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Silicon Solar Cell Grid And Bus Bar Design Engineering Essay

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In this paper, we shall be designing and explaining the process for fabricating the top grid and busbar electrodes for a crystalline silicon solar cell using commercial processing techniques, such as screen-printing. The solar cell must be kept at a minimum of 20.8% conversion efficiency and we must keep our power losses under 8% using commercially available technology and inks.

INTRODUCTION

Solar cell technology is a rapidly growing form of power generation. There are new and improved processes being developed and understood all the time. In this paper, we will introduce, design, and calculate the properties and a process of fabricating a crystalline silicon solar cell.

A solar cell is an electronic device that converts sunlight into electricity, using what is known as the photovoltaic effect. The photovoltaic effect is the basic physical process in which a PV cell converts sunlight into energy. The sunlight, containing photons, strikes the PV cell and are either reflected or absorbed. When the PV cell absorbs a photon with an adequate energy level, an electron is excited to the conduction band, which produces a "hole" in the valence band. Once the electron-hole pair is formed, the built-in electric field separates the electron from the hole and then the two charges can contribute to current flow in the PV cell. Once this "hole" is formed a built-in electric field is needed to drive the current to an external load, assuming one is connected.

The built in electric field is a property of the PV cell consisting of an n-type (negative) and a p-type (positive) semiconductor formed together. When these two materials, normally positively or negatively doped silicon, are together, they create a pathway for excess electrons in the n-type material to flow to the p-type material. This causes the "holes" to flow into the n-type material from the p-type material. Through this process of electron and "hole" flow, the electric field is created and needed to force the electrons to jump to the surface in order to become available for use in the electrical circuit as shown below in figure 1. [1]

With light, charge imbalance, generates voltage across terminals

No voltage across diode, No Current

Load connected, Current flows

Figure 1: Different stages of PV cell [2].

Problem:

Our job was to design grids and busbars, outline a fabrication process for putting down the grids and busbars on top of the crystalline silicon solar cell using commercial printing inks for making ohmic contacts, and describe the commercial processing techniques we are using, such as screen-printing. Our initial constraints are that the silicon solar cell is a square such that its dimensions are 10cm by 10cm. Also, the cell is such that, when there are no resistive or grid-shadowing losses, it produces a current density of 40 mA/cm2. The cell also produces a voltage of 0.65 V, a fill factor of 80% and an energy conversion efficiency of 20.8%. We are assuming that the sheet resistance of the p layer is 100 ohms/square. We also must calculate the shadowing loss due to the blocked sunlight from the grids and the busbar on the solar cell. In addition to those losses, we must take into account the resistive losses in the top junction layer with the cell's sheet resistance, grids and busbars. While fabricating this solar cell, the losses are to be minimized such that the sum is to be less than 8%.

MATHEMATICAL DERIVATIONS

In this section the mathematical equations for calculating various losses in the solar cell will be derived. Below in table 1 is the design parameters and constraints given for this solar cell.

Parameters given in the design project specifications

Parameter

Value

Description

Light-generated current density

Open-circuit voltage

Fill factor

Energy conversion efficiency

Sheet resistance of the p-layer

Area of the solar cell

Table 1: Design parameters for solar cell

Using the parameters given above, the optical power incident on the solar cell is given by manipulating the equation into the form which gives

.

The power generated by the cell is given by

.

The voltage at the maximum power point Vm is determined by using the following equation:

[2].

Using this value of Vm, the current at the maximum power point Im was then determined with the following equation:

The losses in the solar cell are caused by resistance in the bus bar, resistance in the fingers, resistance in the base and emitter layers, and shadowing by the bus bar and fingers. The resistivity per square of the ink used in this cell is measured at 25µm thick [5]. The resistivity of the ink is then calculated by

.

A MATLAB program was written to help determine the optimum spacing between the fingers of the solar cell. The code that was written can be seen in the back of this paper located in the appendix. The graph generated by the program is in figure 2. C:\Users\Gen\Documents\EE 332\Final Project\Finger Spacing Chart.png

Figure 2: Total Power Loss versus Finger Spacing.

