critical environment for storage, fabrication, and device operation. The notorious problems of photo and thermal stabilities in omh-perovskite materials are also observed due to the instability of organic groups. Moreover, the unavoidable generation of defects and grain boundaries which formed in the process of perovskite film formations reduce the quality of perovskite films and further affect optoelectronic properties of the resulting films. Metal halide perovskite nanocrystals (NCs) have gathered immense attention as materials with highly tunable chemistry and unique optoelectronic properties such as the so-called defect tolerance. They have been implemented in a variety of optoelectronic applications, like light-emitting diodes, solar cells, white phosphors, and solar concentrators. Among them, the Cs based all-inorganic lead halide perovskites, CsPbX3 (X = I, Br, Cl), without a volatile organic component, has resulted of particular interest due to their high potential in terms of thermal stability. However, they suffer of severe chemical and phase stability, especially when targeting narrower bandgap semiconductors because  iodide-containing inorganic perovskite could be easily degraded to nonphotoactive δ-phase under illumination in ambient conditions.

 The most widely used synthesis method for mixed halide inorganic perovskite NC has been hot-injection (HI) synthesis. The HI holds all the control over the reaction environment such as temperature, vacuum, and inert gas-filling, so it is effective to stabilize the desirable perovskite phase of CsPbX3 perovskite NCs. However, at the same time, the HI method presents limitations in terms of scalability and an absolute yield due to the need for vacuum and inert-conditions equipment, a limited solubility in an inert solvent, and the temperature inhomogeneity in the reaction vessel. Thus, a low-temperature synthetic method compatible with ambient conditions would be ideal to improve the productivity of NC synthesis. For a suitable replacement, ligand-assisted reprecipitation (LARP) method has been employed to synthesize various types of perovskite NCs. The LARP procedure simply includes dropping a low amount of perovskite precursors directly into antisolvent which can crystallize perovskite during a strong agitation. In particular, this LARP method can be carried out in an ambient condition and at room temperature and showed high optoelectric properties of the perovskite semiconductor. However, LARP also has low reaction yield because it is based on the principle of solubility difference between antisolvent and perovskite precursor solution. This has hampered so far the use of such methodology for the production of materials and for photovoltaic applications.

- The overall objectives

 FASTEST project aims to develop fully air-stable inorganic metal halide perovskite NCs-based solar cells. It developed strategies for producing air-stable inorganic metal halide perovskite NCs using RT synthesis method. The ligand structure was controlled to make crystal structure suitable for absorption and charge transport. Along with that, chemical doping with a mixed halide ions was incorporated to retain the phase stability and to access the optimized band gap energy for single junction solar cells around 1.9 eV. The synthesized NCs are applied for photovoltaic devices to demonstrate its enhanced operational stability under continuous illumination.

 - Importance of this project for society

 A perovskite nanocrystal solar cell is a type of solar cell which includes a perovskite structured nanocrystals as the light-harvesting active layer. Perovskite nanomaterials are cheap to produce and relatively simple to manufacture especially in the shape of nanocrystals. Perovskites provides a bright future promise for solid-state solar cells due to the intrinsic properties: for example, tunable absorption spectrum, fast charge separation, long transport distance of electrons and holes. Therefore, perovskite solar cells are the rising star in the field of photovoltaics. They are causing excitement within the solar power industry with their ability to absorb light across almost all visible wavelengths, exceptional power conversion efficiencies already exceeding 20% in the lab, and relative ease of fabrication. Perovskite solar cells still face several challenge, but much work such as this FASTEST project is put into facing them and some companies, are already talking about commercializing them in the near future.

FASTEST project

FASTEST project (Fully Air-Processable and Air-Stable Perovskite Solar Cells Based on Inorganic Metal Halide Perovskite Nanocrystals)

Hybrid perovskites represent a new paradigm for photovoltaics, showing the potential of cost-effective fabrication, viable integration for a multi-junction device, and flexible device applications. However, the viability of perovskite solar cells is still far behind commercialization due to difficulties arising from little air-stability and inconsistent power output. The FASTEST project aims to synthesize air-stable inorganic perovskite nanocrystals (NCs) for their application in high-performance photovoltaics. Inorganic perovskite NCs exhibited outstanding optical properties, with photoluminescence quantum yield above 80%, i.e. low charge recombination losses. However, current nanoparticle synthesis methods use bulky, high-boiling point ligands which hamper the formation of high quality optoelectronic thin films, i.e. films with high charge transport and limited recombination, which severely limits possibilities of applications. This project will overcome these hurdles by engineering perovskite NCs by introducing short ligands for room temperature (RT) synthesis and compositional substitution with second metallic ions to stabilize perovskite NCs with an optimal bandgap. Furthermore, to attain air-durability as well as a good dispersion in solution states, novel polymeric passivating materials which protect perovskite NCs from degradation will be incorporated. These will develop effective strategies for enhancing the durability of metal halide perovskite nanoparticles from synthesis scheme to device operations. The technological advancement will be supported by fundamental studies on the photophysical properties of perovskite NCs related with physics of defect and perovskite degradation under controlled conditions of humidity, light, and temperature. This will lead to an understanding of the degradation mechanisms in the perovskite NCs, finally a demonstration of the solution-processable perovskite NCs for flexible large-area PV applications.