One-step in situ solution growth for lead halide perovskite

Abstract

A method of forming lead halide perovskite crystals in a solvent. The perovskite is form by solution processing of an organic and inorganic precursor in a polar protic solvent.

Claims

1. A method of forming a lead halide perovskite comprising: dissolving an organic halide precursor in a polar protic solvent; dissolving a lead halide precursor in the solvent to form a reaction solution, the reaction solvent has a concentration of organic halide precursor plus lead halide precursor of at least 40 wt %; reacting the organic halide precursor and the lead halide precursor at a reaction temperature of the solvent's boiling point between 60° C. and 85° C. for at least 48 hours; and precipitating tetragonally-orientated lead halide perovskite crystals free of a substrate.

2. The method of claim 1, wherein the organic halide precursor is selected from methylammonium chloride, methylammonium iodide, and methylammonium bromide.

3. The method of claim 1, wherein the lead halide precursor is selected from lead chloride, lead iodide, and lead bromide.

4. The method of claim 1 wherein the organic halide precursor and the lead halide precursor each comprise an identical halide.

5. The method of claim 1, wherein the organic halide precursor and the lead halide precursor are present in substantially a 1:1 molar ratio in the solvent prior to reacting.

6. The method of claim 1, wherein the polar protic solvent is an alcohol.

7. The method of claim 1, further comprising evaporating the solvent after precipitation of lead halide perovskite crystals.

8. The method of claim 7, wherein the evaporation is in an inert environment.

9. The method of claim 1, wherein within 20%+/− of the boiling point comprises at the boiling point.

10. A method of forming a perovskite comprising: dissolving an organic halide precursor in an alcohol solvent, the alcohol solvent comprising methanol, ethanol, 2-propanol, 1-butanol, or 2-butanol; dissolving a lead halide precursor in the alcohol solvent; forming a reaction solution having a concentration of dissolved organic halide precursor plus dissolved lead halide precursor of at least 40 wt % and the reaction solution having a temperature about an alcohol solvent boiling point, the alcohol solvent boiling point being: a) between 60° C. and 85° C. for methanol, ethanol, and 2-propanol; or b) at least 20° C. for 1-butanol or 2-butanol; reacting the organic halide precursor and the lead halide precursor for at least 48 hours; and precipitating tetragonally-orientated lead halide perovskite crystals free of a substrate.

11. The method of claim 10, wherein the organic halide precursor is selected from methylammonium chloride, methylammonium iodide, and methylammonium bromide.

12. The method of claim 10, wherein the lead halide precursor is selected from lead chloride, lead iodide, and lead bromide.

13. The method of claim 10 wherein the organic halide precursor and the lead halide precursor each comprise an identical halide.

14. The method of claim 10, wherein the organic halide precursor and the lead halide precursor are present in substantially a 1:1 molar ratio in the solvent prior to reacting.

15. The method of claim 10, further comprising evaporating the solvent after precipitation of lead halide perovskite crystals.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The foregoing and other features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several implementations in accordance with the disclosure and are therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings.

(2) FIG. 1 illustrates one embodiment of a method of forming MAPbI.sub.3 perovskite material.

(3) FIG. 2. Illustrates a graph of high-energy synchrotron XRD data for MAPbI.sub.3 perovskite growth in a number of different solvents (A-F) at the noted temperatures for 48 hours.

(4) FIG. 3A shows Pb4f X-ray photoelectron spectrum (XPS) with 4f.sub.7/2 and 4f.sub.5/2 bands at 138.8 and 143.7 eV, respectively, for one embodiment of in situ growth of MAPbI.sub.3 perovskite in 2-propanol at 80° C.; FIG. 3B shows Pb4f XPS for MAPbI.sub.3 perovskite growth using a conventional one-step method for crystal formation prepared from DMF deposited on a Si/SiO.sub.2 substrate and annealed at 130° C., with 4f.sub.7/2 and 4f.sub.5/2 bands at 138.8 and 143.7 eV, respectively. Pb.sup.0 bands are given at 136.9 eV and 141.8 eV.

(5) FIG. 4 (i-iii) compares solid-state NMR (.sup.13C, .sup.1H, .sup.207Pb) quantitative results for MAPbI.sub.3 perovskite crystals showing CH.sub.3, CH.sub.3 and NH.sub.3 and PbI.sub.3 and PbI.sub.2 components, respectively.