From the graph we determined the optimal spacing between fingers to be 2.5mm. The power lost in each finger is given by

where Lf is the length of the finger, Sf is the spacing between fingers, ρf is the resistivity of the finger, wf is the width of the finger, and df is the height of the finger. A summary of the design parameters used here are shown in Table 2. Once the power lost in each finger is known, the total resistive losses in the fingers can be calculated by multiplying by the number of fingers

.

The power loss in the bus bar is calculated by

where Lb is the length of the bus bar, ρb is the resistivity of the bus bar, wb is the width of the bus bar, and db is the height of the bus bar.

The power loss in the emitter of the solar cell is calculated by

where ρem is the resistance per square of the emitter.

There are losses due to shadowing of the solar cell caused by the fingers and bus bar blocking light. The shadowing losses is calculated by

where Nb is the number of bus bars.

The total power losses in the solar cell is then calculated by

.

We can then determine the percentage of power losses to be

which is below the 8% loss specified for this design project.

CELL FABRICATION TECHNOLOGIES

Cell Fabrication:

When it comes to fabrication techniques of solar cells there are a few techniques to pick from. These choices mainly consist of screen printed solar cells, buried contact solar cells, high efficiency solar cells, and rear contact solar cells. For the purpose of this paper the method of choice that will be used is screen printing. The screen printing process can be a cheaper process and is often comprised of fewer steps than other methods.

Screen printing has been around for quite some time, though originally designed for printing graphics, advertisements, flyers, etc.; one of these screen printing machines can be seen in figure 2.

http://www.phoseon.com/img/Screen_Print_iStock_000008690558XSmall.jpg

Figure 2: One of the arms of a machine that holds a screen, squeegee, and ink. [3]

As far as the physical screen printing portion of semiconducting devices or in our case a solar cell it is a pretty simple idea. A screen is created in the form of a negative just as film in a camera. Certain areas of the screen allow ink to pass through and others do not. This screen can be made of different materials such as silk, metal, plastics. The screen is used to hold the ink that will be transferred to the target object, in this particular scenario a wafer of crystalline silicon or c-Si.

Saw damage etch:

After the blocks of silicon are cut into wafers, roughly .5mm thick, the saw leaves behind a residue that was used as a coolant during the cutting process. [2] The wafers get loaded into cassettes so that they can be cleaned as show in figure 3. The wafers are then cleaned in a hot sodium hydroxide bath to remove the contamination left behind. This process removes the first 10 micrometers of silicon which was damaged during the cutting process. Once this has taken place the silicon is then placed into another, more diluted, sodium hydroxide bath which has been mixed with isopropanol as a wetting agent to etch the surface of the silicon wafers. This process gives a very uniform etch rate when c-Si wafers are used as the primary substrate.

http://www.pveducation.org/sites/default/files/PVCDROM/Manufacturing/Images/MVC-420F.JPG

Figure 3: Multicrystalline silicon wafers in cassettes are ready to be cleaned [2].

Junction formation by doping:

The wafers have now been cleaned and are ready to be doped. This process will create an n-type layer on the c-Si. This n-type layer is created by applying a phosphorus coating to the silicon and then firing the wafers. The wafers are in what is called a diffusion coating furnace where the layer of phosphorus is deposited on to the wafers. The wafers are then moved to a different furnace where they are fired at a temperature roughly 800-1000 degrees Celsius. The firing process incorporates the phosphorus layer into the outer surface of the silicon wafer.

Edge isolation:

Edge isolation is a process in which the newly doped silicon wafers have the front and back sides isolated from each other. This is done by first stacking the wafers together and then loading them in to a plasma etching system. In this system the sides of the wafers will be plasma etched by using CF4 and O2. This process will remove the phosphorus dopant from the sides of the wafers thus separating the two n-type surfaces encompassing the silicon.

