Abstract
An overview is given on the fabrication of the buss bars electrodes and the top grid for a crystalline Si solar cell. The technique used in the fabrication of the Solar Cell is chosen to be buried contacts. This paper demonstrates the physics and techniques used in today’s industry up to standards with the available resources. The most cost effective and most efficient way in fabricating a Si solar cell that is a square, 10 cm on each side. Given certain specification like the total loss accepted in percentage of 8 % and that the only materials that are allowed are commercial available. This paper also shows and demonstrates the physics and mathematical formulation of the Solar cell design.
Introduction
First looking on the design of the top grid we look at the impact of finger spacing on the emitter resistance, the grid resistance and the shading losses. This is an important concept in our design of the top grid since the total loss has to be below 8%. The top contact design involves minimizing the busbar resistance and finger resistance. Below is the schematic shown for a top surface contacting scheme [1]. For our design we choose a 100 cm^2 total area for the solar cell which has 2 busbars and 2400 fingers. Our mission was to design an efficient solar cell using industry techniques that are available. We choose to build the top grid and busbars by using buried contacts over conventional screen printed solar cell design because of the efficiency that was asked by our client.

Mathematics section
The given Si solar cell is a square 10 by 10 cm dimension. Our mission is to keep the sum losses approximately below 8%. Our design for the top grid with busbars is as follows. The total area is
100 cm^2. Each finger is 3.3 cm in length and is exactly 0.002 cm in width. The total amount of fingers we used is 2400 fingers. The spacing between the fingers is 0.003 cm. The graphic below shows the fingers that we used in our Solar Cell design. We only used two busbars in our design to have most efficiency in our Solar Cell design. The calculations used in our design are shown below. We looked at the efficiency of the Solar Cell which is affected by the parameters in Table of “losses affecting the parameters” shown also below [2].
Calculations:
Pmax = Voc Isc FF
Pmax = (0.65)(40)(.80) = 2.08 W
Power In = Pin = 100mW/cm^2 * 100cm^2 = 10W

Efficiency n = Voc Isc FF / Pin = (0.65) (40) (.80) / 10 = 2 mW/cm^2
Finding Characteristic Resistance:
Rch = Voc/Isc
Rch = 0.65/40 = 16.25 ohms
Graphical Illustrations of Top Grid Design
Fingers used in our design

Top View Top Grid
Tapered Buss-bars and Fingers

Calculations continued
Reflection Loss:
The wavelength of the Sun is lambda = 483 nm
R (lambda) Si = (nsi (lambda)-1) ^2 + k (lambda) ^2 / (nsi (lambda) +1) ^2 +k (lambda) ^2
Where nsi = 3.9 and k = 0.05
R (lambda) Si = (3.9(483nm)-1) ^2 + 0.05(483nm) ^2 / (3.9 (483 nm) +1) ^2 +0.05(483nm) ^2
Losses of affecting the parameter
Design and Technique Used
We first introduce the design tactics that we are going to use to design the square 10 cm Si solar cell. The chosen commercial processing technique is using buried contacts. This is placing down number of fingers and buss bars which we calculated to deem fit the given parameters. The buried contact commercial technology allows for high performance and low cost. In recent years

silicon nitride had become popular to be used in the buried technique because of low rear effective recombination velocities on lightly doped p-type substrates [1]. The simple structure buried contact solar cell structure is shown below. Since we are trying to meet the given specifications we chose to use a single crystalline which gives better quality and is more efficient than a multi-crystalline which is less expense but we get less efficiency. Table 1 shows the available options for the use of the silicon nitride in buried contacted solar cells [1].
The buried contact solar cell allows for large metal-to-width aspect ratio which is very desirable since this lets you use a larger volume of metal in the contact finger, without having a wide strip of metal on the top surface [2]. The buried contact technology gives low parasitic resistance losses because of the high metal aspect ratio.

What we selected and Why
For our solar cell we elected to us single crystalline silicone as the material used to fabricate the cell. We would then put all of the contacts (buss-bars and grids) down using the buried contact technique. The buss-bars and fingers that make up the grid would be tapered compared to the normal rectangular. The surface of the solar cell would then be texturized in the fabrication process.
The silicon wafer could be constructed out of two different types of silicon: single crystalline silicon and multi-crystalline silicon. Single crystalline silicon has a much more uniform cell structure which allows it to be a much more efficient in the solar cell application due to its higher quality. Multi-crystalline silicon tends to be a lower quality in comparison to single crystalline but the process to create a multi-crystalline wafer is simpler and therefore more cost effective.
[4] We chose to use the single crystalline structure due to its benefits in efficiency and the applications that are applicable to reduce the reflection losses.
To reduce the loss of reflected light multiple techniques can be implemented but some depend on the type of silicon that is used. All solar cells can and should have a rear metal contact to reflect the light back towards the top of the cell that is not absorbed. This in combination with surface texturing both the top surface and the rear reflector can significantly increase the amount of time the light ray is within the solar cell. As you can see in the two images below [5], when the wafer is textured both on the top contact and the rear reflector the chance of being totally internally reflected is much greater. This allows more of the light to be absorbed and used to create electricity which increases the efficiency of the solar cell. The angle at which the light rays

