Lab 11 Silicon Photovoltaics

supplies for this lab you will need cleaved pieces of boron doped silicon spin-on phosphorus dopent a pipetter or dropper glass etent cream cotton swabs solder wires Gallum indiia mctic and tweezers step step one applying the spin-on dopent this portion of the lab requires a spin coater and a hot plate and should be performed in a fume Hood first open the lid to the spin coater you will see a black o-ring on a vacuum Chuck that will hold your sample as it spins grab a piece of boron do silicon with your tweezers now place the Silicon piece on top of the vacuum Chuck the piece should be roughly centered on the Chuck and and must completely cover the black o-ring to ensure a good vacuum seal press the blue vacuum soft key to turn on the vacuum next open up the bottle of spin-on dopent and pipe it out a small volume of solution apply several drops of the spinon dopen to the Silicon piece the drops should completely cover the surface close the spin coater lid and press the start soft key to begin spin coding for our application we have chosen a spin rate of 4,000 RPM and a spin time of 30 seconds after the spin coding is complete open up the spin coater lid and turn off the vacuum now remove the sample from the chuck with your tweezers and place it on a hot plate set to 220° C the sample should remain on the hot plate for 15 minutes the baking removes excess solvent from the applied film the spin-on film should be a blue or green tint depending on viewing angle this color is due to thin film interference step two diffusion doping using a tube furnace the furnace assembly consists of the tube furnace and the tube furnace controller the controller is on the right it Powers the tube furnace and monitors temperature using feedback from a thermocouple the tube furnace is on the left the furnace is a split tube furnace and can be opened using a latch on the side heat is provided by resistive heaters that are encased in ceramic a hollow process tube rests in between the heaters here here we are using a Mite tube quartz tubes may also be used but are more expensive the sample is loaded into the front end of the tube the sample may be loaded directly into the end of the tube using your tweezers use a rod to push the sample into the center of the tube the center of the tube is where the temperature is probed and is also the area of the Furnace that offers the best temperature uniformity after you've loaded your sample close and latch the furnace now place the cover on the tube adapter and turn on the furnace controller use the arrow soft keys to change the furnace temperature set point to 1,000° c press the blue set enter soft key to confirm the new temperature set point point the temperature should rapidly start to rise it takes the furnace approximately 15 minutes to rise from room temperature to 1,000° C at high temperatures the tube will glow a bright red diffusion do the sample at 1,000° for 2 hours once 2 hours have elapsed set the controller temperature to 50° and and allow the furnace to cool cooling will take another 2 to 3 hours once the furnace has cooled you may open it up to remove your sample remove the mulite process tube from the furnace and use the loading rod to nudge the sample the sample should easily slide out of the tube step three making electrical contact you will see that both sides of your silicon piece have coloration the color on the top side is the thin film from the spin-on dopent the color on the back side is due to a thermal oxide thin film that grew on the Silicon during the high temperature diffusion doping in order to make electrical contact to the ptype and N type sides of your cell you will need to remove these insulating dialectric films since both films are glass we can remove them by etching them with a glass Ed cream use a cotton swab to apply a small amount of etent cream to the front and back sides of your sample allow the cream to etch for at least 30 seconds and then rinse it off using deionized water you can dry the sample using compressed air there should be shiny areas of silicon where the glass has been successfully etched away set a hot plate at 300° C and place the sample on the hot plate backside facing up the back is the Boron rich ptype side of the cell next apply solder to the exposed silicon surface apply solder flux to one end of a piece of electrical wire and place it in the pool of solder now gently slide the sample to the side of the hot plate to remove it the solder will solidify as the sample cools at the edge of the hot plate the wire is now connected to the ptype side of the solar cell in order to make electrical contact to the end type side we will use gallium indium tic since the TIC is a liquid at room temperature it will allow good electrical contact with minimal damage to the PN Junction side of the cell use a cotton swab to apply a small amount of tic to the exposed n type silicon step four testing your solar cell use alligator clips to make electrical contact to the front and back of the cell here the black cable is connected from the ptype side of the cell to the common Port of the multimeter the yellow cable is connected from the end type side of the cell to the low current input Port of the multimeter the multimeter is set to measure DC current we can use a visible light source such as an LED lamp to test if the cell is working when I remove the direct source of light from the cell the multimeter measures about 60 microamps of current however when the light is shined directly at the cell the multimeter measures greater than a milliamp of current we can also measure the open circuit voltage of the cell by changing the connections to the multimeter this cell provides about 300 molts when illuminated compared to 30 microvolts when the light source is taken away

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