Anti-reflection coating:

One more step is needed before the wafers can begin the screen printing process and this is the application of an anti-reflection coating. This is a very important step because it is this coating that helps to reduce the amount of reflected photons, which in turn helps increase the efficiency of the solar cell.[4]

For this step silicon nitride is used via a process called chemical vapor deposition process or CVD. During this process the chemicals are fed into the deposition chamber and they break down and adhere to the wafer. The actual chemical process for this is 3SiH4 + 4NH3 -> Si3N4 + 12H2. [2]

Screen printing the rear contact:

Now that the wafers have been doped, the front and back have been isolated from each other, and they have been coated with an anti-reflection coating they are now ready to receive the rear contacts. This is done through the screen printing process discussed earlier on in this paper. A screen carrying a metal paste is lowered over the top of the solar cell and then a squeegee pushes the paste through holes, transferring it to the cell in the desired pattern, this can be seen below in figure 4. This process is usually repeated twice for both the front and back sides of the solar cell, each of which both distributing different patterns to the solar cell. The first usually deposits an aluminum mask across the back of the cell to create the back surface field. [2]

Figure 4: Squeegee pushing metallic ink across a screen. [2]

The cell is then dried in an oven to remove all of the organic solvents and binders before the next screen takes place. The second screen on the back of the solar cell deposits a silver paste which is used for the contacts on the solar cell.

After performing the second screen print on the back of the solar cell it is then placed in to an oven and fired at a higher temperature. During this process the metal and wafer are heated to a high enough temperature to destroy the n-layer allowing the metals printed on the cell to become in contact with the p-type silicon itself.

Screen printing the front contact:

Now that we have printed the back side of the solar cell it gets flipped over for the printing of the conductive fingers and the main busbars. This is the exact same process as explained earlier when printing the rear side of the solar cell except for the fact that the screens will have different designs to create the buss bar and finger system as mentioned. For the purpose of this paper we will be using a DuPont Solamet PV412 ink. The properties of this ink can be seen below in figure 5.

Figure 5: Specification sheet for DuPont Solamet PV412 ink. [5]

In between these two steps the wafers will be put into a drier around 200 degrees Celsius to dry the ink as said before on the back side. Once the second screen is transferred the wafer will again go back into an oven to fire the chosen metal paste into the silicon. A finished image of the front of a solar cell is shown in figure 6.

http://www.ecn.nl/typo3temp/pics/db94f2dc2b.png

Figure 6: Front of a finished solar cell [6]

TECHNOLOGIES CHOSEN FOR THIS APPLICATION

Why screen printing is used:

Screen printing technology, where a metal-containing conductive paste is forced through the openings of a screen onto a wafer to form the circuits or contacts, is one of the best established and one of the most mature forms of solar cell fabrication technology. [7] First developed in the 1970's, screen-printing is currently the dominant form of fabrication of photovoltaic modules. [2] While screen printing does reduce the efficiency of a solar cell by 3.5-4% when comparing it to the best and more expensive methods, however, there are advantages to this fabrication method. [6] Some of the key advantages are the technology's simplicity in fabricating the solar cell, its cost-effectiveness and its ease of control.

Why silicon is used:

Currently, crystalline silicon is the dominant material used in the photovoltaic market, even though silicon does not have optimum material parameters. Specifically, its band gap is a little too low for an optimum solar cell and it has a low absorption coefficient. Due to silicon's abundance and its prominence in the manufacturing industry, silicon still makes it difficult for other materials to compete in this market. [8]

Why silver is used:

Many factors come into play when deciding the material to be used in the solar cell. Some factors include the materials efficiency, its conductivity, its adhesion strength, as well as its cost effectiveness. Silver, being a great conductor of electricity, is used in the contact points. "A well-formulated silver paste could have almost 50% higher conductivity compared to other pastes." [9] Silver also has great adhesion strength that meets the general costumer requirements.

FINAL FABRICATION PROCESS DESCRIPTION

Table 2 is a summary of the final design parameters for this solar cell.