(6) FIGS. 5A-5F show SEM images of MAPbI.sub.3 crystal morphology prepared from methanol (FIG. 5A), ethanol (FIG. 5B), 2-propanol (FIG. 5C), 1-butanol (FIG. 5D), 2-butanol (FIG. 5E), and toluene (FIG. 5F), respectively.

(7) FIGS. 6A-6F show SEM images of MAPbI.sub.3 on a large area scale prepared from (a-f) methanol (FIG. 6A), ethanol (FIG. 6B), 2-propanol (FIG. 6C), 1-butanol (FIG. 6D), 2-butanol (FIG. 6E), and toluene (FIG. 6F), respectively.

(8) FIG. 7 is a graph of Fourier transform infrared absorption spectrum in transmission mode for various formulations demonstrating relative chemical stability in air.

(9) FIG. 8 is a graph of thermogram from thermogravimetric analysis (normalized mass in percentage (%) as a function of temperature at 50-450° C.) for various formulations.

(10) FIG. 9 is an image of MAPbI.sub.3 crystals in 2-propanol (a) as a dispersion at room temperature, (b) after tetragonal orientation at 80° C. and precipitated after cooling to room temperature, (c) after separation of 2-propanol resulting in dry powders, and (d) spin-coated MAPbI.sub.3 particulates from 2-propanol solution on a glass substrate (without annealing).

(11) FIG. 10A shows powder XRD studies for MAPbI.sub.3 growth prepared in DMF as a control experiment. FIG. 10A illustrates XRD studies performed at different temperatures (20-150° C.) and reaction time (4-48 hrs).

(12) FIG. 10B illustrates powder XRD studies for MAPbI.sub.3 growth prepared in DMSO as a control experiment. FIG. 10B represents XRD studies performed at different temperatures (20-80° C.) and reaction time (1 h-48 hrs).

(13) FIG. 11 illustrates powder XRD studies for MAPbI.sub.3 growth in methanol prepared at various solution concentration (10-40 wt. %) at 20-60° C. for 48 hrs.

(14) FIG. 12 illustrates powder XRD studies for MAPbI.sub.3 growth in 2-propanol prepared at different temperatures (20-80° C.) and growth time (1 h-48 hrs), given as an example for process optimization.

(15) FIG. 13 illustrates in situ transmission infrared absorbance spectra of the spin-coated MAPbI.sub.3 on a Si/SiO.sub.2 substrate at room temperature (a), further annealed at 40-300° C. in argon (b-j). Each spectrum is referenced to the initial Si/SiO.sub.2 spectrum collected at room temperature.

(16) Reference is made to the accompanying drawings throughout the following detailed description. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative implementations described in the detailed description, drawings, and claims are not meant to be limiting. Other implementations may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and made part of this disclosure.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

(17) As used herein, “micron scale”, “micro scale”, and the like mean having a size of 1.0 microns to 1000 microns. As used herein, “nanometer scale”, “nano scale”, and the like mean having a size of 1.0 nanometers to 1 micron.

(18) Embodiments described herein relate to methods for forming perovskite material by solution processing an organic precursor with an inorganic precursor in a polar protic solvent (i.e. alcohols). For example, described herein are methods for producing methyl ammonium lead halide in polar protic alcohols by focusing on methyl ammonium lead iodide (MAPbI.sub.3) perovskite material. More specifically, some embodiments relate to a “one-step solution” method. In one embodiment, the one-step solution processing method via in situ substrate-free growth to produce MAPbI.sub.3 perovskite crystals. The MAPbI.sub.3 crystals may be micron to nano scale, such as dispersable MAPbI.sub.3 crystals. The same method can be applied to produce other type of methyl ammonium lead halides using appropriate precursors resulting in perovskites such as; methyl ammonium lead bromide (MAPbBr.sub.3) and methyl ammonium lead chloride (MAPbCl.sub.3). The growth method is explained relative to perovskite growth in toluene, which is provided for reference. The materials produced by the methods described herein may be used, for example, in a broad set of solar-layered materials for photovoltaic applications, as semiconductors for the organic LEDs in optoelectronics and nanoscale photonics, photodetectors, lasers, water splitting photocatalysts and thermoelectrics, derived from inexpensive precursors. This method produces tetragonally-oriented MAPbI.sub.3 powders from an alcohol-assisted processing method in one-step. Films can be formed from such MAPbI.sub.3 crystals.

