Solution-Processed “Silver-Bismuth-Iodine” Ternary Thin Films for Lead-Free Photovoltaic Absorbers

Herein, we present detailed protocols for solution-processed silver-bismuth-iodine (Ag-Bi-I) ternary semiconductor thin films fabricated on TiO₂-coated transparent electrodes and their potential application as air-stable and lead-free optoelectronic devices.

This method can help you answer key questions about how to make solution-processable silver-bismuth-iodine ternary semiconductors for environmentally friendly thin film solar cells and adopt applications. The main advantage of this technique is the solution fabrication of silver-bismuth-iodine which is then used as a lead-free photovoltaic absorber in thin film solar cells with a microscopic device architectures. This technique has potential applications in the production of environmentally-friendly thin film solar cells because the silver-bismuth-iodine ternary semiconductors are lead-free, air-stable photovoltaic absorbers.

To begin preparing the precursor solution for compact titanium dioxide layers, place 8 milliliters of anhydrous ethanol in a 20 milliliter glass vial and start stirring it vigorously. Add 0.74 milliliters of titanium isopropoxide to the stirring ethanol dropwise. And then rapidly add 0.06 milliliters of concentrated hydrochloric acid.

Stir the mixture for between 12 and 24 hours at room temperature to form the precursor solution. Next, sonicate a bare one inch by one inch FTO substrate for 15 minutes each in 2%aqueous octoxynol-9, acetone, and isopropyl alcohol. Dry the clean substrate in an oven at 70 degrees Celsius for one hour and let it cool to room temperature in air.

Then, fix the substrate on a spin coater chuck. Fill a 1 milliliter or 3 milliliter syringe with compact titanium dioxide layer precursor solution and attach a 0.2 nanometer syringe filter. Filter the solution into a small vial.

Apply 200 microliters of the filtered precursor solution to the substrate to fully cover it. Spin coat the substrate at 3000 RPM for 30 seconds. Anneal the film in an oven at 500 degrees Celsius for one hour.

Then turn off the heat and let the substrate cool in air to room temperature, which usually takes about six hours. Next, immerse the coated substrate in an aqueous 0.12 molar solution of titanium tetrachloride. Soak the substrate in a 70 degree Celsius oven for 30 minutes.

Thoroughly rinse the substrate in deionized water afterwards to remove residual titanium tetrachloride. Anneal the film at 500 degrees Celsius for one hour and then allow it to cool to room temperature in air. Once cool, store the compact titanium dioxide-coated substrate under nitrogen gas for later use.

To begin preparing the precursor solution for a mesoporous titanium dioxide nanoparticle layer in a 5 milliliter glass vial, combine 0.5 grams of 50 nanometer titanium dioxide nanoparticle paste with 1.75 grams of isopropyl alcohol and 0.5 grams of terpineol. Add a stir bar to the vial and stir until the paste has completely dissolved. This usually takes about one hour.

Next, fix a compact titanium dioxide-coated FTO substrate on a spin coater and apply 200 microliters of the nanoparticle solution to the substrate surface. Spin coat the substrate at 5000 RPM for 30 seconds. Anneal the coated substrate in an oven at 500 degrees Celsius for one hour and allow it to cool to room temperature.

Then soak the substrate in an aqueous 0.12 molar solution of titanium tetrachloride at 70 degrees Celsius for 30 minutes. Thoroughly rinse the substrate with deionized water. Anneal it at 500 degrees Celsius for one hour and let it cool to room temperature in air.

Store the substrate coated with compact and mesoporous titanium dioxide layers under nitrogen gas for later use. To begin preparing thin films of the silver iodobismuthate, silver dibismuth heptaiodide, in a nitrogen-filled glovebox at low humidity combine 0.3 grams of bismuth-three iodide, 0.06 grams of silver iodide, and 3 milliliters of n-butylamine. Vigorously vortex the mixture until the solids have mostly dissolved, and then syringe filter the precursor solution through a 0.2 micrometer polytetrafluoroethylene filter.

Next, fix the desired substrate on a spin coater and apply 200 microliters of the filtered precursor solution. Spin coat the substrate at 6000 RPM for 30 seconds. Place the substrate on a hot plate and heat it to 150 degrees Celsius.

Anneal the film at that temperature for 30 minutes and then quickly remove it from the hot plate to quench it. In a nitrogen-filled glove box, combine 10 milligrams of P3HT and 1 milliliter of chlorobenzene. Stir the mixture at 50 degrees Celsius for 30 minutes to completely dissolve the P3HT and then filter the mixture with a 0.2 micrometer PTFE syringe filter.

Next, fix an FTO substrate coated with silver dibismuth hepatiodide on compact and mesoporous titanium dioxide on a spin coater. Apply 100 microliters of the P3HT solution to the substrate and spin coat the substrate at 4000 RPM or 30 seconds. Anneal the P3HT film on a hot plate preheated to 130 degrees Celsius for 10 minutes.

Let the substrate cool to room temperature in the glove box. Lastly, use a thermal evaporator to deposit 100 nanometers of gold at 0.5 angstroms per second on the substrate to form the top gold contacts of the solar cell. silver-bismuth-iodine ternary thin films, 1:2, 1:1, and 2:1 molar ratios of silver iodide to bismuth-3 iodide were fabricated with this method.

The 1:2 film showed a single peak at about 42 degrees, indiciating a cubic structure. Peak splitting was observed for the 1:1 and 2:1 films, indicating a hexagonal structure. The 1:2 film absorbed longer wavelengths than the 2:1 film did.

Further, the 1:2 film had a smooth surface with large grains whereas particles of excess silver iodide were observed on the 2:1 film. The 1:2 film was thus chosen for further study. X-ray diffraction indicated that an annealing temperature of 150 degrees Celsius was required for the 1:2 film to crystallize entirely in the cubic phase.

The film was stable in the air for at least 10 days. FTIR spectroscopies suggested that residual n-butylamine remained weakly complexed to bismuth-3 iodide and silver iodide at lower annealing temperatures, suppressing the formation of silver-bismuth-iodine building blocks. The grains became larger and more densely packed as the annealing temperature increased.

Films annealed at 150 degrees Celsius also had the most suitable absorption properties for use in solar cells. Overall, the silver dibismuth heptaiodide film annealed at 150 degrees Celsius showed suitable energetic properties for solar cell use. This technique paves the way for researchers of solution-processable thin film solar cells to further develop methodologies for silver-bismuth-iodine ternary semiconductors in applications such as lead-free and air-stable thin film solar cells.

While attempting this procedure, you may want to use controlled humidity of under 20%for spin coating the substrate with the silver-bismuth-iodine precursor solution. If you spin coat the precursor solution at or above 30%humidity, you will see yellowish because of the highly reactive and photo If you cannot control the humidity in the spin coater, you can spin coat the precursor solution in a N-filled glove box. However, please keep in mind that you must completely purge the glove box after it's done.