Engineering
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Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices
Chapters
Summary July 8th, 2016
This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.
Transcript
The overall goal of this procedure is to deposit spin and spray-coated films of inorganic nanocrystals under ambient conditions to build fully solution-processed solar cell devices after ligand exchange and annealing. This method can help answer key questions in the fields of nanoscience and applied materials interfaces such as can we build functional electronics out of inorganic materials by preparing them on the nanoscale so that we can solution-process entire devices from the bottom up. The main advantage to this technique is that we can start thinking about spraying electronics onto new and unconventional surfaces.
This includes spraying large and irregular areas in a short time thanks to the additional freedoms of deposition. Though this method can specifically provide insight into building photovoltaic devices, it can also be applied to other systems such as spray processing of LEDs, transistors and capacitors. When synthesizing the cadmium selenide and cadmium telluride inks, follow the text protocol up to the pyridine synthesis step.
Decant the supernatent and add back 5 milliliters of distilled of pyridine and 5 milliliters of 1-propynol. Then, flush the flask with an inert gas and sonicate the mixture at 40 kilohertz for 30 minutes. It is also important to filter the ink through a one micron PTFE syringe filter and measure the concentration of the ink by desiccating 200 microliters.
Finally, dilute the ink with pyradine and 1-propynol as needed and store the ink under inert gas. In a 500 milliliter Erlenmeyer flask Stir together gold chloride trihydrate and water. Next, add prepared tetraoctylammonium bromide to the yellow mixture.
Next, add the hexanethiol ligand. Separately, mix sodium borohydride into water. Immediately add this bubbling reducing solution dropwise to the reaction flask.
After three hours of stirring, separate the organic phase with a separatory funnel. Then, use a rotovap to reduce the volume to 20 milliliters. Wash the collected ink with 50 milliliters of hexanes and 200 milliliters of methanol.
Precipitate the solids and decant the colorless supernatent. Then, air dry the solids and redisburse them in chloroform at 70 milligrams per milliliter. Preparing the proper ratios of indium tin solids for the ITO ink is crucial for this electrode to be highly conductive.
The oxide film formed after annealing is very sensitive to minor changes in relative concentrations of the precursors. To begin, in a 50 milliliter polypropylene tube combine solid salts of indium nitrate hydrate and tin chloride dihydrate with 10 milliliters of 2-methoxyethanol. To this mixture, add ammonium hydroxide to buffer the pH.
Then mix in ammonium nitrate solid to serve as an oxidizer. Sonicate the tube at 40 kilohertz in warm water for 20 to 60 minutes or until the ink changes from hazy and white to colorless and transparent. Cut a square glass slide and clean it with sonication then ethynol and acetone.
Next, put the glass into concentrated sodium hydroxide for one minute. Briefly rinse the glass with water and then spin coat ITO ink onto it. Then, immediately transfer the slide to a hot plate set to 400 degrees Celsius.
After five minutes, remove it and let it cool to room temperature on a ceramic plate. Continue layering ITO ink onto the slide until the sheet resistance is below 1, 000 ohms as measured by a multimeter or a four point probe. Finally, briefly dip the film in dilute aqua regia and rinse it with distilled water.
Once dry, the resistance should be below 500 ohms. Now, using a printed grid as a reference, arrange strips of cellophane tape to mask the layers for etching with acid. Where the tape is placed, the ITO will be retained on the glass.
Without the correct pattern, the devices will not have the desired area and the efficiency or current measurements will be incorrect. In addition, patterning assures that neighboring devices are not in contact with each other and this avoids device shorting. Next, soak the film in dilute aqua regia at 60 degrees Celsius to dissolve the exposed ITO.
After a quick rinse and dry with water, remove the tape. Then sonicate the film with acetone and ethynol to remove any tape residue. Now, add contact point for taking measurements.
Place a small drop of silver epoxy on the edge of each ITO strip at one side of the substrate square. Finally, heat the substrate at 150 degrees Celsius for two minutes and let it cool. Begin with placing the patterned ITO glass substrate on a spin coater and coating it with cadmium selenide crystal ink.
Let it dry at 150 degrees Celsius for two minutes and then dip it in an ammonium chloride with methynol solution heated to 60 degrees Celsius for 15 seconds. Then, dip the film into isopropanol. Next, dry the film under inert gas and heat it at 380 degrees Celsius for 25 seconds.
Once cooled, rinse off the excess salt with distilled water and dry the substrate under inert gas. Repeat this process until the desired thickness is reached. Unlike spin coating, spray coating has nuances of inconsistencies depending on the person spraying.
However, after months of trial and error, we have found a method that works well. Mount the ITO glass substrate vertically with tape or clips onto a flat, solid backing. Load 0.25 to one milliliter of diluted cadmium ink into a gravity-fed airbrush.
Set a higher pressure for thin, smooth films. Now, 60 millimeters away from the substrate surface, start spraying nanocrystal ink off the substrate Then, move over the substrate in rapid side to side motions keeping the spray stream perpendicular to the surface. We have much more control over the film thickness, roughness, and overall morphology with spraying.
However, due to these added freedoms, one must carefully monitor the spray distance, solution concentration, delivery pressure, and duration of the deposition to achieve consistency. Continue adding layers of nanocrystal ink until the desired thickness is achieved. Then, tape pattern the active layers.
Finally, a film of gold nanocrystals can be sprayed onto the substrate. Scanning electron microscopy was used to monitor the extent of grain growth in the annealed films. After depositing a single layer of cadmium dye and heating in the presence of ammonium chloride, the grain size was optimized by adjusting the temperature and duration of heating, the ink concentration, the spray pressure and duration, or the spin speed.
Typically, larger grains indicate devices with higher short circuit currents. UV-Vis spectroscopy is used to estimate the nanocrystal's size based on absorbence peak correlation with quantum confinement effects. Crystal size was tuned by modifying the concentration of precursors, reaction temperature, and the duration of the ink sythesis.
Optical profilometry was used to measure the film thickness and roughness. This was used on a single layer of each material and on completed devices. Fourier transform infrared spectra was taken to monitor the degress of ligand exchange during the ammonium chloride methynol treatment as measured by the disappearance of the C-H alchol stretching bands at 2, 924 and 2, 852.
Current voltage characteristics were obtained in the dark and under simulated one sun illumination from a calibrated solar simulator. A detailed explanation is provided in the text protocol. After watching this video, you should have a good understanding of how to spray deposit inorganic nanocrystals which are not typically soluble onto non-conductive substrates to build working electronic devices simply using an airbrush, prepared inks, and a heat source.
Once mastered, this technique can be done in one to two hours if it is performed properly. While attempting this procedure, it's important to remember to slowly cool your device substrates after heating to avoid cracking the glass. Don't forget that working with nanomaterials can be extremely hazardous.
Always take precautions such as conducting all spray processes in a fume hood while wearing personal protective equipment.
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