(19) More particularly, the invention essentially introduces an in situ preparation technique for a group of methylammonium lead halide (for example, iodide, bromide or chloride) perovskite crystals or their mixed halides through a one-step growth method which presents a substrate-free approach leading to a crystal pre-growth without annealing for a large-area deposition and a high scale production. One embodiment relates to a temperature-assisted in situ crystallization method based on varying experimental conditions such as temperature, time, solution concentration and choice of the solvent.

(20) In one embodiment, an immediate crystallization occurs in solution with a control of homogeneous crystal growth upon mixing precursors in a solvent followed by a direct precipitation of the perovskite material in solution resulting in dispersions. The solvent is removed leaving perovskite powder.

(21) In particular, the one-step crystallization method may utilize a combination of organic, such as an organic halide, (e.g., methyl ammonium iodide, MAI, CH.sub.3NH.sub.3I) and inorganic, such as an inorganic halide (e.g., lead iodide, PbI.sub.2) precursors. The same method can be used to make MAPbBr.sub.3 perovskite using methyl ammonium bromide (MABr) and lead bromide (PbBr.sub.2), and MAPbCl.sub.3 perovskite using methyl ammonium chloride (MACl) and lead chloride (PbCl.sub.2). The precursors are dissolved in an alcohol, including methyl, ethyl, isopropyl, 1-butyl and 2-butyl. For the reference samples the solvent used was toluene (a non-polar solvent), dimethylformamide (DMF) and dimethylsulfoxide (DMSO).

(22) In one embodiment, methylammonium iodide (CH.sub.3NH.sub.3I) and lead (II) iodide (PbI.sub.2) are introduced at a molar ratio of 1:1 into each anhydrous solvent (for example, methanol, ethanol, 2-propanol, 1-butanol, 2-butanol, and, for comparison, toluene) at a dispersion concentration of 40 wt. %, stirred for 48 hrs. at the boiling points except for 1-butanol and 2-butanol. The black color is observed as soon as mixing precursors at room temperature, however unreacted PbI.sub.2 is monitored by XRD analysis. In order to achieve MAPbI.sub.3 growth at a high yield, the reaction needs to take place at the boiling point of each alcohol. The resulting MAPbI.sub.3 dispersion is measured as pH˜3-4 without adding any acid. Later, the solvent is discarded from the resulting dispersion at room temperature after cooling the solution in nitrogen environment. The powders are further dried in nitrogen, and also kept in vacuum for 3-4 days to obtain powder crystals. The reaction does not happen at a high MAPbI.sub.3 yield when the mass concentration is <40 wt. % in alcohols. When polar aprotic solvents such as dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) are employed, only the intermediate phases and unreacted PbI.sub.2 are observed with conditions of 40 wt. % of concentration at a 1:1 molar ratio of MAI and PbI.sub.2 precursors that is heated at 150° C. (DMF, FIG. 10A) and at 80° C. (DMSO, FIG. 10B). In contrast to the alcohols, perovskite conversion efficiency is found to be relatively low in toluene. MAPbI.sub.3 is found in toluene as soon as the precursors are mixed at room temperature that are visually observed with a black color. However, intermediate phase compounds and significant amount of unreacted PbI.sub.2 (˜41% from ssNMR spectral fits, Table 1) are present, even for those prepared at 110° C. MAPbI.sub.3 growth in DMF and DMSO also resulted in a very low yield having unreacted PbI.sub.2 and intercalation compounds at a large amount (FIGS. 10A-B).

(23) TABLE-US-00001 TABLE 1 .sup.207Pb NMR spectral parameters. CH.sub.3NH.sub.3PbI.sub.3 PbI.sub.2 FWHM (kHz) Fraction FWHM (Hz) Fraction Sample δ (ppm) [FWHM (ppm)] (%) δ (ppm) [FWHM (ppm)] (%) Methanol 1415 ± 5 26.6 [318] 90 Fixed Fixed 10 Ethanol 1413 ± 5 24.6 [293] 85 Fixed Fixed 15 2-Propanol 1416 ± 5 25.2 [300] 96 Fixed Fixed 4 1-Butanol 1434 ± 5 24.9 [298] 99 Fixed Fixed 1 2-Butanol 1414 ± 5 25.5 [305] 89 Fixed Fixed 11 Toluene 1416 ± 5 25.6 [306] 59 −32.7 25.6 [305] 41

