Monovalent Cation Doping of CH₃NH₃PbI₃ for Efficient Perovskite Solar Cells

The overall goal of this procedure is to enhance the optoelectronic properties of lead halide perovskite through monovalent cation doping to fabric highly efficient perovskite solar cells. Incorporation of monovalent cations into lead halide perovskite significantly improves the semiconductor quality and photo effect behavior of this material. Adding a rational amount of low cost monovalent cation dopants to perovskite materials enhances charge mobility and reduces energetic disorder by an order of magnitude.

To begin comparing the substrate, use semi-transparent acrylic adhesive tape to cover 2/3 of the conductive side of an FTO coated glass slide. Then coat the uncovered areas with zinc powder. Pour a solution of 2 molar hydrochloric acid in distilled water over the slide to etch the substrate.

Use a cotton swab to wipe the residue from the exposed sections of the slide. Rinse the etched FTO substrate with distilled water and then remove the tape. Wash the etched surface in a 2%weight by volume solution of alkaline liquid detergent concentrate and water.

Sonicate the FTO substrate for 10 minutes each in acetone and isopropanol baths in sequence. Treat the FTO substrate in an oxygen plasma cleaner for 15 minutes to finish cleaning the etched substrate. To deposit the whole blocking compact titanium oxide layer, first place the etched and cleaned FTO substrate on hot plate at 450 degrees Celsius.

Immediately cover the contact area with a precut glass slide and allow the substrate to heat to 450 degrees Celsius. Mix 0.6 milliliters of TAA with 7 milliliters of isopropanol. With the substrate is at 450 degrees Celsius, apply the TAA solution to the substrate by spray pyrolysis using air as the carrier gas.

Allow the sample to remain at 450 degrees Celsius for 30 minutes. Then allow the sample to cool to room temperature. Remove the glass cover once the substrate has cooled.

To deposit the electron transport layer, first dilute 30 nanomolar titanium oxide paste with ethanol to a 2:7 weight ratio. Sonicate the suspension for 30 minutes. Then spin coat the sample with the suspension for 30 seconds at 5, 000 RPM with a ramp rate of 2, 000 RPM per second.

Annihile the titanium film at 500 degrees Celsius for 30 minutes to yield a mesoporous titanium oxide film. Then immerse the sample in a 40 millimolar solution of titanium chloride and distilled water. Treat the samples at 70 degrees Celsius for 20 minutes.

Annihile the titanium chloride treated sample at 450 degrees Celsius for another 30 minutes. Transfer the sample to a nitrogen-filled glove box with a humidity of less than 1%In a low-water, low-oxygen, inert atmosphere, add 1 milliliters of DMF to 553 milligrams of lead iodide. Heat the mixture to 80 degrees Celsius while stirring continuously until the lead iodide has dissolved.

Then prepare a 0.02 molar solution of the chosen monovalent cation halide in the lead iodide solution. Apply 80 microliters of the monovalent cation halide in lead iodide solution to the substrate. Spin coat the substrate for 30 seconds at 6, 500 RPM with a ramp rate of 4, 000 RPM per second.

Bake the resulting thin film at 80 degrees Celsius for 30 minutes. Next, dissolve 40 milligrams of methylammonium iodide in 5 milliliters of isopropanol. Place the lead iodide coated substrate in a spin coater and apply 120 microliters of the MAI solution.

Allow the solution to sit on the substrate for 45 seconds and then spin coat the substrate with MAI for 20 seconds at 4, 000 RPM. Annihile the perovskite film at 100 degrees Celsius for 45 minutes. To deposit the whole transport layer, first add 1 milliliter of chlorobenzene to 72.3 milligrams of spiro-methoxyTAD.

Shake the mixture until the solution appears transparent. Prepare a stock solution of 520 milligrams of lithium TFSI in 1 milliliter of acetonitrile. Then mix 17.5 microliters of the lithium TFSI solution in 28.8 microliters of 4-tert-butylpyridine with the spiro-methoxyTAD solution.

Spin coat the sample with this mixture for 30 seconds at 4, 000 RPM with a ramp rate of 2, 000 RPM per second. Cover the sample with a mask pattern for the top contacts of the solar cell. Deposit an 80 nanometer layer of gold onto the sample at a rate of 0.01 nanometers per second to finish fabricating the solar cell.

Thin films of pristine and additive-based perovskites were compared to the corresponding lead iodide films. When sodium iodide was used as an additive, large branch-shaped crystals of lead iodide were observed by FESEM along with large, asymmetric perovskite crystals. When cooper iodide and silver iodide were used as additives, uniform, pinhole-free perovskite layers were observed.

X-ray diffraction showed that the cation doping had no effect on the perovskite crystal structure. No peak corresponding to unconverted lead iodide was observed when sodium iodide or copper bromide was used as an additive. Kelvin probe force microscopy showed that the doped perovskite fermi levels shifted toward the valence band.

The doped samples showed increased conductivity in electron and whole mobilities, particularly for the sodium iodide and copper bromide samples. Increases were also observed for the short circuit current in the fill factor. The greater short circuit current increase in copper bromide and sodium iodide based cells was attributed to the full conversion of lead iodide and improved charge mobility.

Improvements in open circuit voltage were observed for copper iodide and silver iodide based solar cells, likely because of their uniform pinhole-free surfaces. The power conversion efficiencies of sodium iodide, copper bromide, and copper iodide based solar cells were all notably higher than that of a pristine perovskite cell. We have demonstrated a facile doping method that is compatible with the solution processing of methylammonium lead halide perovskite as an absorber layer in the mesoscopic perovskite solar cell structure.

The optoelectronic and structural properties of lead halide perovskite materials can be tuned by control amounts of monovalent cation dopants, which can lead to superior photovoltaic performance. Our method paved the way for researchers in the field of photovoltaics to explore this technique in other configurations of perovskite solar cells to further improve the electronic quality of perovskite thin films.