Low Pressure Vapor-assisted Solution Process for Tunable Band Gap Pinhole-free Methylammonium Lead Halide Perovskite Films

Here, we present a protocol for the synthesis of CH₃NH₃I and CH₃NH₃Br precursors and the subsequent formation of pinhole-free, continuous CH₃NH₃PbI_3-xBr_x thin films for the application in high efficiency solar cells and other optoelectronic devices.

the overall goal of this process is to synthesize homogenous pinhole-free and high quality methylammonium lead halide perovskite films over the full halide composition space for application in optoelectronics such as solar cells, lasers and LEDs. This synthetic protocol is designed such that it can be adapted to different laboratories and it provides a synthesis route to make faster and reproducible lead halide perovskites over the full highlight composition space. The main advantage of this technique is the use of low processing temperature and standard equipment available in most laboratories such as, fume hoods and schlenk lines.

Demonstrating the procedure will be Aya Buckley and Phuong Ngo, students from UC Berkeley. To begin this procedure, add 100 milliliters of ethanol to a 250 milliliter round bottom flask equipped with the stir bar. Add 16.5 milliliters of 40%methylamine in water.

Using an ice bath, cool the flask to zero degree celsius. Add 10 milliliters of 76 millimolar hydroiodic acid drop wise, while stirring. Then, seal the flask with a septum.

Stir the reaction for two hours at zero degree celsius. Then, remove the flask from the ice bath using a rotary evaporator equipped with a water bath, evaporate the solvent in unreacted volatile components at 50 Torr and 60 degree celsius for four hours or until the volatiles are removed. Once the solvent has evaporated, add 100 milliliters of warm ethanol to the flask to dissolve the residual material.

Slowly add 200 milliliters of diethyl ether to induce the crystallization of a white solid. After this, vacuum filter the mixture over a 50 millimeter glass frit filter. Recover the supernatant and add 200 milliliters of diethyl ether to induce additional crystallization.

Vacuum filter the mixture again over the 50 millimeter glass frit filter. While vacuum filtering, wash the solid three times with diethyl ether. Dry the washed solid under vacuum.

When cleaning the apparatus, the quality of starting material is also important. Therefore, careful recrystallization of the material volume precursor is particularly important. Store the dried solid in the desiccator in the dark at room temperature, to minimize decomposition.

To begin, preheat a silicone oil bath equipped with a magnetic stir bar to 120 degree celsius. Next, use a weighing paper cylinder to transfer 0.1 grams of methylammonium halide to a 50 milliliter schlenk tube. To precondition the methylammonium halide, attach the tube to a schlenk line equipped with the rotary pump and evacuate the tube.

Adjust the pressure to 0.185 Torr. Then, immerse the tube in the silicone bath for two hours while stirring the bath at 600 rpm. After this, remove the schlenk tube from the oil bath.

Leave the tube under an overpressure of flowing nitrogen gas, to prevent moisture intake. Then make the precursor solutions by dissolving 0.146 grams lead bromide in one milliliter DMF, and then 0.369 grams of lead iodide and 0.073 grams of lead bromide in one milliliter of DMF. Sonicate these for approximately five minutes at 35 kilohertz to fully dissolve the precursor.

Using a 0.2 micron PTFE filter, filter the precursor solution. Heat the filtered solution to 110 degree celsius for five minutes. Using a micropipette, deposit 80 microliters of heated lead halide precursor solution onto the substrate.

Then, spin the substrate at 500 rpm for five seconds with an acceleration rate of 500 rpm/s and at 1500 rpm, for three minutes with an acceleration rate 1500 rpm.s. Next, transfer the substrate onto a hot plate in a fume hood. Dry the precursor film at 110 degree celsius for 15 minutes under flowing nitrogen gas.

Load the dried substrate into the prepared schlenk tube orienting the lead halide surface away from the methylammonium halide in the tube. Adjust the pressure in the schlenk tube to 0.185 Torr. Then, immerse the tube in the silicone oil bath heated to 120 degree celsius for two hours.

After this, remove the tube from the oil bath. Remove the sample from the schlenk tube and quickly dip it into a beaker containing isopropyl alcohol. Use an N2 gun to immediately dry the rinsed sample.

In this study, pinhole-free methylammonium lead halide thin films are synthesized by LP vasp and subsequently characterized. NMR analysis of the precursors shows methyl group shifts at delta 2.35 pbm and delta 2.37 pbm. An ammonium shift centered at delta 7.65 pbm and delta 7.45 pbm, for methylammonium bromide and methylammonium iodide respectively, confirming their identity and purity.

Scanning electron microscopy is then used to analyze mixed flat halide films and kneeled in 100%50%and 30%methylammonium iodide. Each of the faceted films is seen to be pinhole free with grain size up to 700 nanometers. While the standard thickness of the perovskite films is about 400 nanometers, the thickness can be changed by varying the spin coating rotational speed with higher speeds yielding thinner films and vice versa.

X ray diffraction patterns are then collected to confirm phase purity and conversion of the precursors to methylammonium lead halide perovskite films. Residual or not converted lead iodide phase is seen to show a peak at approximately 12.7 degrees. The methylammonium lead halide films are seen films to exhibit a shift to higher angles due to a gradual replacement of the larger iodine atoms by smaller bromide atoms.

While attempting this procedure, please keep in mind that the use of a schlenk line includes explosion and implosion risks. So, before you perform this experiment, please check the integrity of the apparatus. The development of this technique paves the way for researchers in the field of thin film solar cells, as well as, optoelectronics to synthesize high quality organometal highlight perovskite thin films.

With this process, you can make thin film solar cells in planar geometry, reaching efficiencies of up to 19%and they show enhanced long term stability. After watching this video, you should have a good understanding of how to synthesize homogenous, phenofree, high optoelectronic quality, material volume lead halide film by LP vasp.