What is a Solar Wafer?

A solar wafer is a thin slice of a crystalline silicon (semiconductor), which works as a substrate for microeconomic devices for fabricating integrated circuits in photovoltaics (PVs) to manufacture solar cells. This is also called as Silicon wafer. This wafer is very vital to photovoltaic production as well as to the power generation system of PV to convert sunlight energy directly into electrical energy.

The formation of wafers happens with highly pure (99.9999999% purity), almost defect-free single crystalline material. The solar market predominantly has polysilicon and silicon wafers. However, other types of wafers such as Monocrystalline and Multicrystalline are also used to fulfill the specific demand of customers.

When used for solar cells, after cleaning up the particles. wafers are textured to make a rough surface to increase their efficiency.

For solar system application, the wafer is made into a circular disk with high purity silicon material. When it is used for solar cells, after cleaning up the particles, wafers are being textured to make a rough surface to increase their efficiency. 

Solar batteries have silicon semiconductor, compound semiconductor, and an organic compound group. Each solar battery group includes crystal battery types such as multi-crystal solar battery and mono-crystal solar battery whereas the non-crystal battery types comprise of amorphous solar battery, 2 elements compound battery, 3 elements compound battery, and complex type of battery.

The solar market predominantly has polysilicon and silicon wafers. However, other types of wafers such as Monocrystalline and Multicrystalline are also used to fulfill the specific demand of customers.

History of Solar Wafer

Solar Wafer started when Mohamed Atalla examine and study the surface properties of silicon semiconductors at Bell Labs, during the 1950s. He adopted a new method of a semiconductor device fabrication, wherein the coating is made by a silicon wafer with a silicon oxide insulating layer. It was done to effectively penetrate the electricity to the below surface of the conducting silicon while overcoming the surface states to prevent electricity from reaching the semiconducting layer. This method is called surface passivation, which later turned critical to the semiconductor industry as it successfully made silicon mass-production with the integration of the circuits (ICs). In 1957, Atalla presented the surface passivation method and this became the basis for the metal-oxide-semiconductor (MOS) process which was then invented by Dawon Kang together with Atalla in 1959. During the year 1960, Silicon wafers were being produced on larger scales in the U.S. In some companies like MEMC and SunEdison. 

Moreover, the Wafers are formed with 99.9999999 percent highly purity which almost defect-free single-crystalline materials. There’s a process called Czochralski growth which helps in the formation of crystalline wafers. This process was invented by Jan Czochralski–a Polish chemist. During this process, the germanium or silicon called “Boule” which is a cylindrical ingot of high monocrystalline semiconductor purity is being formed by simply pulling the crystal seed from a melt. Then, an ample amount of Boron or Phosphorus (donor impurity atoms) in the silicon case will be added into the molten intrinsic material to tamper the crystal and turn it into an extrinsic semiconductor of p-type or n-type. Afterward, the boule will be sliced using a wafer saw–a type of wire saw and then polished to form a wafer. 

As to photovoltaic wafers, its typical size is 100 to 200 mm square while it has 100 to 500 μm width. On the other hand, electronics use wafer sizes ranging from 100 to 450 mm in diameter. In fact, the largest wafers that had been made have a diameter of 450 mm but these wafers are not yet used in general works. 

Solar Wafer Production Process

The manufacturing and production process of solar cells from a single crystal p-type silicon wafer has different patents and company trade processes, however, the steps below are the generalized method and process of most number of Silicon/Solar Wafer manufacturers. 

  1. Texturing- after the initial cleaning procedures, the wafer is being textured to create pyramid-like structures on the silicon surface. These pyramid-like structures made the incoming sunlight to reflect and bounce into other pyramids on the surface to improve the overall rate of sunlight absorption. 
  2. N doping (usually Phosphorous)- after texturing, a variety of methods are being utilized to dope the top surface of the p-type solar wafer to produce n-type regions. This process typically uses gas diffusion in a high-heat furnace, can create a critical p-n junction which will form as the permanent electrical grid.  
  3. Edge diffusion cleaning- the process of doping the surface of the solar wafer, causes the Phosphorous dopant to disperse to the wafer edges, and if the excess dopant remained it can cause short-circuits between the negative and positive contacts of the solar cell. So the excess dopant should be removed by an acid-etching procedure.
  4. Anti-reflective coating- to improve its light absorption, the wafer will be coated by an anti-reflective coating which is usually a silicon nitride coating. 
  5. Screen printing of front and rear surface contacts- this is the final step of the production process, the front, and rear surface contacts are being screen printed into the wafer surface to produce positive and negative contacts of the solar cell. Then, the solar cells are now ready to be wired altogether to make solar panels.

