Fabrication of Semiconducting Methylammonium Lead Halide Perovskite Particles by Spray Technology
© The Author(s) 2018
Received: 10 October 2017
Accepted: 28 December 2017
Published: 10 January 2018
In this “nano idea” paper, three concepts for the preparation of methylammonium lead halide perovskite particles are proposed, discussed, and tested. The first idea is based on the wet chemistry preparation of the perovskite particles, through the addition of the perovskite precursor solution to an anti-solvent to facilitate the precipitation of the perovskite particles in the solution. The second idea is based on the milling of a blend of the perovskite precursors in the dry form, in order to allow for the conversion of the precursors to the perovskite particles. The third idea is based on the atomization of the perovskite solution by a spray nozzle, introducing the spray droplets into a hot wall reactor, so as to prepare perovskite particles, using the droplet-to-particle spray approach (spray pyrolysis). Preliminary results show that the spray technology is the most successful method for the preparation of impurity-free perovskite particles and perovskite paste to deposit perovskite thin films. As a proof of concept, a perovskite solar cell with the paste prepared by the sprayed perovskite powder was successfully fabricated.
Various forms of the organometal halide perovskites utilizing various cations, such as methylammonium (MA), formamidinium (FA), cesium (Cs), or a combination of thereof, are very attractive photovoltaic materials and are currently widely explored to develop conventional thin-film perovskite solar cells, e.g., [1–4], as well as flexible and low weight to power  and tandem perovskite-based solar cells . MA and FA cations are organic, less stable, and cheaper than Cs, which is a rare metal. While the majority of the research activities on the perovskites focus on thin-film solar cells, such molecular semiconductors could play a role in other similar fields, such as field effect transistors , perovskite light-emitting diodes , and high-energy radioactive radiation sensors .
In most perovskite-based devices, the perovskites are directly deposited in the form of thin films. However, several recent works have reported the fabrication of the perovskite semiconductors in the nanocrystal or particulate form. Perovskite nanocrystals exhibit high photoluminescence quantum yields and quantum confinement effects, analogous to the conventional quantum dots, when their dimensions are reduced to sizes comparable to their respective exciton Bohr radii, bringing about new opportunities for the development of new devices [10–12]. Most of such studies are centered around all-inorganic Cs-based perovskites, owing to their higher stability, e.g., [13–30], followed by organic-inorganic MA-based perovskites, e.g., [31–41], and very few on the FA-based perovskites, e.g., . Most of the abovementioned works have focused on the properties of the perovskite nanocrystals. Some works have fabricated perovskite devices such as perovskite light-emitting diodes that incorporate the nanocrystals in the form of thin films, e.g., [21, 27, 29]. Few works have proposed formulations to prepare perovskite inks such as inks containing lead halide nanocrystals mixed with MA precursors  for the deposition of the thin films for solar cell applications.
The perovskite nanocrystals with rather small sizes and controlled morphology, as reported by the abovementioned works, are commonly grown in the solution (wet chemistry) . Schmidt et al.  prepared colloidal MAPbBr3 nanocrystals with the size of 6 nm by mixing the perovskite precursors with organic solvents. They also prepared homogeneous thin films of these nanoparticles by spin-coating. Hassan et al.  used a two-step solution method to prepare mixed MA-based perovskite nanodots, where first the lead halide seed particles form in the solution and then the MA solution is added in order to complete the process. All-inorganic Cs-based perovskite nanoparticles have been prepared using similar wet chemistry methods, such as injection of Cs precursors into the lead halide precursor solution containing hot, high boiling point solvents . Most of the aforementioned works focus on the fabrication of perovskite nanocrystals, which show a quantum confinement effect. However, for most thin-film devices such as solar cells, the quantum confinement effect is immaterial, and the preparation of polycrystalline micro- and nano-perovskite particles and thin films with facile techniques is desirable.
Presentation of the Hypothesis
In this work, we report the idea and successful preparation of MAPbI3 perovskite particles by low-cost and facile spray technology, for the first time. In this proposed method, following the well-known process of droplet-to-particle formation of pharmaceuticals and ceramics by spray drying and spray pyrolysis, e.g., [43–46], a spray nozzle atomizes the perovskite solution, where the droplets in the form of a mist are introduced into a single- or multi-stage hot wall (tubular) reactor. As the droplets travel along the reactor, the solvent evaporates, a chemical conversion occurs to convert the precursor droplets into the perovskite particles. Therefore, as a result of the presence of a chemical reaction, the process may be called spray pyrolysis. The produced perovskite particles are collected at the outlet of the reactor. The method is capable of producing small particles in the nanometer range, i.e., nanocrystals, if the solution is atomized using specialty atomization techniques, such as electrospray nozzles or low-concentration solutions . In addition, the fragile as-prepared perovskite particles may break down to form nano-sized perovskite particles, to be elaborated on later in this paper.