(24) The solution then experiences a direct precipitation of the perovskite material in a dispersion leading to the formation of perovskite crystal. The crystal material may be isolated as a powder material once the solvent is removed, such as by first discarding by a Pasteur pipette in an inert environment, such as a nitrogen environment, and then the alcohol is vaporized to dry the powder in vacuum for 4-5 days. The crystal morphology of MAPbI.sub.3 is characterized by SEM (See FIGS. 5A-F and 6A-F), which is highly related to the alcohol reactivity order. The observed grain size variation indicates that as the binding affinity of the alcohol is lowered, the crystal size is reduced from microns to nano scales. For SEM, MAPbI.sub.3 dispersions are spin-coated from each dispersion on a carbon tape at a large area without sonication. The size of the grains is largest when methanol is used (˜≥10 μm), while their grain size is substantially smaller for all other alcohols. The crystals from ethanol are in a range less than ≤10 μm that are mostly in the range of ≥200 nm up to 1 μm, ≥1 μm (with small crystals between 200-500 nm) for 2-propanol, and 100-500 nm for 1-butanol, respectively. MAPbI.sub.3 grown in relatively lower reactivity solvents, 2-butanol and toluene; platelets or fiber-like crystals are obtained. Considering the fiber-like morphology of PbI.sub.2, which is the precursor, the presence of fibers in MAPbI.sub.3 powder from toluene indicates the large amount of unreacted PbI.sub.2.

(25) In one specific implementation, a solution growth technique thereby allows formation of methylammonium lead halide perovskites. In one such embodiment, the perovskite are formed in solution resulting a pH −3-4), when the precursors are mixed at a concentration of 40 wt. %, heated to the solvent boiling point (60° C. for methanol, 85° C. for ethanol, 80° C. for 2-propanol, 110° C. for toluene), for example in a glass vial sealed with a teflon cap, and are further cooled down to the room temperature. At 10 wt. % of solution concentration, MAPbI.sub.3 crystals are not present in methanol even though heated at the boiling point of 60° C. (slide 12). MAPbI.sub.3 crystals form at 20-40 wt. % of solution concentration, however the reaction results in formation of intermediate phases and unreacted PbI.sub.2. To minimize unreacted PbI.sub.2 and increase the product yield, 40 wt. % concentration is found to be the possible highest saturated concentration for MAPbI.sub.3 growth. Although MAPbI.sub.3 crystals form at ≤60° C. in 2-propanol as an example (slide 13), intermediate by-products and unreacted PbI.sub.2 are present. The crystallization occurs at a higher yield at 60° C. without showing any intermediate phase however there is remaining unreacted PbI.sub.2. For this reason, the solvent boiling point (80° C.) is the best process condition to obtain a tetragonal orientation of MAPbI.sub.3 in 2-propanol with minimized unreacted PbI.sub.2. For MAPbI.sub.3 growth in 1-butanol and 2-butanol, tetragonal orientation of crystals is observed at room temperature after mixing for 48 hrs. Therefore, these reaction only takes place without applying any heat. The purity of the precursors, their anhydrous nature and solubility, polarity of the alcohol, the reaction concentration, time and type of alcohol are critical parameters for the reaction yield to increase. In one embodiment, the reaction temperature is kept at the boiling point of the alcohol(s) (or, generally, the solvent), in one embodiment the reaction temperature is plus/minus 0%-25% (inclusive) of the boiling point.

(26) Perovskite growth is also possible at room temperature using 1-butanol or 2-butanol under continuous stirring via a continuous nitrogen purge in a glove box. It has been observed from the remaining amount of alcohol after the reaction indicates that the alcohol reacts with the precursors, particularly with MAI through a hydrogen bond formation for MAPbI.sub.3 growth. Nevertheless, the alcohol is not consumed in the reaction, instead catalyzes the reaction to proceed with a high yield of perovskite formation. Once the solvent is removed, such as by evaporation through a continuous stirring in nitrogen environment in the glass vial for 4-5 days, the resulting dry powder is further dried in a vacuum desiccator overnight for a complete solvent removal. Consequently, the color of the resulting perovskite solution and the dry powder are black for methylammonium lead iodide. The same general reaction can form methylammonium lead bromide and methylammonium lead chloride, with the appropriate precursors as noted herein, and result in orange and pale yellow colors, respectively.

