The overall goal of this comprehensive study is to present how different fabrication methods for organic inorganic perovskite thin films influence crystal structures, density of states, energy levels, and solar cell performance. This study can help answer key questions in the field of hybrid organic inorganic perovskites, specifically how preparation methods could influence various thin layer properties. Our approach is to categorize differently fabricated perovskite layers using methods such as photoelectron spectroscopy, scanning electron microscopy in order to monitor film composition morphology and or density of states.
A visual demonstration of the processing and characterizing steps as critical as minor variations in film fabrication can have major effects on the quality of the thin film. Demonstrating the procedure will be Ines Schmidt, Tobias Schnier, and Jennifer Emara, one master and two PhD students from my group. Ines and Tobias are focusing on solution processing, whereas Jenny studies vacuum deposited layers.
To prepare the conductive anode material to be used as the bottom contact, first etch, clean, and ozonize 2.5 by 2.5 square centimeter pieces of Indium Tin Oxide coated glass. Next, draw at least 150 microliters of 1.5%PEDOT:PSS in water into a syringe. Attack a 0.45 micron filter to the syringe tip.
Mount an ITO glass substrate on a spin coater in air with a relative humidity of 30 to 50%Dispense 150 microliters of the filtered PEDOT:PSS suspension onto the substrate's surface. Spin coat the substrate at 2, 500 RPM for 25 seconds, and 4, 000 RPMs for five seconds, with an acceleration of 4, 000 RPM per seconds. Heat the spin coated substrate at 150 degrees Celsius on a hot plate for 10 minutes to remove residual water from the PEDOT:PSS film.
To prepare the perovskite coated substrate by vapor deposition, first place the PEDOT:PSS coated substrate into the instrument vacuum chamber, and evacuate the chamber. Once the chamber pressure reaches 10 to the negative seven millibar, move the substrate to the evaporation chamber. Ensure that the sample shutter is closed, and move the tooling quartz crystal microbalance to in front of the sample position.
Heat the lead iodide source to 320 degrees Celsius, and the methylammonium iodide or MAI source to 190 degrees Celsius. Monitor the increase in chamber pressure caused by MAI evaporation. Then for each source deposit a film onto the QCM positioned near the sample, and simultaneous record the thicknesses shown on the source and sample QCMs.
From this, calculate the tooling factors for lead, iodide, and MAI. To perform co-evaporation, adjust the source temperatures so the ratio of deposition rates is approximately one to two lead iodide to MAI. Then open the sample shutter and deposit the perovskite film.
Alternatively, to perform sequential evaporation, adjust the source temperatures to heat only the lead iodide source. Open the sample shutter, deposit a 50 nanometer layer of lead iodide on the substrate, and close the sample shutter. After deposition of the lead iodide layer, reduce the lead iodide source temperature, and head the MAI source to the evaporation temperature.
Deposit a 50 nanometer layer of MAI onto the pre-covered substrate. Alternate deposition of lead iodide and MAI layers until the intended thickness is reached. Once the film reaches the intended thickness by either method, close the shutter and turn off the source heaters.
Move the sample to a heating stage in the vacuum chamber, and anneal the film at 70 degrees Celsius for one hour to remove volatiles and complete the perovskite formation. To create the perovskite by sequential deposition, first place a PEDOT:PSS coated substrate in a spin coater in a nitrogen filled glove box. Using a pipette, drop 150 microliters of a 400 milligram per milliliter solution of lead iodide and DMF on the substrate.
Spin coat the substrate at 3, 000 RPM for 30 seconds. Then either drop 150 microliters of a 10 milligram per milliliter solution of MAI and isopropanol onto the substrate's surface and allow it to sit for 40 seconds, or submerge the substrate in the MAI solution for 40 seconds. Remove excess solution from the substrate by spin coating for 30 seconds at 3, 000 RPM.
Anneal the substrate on a hot plate at 100 degrees Celsius for 15 minutes to finish the sequential deposition process. To deposit the perovskite film with the co-solution method, in a nitrogen filled glove box, dissolve sufficient lead iodide and MAI in the desired ratio in DMF to obtain a 250 milligram per milliliter precursor solution. If a molecular additive is needed, add sufficient ammonium chloride to achieve a concentration of 18 to 20 milligrams per milliliter in the precursor solution.