Testing the Hypotheses
In the anti-solvent method, the perovskite solution was added to toluene dropwise under stirring condition. After 2 min, yellow perovskite powder precipitated at the bottom and sidewalls of the beaker, and after 20 min of stirring, colloidal perovskite powder was observed in toluene, as well. This product (after 20 min) was annealed in an oven at 150 °C for 60 min. Figure 2a shows the X-ray diffraction (XRD; model D5005, Bruker, Germany) of the perovskite powder prepared by an anti-solvent method, where it is evident that the precursors have converted to the perovskite, although some weak peaks, associated with impurities are present.
Testing the idea of blending and milling of the dry perovskite precursors for the preparation of the perovskite powder requires a well-designed milling machine to provide sufficient forces. Here, in order to test the idea, a simple hot plate magnetic stirrer was used. The MAI and PbI2 powders were mixed with the mass ratios of PbI2/MAI of 1 and 2. The hot plate was kept at 200 °C, and the dry powders were blended and crushed in the container due to the force of the magnetic stirring bar. In wet chemistry preparation of perovskite precursor solution, the mass ratio of PbI2/MAI is around 3 (as mentioned above for the preparation of the perovskite solution), whereas in the milling method, we found that lower mass ratios (less PbI2 than stoichiometric) is more effective, in that the reaction of the precursor powders and conversion to the perovskite is improved. Figure 2b shows the XRD patterns of the produced perovskite powder for the PbI2/MAI mass ratios of 1.0 and 2.0. In general, the mass ratio of 1.0 is more successful in producing the perovskite powders; however, traces of impurities are present. This may be due to the insufficient interacting forces between the two precursors that results in traces of the initial precursors mixed up with the perovskite powder. Therefore, the milling approach was not successful in producing pure perovskite structure. Using a well-designed milling machine and careful control of the process parameters, such as the milling time and temperature, and addition of small amount of proper solvents to facilitate the process may improve the purity and the crystalline structure of the powders.
In the spray method, the perovskite solution was atomized with an air-assisted spray nozzle with a nozzle diameter of 0.2 mm, where the air pressure was set to 2.0 psig. The spray droplets were introduced into two vertically stacked stainless steel tubular heaters with a diameter of 10 cm, a length of 30 cm, with the maximum power of 800 W, each (Yancheng Huabang Electric Equipment Co., Ltd). The first heater was kept at 275 °C, so as to quickly evaporate the solvent, and the second or bottom heater was kept at either 275 °C or a lower temperature of 175 °C, where the latter was used to avoid the decomposition of the perovskite powders that had already formed. As Fig. 2c shows, the powder produced when the temperature of both heaters is kept at 275 °C contains high intensity peaks of PbI2, whereas when the temperature of the second heater is reduced to 175 °C, the impurities are nearly disappeared and the crystallinity of the perovskite is increased. In summary, the XRD results of the powders produced using the three abovementioned methods (Fig. 2) substantiate the merit of the spray method for producing pure and crystalline perovskite powders.
Implication of the Hypothesis
In this work, we introduced three ideas for the preparation of perovskite particles and perovskite pastes to produce thin films. It was demonstrated that the powder prepared by spraying of the perovskite solution is crystalline and impurity-free, and has a small particle size and size distribution. Perovskite pastes and thin films were prepared using the aforementioned perovskite powders, where the perovskite film prepared using the spraying technique showed a standard morphology and light absorbance. A mesoporous perovskite solar cell was fabricated using the perovskite film prepared by the sprayed particles, where an efficiency of 2.05% was measured.
The research funding by the Shanghai Municipal Educational Commission and by the National Natural Science Foundation of China (NSFC) are acknowledged. The funding was used to purchase equipment and materials, perform the experiments and analyses, and pay for the publication fees.
Both authors conceived the ideas. MRAY performed the experiments and analyses. Both authors have contributed to the preparation of the manuscript.
The authors declare that they have no competing interests.
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