(27) In one embodiment, a solution growth technique results in dispersive perovskite crystals in alcohols that not only eliminates deposition of a film onto a substrate and avoids heating to induce crystallization on the substrate, and air/thermal stability of the perovskite crystals.

(28) In one embodiment, the method utilizes a solvent. The use of a variety of low boiling point alcohols (20-80° C.) is also advantageous over high boiling point solvents such as dimethylformide (“DMF”). For example, due to the low boiling point of methanol, ethanol and 2-propanol, the preparation method is easy to dry, attributed to high volatility of such alcohols, environmentally benign, free from high levels of solvent toxicity as compared to other solvents such as benzene, chloroform, etc.

(29) While a substrate is not required for the formation of perovskite as described above, the resultant perovskite materials can be deposited on a substrate. In one embodiment, perovskite crystal dispersions can be deposited on a flexible or bendable substrates (for example, but not limited to plastics, mica, etc.) through inexpensive techniques such as, but not limited to, spin-coating, spray-coating, paint brush, ink jet printing, drop casting, stamping techniques, etc. Further, the ability to be used on a flexible substrate opens the possibilities for flexible products such as wearables, clothing, smart textiles and the like. Thus, in some embodiments, the in situ technique also provides a complete surface coverage of any desirable substrate with a controlled film thickness and excellent uniformity for flexible solar cell fabrication. Such perovskite dispersions in alcohols can be also readily deposited on rigid substrates such as Si/SiO.sub.2, metals, or crosslinked polymers by using the same techniques. To fabricate thin films from these perovskite powders, new methods need to be further explored for various applications.

(30) In addition to use in traditional deposition techniques on flexible or rigid substrates, the perovskite dispersions allow for other uses. For example, the dispersions can be dried and the perovskites stored in the forms of dry powder. Further, perovskites created according to some embodiments can be used with sticky carbon tapes for SEM measurements or on tantalum foils for XPS studies. The perovskite crystals in powders can be redispersed in each specific solvent and used as dispersions as a reverse engineering for required applications.

(31) Further, as embodiments herein can be used to form different perovskites, each as a powder, they can be possibly mixed with other powder materials of TiO.sub.2 (electron transport “ETL” layer) or meso-structured TiO.sub.2/AlO.sub.3 or even direct use with other hole transport layer (“HTL”) materials such as graphene oxide or reduced graphene oxide for solar cell applications.

(32) Further, the methods of perovskite material formation may be utilized to provide a perovskite layer on a graphene oxide or reduced graphene oxide substrate, such as a GO or RGO thin film in a solar cell stack. Perovskites, particularly lead halide perovskites, have been shown to exhibit poor formation on GO thin films. It is believed that the graphene oxide reacts with the perovskite during the annealing step of traditional perovskite growth on a thin film. However, the perovskite crystal materials can be deposited on a GO thin film, such as by solution processing through deposition in a solvent and evaporation of the solvent or by other known deposition techniques. Thus, the perovskite is deposited onto the GO rather than grown from precursors on the GO, without the need for an annealing step and voiding the reactive problems presented by such.

(33) The availability of perovskite materials as both dispersions and as dry powder materials provides for flexibility in analytics and characterization as well. For example, the perovskite materials may be solution processed to broaden the applications available for characterization. For instance, solids are excellent sources for the solid state .sup.1H, .sup.13C, .sup.15N, and .sup.207PbNMR techniques to understand their molecular structures, and the powder materials are easy to use for XRD, TGA, ATR and photoluminescence studies. The easy processability as dispersions also provides opportunity for in situ High Energy XRD studies. FIGS. 10A-10B illustrate powder XRD studies for MAPbI.sub.3 in DMF (FIG. 10A) and DMSO (FIG. 10B), MAPbI.sub.3 in 2-PrOH as a function of solution concentration (FIG. 11), and MAPbI.sub.3 in methanol (FIG. 12) to test different temperature and reaction time. The micron-sized particles of these perovskites allow large grain boundaries in a film-like morphology with particulates by simply varying the spin-coating parameters that also leads to a better control of the perovskite deposition. As the binding site availability increases (most difficult with methanol and the easiest with 2-butanol), the particle size is reduced to nanometer scales (−<500 nm). Room temperature formation of perovskites in 1-butanol and 2-butanol also results in avoidance of the heat during crystal growth in solution.