A video on how Solar Wafer is being produced: 

The Limiting Efficiency of Solar cells

The Efficiency limitation of single crystals fall at about 29 percent, it has been well established in the seminal work by Tiedje. There are two approaches used to calculate the efficiency and the limiting performance of Silicon solar cells in the past. 

The first approach calculates the efficiency with the function fo the bandgap for hypothetical semiconductors with optical absorption functional steps as well as the radiative recombination. With this approach, the limiting efficiency of silicon is made the same as the hypothetical material with a 1.12 eV bandgap. Whereas, the second approach, uses a particular structure device such as an N or P-type diffused junction cell. Then the limiting performance of the optimized device is being calculated through the known properties of Silicon. 

The typical solar cell production can achieve about 20 percent of limiting efficiency, while the solar cells from the best laboratory efforts have obtained about 25 percent. Green provided an excellent summary of the current progress of single-crystal silicon solar cell high-efficiency and attested the limiting efficiency of solar silicon is 29 percent.

Solar Wafer's Progress

Due to insufficient return on the Solar wafer investment, there will be a possible improvement in the production. Also, the proposed 450 mm in the Solar wafer size was being considerately resisted but eventually manufactured. In 2012, Chris Mack–a lithographer acclaimed that the overall price of 450 mm solar wafer per die was reduced by only 10 to 20 percent compared to the 300 mm wafers. This is due to lithography-related which made a total of 50 percent wafer processing costs. Thus, converting the wafers into 450 mm would reduce the price of each die for process operations only like the ETCH which cost is related to wafer count instead of the wafer area. On the other hand, the cost of the lithography process is only proportional to the wafer area and the larger wafers (450 mm) would not reduce its lithography contribution to the cost of the die. 

November 2013, when ASML paused the development of the 450-mm lithography equipment due to the uncertain timing of chipmaker demand. Whereas, in February 2014, the CEO of Micron Technology–Mark Durcan said that he expected that the 450 mm adoption will be totally delayed or discontinued because of the large amount of money and development that are needed to invest in the equipment to continue its production in the market. While the Intel corporation expected that 450-mm wafer will be deployed by the year 2020 and others expected it to happen between 2018 to 2020. In contrast, Dan Hutcheson, chief executive of VLSI Research did not foresee the deployment of 450 mm fabs until the year 2025.

The huge investments allotted for the 200mm wafers and 300 mm wafers, results in huge resistance to develop and adopt the 450 mm. Also, with the thought that crystal ingots will be 3 times heavier and will take a longer time to undergo cool down process compared to the smaller sizes of wafers, the 450 mm adoption is being resisted more to develop.  Also, it has been stated that the development of 450 mm wafers will require significant process, engineering works, time, and cost to deploy in the market.

Reviews on Solar Wafer

Monocrystalline vs. Polycrystalline Solar Panels – What’s the Difference?

altE Store explained the difference between Monocrystalline and Polycrystalline solar panels.

Little Green Energy Company – Wafer Production

The Little Green Energy Company Ltd. discussed how the silicon or solar wafer being processed and produced. 

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Sources

  • https://en.wikipedia.org/wiki/Wafer_(electronics)
  • http://universitywafer.com/solar-silicon-wafer.html
  • http://eandmint.co.jp/eng/solar/product_detail/product_solarwafer.html
  • http://optoelectronics.eecs.berkeley.edu/ey1984ieeeed315.pdf
  • https://www.youtube.com/watch?v=3TOpg1niATg
  • https://www.youtube.com/watch?v=TCq0K3DlFdc
  • https://www.youtube.com/watch?v=jKXtc0ADUOk

 

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