The overall goal of the following experiment is to demonstrate how light scattering by plasmonic nanoparticles enhances light trapping in thin film solar cells and improves their performance. This is achieved by depositing a precursor silver film on the rear thin film solar cell surface, and then kneeling it to fabricate a light scattering random silver nanoparticle array. As a second step, the solar cell with a nanoparticle array is coated with a magnesium fluoride dielectric layer followed by a white paint to add a diffuse rear reflector, which captures the light transmitted through the nanoparticle array To further enhance the cell photo current, the light which enters the solar cell and which is not absorbed in the first pass, gets scattered back to the cells by both the nanoparticle array and by the rear diffuse reflector at oblique angles, which enhances the optical cell thickness and thus improves light absorption Results are obtained that show the solar cell short circuit current increases by 45%in presence of the plasmonic light scattering reflector.
The main advantage of this technique of a conventional light trapping approaches based on texturing is that it can be applied to planner and fully fabricated devices, thus avoid impossible complications due to either texture related defects or incompatibility with device fabrication processes. Although this method is applied to crystalline silicon syndrome cell cells, it can also be applied to other types of sun cells and optoelectronic devices to improve their performances such as amorous, silicon, and microm film cells, organic sun cells, and even light emit diodes. Begin this protocol with fabrication of polycrystalline silicon solar cells as described in the written protocol accompanying this video.
This closeup view of a resulting cell from the two week long fabrication process shows the cell silicon surface between the metalization pattern where silicon nanoparticles will be formed. Blow the metalized cell surface with dry nitrogen to remove dust and load the sample into a thermal evaporator containing a tungsten boat filled with 0.3 to 0.5 grams of silver granules. Pump down the evaporator chamber to the base pressure of two to three by 10 to the minus five tor Next program.
The quartz crystal monitor abbreviated QCM with parameters for silver. Ensure that the sample shutter is closed and turn the tungsten boat heater on. Increase the current slowly enough to avoid a pressure rise above eight by 10 to the minus five tor until the silver granules melt as observed through a viewport After pressure stabilizes, set the current to the set point that corresponds to the silver deposition rate of 0.1 to 0.2 angstroms per second.
Open the shutter to start the deposition process. A critical aspect in the fabrication of a plus mono reflector is to precisely control the silver film thickness and a knee conditions. To form the best performing nanoparticle arrays.
Monitor the growing silver film thickness using QCM and close the shutter when a thickness of 14 nanometers is reached, allow the tungsten boat to cool down for about 15 minutes and then unload the sample the cell with a freshly deposited silver film is placed into a nitrogen purged oven preheated to 230 degrees Celsius and a kneeled for 50 minutes. Following a kneeling, a change in surface appearance is apparent due to the presence of nanoparticles. The rear reflector consists of approximately 300 nanometer thick magnesium fluoride dielectric cladding with a coat of commercial white ceiling paint.
Before fabricating the rear reflector, protect the cell contacts by applying black marker ink on them. This allows exposure of the contacts from under the dielectric by a liftoff process. Use a nitrogen gun to blow the nanoparticle array and painted contacts to remove dust.
Utilize a moderate nitrogen pressure to avoid removing weekly adhered nanoparticles. Place the sample into the thermal evaporator containing a tungsten boat filled with magnesium. Fluoride pieces.
Pump down the evaporator to pressure of two to three by 10 to the minus five tor set. QCM parameters for magnesium fluoride, ensure that the sample shutter is closed and turn on the boat. Heater slowly increase the current to avoid excessive pressurize until the magnesium fluoride melts as seen through a viewport.
After the pressure stabilizes, set the current to the set point that corresponds to the magnesium fluoride deposition rate of 0.3 nanometers per second and open the sample shutter. Monitor the deposited thickness using QCM and close the shutter when 300 nanometers is reached, turn off the heater after the tungsten boat is cooled for about 15 minutes, unload the sample. Note the change in the cell appearance with the magnesium fluoride cladding.
To remove the ink mask from the cell contacts, immerse the cell with the dielectric cladding into acetone. Wait until the dielectric above the ink starts cracking and lifting off. Keep the cell in acetone until all the ink with the dielectric is removed and the metal contacts are fully exposed.
Remove the sample from acetone. Try with a nitrogen gun. Apply a layer of white paint with a fine soft brush on the whole cell surface.
Carefully avoiding the metal contacts, the paint layer has to be thick enough to be completely opaque so that no light can be seen when looking through the painted cell at a bright light source, let the paint dry for a day. The solar cell short circuit current is calculated by integrating the external quantum efficiency or EQE curve over the standard global solar spectrum. Both the cell current and its enhancement due to light trapping depend on the cell absorber layer thickness.
The current itself is higher for thicker cells, but the current enhancement is higher for thinner devices. The original two micron thick cells without light trapping have short circuit current density measured at approximately 15 milliamps per square centimeter with diffuse back reflector. The current can be around 20 milliamps per square centimeter or 25 to 31%higher after fabrication of a nanoparticle array on the rear cell surface.
Short circuit current density increases up to about 20 milliamps per square centimeter, which is 32%enhancement, slightly better than the enhancement effect of the diffuse back reflector. Only after adding the rear diffuse reflector on the magnesium fluoride cladding to the cell with the plasmonic nanoparticle array, the short circuit current density is increased further to 22.3 milliamps per square centimeter or about 45%enhancement. Note that for the three micron thick cell, all currents are higher up to 25.7 milliamps per square centimeter while the relative enhancement is slightly lower at 42%Light trapping has a relatively larger effect in thinner devices Once mastered.
This procedure can be done within four to five hours if it is performed properly. Excluding, join the reflective paint which will take around 12 hours and room temperature. After watching this video, you should have a good idea of how plasmonic clay trapping works for solar cells.
Furthermore, you should have good understanding of how to fabricate a plasmonic like scattering reflector on the solar cells to improve light trapping in the cell photo current.