Examples

(34) A number of experiments were undertaken with regard to perovskite formation. Several lead halide perovskites were investigated. Methylammonium lead iodide perovskite is characterized by a black color, Methylammonium lead bromide perovskite is characterized by an orange color. Methylammonium leadchloride perovskite is characterized by a pale yellow color. Mixed halides could also be observed and identified based on color combinations.

(35) Materials:

(36) The precursors, namely, methylammonium iodide (CH3NH3I, Assay: 98%, CAS #14965-49-2, SKU #793493-5g or 806390-25G), methylammonium bromide (CH.sub.3NH.sub.3Br, Assay: 98%, CAS #6876-37-5, SKU #793507-5g or 806498-25g), methylammonium chloride (CH.sub.3NH.sub.3Cl, CAS #593-51-1, SKU #8.06020.1000), lead (II) iodide (PbI.sub.2, CAS #10101-63-0, SKU #203602-50g, 99.999% trace metals basis from Sigma-Aldrich or CAS #14965-49-2, ITEM #MSIOI000-50 from Dyesol Inc.), lead (II) bromide (PbBr.sub.2, CAS #10031-22-8, SKU #398853-5G, 99.999% trace metals basis), and lead (II) chloride (PbCl.sub.2, CAS #7758-95-4, SKU #203572-50g, 99.999% trace metals basis) have been purchased from Sigma-Aldrich or Dyesol Inc. The solvents, namely, methyl (CH.sub.3OH, anhydrous, Assay: 99.8%, CAS #67-56-1, SKU #322415-100 ml), ethyl (CH.sub.3CH.sub.2OH, anhydrous, non-denatured, absolute, 200 proof, Assay: 99.5%, CAS #64-17-5, SKU #459836-100 ml), isopropyl (CH.sub.3CH(OH)CH.sub.3, anhydrous, Assay: 99.5%, CAS #67-63-0, SKU #278475-100 ml), 1-butyl (CH.sub.3CH.sub.2CH.sub.2CH.sub.2OH, anhydrous, Assay: 99.8%, CAS #71-36-3, SKU #28 1549-100 ml), and 2-butyl (CH.sub.3CH.sub.2CH(OH)CH.sub.3, anhydrous, Assay: 99.5%, CAS #78-92-2, SKU #294810-100 ml) alcohols and toluene (C.sub.6H.sub.5CH.sub.3, Assay: 99.8%, CAS #108-88-3, SKU #244511-100 ml) have been purchased from Sigma-Aldrich.

(37) The first experiment was performed with trials in DMF for all methylammonium lead iodide/bromide/chloride perovskites and studies continued by searching the role of DMF and understanding the chemical interactions with the precursors (methylammonium iodide/bromide/chloride and lead (II) iodide/bromide/chloride) in DMF, followed by the conventional approach deposited on Si/SiO.sub.2 and annealed on a hot plate.

(38) An experiment was undertaken with methylammonium iodide with lead (II) iodide at a concentration of 40 wt. % in 2-propanol at room temperature with overnight stirring in nitrogen environment. A black color, indicative of perovskite formation, was observed. The solvents such as DMSO and DMF were also tested at room temperature with overnight stirring in nitrogen environment but the perovskite formation could not be observed at room temperature.

(39) Additional experiments were done in methanol and ethanol following the same experimental procedure in the case of 2-propanol. The solutions were heated to 60° C. and the time of the experiment was tested for 4 hours and 20 hours of continuous stirring in nitrogen environment. First in situ High Energy Syncrothron XRD tests were performed to evaluate formation of methylammonium lead iodide/bromide/chloride in DMF solution (yellow, colorless, and white, respectively) together with methylammonium lead iodide (black) solutions in 2-propanol prepared at room temperature. Particles of methylammonium lead iodide were noticed in these black solutions in 2-propanol.

(40) Next, methylammonium lead bromide and methylammonium lead chloride perovskite solutions were prepared in methanol, ethanol and 2-propanol at room temperature with an overnight continuous stirring. Orange and pale yellow solution colors were observed.

