Organic-inorganic halide perovskites, such as, for example, the methylammonium lead halides, i.e., (CH3NH3)PbX3, where CH3NH3 corresponds to the methylammonium cation and X is a halide, are a broad class of photoactive materials with applications to solar energy with device efficiencies typically exceeding 20%. The class of materials, hereafter referred to as halide perovskites, are distinguished by their AMX3 perovskite structure, wherein A is an organic molecule, typically methylammonium, tetramethylammonium, guanidinium, or formamidinium; M is typically tin or lead; and X is a halide, such as fluoride, chloride, iodide, or bromide.
A particular advantage of these materials is that they can be produced and processed at or near ambient conditions from solution. The facile nature of the process makes these photoactive materials relatively inexpensive to produce, which is a critical advantage when considering these materials for large-scale commercial production. In the conventional process, the precursor components are mixed in a solvent containing a volatile organic component, hereafter referred to as a VOC, and the resulting precursor solution are deposited on a substrate, followed by heating the precursor solution at a temperature sufficient to react and convert the precursor species into the perovskite composition.
However, there are several challenges and drawbacks in the VOC process, all of which result in substantial imperfections in the final perovskite film, particularly in crystallinity, grain size, and crystal orientation. These imperfections lead to non-optimal or poor device performance. Thus, proper control of the growth of the halide perovskite is critical for producing high-quality crystalline films, which would improve device performance. However, the VOC process poses significant limitations in the effort to achieve high-quality crystalline films. The VOC process is also generally hampered by incomplete coverage of the substrate, low crystallinity, and inconsistent (non-uniform) film thickness.
Generally, the conventional VOC process begins with dissolving two or more salts in a common solvent. For example, to make methylammonium lead iodide, lead chloride (PbCl2) and methylammonium iodide (CH3NH3+I−) in a 1:3 molar ratio can be dissolved in a polar aprotic solvent, such as dimethylformamide (DMF). The resulting solution is deposited on a substrate and then heated to about 100° C. for about 45 minutes. At 100° C., films of acceptable quality generally need to be grown for at least 40 minutes but no longer than 60 minutes. Generally, longer processing times are not possible in the VOC process because the volatility of the organic solvent limits the time available for processing. For any given combination of salts in any specific solvent, the length of time is substantially limited by the time available until the solvent completely evaporates. The crystallization and growth of thin films from solution is a function of supersaturation; for the VOC process this supersaturation is due to evaporation of the solvent. The growth time available is the time between enough evaporation for supersaturation to occur and the time at which evaporation is substantially complete, after which, the perovskite film begins decomposing if the processing temperature is continued. Thus, using the VOC process, the processing time is generally fixed, which in turn hampers any effort to employ longer processing times with the precursor components remaining in solution.
In the case of (CH3NH3)PbI3, the crystallization completes at about 45 minutes, but decomposition begins within 5-10 minutes after that if left at the processing temperature. This is the source of the challenges discussed above. Although the growth time can be altered by changes to the salt ratios, solvent mixture, and temperature, for any given combination of these three processing conditions, there will be an exact crystallization time, and thus, the length of time for which a film can be continued to be processed is very short. The stability of the films, particularly with regard to decomposition, is dramatically affected by the short time window. Moreover, slight variations in processing time result in large changes in the resulting film quality. For this reason, the VOC process results in perovskite films of inconsistent film quality.
Another problem with the VOC process is the tendency of the solvent to dewet the substrate. Dewetting of the substrate contributes to incomplete film coverage and lack of uniformity. Earlier efforts focused on tailoring the solvent-salt and/or solution-substrate interactions to limit the dewetting effect. These approaches sought to induce homogeneous nucleation using a high nucleation density. The higher density of the crystal nuclei was expected to allow the crystal grains to grow to impingement before the solvent evaporation was substantially complete, which would improve substrate coverage. However, the nucleation density also determines the maximum grain size; i.e., if the nuclei are spaced 500 nm apart, then the grains can only grow to 500 nm before they impinge on each other and grain growth stops.
In addition to the above concerns, there is the problem of a solid-state intermediate phase that occurs in the VOC process. The existence of this intermediate defines the perovskite growth as a solid-state transformation. The solid-state transformation makes the process even more challenging since rearranging atoms and molecules becomes significantly more difficult in the solid state than in solution.
Lastly, the VOC process generates byproducts, such as unreacted reagent salts and solvent vapor, that are generally not recoverable. Particularly when considering commercial scale production, such byproduct formation amounts to significant waste and financial losses.
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