Table 2: Final Design Parameters

Parameter

Value

Description

Voltage at maximum power output

Current density at max power output

Maximum power output without losses

Resistance per square of the emitter [10]

Resistance per square of the ink [5]

Resistivity of the ink

Spacing between fingers

Width of the fingers

Height of the fingers

Length of the fingers

80

Number of fingers

Length of the bus bar

Width of the bus bar

Table 3 is a summary of the calculated losses of the solar cell associated with the grid and bus bars.

Table 3: Calculated Power Losses

Power losses due to shadowing

Power losses in the emitter

Power losses in the fingers

Power losses in the bus bar

Total power losses

Percentage power losses

The Final Fabrication

After all of the multiple steps of screen printing are completed and the wafers have been fired at the appropriate temperatures for the correct amount of time, the solar cells are complete. Keep in mind that the stages of manufacturing where the wafers are fired must be done correctly to get the desired solar cell. If the temperature is to high this will cause the metals to melt together and make contact with others causing possible short circuit or giving the wafer improper properties. After this process is successfully completed, the cells can then have leads soldered to the back sides of them. This completes production of the physical solar cell and just leaves the process of soldering leads to the back side of the wafers which will allow the cells to be wired in series to achieve the desired output voltage.

In conclusion the initial parameters included a minimum of 20.8% conversion efficiency and the power loss was to be kept under 8% by using commercially available inks and technology. We were able to achieve a conversion efficiency of 20.8% and we were able to reduce our power lost to 4.64% by screen printing our c-Si solar cells and also reducing the width while increasing the height of the fingers and busbars.

REFERENCES

[1] Reprinted from U.S. Department of Energy Photovoltaics Program. [Online]. Available:

http://inventors.about.com/library/inventors/blsolar3.htm [Nov. 15, 2012]

[2] Christiana Honsberg, Stuart Bowden. Photovoltaic Education Network [Online]. Available: http://www.pveducation.org/ [Nov. 1, 2012] .

[3] "Phoseon Technology Screen Printing". [Online]. Available: http://www.phoseon.com/applications/screen-printing.htm [Nov. 4, 2012]

[4] Daniel Nilsen Wright. (2008, Jan, 16) Anti-reflection coatings for silicon solar cells [Online]. Available: http://www.sintef.no/uploadpages/24509/Daniel%20N%20Wright.pdf [Nov 3, 2012]

[5] DuPont Solamet PV412 photovoltaic metallization [Online]. Available: http://www2.dupont.com/MCM/en_US/assets/downloads/prodinfo/Solamet_PV412.pdf

[6] Jaap Hoornstra. "Better printing delivers more solar power." ECN (Sep. 2010). [Online]. Available: http://www.ecn.nl/nl/nieuws/newsletter-en/2010/september-2010/two-stage- screen-printing-improves-solar-cell/ [Nov. 6, 2012]

[7] Applied Materials. (April 2011). Screen Printing For Crystalline Silicon Solar Cells. [Online]. Available: http://www.appliedmaterials.com/sites/default/files/ [Nov. 15, 2012]

[8] Jessika Toothman and Scott Aldous. How Solar Cells Work. [Online]. Availabe: http://science.howstuffworks.com/environmental/energy/solar-cell2.htm [Nov. 15, 2012]

[9] Dr. Weiming Zhang. How Silver Paste Can Improve Silicon Solar Cell Performance/Cost Ratio. [Online]. Available: http://pvsilverpaste.com/media/webmedia_local/media/whitepapers/HowSilverPasteCanI mproveYourPerformance.pdf [Nov. 15, 2012]

[10] Yelundur, V.; Nakayashiki, K.; Hilali, M.; Rohatgi, A.; , "Implentation of a homogeneous high-sheet-resistance emitter in multicrystalline silicon solar cells," Photovoltaic Specialists Conference, 2005. Conference Record of the Thirty-first IEEE , vol., no., pp. 959- 962, 3-7 Jan. 2005 URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1488291& isnumber=31426


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