(41) Solutions were also studied at a concentration of 10, 20 and 30 wt. %. These did not result in a perovskite formation in methanol, ethanol and 2-propanol at a high yield. Further, the experiments were tested for methylammonium lead iodide in toluene at room temperature which did not result perovskite growth at a high yield. However, after heating toluene to near a boiling point of 100-110° C., a black color was observed.

(42) A further experiment prepared methylammonium lead iodide/bromide/chloride solutions in methanol, ethanol, 2-propanol and toluene, prepared at the solvent boiling point overnight. The resultant solution was tested with in situ High Energy Syncrothron XRD.

(43) Methylammonium lead iodide/bromide/chloride perovskite growth was studied in methanol, ethanol and 2-propanol at their boiling points of 60° C., 75° C., and 80° C., respectively for 48 hours of continuous heating and stirring in nitrogen environment which was confirmed as the optimum process condition using XRD upon spin-coating on Si/SiO.sub.2.

(44) Methylammonium lead iodide/bromide/chloride perovskite formation was also confirmed in toluene at −110° C. with a 2-day (48 hours) heating and stirring.

(45) Another set of experiments was performed by mixing halides of methylammonium lead iodide/bromide (at a concentration of 40 wt. %, with molar ratios of I to Br at 1:2) methanol, ethanol and 2-propanol at 60° C. for an overnight heating and stirring in nitrogen environment. A black/brown color was followed a dark red color in solution.

(46) In another experiment, methylammonium lead iodide perovskite formation was first observed in 1-butanol and 2-butanol with a black color after mixing at room temperature for 48 hours in nitrogen. Further experiments were performed for methylammonium lead bromide/chloride in 1-butanol and 2-butanol at room temperature with orange and pale yellow colors, respectively. Additional experiments with all samples prepared at their solvent boiling points for 48 hours were tested in solution using High Energy XRD. Mixed halides such as methylammonium lead iodide/bromide, iodide/chloride and bromide/chloride were also studied in all the solvents heated at the boiling point for 48 hours at a solution concentration of 40 wt. % at a halide molar ratio of 1:2, and tested using High Energy XRD. Mixing iodide/bromide, iodide/chloride or bromide/chloride resulted in red, gray, and bright yellow colors in solution, confirming the perovskite formation.

(47) The resulting perovskite material was characterized in solution transferred into a capillary tube sealed by an epoxy in a nitrogen glove box by High Energy Syncrothron XRD techniques. The spin-cast perovskite material is also characterized on a Si/SiO.sub.2 substrate by a powder X-Ray technique as well as Fourier Transform Infrared (FTIR) in transmission or reflection geometry (ATR), UV-Vis-NIR (% Reflection), Thermogravimetric Analysis, and Scanning Electron Microscopy. Their air/moisture/humidity tests have been followed by exposing the material to air in a fume hood, and characterized by the FTIR method, see FIG. 13. Indeed, their thermal and chemical stability is studied by in situ FTIR where the material is heated at 20-300° C. for 30 minutes at a ramp rate of 10° C./min. in a continuous nitrogen gas purge. All the measurements were performed at room temperature once the material was cooled down to room temperature under same test conditions with a simultaneous measurements done at a Brewster's angle (70°) with respect to the normal mode operation.

(48) As used herein, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, the term “a member” is intended to mean a single member or a combination of members, “a material” is intended to mean one or more materials, or a combination thereof.

(49) As used herein, the terms “about” and “approximately” generally mean plus or minus 10% of the stated value. For example, about 0.5 would include 0.45 and 0.55, about 10 would include 9 to 11, about 1000 would include 900 to 1100.

(50) It should be noted that the term “exemplary” as used herein to describe various embodiments is intended to indicate that such embodiments are possible examples, representations, and/or illustrations of possible embodiments (and such term is not intended to connote that such embodiments are necessarily extraordinary or superlative examples).

(51) The terms “coupled,” “connected,” and the like as used herein mean the joining of two members directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another.

(52) It is important to note that the construction and arrangement of the various exemplary embodiments are illustrative only. Although only a few embodiments have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter described herein. Other substitutions, modifications, changes and omissions may also be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present invention.

(53) While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what may be claimed, but rather as descriptions of features specific to particular implementations of particular inventions. Certain features described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.