become totally internally reflected can be found by the equation θ = sin^-1( 𝑛2 𝑛1
) where n1 and n2 are the refraction indexes of air and silicon. The critical angle is found to be 14.4 degrees.
Reflection caused internally by surface texturing
The buried contact method creates grooves in wafer so that the buss-bars and fingers are set into the silicon wafer. This process significantly reduces the shadowing effects in comparison to the screen printing method which places the contacts on top of the silicon wafer. Both methods,

screen printing and buried contact, cost the same to operate but since a buried contact solar cell is more efficient than a screen printed solar cell it is effectively more cost effective in comparison.[6]As mentioned above the buried contact method also yields low parasitic resistance losses due to the high metal aspect ratio.
Final Design
Based on our research, parameters, and supplied information we decided that the best process to design the desired solar cell included the strategies followed below. The solar cell wafer should be manufactured out of single crystalline silicon. The single crystalline silicon should then have a rear reflector, both of which should have surface texturing applied during the manufacturing process. Next the buss-bars and finger contacts should be placed down using the buried contact method in order to significantly reduce the effects of shadowing caused by the contacts covering surface. The metal buss-bars and fingers should be tapered as described in the image above labeled “Tapered Buss-bars and fingers”. This reduces resistances lost from the contacts.
Process Used
Single Crystalline Silicon
Surface Texturing
Buried Contacts
Rear Reflector
Tapered Fingers
Reasoning
More Efficient, Higher Quality
Increase chance of internal reflection
Reduce effects of Shadowing, Increase
Efficiency
Reflect light back upward towards the top contact
Reduce losses compared to fingers of constant width

Tapered Buss-bars Reduce losses compared to Buss-bars of constant width
Based on the parameters that were provided (Current density, voltage, fill factor, energy conversion efficiency, sheet resistance, and solar cell size) we calculated different qualities such as the grid design (number buss-bars and fingers) to meet the design specifications. The parameters and calculated values can be found in the tables below.
Property
Current Density
Voltage
Fill Factor
Sheet Resistance
Energy conversion
Efficiency
Solar Cell Size
Given Parameters
Value Units
40 mA/cm^2
0.65 Volts
10 by 10
80 %
100 ohm/square
20.8 % cm
Property
Number of Buss-Bars
Number of Fingers
Finger Length
Finger Width
Total Area Consumed
Calculated Values
Value Units
2
2400
3*10^-3
3.3 cm cm
15.84 cm^2
Power Max
Power In
Efficieny
Characteristic
Resistance
2.08 Watts
10 Watts
2 mW/cm^2
16.25 ohm

Conclusion
In conclusion we have demonstrated an effective way of fabricating a Solar Cell by using the technique described above. Not only did we show how to fabricate a Solar Cell to certain standards and parameters but also showed a most cost effective method. We strongly believe after numerous research like that of Dr. Martin “Buried contacts leave a greater surface area to catch the sun’s rays and this led to a total efficiency of 24.7 % in experimental cells and around
20% for commercial made cells” [7], that buried contact technology is superior over conventional screen printed solar cell design. Buried contact technology decreases the total losses by around 25 percent over the conventional screen printing designs.

References
[1] B. Vogl et al. / Solar Energy Materials & Solar Cells 66 (2001) 17-25.
[2] Jordan D, Nagle JP. Buried contact concentrator solar cells . Progress in Photovoltaic:
Research and Applications. 1994; 2:171-176.
[3] Shard Kr. Gupta. Mathematical Formulation and Comparative Analysis of Losses in Solar cells (2004) 8-9.
[4] Stuart, . "PVeducation: Single Crystalline Silicon." PVeducation . N.p., n.d. Web. 27 Nov
2012. <http://www.pveducation.org/pvcdrom/manufacturing/single-crystalline-silicon>.
[5] Stuart, . Light Trapping: Total Internal Reflection . 2012. Simulation. PVeducation, Internet.
Web. 27 Nov 2012. <http://www.pveducation.org/pvcdrom/design/light-trapping>.
[6] Stuart, . "PVeduacation: Buried Contact Solar Cell." PV education . N.p., n.d. Web. 27 Nov
2012. <http://www.pveducation.org/pvcdrom/manufacturing/buried-contract>.
[7] Martin Green. High Efficiency Photovoltaic Cell 2-5 (1991).