Stir the precursor solution at 50 degrees Celsius for five hours. Then preheat a PEDOT:PSS coated substrate for five minutes on a hot plate at 50 degrees Celsius. Place the warmed substrate in the spin coater.
Add 200 microliters of toluene to the spin coater bowl to create a toluene atmosphere during spin coating. Pipette 150 microliters of precursor solution onto the substrate, and spin coat at 3, 000 RPM for 30 seconds. Heat the spin coated substrate for 30 seconds on a hot plate at 110 degrees Celsius to finish the co-solution process.
The quality of the perovskite film can be assessed visually based on the shininess of the surface. To proceed with solar cell fabrication, in a nitrogen filled glove box, prepare a 20 milligram per milliliter solution of 60 PCBM in chlorobenzene as the acceptor material. Stir the solution at 50 degrees Celsius for at least 24 hours.
Then place a freshly thermally annealed perovskite coated substrate on a metal plate for 30 seconds to cool it to room temperature. Mount the cooled substrate in a spin coater. Place 150 microliters of 60 PCBM solution onto the substrate, and spin coat at 2, 000 RPM for 30 seconds to form the acceptor layer.
Next, place the substrate in a vapor deposition sample holder and cover the substrate with a shadow mask. Scratch one of the uncovered contact locations with a scalpel to expose the ITO layer. Then place the masked substrate in the vacuum chamber, and prepare for vapor deposition of aluminum.
Deposit 10 nanometers of aluminum at a rate of 0.5 angstroms per second at most with a maximum pressure of three times 10 to the negative six millibar. Then increase the rate to 2.5 angstroms per second, and continue deposition until the aluminum layer is 100 nanometers thick to form the aluminum cathode top contact. Transport the cell to a solar simulator setup, and sweep from negative 0.5 to positive 1.5 volts in 0.02 volt steps.
Then scan in the reverse direction to check for hysteresis. Perovskite thin films were prepared with each of the described methods and with varying ratios of lead iodide to MAI. Scanning electron microscopy of the vapor deposited films and the films prepared from co-solution with an additive showed the desired smooth pinhole free surface.
Variation in morphology was observed with changes in the lead iodide to MAI molar ratio. The films prepared from co-solution without an additive, and the films prepared by dip and drop coating showed less desirable voids, surface roughness, and needle like structures. X-ray diffraction of six co-solution samples with different lead iodide to MAI molar ratios all showed the typical tetragonal crystal structure.
No additional phases of MAI or lead iodide were observed, indicating the x-ray diffraction methods could not provide information about film composition. X-ray and UV photo electron spectroscopy were used to assess the stoichiometry and ionization energies respectively of the thin films. A trend of increasing ionization energy with increasing lead iodide content was observed across all methods, indicating that the effect is independent of the preparation method.
The solar cell capabilities of thin films with various lead iodide to MAI molar ratios were assessed. The highest efficiency was observed with a molar ratio of 1.02 which is closest to the stoichiometric perovskite composition. Generally, individuals new to this field of hybrid perovskite will struggle because film formation is very sensitive to the preparation method.
The wide variety of different device efficiencies, morphologies, and especially the energy level values and the literature is what caught our attention and motivated this work. It is crucial to ensure a high degree of reproducibility. All processing steps and characterization methods should be performed under an inert atmosphere to avoid the gradation of the perovskite by water exposure.
And you shouldn't forget that working with lead iodide can be extremely hazardous, and therefore precautions, such as working in a glove box, should always be taken while performing these procedures. After watching this video, you should have a good understanding of the influences the different preparation methods for perovskites have on the crystal structures, density of states, energy levels, and ultimately the solar cell performance. With our approach, what we want to do is provide insight into correlations between film preparation on the one hand, and physical properties on the other to ultimately influence solar cell performance.
Of particular interest is the possibility of adjusting the ionization energy of the perovskite films by intentional incorporation of MAI or lead iodide interstitialis. This can be used for interface optimization in novel device architectures. Future techniques will look at more advanced preparation techniques that aim towards large device areas including methods like slot eye coating, spray techniques, and large scale printing.
